Process for the preparation of acetic acid

Abstract

The present application provides a method capable of reducing the concentration of formic acid in an acetic acid product using a simple means. In the production method of acetic acid of the present application, at least one of the processes having an operation condition selected from the group consisting of (i) an operation condition in which the hydrogen partial pressure is lower than 500 kPa (absolute pressure), the carbon dioxide partial pressure is lower than 70 kPa (absolute pressure), and the operation temperature exceeds 150°C, and (ii) an operation condition in which the hydrogen partial pressure is 5 kPa (absolute pressure) or lower, the carbon dioxide partial pressure is lower than 20 kPa (absolute pressure), and the operation temperature exceeds 100°C is included in the production process of acetic acid, and the oxygen concentration is controlled in such a manner as to satisfy at least one selected from the group consisting of (iii) the oxygen concentration in the gas phase is controlled to be lower than 7% by volume, and (iv) the oxygen concentration in the liquid phase is controlled to be lower than 7 x 10 ‑5 g / g.

Description

Technical Field

[0001] This invention relates to a method for preparing acetic acid. Background Technology

[0002] As an industrial method for preparing acetic acid, the methanol carbonylation process (methanol-acetic acid process) is known. In this process, for example, methanol and carbon monoxide can be reacted in a reaction tank in the presence of a catalyst to produce acetic acid, the reaction mixture can be evaporated in an evaporation tank, and the vapor phase can be purified by a de-boiling tower and then by a dehydration tower to produce acetic acid. Alternatively, acetic acid can be produced by passing through a de-boiling tower after the dehydration tower, or further through a product tower.

[0003] In this acetic acid production process, formic acid is produced as a byproduct in the reaction tank. Since formic acid reduces the purity of the finished acetic acid, it is best to minimize its production. Patent documents 1 and 2 disclose that formic acid is produced by the reaction of carbon monoxide and water; therefore, by controlling the water concentration in the reaction medium to a low level, the formic acid concentration in the finished acetic acid can be reduced. However, when the water concentration in the reaction medium is reduced, there is a problem that the catalyst may become unstable.

[0004] Existing technical documents Patent documents Patent Document 1: U.S. Patent Application Publication No. 2008 / 0293966 Patent Document 2: U.S. Patent Application Publication No. 2008 / 0293967 Summary of the Invention

[0005] The problem the invention aims to solve Therefore, the object of the present invention is to provide a method for reducing the concentration of formic acid in acetic acid products using simple means.

[0006] In addition, other objects of the present invention are to provide a method for preparing acetic acid that can effectively prevent local corrosion of the acetic acid preparation apparatus, and a method for preparing acetic acid that can effectively suppress the coloring of the acetic acid product.

[0007] Problem Solving Methods To achieve the aforementioned objectives, the inventors conducted in-depth research to explore the mechanism of formic acid formation, resulting in the following insights: Formic acid is generated in considerable quantities in reaction tanks, evaporation tanks, and de-boiling towers, where hydrogen and carbon dioxide are primarily present; higher hydrogen and carbon dioxide partial pressures lead to greater formic acid formation; and higher temperatures inhibit formic acid formation. These insights suggest the existence of the equilibrium reaction H₂ + CO₂ ⇔ HCOOH; and so on. Furthermore, the following insights were obtained: It can be inferred that various factors, such as externally introduced components into the process, can lead to the introduction of oxygen into the process stream. In the presence of methanol and oxygen, formaldehyde is generated, and further, in the presence of oxygen, formic acid is formed from the generated formaldehyde. Based on this, further research revealed that: to suppress the formation of formic acid, it is preferable to maintain a low hydrogen partial pressure, a low carbon dioxide partial pressure, a high temperature, and further maintain a low oxygen partial pressure; by recycling the process liquid containing formic acid to the reaction tank, evaporator, and distillation column, and maintaining a low hydrogen partial pressure, a low carbon dioxide partial pressure, and a high temperature, formic acid can be decomposed; since formic acid has a lower boiling point than acetic acid, it will concentrate at the top of each distillation column. Therefore, by recycling the top liquid of the distillation column to the reaction system or to a distillation column located upstream of that distillation column, formic acid can be decomposed; and so on. This invention is based on these insights and was completed through further repeated research.

[0008] That is, the present invention provides a method for preparing acetic acid, wherein the acetic acid preparation process includes at least one step selected from the steps of satisfying the operating conditions of (i) below and the steps of satisfying the operating conditions of (ii) below, and for more than one process, the oxygen concentration is controlled in a manner satisfying at least one of (iii) below and (iv) below.

[0009] (i) Operating conditions with hydrogen partial pressure below 500 kPa (absolute pressure), carbon dioxide partial pressure below 70 kPa (absolute pressure), and operating temperature above 150°C. (ii) Operating conditions where the hydrogen partial pressure is below 5 kPa (absolute pressure), the carbon dioxide partial pressure is below 20 kPa (absolute pressure), and the operating temperature exceeds 100°C. (iii) Control the oxygen concentration in the gas phase to below 7% by volume. (iv) Control the oxygen concentration in the liquid phase to below 7 × 10⁻⁶. -5 g / g In (ii) above, the hydrogen partial pressure can also be below 1 kPa (absolute pressure) and the carbon dioxide partial pressure can be below 2 kPa (absolute pressure).

[0010] In the method for preparing acetic acid of the present invention, a reaction step may be included that satisfies the operating conditions described in (i) above. In this case, the concentration of acetic acid in the reaction mixture of the above reaction step may be 30% by mass or more, and the concentration of formic acid may be 102 ppm by mass or less. Alternatively, the concentration of acetic acid in the reaction mixture of the above reaction step may be 50 to 90% by mass, the concentration of metal catalyst (in metal conversion) may be 200 to 10,000 ppm by mass, the concentration of iodomethane may be 1 to 20% by mass, the concentration of ionic iodide may be 1 to 25% by mass, the concentration of water may be 0.1 to 15% by mass, the concentration of methyl acetate may be 0.1 to 30% by mass, and the concentration of formic acid may be 102 ppm by mass or less.

[0011] In the method for preparing acetic acid of the present invention, an evaporation step or a distillation step may be included, which satisfies the operating conditions described in (ii) above. In the evaporation step, the concentration of acetic acid in the feed liquid to the evaporation tank may be 50-90% by mass, the concentration of the metal catalyst (in metal conversion) may be 200-10000 ppm by mass, the concentration of iodomethane may be 1-20% by mass, the concentration of ionic iodide may be 1-25% by mass, the concentration of water may be 0.1-15% by mass, the concentration of methyl acetate may be 0.1-30% by mass, and the concentration of formic acid may be less than 10000 ppm by mass. In the distillation step, the concentration of acetic acid in the feed liquid to the distillation column may be 30% by mass or more, and the concentration of formic acid may be 5 ppm by mass or more. Alternatively, the concentration of acetic acid in the feed liquid to the distillation column may be 40-85% by mass, the concentration of iodomethane may be 2-50% by mass, the concentration of water may be 0.2-20% by mass, the concentration of methyl acetate may be 0.2-50% by mass, and the concentration of formic acid may be 5-10000 ppm by mass. Furthermore, in the above-mentioned distillation process, the concentration of acetic acid in the feed liquid to the distillation column may be 80-99.9% by mass, the concentration of iodomethane may be 0.01-16% by mass, the concentration of water may be 0.05-18% by mass, the concentration of methyl acetate may be 0.01-16% by mass, and the concentration of formic acid may be 5-10000 ppm by mass. Alternatively, in the above-mentioned distillation process, the concentration of acetic acid in the feed liquid to the distillation column may be 99.1-99.999% by mass, and the concentration of formic acid may be 5-9000 ppm by mass.

[0012] In (iii) above, the ratio of oxygen to carbon monoxide in the gas phase is preferably 2% by volume or less. Furthermore, in (iv) above, the ratio of oxygen to carbon monoxide in the liquid phase is preferably 2% by volume or less.

[0013] In (iii) and / or (iv) above, it is preferable to introduce at least one component selected from oxygen-containing gas, oxygen-containing compound, and oxygen-generating agent, and to control the oxygen concentration in the gas phase in (iii) above to be 1 volume ppt or more, and / or to control the oxygen concentration in the liquid phase in (iv) above to be 0.1 × 10⁻⁶. -9 g / g or more.

[0014] In (iii) and / or (iv) above, the oxygen concentration is preferably controlled to be less than 0.25 moles relative to the total amount of hydrogen iodide and iodomethane.

[0015] In the method for preparing acetic acid of the present invention, the gas phase in (iii) above and / or the liquid phase in (iv) above may be the gas phase and / or liquid phase in the reaction step, evaporation step, or distillation step.

[0016] In the method for preparing acetic acid of the present invention, the acetic acid preparation process may include: a carbonylation reaction step in which methanol reacts with carbon monoxide to generate acetic acid; an evaporation step in which the reaction mixture obtained in the carbonylation reaction step is separated into a vapor stream and a residual liquid stream; and a de-boiling step in which the vapor stream is distilled to separate into a column overhead stream enriched with low-boiling components and a first acetic acid stream enriched with acetic acid; or, in addition to these steps, at least one of the steps (a) to (d) below.

[0017] (a) A dehydration process in which the first acetic acid stream is distilled to separate a water-rich overhead stream and a second acetic acid stream that is more acetic acid-rich than the first acetic acid stream. (b) A de-boiling process that separates the first or second acetic acid stream into a bottoms stream enriched with high-boiling components by distillation, and a third acetic acid stream that is more enriched with acetic acid than the acetic acid stream before distillation. (c) An adsorption removal process in which the first, second, or third acetic acid stream is treated with an ion exchange resin to obtain a fourth acetic acid stream. (d) A process of distilling the first, second, third, or fourth acetic acid stream to obtain a fifth acetic acid stream that is more enriched in acetic acid than the acetic acid stream before distillation. In this case, the carbonylation reaction step described above can satisfy the operating conditions described in (i). Additionally, at least one step selected from the evaporation step, the low-boiling point removal step, the dehydration step, the high-boiling point removal step, and the product step described above can satisfy the operating conditions described in (ii).

[0018] In the method for preparing acetic acid of the present invention, it is preferable that the residence time in the step that satisfies the operating conditions described in (i) above or the step that satisfies the operating conditions described in (ii) above is 1 minute or more.

[0019] In the method for preparing acetic acid of the present invention, the gas phase and / or liquid phase selected from at least one of the carbonylation reaction step, evaporation step, de-low boiling step, dehydration step, de-high boiling step and product step is preferably the gas phase and / or the liquid phase in (iii) above and (iv) above.

[0020] In the acetic acid preparation method of the present invention, the process liquid with a formic acid concentration of 10 ppm or more can be recycled to a process that meets the operating conditions of hydrogen partial pressure below 500 kPa (absolute pressure), carbon dioxide partial pressure below 70 kPa (absolute pressure), and operating temperature above 100°C.

[0021] In the method for preparing acetic acid of the present invention, the acetic acid preparation process may include at least one distillation step, and the overhead liquid of the distillation column in the at least one distillation step is recycled to a step that satisfies the operating conditions described in (i) above and / or a step that satisfies the operating conditions described in (ii) above. In this case, the destination of the recycled overhead liquid of the distillation column may be a reaction step and / or an evaporation step or distillation step located upstream of the distillation step carried out in the distillation column.

[0022] The effects of the invention According to the present invention, since there is a process that satisfies specific operating conditions, it is possible to suppress the formation of formic acid or to effectively decompose the formed formic acid. Therefore, it is possible to easily reduce the formic acid concentration in the acetic acid product. Attached Figure Description

[0023] [ Figure 1 The diagram illustrates a process flow chart for preparing acetic acid according to one embodiment of the present invention.

[0024] [ Figure 2 A schematic flowchart of an example acetaldehyde separation and removal system is shown.

[0025] [ Figure 3 A schematic flowchart of another example of an acetaldehyde separation and removal system is shown.

[0026] [ Figure 4 A schematic flowchart of another example of an acetaldehyde separation and removal system is shown.

[0027] [ Figure 5 A schematic flowchart of another example of an acetaldehyde separation and removal system is shown. Detailed Implementation

[0028] In the method for preparing acetic acid of the present invention, the acetic acid preparation process includes at least one step selected from the steps that satisfy the operating conditions described in (i) below and the steps that satisfy the operating conditions described in (ii) below, and for more than one process, the oxygen concentration is controlled in a manner that satisfies at least one of the conditions described in (iii) below and (iv) below.

[0029] (i) Operating conditions with hydrogen partial pressure below 500 kPa (absolute pressure), carbon dioxide partial pressure below 70 kPa (absolute pressure), and operating temperature above 150°C. (ii) Operating conditions where the hydrogen partial pressure is below 5 kPa (absolute pressure), the carbon dioxide partial pressure is below 20 kPa (absolute pressure), and the operating temperature exceeds 100°C. (iii) Control the oxygen concentration in the gas phase to below 7% by volume. (iv) Control the oxygen concentration in the liquid phase to below 7 × 10⁻⁶. -5 g / g It should be noted that the operating conditions described in (i) above in the process of preparing acetic acid by continuous operation are operating conditions under continuous operation, for example, operating conditions that remain essentially unchanged and stable even when process conditions may sometimes change. The same applies to operating conditions that meet (ii), (iii), and (iv) above.

[0030] In a process that satisfies the operating conditions described in (i) or (ii) above, the formation of formic acid can be effectively suppressed while the formic acid in the supply liquid to that process is efficiently decomposed. This is presumably due to the existence of the equilibrium reaction H₂ + CO₂ ⇔ HCOOH, which shifts to the left under the above operating conditions. The process satisfying the above operating conditions can be a reaction process, any process included in the separation processes described later (e.g., evaporation process, distillation process, etc.), or any process not included in the separation processes. It should be noted that in this specification, "distillation process" refers to the process of distilling acetic acid, and examples include, for instance, the removal of low-boiling-point processes, dehydration processes, removal of low-boiling-point dehydration processes, removal of high-boiling-point processes, and product processes described later.

[0031] It should be noted that, in this specification, "hydrogen partial pressure" and "carbon dioxide partial pressure" refer to the partial pressure of the component in the gas phase of the apparatus or equipment (reactor, evaporator, distillation column, etc.) used in this process. In a distillation column, the partial pressure in the gas phase of at least one section (e.g., the bottom section, the feed section, or the top section) is acceptable if it falls within the aforementioned range, but it is preferable that the partial pressure of the gas phase in each section from the feed section to the top section is within the aforementioned range, and even more preferably, the partial pressure of the gas phase in each section from the bottom section to the top section is within the aforementioned range. Additionally, "operating temperature" refers to the temperature of the liquid phase or gas phase of the apparatus or equipment (reactor, evaporator, distillation column, etc.) used in this process. In a distillation column, the temperature of the liquid or gas phase in at least one section (e.g., the bottom section, the feed section, or the top section) is within the above-mentioned range, but it is preferable that the temperature of the liquid or gas phase in each section from the feed section to the top section is within the above-mentioned range, and even more preferably that the temperature of the liquid or gas phase in each section from the bottom section to the top section is within the above-mentioned range.

[0032] In (i) above, the hydrogen partial pressure (absolute pressure) is preferably below 500 kPa, more preferably below 400 kPa, more preferably below 300 kPa, further preferably below 200 kPa, and particularly preferably below 150 kPa. The lower limit of the hydrogen partial pressure (absolute pressure) is 0 kPa, but from the viewpoint of utilizing hydrogen to improve catalytic activity, the hydrogen partial pressure (absolute pressure) may exceed 1 kPa (or exceed 5 kPa). The carbon dioxide partial pressure (absolute pressure) is preferably below 70 kPa, more preferably below 60 kPa, more preferably below 50 kPa, further preferably below 40 kPa, and particularly preferably below 30 kPa. The lower limit of the carbon dioxide partial pressure (absolute pressure) is 0 kPa. It should be noted that carbon dioxide and hydrogen are present in the carbon monoxide used as a feedstock for the methanol carbonylation reaction and are generated in the reaction vessel through the water-gas shift reaction. Therefore, using carbon monoxide with extremely low partial pressures of carbon dioxide and hydrogen in the feedstock carbon monoxide is economically unfavorable. Therefore, the lower limit of the partial pressure (absolute value) of carbon dioxide can be 2 kPa (or 20 kPa). The operating temperature can be above 150°C, for example above 160°C, preferably above 175°C, more preferably above 178°C, further preferably above 181°C, and particularly preferably above 184°C. The upper limit of the operating temperature is, for example, 250°C, preferably 230°C, and more preferably 200°C.

[0033] In (ii) above, the hydrogen partial pressure (absolute pressure) can be 5 kPa or less, preferably 4 kPa or less, more preferably 3 kPa or less, further preferably 2 kPa or less, and particularly preferably 1 kPa or less. The lower limit of the hydrogen partial pressure (absolute pressure) is 0 kPa, but it is economically undesirable to completely remove hydrogen that may sometimes mix into the reaction mixture; therefore, the hydrogen partial pressure (absolute pressure) can exceed 0.0001 kPa. The carbon dioxide partial pressure (absolute pressure) can be less than 20 kPa, preferably 18 kPa or less, more preferably 16 kPa or less, further preferably 14 kPa or less, and particularly preferably 12 kPa or less. The lower limit of the carbon dioxide partial pressure (absolute pressure) is 0 kPa, but it is economically undesirable to completely remove carbon dioxide that may sometimes mix into the reaction mixture; therefore, the carbon dioxide partial pressure (absolute pressure) can exceed 0.0001 kPa. The operating temperature can be a temperature exceeding 100°C, preferably 102°C or more, more preferably 104°C or more, further preferably 106°C or more, and particularly preferably 112°C or more. The upper limit of the operating temperature is, for example, 250°C, preferably 200°C, and more preferably 175°C.

[0034] In (ii) above, the hydrogen partial pressure (absolute pressure) may be 1 kPa or less, and the carbon dioxide partial pressure (absolute pressure) may be less than 2 kPa. In this case, the upper limit of the hydrogen partial pressure (absolute pressure) is preferably 0.9 kPa, more preferably 0.8 kPa. The lower limit of the hydrogen partial pressure (absolute pressure) is 0 kPa, but it is also possible for the hydrogen partial pressure (absolute pressure) to exceed 0.0001 kPa. The upper limit of the carbon dioxide partial pressure (absolute pressure) is preferably 1.8 kPa, more preferably 1.5 kPa, further preferably 1.0 kPa, and particularly preferably 0.5 kPa. The lower limit of the carbon dioxide partial pressure (absolute pressure) is 0 kPa, but it is also possible for the carbon dioxide partial pressure (absolute pressure) to exceed 0.0001 kPa.

[0035] As a process that satisfies the operating conditions described in (i) above, a reaction process can be cited as an example. In this case, it is preferable that the concentration of acetic acid in the reaction mixture of the above reaction process is 30% by mass or more (e.g., 30 to 90% by mass) and the concentration of formic acid is 102 ppm by mass or less (0 to 102 ppm by mass). Further preferably, the concentration of acetic acid in the reaction mixture of the above reaction steps is 50-90% by mass (e.g., 60-80% by mass), the concentration of metal catalyst (in metal conversion) is 200-10000 ppm by mass (e.g., 300-5000 ppm by mass, preferably 400-2000 ppm by mass), the concentration of iodomethane is 1-20% by mass (e.g., 5-15% by mass), the concentration of ionic iodide is 1-25% by mass (e.g., 5-20% by mass), the concentration of water is 0.1-15% by mass (e.g., 0.8-10% by mass), the concentration of methyl acetate is 0.1-30% by mass (e.g., 1-10% by mass), and the concentration of formic acid is below 102 ppm by mass (e.g., 0-85 ppm by mass).

[0036] Examples of processes that satisfy the operating conditions described in (ii) above include evaporation and distillation processes. It should be noted that the distillation process described above may be a process included in the separation processes described later, or it may be a process not included in the separation processes. In the evaporation process that satisfies the operating conditions described in (ii) above, the concentration of acetic acid in the feed liquid to the evaporation tank may be 50-90% by mass (e.g., 60-80% by mass), the concentration of metal catalyst (in metal conversion) may be 200-10000 ppm by mass (e.g., 300-5000 ppm by mass, preferably 400-2000 ppm by mass), the concentration of iodomethane may be 1-20% by mass (e.g., 5-15% by mass), the concentration of ionic iodide may be 1-25% by mass (e.g., 5-20% by mass), the concentration of water may be 0.1-15% by mass (e.g., 0.8-10% by mass), the concentration of methyl acetate may be 0.1-30% by mass (e.g., 1-10% by mass), and the concentration of formic acid may be less than 10000 ppm by mass (e.g., 0-1000 ppm by mass, preferably 10-500 ppm by mass, more preferably 15-200 ppm by mass, and even more preferably 20-100 ppm by mass).

[0037] In the distillation process that satisfies the operating conditions described in (ii) above, the concentration of acetic acid in the feed liquid of the distillation column is 30% by mass or more (e.g., 30 to 99.999% by mass), and the concentration of formic acid is 1 ppm by mass or more (e.g., 5 ppm by mass or more, preferably 5 to 10,000 ppm by mass). Alternatively, in the distillation process described above, the concentration of acetic acid in the feed liquid of the distillation column may be 40 to 85% by mass (e.g., 50 to 75% by mass), the concentration of iodomethane is 2 to 50% by mass (e.g., 5 to 30% by mass), the concentration of water is 0.2 to 20% by mass (e.g., 1 to 15% by mass), the concentration of methyl acetate is 0.2 to 50% by mass (e.g., 2 to 30% by mass), and the concentration of formic acid is 1 ppm by mass or more (e.g., 5 to 10,000 ppm by mass, preferably 10 to 1,000 ppm by mass, more preferably 10 to 500 ppm by mass, further preferably 15 to 200 ppm by mass, particularly preferably 20 to 100 ppm by mass). Furthermore, in the above-mentioned distillation process, the concentration of acetic acid in the feed liquid of the distillation column is 80-99.9% by mass (e.g., 90-99.9% by mass, preferably 93-99% by mass), the concentration of iodomethane is 0.01-16% by mass (e.g., 0.1-8% by mass, preferably 0.2-5% by mass), the concentration of water is 0.05-18% by mass (e.g., 0.1-8% by mass, preferably 0.2-5% by mass), the concentration of methyl acetate is 0.01-16% by mass (e.g., 0.1-8% by mass, preferably 0.2-5% by mass), and the concentration of formic acid is 1% by mass or more (e.g., 5-10000 ppm by mass, preferably 10-1000 ppm by mass, more preferably 10-500 ppm by mass, further preferably 15-200 ppm by mass, particularly preferably 20-100 ppm by mass). Furthermore, in the above-mentioned distillation process, the acetic acid concentration in the feed liquid of the distillation column in which the distillation process is carried out may be 99.1 to 99.999% by mass, and the formic acid concentration may be 1% by mass or more (e.g., 5 to 9000 ppm by mass, preferably 10 to 1000 ppm by mass, more preferably 10 to 500 ppm by mass, further preferably 15 to 200 ppm by mass, and particularly preferably 20 to 100 ppm by mass).

[0038] Furthermore, by controlling the oxygen concentration in more than one process using the methods described in (iii) or (iv) above, the formation of formic acid can be effectively suppressed. This can be inferred as follows: if methanol originating from a methanol source (e.g., methanol, methyl acetate, dimethyl ether), or methanol within the process reacts with oxygen, formaldehyde will be generated due to an oxidation reaction (CH3OH + 1 / 2O2 → HCHO + H2O). If the generated formaldehyde further reacts with oxygen, an oxidation reaction (HCHO + 1 / 2O2 → HCOOH) will occur, resulting in the formation of formic acid. The aforementioned process for controlling oxygen concentration can be a reaction step, any step included in the separation steps described later (e.g., evaporation step, distillation step, etc.), or any step not included in the separation steps.

[0039] It should be noted that the gaseous or liquid phase for controlling oxygen concentration in (iii) and (iv) above refers to at least one gaseous or liquid phase in the acetic acid preparation process, representing all gaseous and liquid phases. For example, the gaseous phase could be the gaseous phase in at least one of the apparatus or equipment in the acetic acid preparation process, the exhaust gas supplied to the scrubbing system, or the gaseous phase in at least one of the reaction tank, evaporation tank, and distillation column in the acetic acid preparation process. Furthermore, regarding the gaseous or liquid phase, in the distillation column, it is sufficient that the concentration in at least one section (e.g., the bottom section, the feed section, or the top section) of the gaseous or liquid phase is within the aforementioned range. However, it is preferable that the concentration in the gaseous or liquid phase in each section from the feed section to the top section is within the aforementioned range, and even more preferably, the concentration in the gaseous or liquid phase in each section from the bottom section to the top section is within the aforementioned range.

[0040] It should be noted that, in this specification, "process" refers to the steps involved in performing process unit operations such as reaction, evaporation, distillation, cooling, condensation, separation, storage, and absorption in an acetic acid preparation apparatus, or the apparatus or equipment used to perform these process unit operations. Examples of such apparatus and equipment include piping, reaction tanks, evaporation tanks, and distillation columns. Furthermore, "process liquid" refers to the liquid phase in the process, and "process flow" refers to either the liquid or gas phase in the process.

[0041] For the oxygen concentration in the gas phase, known oxygen concentration meters can be used, such as explosion-proof process magnetic pressure oxygen analyzers (trade name "MPA-51d / p", manufactured by Horiba Corporation), separate zirconia oxygen concentration meters (trade names "ZR402G" and "ZR22G", manufactured by Yokogawa Electric Corporation), and near-infrared laser gas analyzers (trade name "SITRANS SL", manufactured by Siemens Corporation).

[0042] For the oxygen concentration in the liquid phase, known oxygen concentration meters (dissolved oxygen sensors) can be used, such as the "DO", "OC", "ODM", and "OBM" types manufactured by Toa DKK-TOA Co., Ltd., the "DO meter" manufactured by Iijima Electronics Co., Ltd., the oxygen concentration meter manufactured by Mettler Co., Ltd. which can also measure the dissolved oxygen concentration in water and solvents (methanol), and the "OX type" manufactured by Yokogawa Electric Corporation which measures the oxygen concentration in gases.

[0043] It should be noted that for gas or liquid phases with oxygen concentrations below the measurement limit, conventional methods (e.g., methods that selectively adsorb oxygen onto an adsorbent, methods that selectively allow oxygen to permeate through selectively permeable membranes such as oxygen-enriched membranes, distillation methods that separate light and heavy components, extraction methods, etc.) can be used to generate a concentrated component with concentrated oxygen from the gas or liquid phase, measure the oxygen concentration of the concentrated component, and convert the measured value into the oxygen concentration in the gas or liquid phase.

[0044] Furthermore, in this specification, the total amount of the mixture forming the gaseous and liquid phases, including impurities, is 100%. Additionally, if the gaseous mixture contains condensable components, even if it is in gaseous form under process conditions (temperature and pressure), the temperature may drop due to sampling, causing the condensable components to liquefy at room temperature and pressure (25°C, 1 atmosphere ≈ 0.1 MPa), making it impossible to accurately determine the composition of the gaseous mixture under process conditions. Therefore, the composition of the gaseous mixture is expressed as a volumetric or mass basis of the gaseous mixture at 25°C. The composition of the liquid mixture (liquid mixture) is expressed as a mass basis.

[0045] In the acetic acid production process, water may be present due to the supply of water or water generated by side reactions. For example, water may be fed into the reaction process, and the overhead stream enriched with low-boiling components from the de-boiling column (split column) may be distilled in the acetaldehyde removal column of the acetaldehyde separation and removal system to generate an acetaldehyde-enriched overhead stream. Water can be utilized in the extraction (extraction column, extractive distillation column, etc.) of this acetaldehyde-enriched overhead stream. Furthermore, in the dehydration column, aqueous solutions of alkali metal hydroxides are sometimes used to remove hydrogen iodide. Trace amounts of oxygen are also dissolved in these waters, and using these waters introduces oxygen into the process stream.

[0046] Furthermore, in the carbonylation-based acetic acid preparation process, various tanks, storage tanks, pumps, and measuring instruments (level gauges, pressure gauges, etc.) are installed between the reaction tank and the product tower. To prevent liquefaction caused by backflow of process flow (acetic acid flow, etc.) at the measuring instruments, or to prevent carbon monoxide leakage from the agitator shaft of the reaction tank, nitrogen is sometimes purged at high-pressure sealing sections. During the purging of nitrogen to these measuring instruments, nitrogen may sometimes be fed into the process, and during the pressure sealing of the agitator shaft, some of the nitrogen may leak into the reaction tank through the seal. This nitrogen also contains trace amounts of oxygen.

[0047] In addition, when oxygen is present in the process, besides the formic acid formation reaction mentioned above, it also reacts with hydrogen iodide and iodomethane in the process, releasing iodine based on oxidation reactions (2HI + 1 / 2O2 → I2 + H2O, 2CH3I + 1 / 2O2 → CH3OCH3 + I2, etc.). Furthermore, it has been found that if the generated iodine adheres to or adheres to the walls of the equipment, devices, or piping, the adhered area will be selectively or locally corroded, resulting in pitting corrosion. Additionally, typically, when the moisture concentration in the gaseous atmosphere is below 5% by mass, hydrogen iodide is concentrated to the top of the dehydration tower, dewatering tower, high-boiling-point dehydration tower, and product tower. On the other hand, it has been found that iodine, due to its higher boiling point than hydrogen iodide, often flows out together with high-boiling fractions from distillation columns (e.g., side streams from dehydration columns, bottom residue streams from dehydration columns, and side streams from product columns). This can sometimes lead to iodine contamination even in the product acetic acid, increasing the iodine concentration in the product or causing the characteristic brown to reddish-brown coloration of iodine. It should be noted that if iodine is contaminated in the product acetic acid, it will inhibit catalytic activity in the preparation of acetic acid derivatives such as vinyl acetate. Therefore, it is generally necessary to manage the iodine concentration in the product acetic acid to an extremely low concentration, below 10 ppb by mass. Furthermore, as mentioned above, sometimes methanol or alkali metal hydroxides (such as potassium hydroxide) are added to equipment such as dehydration columns to remove trace amounts of hydrogen iodide in the form of iodomethane or iodide bases (such as potassium iodide). However, in such methods, if iodine is generated from hydrogen iodide and / or iodomethane, iodine removal becomes impossible. In processes where equipment such as dehydration towers is located downstream, although the concentration of hydrogen iodide is lower, if a process stream containing iodine is exposed to a reducing gas atmosphere, hydrogen iodide can still be generated due to a reverse reaction. Therefore, if the walls of devices, equipment, or piping are formed using metals with low corrosion resistance (e.g., low-grade materials such as SUS, Hastelloy C, etc.), uniform corrosion caused by hydrogen iodide may sometimes occur, rather than localized corrosion caused by iodine.

[0048] The carbonylation process of methanol (especially the reaction system) is usually a pressurized system. Therefore, the oxygen concentration in each process stream of the acetic acid preparation unit can be adjusted by controlling the oxygen concentration of the feedstock and each feed line. For example, the oxygen concentration in carbon monoxide can be controlled by appropriately operating the carbon monoxide preparation process. For instance, the oxygen feed rate and / or steam feed rate relative to the carbon monoxide feedstock (coal, natural gas, heavy oil, asphalt, etc.) can be controlled to achieve complete partial oxidation using oxygen. Alternatively, the oxygen concentration in the purified carbon monoxide can be measured, and its usability can be determined based on the measured value. The oxygen concentration in carbon monoxide can also be controlled by feedback control of the carbon monoxide preparation process based on the measured value. Furthermore, the oxygen concentration in carbon monoxide can be controlled by introducing a non-reactive gas based on the aforementioned measured values.

[0049] The same applies to methanol. The dissolved oxygen concentration can be measured, and its usability can be determined based on this measurement. Alternatively, the dissolved oxygen concentration can be controlled by heating or other methods based on the measured value. Similarly, the dissolved oxygen concentration of water or aqueous solutions (alkaline solutions (aqueous solutions of alkali metal hydroxides), sodium hypophosphite solutions, etc.) fed into the process (reaction system, etc.) can also be measured. The dissolved oxygen concentration can be measured, and its usability can be determined based on this measurement. Alternatively, water or aqueous solutions whose dissolved oxygen concentration has been controlled by heating or other methods (e.g., water or aqueous solutions whose oxygen concentration has been reduced by boiling) can be used based on the measured value.

[0050] Furthermore, the oxygen concentration of gases and liquids fed into the process can also be measured in the same way as described above, and the oxygen concentration of the process flow can be controlled or managed based on the measured value.

[0051] Furthermore, as a method to ensure that the amount of nitrogen purging into the process stream is at the necessary minimum, the oxygen concentration in the process stream can be controlled by switching the purging gas to carbon monoxide or other inert gases.

[0052] It should be noted that in the process of the pressure reduction system, while maintaining airtightness to keep the operating pressure, a non-reactive gas can be introduced and controlled to the target pressure before operation begins. The oxygen concentration in the exhaust gas from the vacuum pump is measured to manage the oxygen concentration in the process flow of the pressure reduction system.

[0053] For both gas and liquid phase oxygen concentrations, continuous monitoring can be achieved by observing the measured values ​​of oxygen concentration meters (oxygen sensors) installed at any location in the acetic acid preparation unit, such as distillation columns and piping. Alternatively, monitoring can be performed by periodically analyzing samples taken from these locations. Furthermore, the measured value of the oxygen concentration meter can be compared with an upper limit reference value (threshold). When the measured value reaches the threshold, the oxygen concentration can be controlled by automatically introducing a fluid with a low oxygen concentration into the process flow, or by switching the introduced fluid to a fluid with a low oxygen concentration. Moreover, when the oxygen concentration drops excessively (reaching the threshold value used as a lower limit reference value), an oxygen source can also be introduced into the process flow.

[0054] In (iii) above, the oxygen concentration in the gas phase is preferably less than 7% by volume, more preferably 6.5% by volume or less (e.g., 6% by volume or less), more preferably 5.5% by volume or less (e.g., 5% by volume or less), further preferably 3% by volume or less (e.g., 1% by volume or less), particularly preferably 0.5% by volume or less (e.g., 0.1% by volume or less), and especially 0.01% by volume or less (e.g., 0.001% by volume or less, 0.0001% by volume or less). The lower limit of the oxygen concentration in the gas phase is 0% by volume, but it can also be 1% by volume ppt or more (e.g., 100% by volume ppt or more), preferably 1% by volume ppb or more (e.g., 100% by volume ppb or more). If the oxygen concentration is too high, iodine may sometimes be generated in the process, leading to corrosion of the equipment and apparatus. In addition, if the oxygen concentration is too high, formaldehyde and formic acid may sometimes be generated in the process, resulting in a higher formic acid concentration in the acetic acid product. In addition, to achieve the condition of extremely low oxygen concentration, the raw materials such as carbon monoxide, methanol, or water introduced into the process, as well as the inert gases (such as nitrogen) purged to measuring instruments such as level gauges and pressure gauges, need to be those that have already reduced the dissolved oxygen or oxygen concentration to an extremely low level, which is sometimes uneconomical.

[0055] In (iv) above, the oxygen concentration in the liquid phase is less than 7 × 10⁻⁶. -5 g / g is acceptable, preferably 2×10 g / g. -5 Below g / g (e.g., 1×10⁻⁶) -5 (less than g / g), more preferably 0.5 × 10 g / g. -5 Below g / g (e.g., 0.1 × 10⁻⁶ g / g) -5 (below g / g), more preferably 0.05×10 -5 Below g / g (e.g., 0.01 × 10⁻⁶ g / g) -5 (below g / g), particularly preferably 0.001×10 g / g. -5 Below g / g (e.g., 0.0001 × 10⁻⁶ g / g) -5 The lower limit of oxygen concentration in the liquid phase is 0 g / g, but it can also be 0.1 × 10 g / g.-9 Above g / g. It should be noted that in pressurized or high-temperature process liquids, the oxygen concentration can sometimes be difficult to determine accurately due to sampling difficulties and oxygen vaporization. In such cases, the oxygen concentration in the liquid phase can be measured under multiple conditions that change temperature and / or pressure, or the oxygen concentration in the liquid phase at the actual process temperature and pressure can be used as an estimate (based on experimental estimates). Alternatively, Aspen+ (Plus) (manufactured by Aspen Technology Inc.) can be used to calculate the oxygen concentration in the liquid phase. If the oxygen concentration is too high, iodine may sometimes be generated in the process, leading to corrosion of equipment and devices. Additionally, excessively high oxygen concentrations may sometimes generate formaldehyde and formic acid in the process, resulting in a higher formic acid concentration in the acetic acid product. In addition, to achieve the condition of extremely low oxygen concentration, the raw materials such as carbon monoxide, methanol, or water introduced into the process, as well as the inert gases (such as nitrogen) purged to measuring instruments such as level gauges and pressure gauges, need to be those that have already reduced the dissolved oxygen or oxygen concentration to an extremely low level, which is sometimes uneconomical.

[0056] The ratio of oxygen to carbon monoxide in the gas phase in (iii) above and / or in the liquid phase in (iv) above is, for example, 2% by volume or less, preferably 1% by volume or less.

[0057] When controlling the oxygen concentration in (iii) and / or (iv) above, it is preferable to introduce at least one component selected from oxygen-containing gas, oxygen-containing compound, and oxygen-generating agent, so as to control the oxygen concentration in the gas phase in (iii) above to 1 volume ppt or more, and / or control the oxygen concentration in the liquid phase in (iv) above to 0.1 × 10⁻⁶. -9 g / g or more.

[0058] It should be noted that although lower oxygen concentrations are generally preferred, excessively low oxygen concentrations can sometimes lead to an overly reducing atmosphere, increasing the corrosion rate of equipment and devices in the acetic acid preparation unit, such as distillation columns and piping. Therefore, to control the oxygen concentration, at least one oxygen source selected from oxygen-containing gases, oxygen-containing compounds, and oxygen-generating agents can be introduced into the process to control the oxygen concentration in the process flow.

[0059] Examples of oxygen-containing gases mentioned above include air. Examples of oxygen-containing compounds mentioned above include ozone. Examples of oxygen-generating agents mentioned above include peracetic acid and hydrogen peroxide. Only one or more of these oxygen sources may be used.

[0060] In (iii) and (iv) above, from the viewpoint of suppressing iodine formation, the oxygen concentration in the process stream relative to the total amount of hydrogen iodide and iodomethane per mole can be, for example, 0.25 moles or less (e.g., 0.2 moles or less), preferably 0.1 moles or less (e.g., 0.05 moles or less), and more preferably 0.01 moles or less (e.g., 1 × 10⁻⁶ moles or less). -3 (less than 1 mole), especially preferred is 1×10 -4 Below 1 mole (e.g., 1 × 10⁻⁶) -5 Around 1×10 (less than a mole), or approximately 1×10 -6 Below 1 mole (e.g., 1 × 10⁻⁶) -7 (below the mole).

[0061] The oxygen-to-carbon monoxide ratio (O2 / CO) in the above process flow is, for example, 7 vol% or less (e.g., 5 vol% or less), preferably 2 vol% or less (e.g., 1 vol% or less), more preferably 0.5 vol% or less (e.g., 0.1 vol% or less), further preferably 0.01 vol% or less (e.g., 0.001 vol% or less), and particularly preferably 0.0001 vol% or less (e.g., 0.00001 vol% or less).

[0062] Regarding (iv) above, the oxygen concentration in the liquid phase is generally low, and the oxygen-to-carbon monoxide ratio (O2 / CO) sometimes varies greatly. The mass ratio of oxygen to carbon monoxide (O2 / CO) in the liquid phase can be, for example, less than 1000 parts by mass (less than 10 times) (e.g., less than 500 parts by mass), less than 250 parts by mass (e.g., less than 100 parts by mass), less than 75 parts by mass (e.g., less than 50 parts by mass), less than 20 parts by mass (e.g., less than 10 parts by mass), less than 5 parts by mass (e.g., less than 1 part by mass), less than 0.1 parts by mass (e.g., less than 0.01 parts by mass), less than 0.001 parts by mass (e.g., less than 0.0001 parts by mass), or less than 0.00005 parts by mass (e.g., less than 0.00001 parts by mass).

[0063] In (iii) and (iv) above, to suppress the byproducts of iodine and formic acid, the gas or liquid phase in the process preferably contains at least one selected from iodomethane, hydrogen iodide, and formic acid. Further, the process stream (e.g., the gas phase of the process) may, depending on the process, contain at least one selected from acetic acid, methyl acetate, methanol, water, acetaldehyde, byproducts derived from acetaldehyde, and dialkyl ethers. The aforementioned byproducts may contain at least one selected from iodoalkanes with 2 or more carbon atoms, alkanes with 4 or more carbon atoms, alkane carboxylic acids with 3 or more carbon atoms, alkanes, and ketones; the dialkyl ethers may contain at least dimethyl ether.

[0064] Examples of gas phases in (iii) above include those in reaction processes, evaporation processes, or distillation processes. Similarly, examples of liquid phases in (iv) above include those in reaction processes, evaporation processes, or distillation processes.

[0065] By controlling the oxygen concentration in the acetic acid preparation process as described in (iii) and / or (iv) above, useful process conditions can be provided that suppress the byproducts of iodine and / or formic acid, eliminate problems such as localized corrosion caused by iodine, increases in the total iodine and / or formic acid concentrations in the acetic acid product, and coloration of the acetic acid product. Furthermore, controlling the oxygen concentration in (iii) and / or (iv) above is also very useful in managing extremely low concentrations such as iodine concentration in the acetic acid product to below 10 ppb by mass and formic acid concentration to below 50 ppm by mass. Further, it is known that advanced corrosion-resistant metals such as zirconium exhibit complete corrosion resistance over a wide range, whether under reducing or oxidizing conditions. However, even such advanced corrosion-resistant metals can sometimes corrode under strong oxidizing conditions. Therefore, in some cases, even if corrosion resistance is exhibited up to a certain level of high oxygen concentration due to the selection of the material of the apparatus or equipment, corrosion may still occur due to the oxygen concentration. Such corrosion can also be suppressed by controlling the oxygen concentration described above.

[0066] In the acetic acid preparation method of the present invention, the acetic acid preparation process may include: a carbonylation reaction step in which methanol reacts with carbon monoxide to produce acetic acid; and a separation step in which the reaction mixture obtained in the carbonylation reaction step is separated into a feed stream containing a metal catalyst, an acetic acid stream enriched with acetic acid, and a feed stream enriched with low-boiling components compared to the acetic acid stream, using one or more evaporation tanks and / or distillation columns. It should be noted that the separation step preferably includes, for example: an evaporation step in which the reaction mixture obtained in the carbonylation reaction step is separated into a vapor stream and a residual liquid stream in an evaporation tank; and a de-low-boiling step in which the vapor stream is distilled to separate into a column overhead stream enriched with low-boiling components and a first acetic acid stream enriched with acetic acid.

[0067] Furthermore, the method for preparing acetic acid according to the present invention may further include at least one of the steps (a) to (d) described below. It should be noted that, in the case of step (a) described below, step (a) is included in the separation step described above.

[0068] (a) A dehydration process in which the first acetic acid stream is distilled to separate a water-rich overhead stream and a second acetic acid stream that is more acetic acid-rich than the first acetic acid stream. (b) A de-boiling process that separates the first or second acetic acid stream into a bottoms stream enriched with high-boiling components by distillation, and a third acetic acid stream that is more enriched with acetic acid than the acetic acid stream before distillation. (c) An adsorption removal process in which the first, second, or third acetic acid stream is treated with an ion exchange resin to obtain a fourth acetic acid stream. (d) A process of distilling the first, second, third, or fourth acetic acid stream to obtain a fifth acetic acid stream that is more enriched in acetic acid than the acetic acid stream before distillation. It should be noted that the above separation process can also replace the above evaporation process and the low-boiling point removal process by including a process that separates the reaction mixture obtained in the above carbonylation reaction process into the above feed stream containing the metal catalyst, the above overhead stream enriched with low-boiling components, and a first acetic acid stream enriched with acetic acid (evaporation low-boiling point removal process). Furthermore, the above separation process can also replace the above low-boiling point removal process and the dehydration process by including a low-boiling point removal process (so-called low-boiling point removal and dehydration process) that also functions as the above dehydration process, i.e., a process that distills the above vapor stream to separate it into an overhead stream enriched with low-boiling components and an acetic acid stream dehydrated to a water concentration equivalent to the above second acetic acid stream. Therefore, the above evaporation low-boiling point removal process can also be a process that also functions as the above dehydration process (evaporation low-boiling point removal and dehydration process). The acetic acid stream enriched with acetic acid obtained by the low-boiling point removal and dehydration process and the evaporation low-boiling point removal and dehydration process is equivalent to the above second acetic acid stream.

[0069] The carbonylation reaction process described above can satisfy the operating conditions described in (i). Alternatively, at least one process selected from the above-described evaporation process, low-boiling point removal process, evaporation-to-low-boiling point removal process, dehydration process, low-boiling point removal and dehydration process, evaporation-to-low-boiling point removal and dehydration process, high-boiling point removal process, and product process (preferably low-boiling point removal process, evaporation-to-low-boiling point removal process, low-boiling point removal and dehydration process, evaporation-to-low-boiling point removal and dehydration process, more preferably low-boiling point removal and dehydration process, evaporation process and low-boiling point removal process, low-boiling point removal and dehydration process, evaporation-to-low-boiling point removal process, or evaporation-to-low-boiling point removal and dehydration process, even more preferably evaporation process, low-boiling point removal and dehydration process) can satisfy the operating conditions described in (ii). Alternatively, at least one process selected from the above-described evaporation process, low-boiling point removal process, high-boiling point removal process, and product process can satisfy the operating conditions described in (ii).

[0070] When the oxygen concentration in the above-described process stream increases, iodine and / or formic acid are easily generated in the process stream. Therefore, processes that include a gaseous or liquid phase as the object of oxygen concentration control in (iii) and (iv) above are preferably processes in which hydrogen iodide, iodomethane, methanol, or formaldehyde are readily present. Therefore, the gaseous phase in (iii) above and / or the liquid phase in (iv) above are preferably gaseous and / or liquid phases selected from one or more of the following steps: reaction steps, steps included in separation steps (evaporation steps, de-boiling point removal steps, dehydration steps, evaporation de-boiling point removal steps, de-boiling point removal and dehydration steps, evaporation de-boiling point removal and dehydration steps), aqueous and organic phases in the decanter 4 described later, steps in the acetaldehyde separation and removal system (extraction steps, distillation steps, extractive distillation steps, etc.), high-pressure absorption steps, low-pressure absorption steps, and diffusion steps. From the viewpoint that hydrogen iodide, iodomethane, methanol, or formaldehyde are more likely to be present, it is more preferable to select the gas phase and / or liquid phase from one or more of the following steps: reaction process (e.g., reaction mixture, gas phase in reaction tank), evaporation process (especially volatile phase), de-boiling process (especially the top of de-boiling tower), de-boiling and dehydration process, aqueous phase and organic phase in decanter 4 (described later), high-pressure absorption process, and low-pressure absorption process. It is particularly preferred to select the gas phase and / or liquid phase from one or more of the following steps: reaction process (e.g., reaction mixture, gas phase in reaction tank), evaporation process (especially volatile phase), and de-boiling process (especially the top of de-boiling tower), specifically the gas phase in (iii) above and / or the liquid phase in (iv) above.

[0071] Furthermore, in the method for preparing acetic acid of the present invention, the gas phase in (iii) and / or the liquid phase in (iv) above may be the gas phase and / or liquid phase in the reaction step, evaporation step, or distillation step. For example, the gas phase and / or liquid phase selected from at least one of the above-mentioned reaction step, evaporation step, de-low boiling point removal step, dehydration step, de-high boiling point removal step, and product step may be the gas phase in (iii) above and / or the liquid phase in (iv) above.

[0072] Furthermore, in the method of the present invention, it is preferable to control the formic acid concentration in at least one process solution to be 500 ppm by mass or less, more preferably 400 ppm by mass or less, even more preferably 300 ppm by mass or less, further preferably 200 ppm by mass or less, particularly 100 ppm by mass or less, even more preferably 50 ppm by mass or less, and 30 ppm by mass or less. Additionally, the formic acid concentration in the liquid phase may be 0 ppm by mass or more, for example, 0.1 ppm by mass or more (e.g., 1 ppm by mass or more), preferably 3 ppm by mass or more (e.g., 5 ppm by mass or more), more preferably 10 ppm by mass or more, further preferably 15 ppm by mass or more, particularly 20 ppm by mass or more, or below the measurement limit.

[0073] When the concentration of formic acid in the process liquid increases, it is easy for formic acid to be mixed into the acetic acid product. Therefore, the process that includes a liquid phase as the object of formic acid concentration control is preferably a process in which methanol or formaldehyde is readily present, preferably a liquid phase selected from one or more of the following steps: reaction step, separation step (e.g., evaporation step, low-boiling point removal step, evaporation to remove low-boiling point removal step, evaporation to remove low-boiling point dehydration step), high-boiling point removal step, aqueous and organic phases in decanter 4 (described later), acetaldehyde separation and removal system (extraction step, distillation step, extractive distillation step, etc.), alkane separation step, high-pressure absorption step, low-pressure absorption step, and diffusion step. From the viewpoint that methanol or formaldehyde is more likely to be present, a liquid phase selected from one or more of the following steps is preferred: a reaction step (e.g., a reaction mixture), an evaporation step, a de-boiling step (especially the top of a de-boiling tower), an aqueous phase and an organic phase in the decanter 4 described later, a high-pressure absorption step, and a low-pressure absorption step. A liquid phase selected from one or more of the following steps is particularly preferred: a reaction step (e.g., a reaction mixture), an evaporation step, and a de-boiling step (especially the top of a de-boiling tower).

[0074] It should be noted that the process that satisfies the operating conditions (i) and / or (ii) above can be a process that controls the oxygen concentration to satisfy (iii) and / or (iv) above, or it can be a process that does not satisfy (iii) and / or (iv) above. Furthermore, in (iii) or (iv) above, the process that includes a gaseous or liquid phase as the object of oxygen concentration control, and the process that includes a liquid phase as the object of formic acid concentration control, can be the same or different.

[0075] It should be noted that acetaldehyde, and byproducts derived from acetaldehyde that shorten the permanganate time in the permanganate reducing agent test (permanganate time) (aldehydes, iodoalkanes with more than 2 carbon atoms, etc.), are sometimes abbreviated as PRC class. Furthermore, unless otherwise specified, the aqueous phase containing acetaldehyde obtained by liquid-liquid extraction is used with the same meaning as the light phase or upper phase, and the organic phase containing iodomethane is used with the same meaning as the heavy phase, iodomethane phase, or lower phase. The aqueous phase obtained by extraction is used with the same meaning as the extract, and the organic phase is used with the same meaning as the raffinate.

[0076] The following describes one embodiment of the present invention. Figure 1This is an example of an acetic acid preparation process (methanol carbonylation process) according to one embodiment of the present invention. The acetic acid preparation apparatus involved in this process includes a reaction tank 1, an evaporation tank 2, a distillation column 3, a decanter 4, a distillation column 5, a distillation column 6, an ion exchange resin column 7, a washing system 8, an acetaldehyde separation and removal system 9, condensers 1a, 2a, 3a, 5a, and 6a, a heat exchanger 2b, reboilers 3b, 5b, and 6b, circuits 11-56, and a pump 57, configured to continuously produce acetic acid. In the acetic acid preparation method of this embodiment, the reaction process, evaporation process (flash evaporation process), first distillation process, second distillation process, third distillation process, and adsorption removal process are respectively performed in the reaction tank 1, evaporation tank 2, distillation column 3, distillation column 5, distillation column 6, and ion exchange resin column 7. The first distillation process is also called a low-boiling point removal process, the second distillation process is also called a dehydration process, and the third distillation process is also called a high-boiling point removal process. It should be noted that the process in this invention is not limited to the above-described steps, and in particular, it includes cases where the equipment does not include the distillation column 5, the distillation column (high-boiling point removal column) 6, the ion exchange resin column 7, and the acetaldehyde separation and removal system 9 (acetaldehyde removal column, etc.). Alternatively, as described later, a product column may be installed downstream of the ion exchange resin column 7.

[0077] Reactor 1 is a unit for carrying out a reaction process. This reaction process is for the continuous generation of acetic acid through the reaction shown in the following reaction formula (1) (carbonylation of methanol). Under normal operating conditions of the acetic acid preparation apparatus, a reaction mixture, for example stirred by a stirrer, exists in reactor 1. The reaction mixture contains methanol and carbon monoxide as raw materials, a metal catalyst, a co-catalyst, water, acetic acid as the target of preparation, and various by-products, with the liquid and gas phases in equilibrium.

[0078] CH3OH + CO → CH3COOH (1) The reactants in the reaction mixture are liquid methanol and gaseous carbon monoxide. Methanol is continuously supplied to reaction vessel 1 from a methanol storage unit (not shown) via line 11 at a given flow rate. Carbon monoxide is continuously supplied to reaction vessel 1 from a carbon monoxide storage unit (not shown) via line 12 at a given flow rate. The carbon monoxide does not have to be pure carbon monoxide; it may also contain small amounts (e.g., less than 5% by mass, preferably less than 1% by mass) of other gases such as nitrogen, hydrogen, carbon dioxide, oxygen, and helium. It should be noted that, in order to improve catalytic activity, hydrogen may also be supplied via a line (not shown) that merges with line 12 for supplying carbon monoxide, and sometimes carbon monoxide is supplied to reaction vessel 1 in the form of a gas mixture with hydrogen.

[0079] It should be noted that methanol and carbon monoxide, as raw materials, can be obtained by purifying the syngas (CO, H2, CO2, and trace amounts of O2) generated from carbon sources (carbons and hydrocarbons) such as fossil fuels (coal, oil, etc.) and natural gas through oxygen- and air-based partial oxidation, such as steam methane reforming (SMR), autothermal reforming (ATR), and partial oxidation (POX). Oxygen-based partial oxidation is self-evident; in SMR, oxygen is also present in the carbon source and steam. Therefore, trace amounts of oxygen may be introduced into the process due to the introduction of raw material carbon monoxide and raw material methanol into the reactor, or the supply or addition of methanol to the process to remove hydrogen iodide in the conversion to iodomethane. Therefore, using methanol and carbon monoxide with low oxygen concentrations as raw materials is useful.

[0080] Therefore, methanol that has been pre-oxygenated is preferred as the raw material. As for carbon monoxide, exhaust gas containing carbon monoxide obtained from downstream processes can be recycled back to the reaction vessel. Pre-oxygenated carbon monoxide or exhaust gas is preferred as such raw material.

[0081] The metal catalyst in the reaction mixture is a substance used to promote the carbonylation reaction of methanol, and can be, for example, a rhodium catalyst, an iridium catalyst, a cobalt catalyst, etc. As a rhodium catalyst, for example, [Rh(CO)₂I₂] can be used. - The rhodium complex is represented. As an iridium catalyst, for example, [Ir(CO)3I3] can be used. - The iridium complex is indicated. As a metal catalyst, a metal complex catalyst is preferred. The concentration of the catalyst in the reaction mixture (in metal terms) relative to the overall liquid phase (reaction mixture) of the reaction mixture is, for example, 200 to 10000 ppm by mass, preferably 300 to 5000 ppm by mass, and more preferably 400 to 2000 ppm by mass.

[0082] The co-catalyst is an iodide used to assist the action of the catalyst described above, and can be, for example, iodomethane or an ionic iodide. Iodomethane can promote the catalytic action of the catalyst. The concentration of iodomethane relative to the overall liquid phase of the reaction mixture is, for example, 1 to 20% by mass (preferably 5 to 15% by mass). The ionic iodide is an iodide that generates iodide ions in the reaction solution (especially an ionic metal iodide), and can stabilize the catalyst and suppress side reactions. Examples of ionic iodides include alkali metal iodides such as lithium iodide, sodium iodide, and potassium iodide. The concentration of the ionic iodide in the reaction mixture is, for example, 1 to 25% by mass, preferably 5 to 20% by mass, relative to the overall liquid phase of the reaction mixture. Furthermore, when using catalysts such as iridium, ruthenium compounds or osmium compounds can also be used as co-catalysts. The amounts of these compounds used are total, for example, 0.1 to 30 moles (in metal terms) relative to 1 mole of iridium, preferably 0.5 to 15 moles (in metal terms).

[0083] Water in the reaction mixture is essential for the formation of acetic acid in the carbonylation reaction of methanol, and is also necessary for the solubility of water-soluble components in the reaction system. The concentration of water in the reaction mixture relative to the overall liquid phase is, for example, 0.1 to 15% by mass, preferably 0.8 to 10% by mass, more preferably 1 to 6% by mass, and particularly preferably 1.5 to 4% by mass. The water concentration is preferably 15% by mass or less to suppress the energy required for water removal during acetic acid purification and to promote the efficiency of acetic acid preparation. To control the water concentration, a given flow rate of water can be continuously supplied to the reaction tank 1. It is preferable to remove oxygen from the catalyst mixture and water beforehand by heating or boiling.

[0084] The acetic acid in the reaction mixture includes acetic acid that has been pre-feeded into reaction tank 1 before the acetic acid preparation apparatus is operated, and acetic acid generated as the main product of the carbonylation reaction of methanol. This acetic acid can function as a solvent in the reaction system. The concentration of acetic acid in the reaction mixture is, for example, 50-90% by mass, preferably 60-80% by mass, relative to the overall liquid phase of the reaction mixture.

[0085] A major byproduct included in the reaction mixture can be methyl acetate, for example. Methyl acetate can be generated by the reaction of acetic acid with methanol. The concentration of methyl acetate in the reaction mixture is, for example, 0.1 to 30% by mass, preferably 1 to 10% by mass, relative to the overall liquid phase of the reaction mixture.

[0086] Hydrogen iodide can also be listed as a byproduct contained in the reaction mixture. Considering the reaction mechanism of the methanol carbonylation reaction, the generation of hydrogen iodide is unavoidable when using catalysts and co-catalysts as described above. The concentration of hydrogen iodide in the reaction mixture relative to the overall liquid phase is, for example, 0.01 to 2% by mass. Other byproducts include, for example, hydrogen, methane, carbon dioxide, acetaldehyde, crotonaldehyde, 2-ethylcrotonaldehyde, dimethyl ether, alkanes, formic acid, propionic acid, and iodoalkanes such as iodohexane and iododecane. It should be noted that in this invention, since acetaldehyde can be effectively removed using the acetaldehyde separation and removal system described later, the concentration of acetaldehyde in the reaction vessel can be reduced even in continuous reactions, and the generation of byproducts originating from acetaldehyde can be significantly suppressed. The concentration of acetaldehyde in the reaction mixture relative to the overall liquid phase can be, for example, below 1500 ppm by mass, 10 to 1000 ppm by mass, 50 to 500 ppm by mass, or 100 to 400 ppm by mass.

[0087] Examples of byproducts derived from acetaldehyde include: aldehydes such as butyraldehyde, crotonaldehyde, 2-ethylcrotonaldehyde, and 2-ethylbutyraldehyde; ketones such as acetone and methyl ethyl ketone; their aldehyde-alcohol condensation products; and iodocarbon compounds such as iodoethane, iodopropane, iodobutane, iodopentane, and iodohexane. 2-12 Alkanes, etc. Additionally, examples include formic acid, carboxylic acids with 3 or more carbon atoms (propionic acid, butyric acid, valeric acid, hexanoic acid, heptanoic acid, octanoic acid), and higher fatty acids with 9 or more carbon atoms, such as C42-C ... 3-12 Alkane, carboxylic acid, etc.); alkyl alcohols (ethanol, butanol, 2-ethylbutanol, hexanol, heptanol, octanol, and alkyl alcohols with 9 or more carbon atoms, such as C45, 2-ethylbut ... 3-12 Alkyl alcohols, etc.; hydrocarbons with 2 or more carbon atoms (e.g., C40, C50, C60 ... 2-12 Alkanes, etc. Furthermore, methanol or esters of these alkyl alcohols with acetic acid or the aforementioned carboxylic acids (ethyl acetate, etc.) may also be produced as byproducts in the liquid phase system; dimethyl ethers and other dialkyl ethers, etc. Regarding the concentration of these byproducts, throughout the entire process including the liquid phase system, it can be approximately 0.1 ppb to 100 ppm (e.g., 0.5 ppb to 50 ppm), preferably around 1 ppb to 10 ppm (e.g., 2 ppb to 1 ppm).

[0088] The concentration of iodoalkanes with 2 or more carbon atoms, such as iodohexane, is, for example, 0.1 ppb to 1 ppm (e.g., 0.5 to 500 ppb), preferably 1 to 100 ppb. The concentration of carboxylic acids with 3 or more carbon atoms is, for example, 0.1 to 500 ppm (e.g., 1 to 500 ppm), preferably 3 to 100 ppm.

[0089] The concentration of dimethyl ether is, for example, less than 0.5% by mass (e.g., 0.1 to 1000 ppm by mass), preferably 1 to 500 ppm by mass (e.g., 2 to 300 ppm by mass), and more preferably 3 to 200 ppm by mass (e.g., 5 to 100 ppm by mass).

[0090] Furthermore, 3-hydroxyalkanes (such as 3-hydroxybutanol) are sometimes produced as byproducts derived from acetaldehyde. The concentration of 3-hydroxyalkanes in the reaction mixture is, for example, less than 100 ppm by mass (e.g., 0.1 ppb to 100 ppm by mass), preferably 0.5 ppb to 50 ppm by mass. In many cases, these byproducts are produced in proportion to the 2nd to 3rd power of the acetaldehyde concentration.

[0091] Furthermore, acetaldehyde and its byproducts can form permanganate reducing agents (PRCs). Therefore, it is preferable to separate and remove acetaldehyde, the main component of the byproducts, from the reaction mixture, and to recover and efficiently utilize useful components (e.g., iodomethane, etc.) from the process stream. It should be noted that this includes iodomethane and iodoform C... 2-12 Alkanes and the like also belong to the PRC category, but iodomethane is not included in the PRC category in this specification.

[0092] In addition, the reaction mixture may include metals such as iron, nickel, chromium, manganese, and molybdenum (hereinafter also referred to as "corrosive metals") produced due to corrosion of the apparatus, as well as other metals such as cobalt, zinc, and copper. These corrosive metals, along with other metals, are collectively referred to as "corrosive metals, etc." The total content of these byproducts, corrosive metals, and other impurities relative to the overall liquid phase of the reaction mixture is, for example, 1 ppm to 1% by mass. Therefore, the process liquid in this acetic acid preparation process may contain, for example, about 1 ppm to 1% by mass of the aforementioned impurities in total. It should be noted that, relative to the overall liquid phase of the reaction mixture, the concentration of formic acid in the reaction mixture is, for example, 0 to 102 ppm by mass, preferably 0 to 85 ppm by mass, and more preferably 0 to 50 ppm by mass.

[0093] In the reaction vessel 1 containing the reaction mixture as described above, the reaction temperature can be set to, for example, 150~250°C, the reaction pressure measured by the overall pressure gauge can be set to, for example, 2.0~3.5 MPa (absolute pressure), and the partial pressure of carbon monoxide can be set to, for example, 0.4~1.8 MPa (absolute pressure), preferably 0.6~1.5 MPa (absolute pressure).

[0094] The vapor in the gas phase of the reaction tank 1 during operation includes, for example, carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, iodomethane, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid. This vapor can be extracted from the reaction tank 1 via line 13. By adjusting the amount of vapor extracted, the pressure within the reaction tank 1 can be controlled, for example, the pressure within the reaction tank 1 can be kept constant. The vapor extracted from the reaction tank 1 is introduced into the condenser 1a.

[0095] Condenser 1a partially condenses the vapor from reaction tank 1, separating it into condensate and gaseous components. The condensate includes, for example, iodomethane, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid, and is introduced from condenser 1a into reaction tank 1 via line 14 for recirculation. The gaseous components include, for example, carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, iodomethane, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid, and are supplied from condenser 1a to scrubbing system 8 via line 15. It should be noted that the gas discharged from line 49 can be used as a CO source introduced into the bottom or residual liquid recirculation lines 18 and 19 of evaporation tank 2 (described later).

[0096] Acetic acid is continuously generated in reaction tank 1 as described above during operation of the apparatus. The reaction mixture containing acetic acid is continuously extracted from reaction tank 1 at a given flow rate and then introduced into subsequent evaporation tank 2 via line 16.

[0097] In this invention, the reaction process using reaction tank 1 preferably meets the operating conditions described above (i) of a hydrogen partial pressure below 500 kPa (absolute pressure), a carbon dioxide partial pressure below 70 kPa (absolute pressure), and an operating temperature above 150°C. In this case, a hydrogen partial pressure (absolute pressure) below 500 kPa is sufficient, preferably below 400 kPa, more preferably below 300 kPa, further preferably below 200 kPa, and particularly preferably below 150 kPa. In the carbonylation reaction described above, hydrogen is generated due to the reaction of carbon monoxide with water. This hydrogen enhances catalytic activity. Therefore, hydrogen can be supplied to the reaction tank as needed. Alternatively, hydrogen can be supplied by purifying the gas components (including hydrogen, carbon monoxide, etc.) discharged in the downstream process and then recycling them back to the reaction tank as needed. As such hydrogen, hydrogen with a low oxygen concentration is preferred. Therefore, the lower limit of the hydrogen partial pressure (absolute pressure) is 0 kPa, but it is also possible for the hydrogen partial pressure (absolute pressure) to exceed 1 kPa (or exceed 5 kPa). The partial pressure (absolute pressure) of carbon dioxide can be below 70 kPa, preferably below 60 kPa, more preferably below 50 kPa, further preferably below 40 kPa, and particularly preferably below 30 kPa. The lower limit of the partial pressure (absolute pressure) of carbon dioxide is 0 kPa, but it can also be 2 kPa (or 20 kPa). The operating temperature can be above 150°C, for example above 160°C, preferably above 175°C, more preferably above 178°C, further preferably above 181°C, and particularly preferably above 184°C. The upper limit of the operating temperature is, for example, 250°C, preferably 230°C, and more preferably 200°C. By ensuring that the reaction process using reaction tank 1 meets the above operating conditions (i), the formation of formic acid in reaction tank 1 can be suppressed. In addition, if a liquid containing formic acid is introduced into reaction tank 1, the formic acid can be effectively decomposed.

[0098] The gas phase in the reaction process using reaction tank 1 described above can be a gas phase that satisfies (iii) above. The oxygen concentration in the gas phase of reaction tank 1 (the gas phase extracted from line 13) can be, for example, less than 10 volume% (e.g., 10 volume ppb to 10 volume%), preferably 10 volume ppb to 3.6 volume% (e.g., 20 volume ppb to 2 volume%), more preferably 30 volume ppb to 1 volume% (e.g., 100 volume ppb to 0.1 volume%), and even more preferably 500 volume ppb to 500 volume ppm (e.g., 1 to 100 volume ppm). In addition, when the gas phase in the reaction process satisfies (iii) above, the oxygen concentration is less than 7 volume% (e.g., 1 volume ppb to 5 volume%), preferably 1 volume ppb to 1 volume% (e.g., 10 volume ppb to 0.5 volume%), more preferably 20 volume ppb to 0.3 volume%, and even more preferably 50 volume ppb to 0.1 volume% (e.g., 100 volume ppb to 200 volume ppm).

[0099] The liquid phase in the reaction process using reaction tank 1 described above can be a liquid phase that satisfies (iv) above. The oxygen concentration in the reaction mixture is, for example, less than 10 vol% (e.g., 0.1 vol ppb to 10 vol%), preferably 0.2 vol ppb to 3.6 vol% (e.g., 1 vol ppb to 2 vol%), more preferably less than 1 vol% (e.g., 1 vol ppt to 1000 vol ppm), further preferably less than 700 vol ppm (e.g., 1 vol ppt to 500 vol ppm), and particularly preferably 10 vol ppt to 300 vol ppm (e.g., 100 vol ppt to 100 vol ppm).

[0100] The vapor from the reaction vessel 1 is introduced from the condenser 1a to the condensate of the reaction vessel 1 via line 14. The condensate can be a composition that satisfies (iv) above. The oxygen concentration in the condensate is, for example, less than 10 vol% (e.g., 0.1 vol ppb to 10 vol%), preferably 0.2 vol ppb to 3.6 vol% (e.g., 1 vol ppb to 2 vol%), more preferably less than 1 vol% (e.g., 1 vol ppt to 1000 vol ppm), further preferably less than 700 vol ppm (e.g., 1 vol ppt to 500 vol ppm), and particularly preferably 10 vol ppt to 300 vol ppm (e.g., 100 vol ppt to 100 vol ppm).

[0101] The gas composition from condenser 1a (the non-condensable gas sent to line 15) can be the composition that satisfies (iii) above. The oxygen concentration in the gas composition can be, for example, less than 10% by volume (e.g., 10% to 10% by volume), preferably 10% to 3.6% by volume (e.g., 20% to 2% by volume), more preferably 30% to 1% by volume (e.g., 100% to 0.1% by volume), and even more preferably 500% to 500% by volume (e.g., 1% to 100% by volume). Furthermore, when the above-mentioned gas composition satisfies (iii) above, the above-mentioned oxygen concentration is less than 7 volume% (e.g., 1 volume ppt to 5 volume%), preferably less than 3.6 volume% (e.g., 0.1 volume ppb to 2 volume%), more preferably 1 volume ppb to 1 volume% (e.g., 10 volume ppb to 0.5 volume%), even more preferably 20 volume ppb to 0.3 volume%, particularly preferably 50 volume ppb to 0.1 volume% (e.g., 100 volume ppb to 200 volume ppm).

[0102] Evaporation tank 2 is a unit used for performing an evaporation process (flash evaporation). This evaporation process is used to partially evaporate the reaction mixture continuously introduced into evaporation tank 2 via line 16 (reaction mixture supply line), separating it into a vapor stream (volatile phase) and a residual liquid stream (low volatile phase). Evaporation can occur by depressurizing the reaction mixture without heating, or by heating the reaction mixture while depressurizing it. In the evaporation process, the temperature of the vapor stream is, for example, 100~260°C, preferably 120~200°C, the temperature of the residual liquid stream is, for example, 80~200°C, preferably 100~180°C, and the tank pressure is, for example, 50~1000 kPa (absolute pressure). Furthermore, the ratio of the vapor stream to the residual liquid stream separated in the evaporation process is, for example, 10 / 90~50 / 50 (vapor stream / residual liquid stream) by mass. The vapor generated in this process contains, for example, iodomethane, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid, and is continuously extracted from evaporation tank 2 to line 17 (vapor discharge line). A portion of the vapor stream extracted from evaporation tank 2 is continuously introduced into condenser 2a, and the remaining portion of the vapor stream is continuously introduced into subsequent distillation column 3 via line 21. The acetic acid concentration of the above vapor stream is, for example, 50-85% by mass, preferably 55-75% by mass. The residual liquid stream generated in this process contains catalysts and co-catalysts (iodomethane, lithium iodide, etc.) contained in the reaction mixture, water, methyl acetate, acetic acid, formic acid, and propionic acid that have not volatilized in this process, and is continuously introduced from evaporation tank 2 to heat exchanger 2b via line 18 using pump 57. Heat exchanger 2b cools the residual liquid stream from evaporation tank 2. The cooled residual liquid stream is continuously introduced from heat exchanger 2b to reaction tank 1 via line 19 for recirculation. It should be noted that lines 18 and 19 are collectively referred to as the residual liquid flow recycling lines. Additionally, to suppress catalyst sedimentation, carbon monoxide (not shown) may be supplied to the aforementioned residual liquid flow. The acetic acid concentration in the aforementioned residual liquid flow is, for example, 55-90% by mass, preferably 60-85% by mass.

[0103] Condenser 2a partially condenses the vapor stream from evaporator 2, separating it into condensate and gaseous components. The condensate includes, for example, iodomethane, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid. This condensate is introduced from condenser 2a through lines 22 and 23 into reaction tank 1 for recirculation. The gaseous components include, for example, carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, iodomethane, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. This gaseous component is supplied from condenser 2a through lines 20 and 15 to washing system 8. When the acetic acid formation reaction in the above reaction steps is exothermic, a portion of the heat accumulated in the reaction mixture is transferred to the vapor generated by the reaction mixture during the evaporation step (flash evaporation step). The condensate generated by cooling this vapor in condenser 2a is recirculated into reaction tank 1. In other words, in this acetic acid preparation apparatus, the heat generated in the carbonylation reaction of methanol can be efficiently removed using condenser 2a.

[0104] In this invention, the evaporation process using the evaporation tank 2 preferably meets the operating conditions described in (ii) above: a hydrogen partial pressure of 5 kPa (absolute pressure) or less, a carbon dioxide partial pressure of less than 20 kPa (absolute pressure), and an operating temperature exceeding 100°C. In this case, the hydrogen partial pressure (absolute pressure) is preferably 4 kPa or less, more preferably 3 kPa or less, further preferably 1 kPa or less, and particularly preferably 0.8 kPa or less. The lower limit of the hydrogen partial pressure (absolute pressure) is 0 kPa, but it may exceed 0.0001 kPa. The carbon dioxide partial pressure (absolute pressure) is preferably 12 kPa or less, more preferably 8 kPa, further preferably 3 kPa or less, and particularly preferably 1 kPa or less. The lower limit of the carbon dioxide partial pressure (absolute pressure) is 0 kPa, but it may exceed 0.0001 kPa. The operating temperature is preferably 112°C or more, more preferably 120°C or more, and further preferably 130°C or more. The upper limit of the operating temperature is, for example, 260°C, preferably 200°C, and more preferably 180°C (or 170°C or 160°C). Furthermore, under such high temperature (and high pressure) conditions, hydrogen iodide is easily generated, and iodine is easily generated depending on the oxygen concentration. However, in this invention, even if hydrogen iodide is generated, the generation of iodine can be effectively suppressed.

[0105] In the evaporation process that meets the operating conditions described in (ii) above, the concentration of acetic acid in the feed liquid to the evaporation tank 2 may be, for example, 50-90% by mass (preferably 60-80% by mass), the concentration of metal catalyst (in metal terms) may be, for example, 200-10000 ppm by mass (preferably 300-5000 ppm by mass, more preferably 400-2000 ppm by mass), the concentration of iodomethane may be, for example, 1-20% by mass (preferably 5-15% by mass), the concentration of ionic iodide may be, for example, 1-25% by mass (preferably 5-20% by mass), the concentration of water may be, for example, 0.1-15% by mass (preferably 0.8-10% by mass), the concentration of methyl acetate may be, for example, 0.1-30% by mass (preferably 1-10% by mass), and the concentration of formic acid may be, for example, less than 10000 ppm by mass (preferably 0-1000 ppm by mass, more preferably 10-500 ppm by mass, further preferably 15-200 ppm by mass, and particularly preferably 20-100 ppm by mass). By ensuring the evaporation process using evaporation tank 2 meets the aforementioned operating conditions, the formation of formic acid in evaporation tank 2 can be suppressed. Furthermore, if a liquid containing formic acid is introduced into evaporation tank 2, the formic acid can be effectively decomposed.

[0106] The gas phase in the evaporation process using evaporator 2 described above can be the gas phase that satisfies (iii) above. The oxygen concentration in the vapor stream (lines 17, 21) is, for example, less than 10 volume % (e.g., 10 volume ppb to 10 volume %), preferably 10 volume ppb to 3.6 volume % (e.g., 20 volume ppb to 2 volume %), more preferably 30 volume ppb to 1 volume % (e.g., 100 volume ppb to 0.1 volume ppm), and even more preferably 500 volume ppb to 500 volume ppm (e.g., 1 to 100 volume ppm). Furthermore, when the gas phase in the evaporation process satisfies condition (iii) above, the oxygen concentration is less than 7 volume% (e.g., 1 volume ppt to 5 volume%), preferably less than 3.6 volume% (e.g., 0.1 volume ppb to 2 volume%), more preferably 1 volume ppb to 1 volume% (e.g., 10 volume ppb to 0.5 volume%), even more preferably 20 volume ppb to 0.3 volume%, and particularly preferably 50 volume ppb to 0.1 volume% (e.g., 100 volume ppb to 200 volume ppm).

[0107] The liquid phase in the evaporation process using evaporation tank 2 described above can be a liquid phase that satisfies (iv) above. The oxygen concentration in the residual liquid stream is, for example, less than 10% by volume (e.g., 0.1 ppb to 10% by volume), preferably 0.2 ppb to 3.6% by volume (e.g., 1 ppb to 2% by volume), more preferably less than 1% by volume (e.g., 1 ppt to 1000 ppm by volume), further preferably less than 700 ppm by volume (e.g., 1 ppt to 500 ppm by volume), and particularly preferably 10 ppt to 300 ppm by volume (e.g., 100 ppt to 100 ppm by volume).

[0108] The vapor stream from the evaporation tank 2 is partially condensed by cooling in the condenser 2a, thereby separating it into condensate and gaseous components. The gaseous components can be further cooled by other condensers to separate them into liquid and gaseous components. Alternatively, the two condensate components can be temporarily stored in a storage tank and then recycled to the reaction tank 1 (not shown) via a recirculation line.

[0109] The condensate supplied from condenser 2a or the other condenser described above to the storage tank may be the composition that satisfies (iv) above. The oxygen concentration in the condensate is, for example, less than 10 vol% (e.g., 0.1 vol ppb to 10 vol%), preferably 0.2 vol ppb to 3.6 vol% (e.g., 1 vol ppb to 2 vol%), more preferably less than 1 vol% (e.g., 1 vol ppt to 1000 vol ppm), further preferably less than 700 vol ppm (e.g., 1 vol ppt to 500 vol ppm), and particularly preferably 10 vol ppt to 300 vol ppm (e.g., 100 vol ppt to 100 vol ppm).

[0110] The gas composition fed from condenser 2a to the other condensers and the gas composition obtained after separation in the other condensers can satisfy the above (iii). The oxygen concentration in the gas composition is, for example, 10 volume% or less (e.g., 10 volume ppb to 10 volume%), preferably 10 volume ppb to 3.6 volume% (e.g., 20 volume ppb to 2 volume%), more preferably 30 volume ppb to 1 volume% (e.g., 100 volume ppb to 0.1 volume ppm), and even more preferably 500 volume ppb to 500 volume ppm (e.g., 1 to 100 volume ppm). Furthermore, when the above-mentioned gas composition satisfies (iii) above, the above-mentioned oxygen concentration is less than 7 volume% (e.g., 1 volume ppt to 5 volume%), preferably less than 3.6 volume% (e.g., 0.1 volume ppb to 2 volume%), more preferably 1 volume ppb to 1 volume% (e.g., 10 volume ppb to 0.5 volume%), even more preferably 20 volume ppb to 0.3 volume%, particularly preferably 50 volume ppb to 0.1 volume% (e.g., 100 volume ppb to 200 volume ppm).

[0111] Distillation column 3 is a unit for performing the first distillation step, and in this embodiment can be considered a so-called low-boiling-point removal column. The first distillation step is a process for separating and removing low-boiling components by distilling a continuously introduced vapor stream into distillation column 3. More specifically, in the first distillation step, the vapor stream is distilled to separate a column overhead stream enriched with at least one low-boiling component selected from iodomethane and acetaldehyde, and an acetic acid stream enriched with acetic acid. Distillation column 3 includes, for example, tray columns and packed columns. When a tray column is used as distillation column 3, its theoretical number of trays is, for example, 5 to 50, and the reflux ratio is, for example, 0.5 to 3000 depending on the theoretical number of trays. Inside distillation column 3, the top pressure is set to, for example, 80 to 160 kPaG (gauge pressure), and the bottom pressure is set to be higher than the top pressure, for example, 85 to 180 kPaG. Inside the distillation column 3, the top temperature is set to, for example, a temperature 90 to 130°C lower than the boiling point of acetic acid at the set top pressure, and the bottom temperature is set to, for example, a temperature 120 to 165°C (preferably 125 to 160°C) higher than the boiling point of acetic acid at the set bottom pressure.

[0112] The vapor stream from evaporator 2 is continuously introduced into distillation column 3 via line 21, while the vapor from the top of distillation column 3, as a top stream, is continuously extracted to line 24. From the bottom of distillation column 3, the bottom residue is continuously extracted to line 25. 3b is a reboiler. From a height position between the top and bottom of distillation column 3, an acetic acid stream (first acetic acid stream; liquid) as a side stream is continuously extracted from line 27.

[0113] Compared to the bottom liquid and side stream from distillation column 3, the vapor extracted from the top of distillation column 3 contains more components with boiling points lower than acetic acid (low-boiling components), including, for example, iodomethane, hydrogen iodide, water, methyl acetate, dimethyl ether, methanol, acetaldehyde, and formic acid. This vapor also contains acetic acid. This vapor is continuously introduced to condenser 3a via line 24.

[0114] Condenser 3a partially condenses the vapor from distillation column 3, separating it into a condensate component and a gaseous component. The condensate component includes, for example, iodomethane, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid, and is continuously introduced from condenser 3a into decanter 4 via line 28. The condensate introduced into decanter 4 is separated into an aqueous phase (upper phase) and an organic phase (iodomethane phase; lower phase). The aqueous phase contains water and, for example, iodomethane, hydrogen iodide, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. The organic phase contains, for example, iodomethane, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. In this embodiment, a portion of the aqueous phase is refluxed back to distillation column 3 via line 29, and the remaining portion is introduced into reaction tank 1 for recirculation via lines 29, 30, and 23. A portion of the organic phase is introduced into reaction tank 1 for recirculation via lines 31 and 23. Other portions of the organic phase, and / or other portions of the aqueous phase, are introduced into the acetaldehyde separation and removal system 9 via lines 31, 50, and / or lines 30, 51.

[0115] The gas phase in the de-boiling step using distillation column 3 described above can be the gas phase that satisfies (iii) above. The oxygen concentrations in the overhead stream (line 24) from the top of distillation column 3 and the uncondensed gas component (line 32) in condenser 3a are, for example, less than 10 volume % (e.g., 10 volume ppb to 10 volume %), preferably 10 volume ppb to 3.6 volume % (e.g., 20 volume ppb to 2 volume %), more preferably 30 volume ppb to 1 volume % (e.g., 100 volume ppb to 0.1 volume ppm), and even more preferably 500 volume ppb to 500 volume ppm (e.g., 1 to 100 volume ppm). In addition, when the de-boiling step satisfies (iii) above, the oxygen concentration is less than 7 vol% (e.g., 1 volpt to 5 vol%), preferably less than 3.6 vol% (e.g., 0.1 volpb to 2 vol%), more preferably 1 volpb to 1 vol% (e.g., 10 volpb to 0.5 vol%), further preferably 20 volpb to 0.3 vol%, and particularly preferably 50 volpb to 0.1 vol% (e.g., 100 volpb to 200 volp).

[0116] The condensed component (line 28) obtained by condensation using condenser 3a can be a component that satisfies (iv) above. The oxygen concentration in the condensed component is the same as the oxygen concentration in the overhead stream from the top of distillation column 3.

[0117] The aqueous and organic phases in the decanter 4 can satisfy the above (iv). The oxygen concentrations in the aqueous and organic phases are, for example, less than 10 vol% (e.g., 0.1 vol ppb to 10 vol%), preferably 0.2 vol ppb to 3.6 vol% (e.g., 1 vol ppb to 2 vol%), more preferably less than 1 vol% (e.g., 1 vol ppt to 1000 vol ppm), further preferably less than 700 vol ppm (e.g., 1 vol ppt to 500 vol ppm), and particularly preferably 10 vol ppt to 300 vol ppm (e.g., 100 vol ppt to 100 vol ppm).

[0118] The first acetic acid stream described above can satisfy the above (iv). The oxygen concentration in the first acetic acid stream is, for example, less than 10 vol% (e.g., 0.1 vol ppb to 10 vol%), preferably 0.2 vol ppb to 3.6 vol% (e.g., 1 vol ppb to 2 vol%), more preferably less than 1 vol% (e.g., 1 vol ppt to 1000 vol ppm), further preferably less than 700 vol ppm (e.g., 1 vol ppt to 500 vol ppm), and particularly preferably 10 vol ppt to 300 vol ppm (e.g., 100 vol ppt to 100 vol ppm).

[0119] The above-mentioned column bottom residue (line 25) can satisfy the above (iv). The oxygen concentration in the above-mentioned column bottom residue is, for example, less than 10 volume% (e.g., 0.1 volume ppb to 10 volume%), preferably 0.2 volume ppb to 3.6 volume% (e.g., 1 volume ppb to 2 volume%), more preferably less than 1 volume% (e.g., 1 volume ppt to 1000 volume ppm), further preferably less than 700 volume ppm (e.g., 1 volume ppt to 500 volume ppm), and particularly preferably 10 volume ppt to 300 volume ppm (e.g., 100 volume ppt to 100 volume ppm).

[0120] It should be noted that the overhead stream from the top of distillation column 6, a portion of the overhead stream from the high-pressure absorption column, and the bottom stream from the low-pressure absorption column can also be recycled back to distillation column 3 (illustration omitted).

[0121] In this invention, the distillation process using the distillation column (low-boiling point removal column) 3 preferably satisfies the operating conditions described in (ii) above: a hydrogen partial pressure of 5 kPa (absolute pressure) or less, a carbon dioxide partial pressure of less than 20 kPa (absolute pressure), and an operating temperature exceeding 100°C. In this case, the hydrogen partial pressure (absolute pressure) is preferably 4 kPa or less, more preferably 3 kPa or less, and even more preferably 1 kPa or less. The lower limit of the hydrogen partial pressure (absolute pressure) is 0 kPa, but it may exceed 0.0001 kPa. The carbon dioxide partial pressure (absolute pressure) is preferably 12 kPa or less, more preferably 8 kPa or less, even more preferably 3 kPa or less, and particularly preferably 1 kPa or less. The lower limit of the carbon dioxide partial pressure (absolute pressure) is 0 kPa, but it may exceed 0.0001 kPa. The operating temperature is preferably 112°C or more, more preferably 114°C or more. The upper limit of the operating temperature is, for example, 165°C, preferably 160°C, and even more preferably 150°C (or 140°C or 130°C).

[0122] When the distillation process using the distillation column (low-boiling column) 3 meets the above operating conditions (ii), the concentration of acetic acid in the feed liquid to the distillation column 3 may be 30% by mass or more (e.g., 30 to 99.999% by mass) and the concentration of formic acid may be 5 ppm by mass or more (e.g., 5 to 10,000 ppm by mass). Furthermore, in the feed liquid to distillation column 3, the concentration of acetic acid is preferably 40-85% by mass (e.g., 50-85% by mass), more preferably 50-75% by mass (e.g., 55-75% by mass), the concentration of iodomethane is preferably 2-50% by mass (e.g., 5-30% by mass), the concentration of water is preferably 0.2-20% by mass (e.g., 1-15% by mass), the concentration of methyl acetate is preferably 0.2-50% by mass (e.g., 2-30% by mass), and the concentration of formic acid is preferably 5-10000 ppm by mass (e.g., 10-1000 ppm by mass, more preferably 10-500 ppm by mass, further preferably 15-200 ppm by mass, and particularly preferably 20-100 ppm by mass). By ensuring that the distillation process using distillation column 3 meets the above operating conditions (ii), the formation of formic acid in distillation column 3 can be suppressed, and the formic acid can be effectively decomposed when a liquid containing formic acid is supplied to distillation column 3.

[0123] In the acetaldehyde separation and removal process using the acetaldehyde separation and removal system 9, acetaldehyde contained in the organic phase and / or aqueous phase is separated and removed using known methods, such as distillation, extraction, or a combination thereof. The separated acetaldehyde is discharged outside the apparatus via line 53. Furthermore, useful components (e.g., iodomethane) contained in the organic phase and / or aqueous phase are recycled back to the reaction tank 1 via lines 52 and 23 for reuse.

[0124] Figure 2A schematic flow chart illustrating an example of an acetaldehyde separation and removal system is provided. According to this process, for example, when the organic phase is treated in the acetaldehyde separation and removal step, the organic phase is fed through line 101 to a distillation column (first acetaldehyde removal column) 91 for distillation to separate it into an acetaldehyde-enriched overhead stream (line 102) and an iodomethane-enriched residual stream (line 103). The overhead stream is condensed using condenser 91a, with a portion of the condensate returned to the top of distillation column 91 (line 104), and the remaining portion supplied to extraction column 92 (line 105). The condensate supplied to extraction column 92 is extracted using water introduced through line 109. The extracted liquid is then fed through line 107 to a distillation column (second acetaldehyde removal column) 93 for distillation to separate it into an acetaldehyde-enriched overhead stream (line 112) and a water-enriched residual stream (line 113). Then, the acetaldehyde-enriched overhead stream is condensed using condenser 93a, with a portion of the condensate being returned to the top of distillation column 93 (line 114), and the remainder being discharged outside the system (line 115). Furthermore, the iodomethane-enriched residual stream from the bottom of the first acetaldehyde removal column 91, the iodomethane-enriched raffinate obtained in extraction column 92 (line 108), and the water-enriched residual stream from the bottom of the second acetaldehyde removal column 93 are each recycled to reaction tank 1 via lines 103, 111, and 113, or recycled to a suitable part of the process for reuse. For example, the iodomethane-enriched raffinate obtained in extraction column 92 can be recycled to distillation column 91 via line 110. The liquid in line 113 is typically discharged to the outside as drainage. The gases that are not condensed in condensers 91a and 93a (lines 106 and 116) are either absorbed or disposed of in the scrubbing system 8.

[0125] In addition, according to Figure 2In the process where the aqueous phase is treated in the acetaldehyde separation and removal step, for example, the aqueous phase is supplied to a distillation column (first acetaldehyde removal column) 91 via line 101 for distillation to separate it into an acetaldehyde-enriched overhead stream (line 102) and a water-enriched residual stream (line 103). The overhead stream is condensed using condenser 91a, and a portion of the condensate is returned to the top of the distillation column 91 (line 104), while the remaining portion is supplied to an extraction column 92 (line 105). The condensate supplied to the extraction column 92 is extracted using water introduced via line 109. The extract obtained after extraction is supplied to a distillation column (second acetaldehyde removal column) 93 via line 107 for distillation to separate it into an acetaldehyde-enriched overhead stream (line 112) and a water-enriched residual stream (line 113). Then, the acetaldehyde-enriched overhead stream is condensed using condenser 93a, with a portion of the condensate being returned to the top of distillation column 93 (line 114), and the remainder being discharged outside the system (line 115). Furthermore, the residual stream of enriched water from the bottom of the first acetaldehyde removal column 91, the raffinate enriched with iodomethane obtained in extraction column 92 (line 108), and the residual stream of enriched water from the bottom of the second acetaldehyde removal column 93 are each recycled to reaction tank 1 via lines 103, 111, and 113, or recycled to a suitable part of the process for reuse. For example, the raffinate enriched with iodomethane obtained in extraction column 92 can be recycled to distillation column 91 via line 110. The liquid in line 113 is typically discharged to the outside as drainage. Gases not condensed in condensers 91a and 93a (lines 106 and 116) are either absorbed in scrubbing system 8 or disposed of as waste.

[0126] The aqueous or organic phase supplied to the distillation column 91 via line 101 can satisfy the above (iv). The oxygen concentration in the aqueous or organic phase is the same as the oxygen concentration in the aqueous or organic phase in the decanter 4.

[0127] The liquid phases in the distillation process of the first acetaldehyde removal tower 91, the extraction process of the extraction tower 92, and the distillation process of the second acetaldehyde removal tower 93 can each be liquid phases that satisfy the above (iv). The oxygen concentrations in the following streams are as follows: the enriched water or iodomethane residue stream from the bottom of the first acetaldehyde removal tower 91 (line 103); the raffinate enriched with iodomethane obtained in the extraction tower 92 (line 108); the enriched water residue stream from the bottom of the second acetaldehyde removal tower 93 (line 113); and the extract obtained by extraction treatment in the extraction tower 92 (line 107). These concentrations are, for example, less than 10% by volume (e.g., 0.1 ppb to 10% by volume), preferably 0.2 ppb to 3.6% by volume (e.g., 1 ppb to 2% by volume), more preferably less than 1% by volume (e.g., 1 ppt to 1000 ppm by volume), further preferably less than 700 ppm by volume (e.g., 1 ppt to 500 ppm by volume), and particularly preferably 10 ppt to 300 ppm by volume (e.g., 100 ppt to 100 ppm by volume).

[0128] The condensate refluxed to the first acetaldehyde removal tower 91 (line 104) and the condensate refluxed to the second acetaldehyde removal tower 93 (line 114) can satisfy the above (iv). The oxygen concentration in the above condensate is the same as the oxygen concentration in the overhead flow of the tower (lines 102, 112).

[0129] The distillation process in the first acetaldehyde removal tower 91, the extraction process in the extraction tower 92, and the distillation process in the second acetaldehyde removal tower 93 can each satisfy the above (iii). The oxygen concentrations in the overhead stream (line 102) from the top of the first acetaldehyde removal tower 91, the gas component that has not been condensed in the condenser 91a (line 106), the overhead stream (line 112) from the top of the second acetaldehyde removal tower 93, and the gas component that has not been condensed in the condenser 93a (line 116) are, for example, less than 10 volume % (e.g., 10 volume ppb to 10 volume %), preferably 10 volume ppb to 3.6 volume % (e.g., 20 volume ppb to 2 volume %), more preferably 30 volume ppb to 1 volume % (e.g., 100 volume ppb to 0.1 volume ppm), and even more preferably 500 volume ppb to 500 volume ppm (e.g., 1 to 100 volume ppm). In addition, when the above-mentioned processes satisfy (iii) above, the oxygen concentration is less than 7 volume% (e.g., 1 volume ppt to 5 volume%), preferably less than 3.6 volume% (e.g., 0.1 volume ppb to 2 volume%), more preferably 1 volume ppb to 1 volume% (e.g., 10 volume ppb to 0.5 volume%), further preferably 20 volume ppb to 0.3 volume%, and particularly preferably 50 volume ppb to 0.1 volume% (e.g., 100 volume ppb to 200 volume ppm).

[0130] For acetaldehyde originating from the aforementioned process stream containing at least water, acetic acid (AC), methyl iodide (MeI), and acetaldehyde (AD), in addition to the methods described above, it can also be separated and removed using extractive distillation. For example, the organic and / or aqueous phases (feed liquid) obtained by separating the process stream can be supplied to a distillation column (extractive distillation column), and the extraction solvent (typically water) can be introduced into a concentration zone (e.g., the space from the top of the column to the feed liquid supply location) within the distillation column where methyl iodide and acetaldehyde are concentrated. The liquid descending from the concentration zone (extract liquid) can be extracted as a side stream, separated into an aqueous and organic phase, and the aqueous phase can be distilled to remove acetaldehyde from the system. It should be noted that if a significant amount of water is present in the distillation column, the liquid descending from the concentration zone can also be extracted as a side stream without introducing the extraction solvent into the distillation column. For example, a unit (such as a chimney tray) capable of receiving the liquid (extract) descending from the concentration zone can be configured in the distillation column, and the liquid (extract) received in this unit can be extracted as a side stream. The extraction solvent is preferably introduced above the feed liquid supply point, more preferably near the top of the column. Regarding the side stream extraction point, it is preferably located below the extraction solvent introduction point and above the feed liquid supply point in the column height direction. According to this method, acetaldehyde can be extracted at a high concentration from the concentrate of iodomethane and acetaldehyde using an extraction solvent (usually water). Furthermore, since the area between the extraction solvent introduction point and the side stream can be utilized as an extraction zone, acetaldehyde can be extracted efficiently with a small amount of extraction solvent. Therefore, compared to methods that extract the extract based on extractive distillation from the bottom of a distillation column (extractive distillation column), the number of distillation column layers can be significantly reduced, and the vapor load can also be reduced. Moreover, due to the use of a small amount of extraction solvent, compared to the method described above… Figure 2Compared to the method of combining aldehyde removal distillation and water extraction, this reduces the ratio of iodomethane to acetaldehyde (MeI / AD ratio) in the water extract, thus allowing acetaldehyde removal while suppressing the loss of iodomethane to the outside of the system. The acetaldehyde concentration in the side stream is significantly higher than that in the feed liquid and the bottom liquid. Furthermore, the ratio of acetaldehyde to iodomethane in the side stream is greater than that in the feed liquid and the bottom liquid. It should be noted that the organic phase (iodomethane phase) obtained by separating the side stream can be recycled to the distillation column. In this case, the recycling position of the organic phase obtained by separating the side stream is preferably below the side stream extraction position and preferably above the feed liquid supply position in the column height direction. In addition, a mixed solvent for the components constituting the organic phase obtained by separating the process stream (e.g., methyl acetate) can be introduced into the distillation column (extractive distillation column). Examples of such mixed solvents include acetic acid and ethyl acetate. Regarding the location of the introduction of the mixed solvent, in the height direction of the column, it is preferably below the side-stream extraction position and preferably above the feed liquid supply position. Furthermore, regarding the location of the introduction of the mixed solvent, if the organic phase obtained from the side-stream separation is to be recycled back to the distillation column, it is preferably below the recycling position. By recycling the organic phase obtained from the side-stream separation back to the distillation column, or by introducing the mixed solvent into the distillation column, the concentration of methyl acetate in the extract obtained from the side-stream extraction can be reduced, and the concentration of methyl acetate in the aqueous phase obtained from the separation of the extract can be reduced, thereby suppressing the mixing of iodomethane into the aqueous phase.

[0131] The theoretical trays of the aforementioned distillation column (extractive distillation column) are, for example, 1 to 100, preferably 2 to 50, more preferably 3 to 30, and even more preferably 5 to 20. Compared with the 80 to 100 trays of existing distillation columns and extractive distillation columns for acetaldehyde removal, acetaldehyde can be separated and removed efficiently with fewer trays. The mass ratio (formula / substrate) of the flow rate of the extraction solvent to the flow rate of the feed liquid (the organic phase and / or aqueous phase obtained by separating the process stream) can be selected from the range of 0.0001 / 100 to 100 / 100, typically 0.0001 / 100 to 20 / 100, preferably 0.001 / 100 to 10 / 100, more preferably 0.01 / 100 to 8 / 100, and even more preferably 0.1 / 100 to 5 / 100. The top temperature of the distillation column (extractive distillation column) is, for example, 15~120°C, preferably 20~90°C, more preferably 20~80°C, and even more preferably 25~70°C. The top pressure is measured in absolute pressure, for example, at 0.1~0.5 MPa. Other conditions of the distillation column (extractive distillation column) can be the same as those of conventional distillation columns and extractive distillation columns used for acetaldehyde removal.

[0132] Figure 3This is a schematic flow chart illustrating an example of the acetaldehyde separation and removal system utilizing extractive distillation described above. In this example, the organic phase and / or aqueous phase (feed liquid) obtained by separating the process stream is supplied to the middle section (between the top and bottom of the column) of distillation column 94 via feed line 201, and water is introduced from near the top of the column via line 202, allowing extractive distillation to take place within distillation column 94 (extractive distillation column). Above the feed liquid supply location in distillation column 94, a chimney tray 200 is provided for receiving the liquid (extract) descending from the concentration zone where iodomethane and acetaldehyde are concentrated within the column. In this extractive distillation, it is preferable to extract the entire volume of liquid from the chimney tray 200 and introduce it via line 208 to decanter 95 for separation. The aqueous phase (containing acetaldehyde) in decanter 95 is introduced to cooler 95a via line 212 for cooling, causing the dissolved iodomethane in the aqueous phase to undergo two-phase separation, and then separated using decanter 96. The aqueous phase in decanter 96 is fed to distillation column 97 (acetaldehyde removal column) via line 216 for distillation. The vapor at the top of the column is introduced to condenser 97a via line 217 for condensation, causing a portion of the condensate (mainly acetaldehyde and iodomethane) to be returned to the top of distillation column 97. The residue is either discarded or supplied to distillation column 98 (extractive distillation column) via line 220. Water from near the top of distillation column 98 is introduced via line 222 for extractive distillation. The vapor at the top of the column is introduced to condenser 98a via line 223 for condensation, causing a portion of the condensate (mainly iodomethane) to be returned to the top of the column. The residue is recycled to the reaction system via line 226, but may also be discharged from the system. For the organic phase (iodomethane phase) in decanter 95, it is preferable to recycle the entire amount through lines 209 and 210 to the position below the chimney tray 200 of distillation column 94. A portion of the aqueous phase in decanter 95 and the organic phase in decanter 96 are recycled to distillation column 94 through lines 213, 210, 214, and 210, respectively, but there are also cases where they are not recycled. A portion of the aqueous phase in decanter 95 can be used as the extraction solvent (water) in distillation column 94. A portion of the aqueous phase in decanter 96 can be recycled to distillation column 94 through line 210. Depending on the situation (e.g., if the feed liquid contains methyl acetate, etc.), a mixed solvent (acetic acid, ethyl acetate, etc.) that constitutes the components (e.g., methyl acetate, etc.) of the organic phase obtained from the process fraction can be fed into distillation column 94 through line 215 to improve distillation efficiency. Regarding the supply location of the mixed solvent to the distillation column 94, it is above the feed liquid supply section (connection of line 201) and below the connection of the recirculation line 210. The bottom residue of the distillation column 94 is recirculated to the reaction system.The vapor at the top of distillation column 94 is introduced to condenser 94a via line 203 for condensation. The condensate is separated in decanter 99. The organic phase is refluxed to the top of distillation column 94 via line 206, and the aqueous phase is introduced to decanter 95 via line 207. The bottom residue of distillation column 97 (mainly water) and the bottom residue of distillation column 98 (extractive distillation column) (water containing a small amount of acetaldehyde) are removed from the system via lines 218 and 224, respectively, or recycled back to the reaction system. Gases that are not condensed in condensers 94a, 97a, and 98a (lines 211, 221, and 227) are either absorbed in scrubbing system 8 or disposed of.

[0133] Figure 4 This is a schematic flow chart illustrating another example of the acetaldehyde separation and removal system utilizing extractive distillation described above. In this example, the condensate of the vapor at the top of distillation column 94 is directed to storage tank 100, and its entire volume is refluxed back to the top of distillation column 94 via line 206. In addition, with... Figure 3 The same applies to other instances.

[0134] Figure 5 This is a schematic flow chart illustrating another example of the acetaldehyde separation and removal system utilizing extractive distillation described above. In this example, the liquid on the chimney tray 200 is fully extracted and directly introduced through line 208 to cooler 95a for cooling without passing through decanter 95, and then supplied to decanter 96. In addition, with... Figure 4 The same applies to other instances.

[0135] The aqueous or organic phase supplied to the distillation column 94 via line 201 can satisfy the above (iv). The oxygen concentration in the aqueous or organic phase is the same as the oxygen concentration in the aqueous or organic phase in the decanter 4.

[0136] The liquid phases in the distillation processes of distillation column 94, distillation column 97, and distillation column 98 can each be liquid phases that satisfy the above (iv). The following are the components of a distillation column: bottom residue of distillation column 97 (line 218), bottom residue of distillation column 98 (line 224), condensate obtained by condensation in condenser 94a (line 205), aqueous and organic phases in decanter 99, organic phase refluxed to distillation column 94 (line 206), aqueous phase supplied to decanter 95 (line 207), aqueous and organic phases in decanter 95, aqueous phase supplied to condenser 95a (line 212), aqueous and organic phases in decanter 96, aqueous phase supplied to distillation column 97, liquid phase condensed in condenser 97a and refluxed to distillation column 97 (line 219), liquid phase supplied to distillation column 98 (line 220), and liquid phase condensed in condenser 98a and refluxed to distillation column 98 (line 225). The oxygen concentrations in the liquid phase obtained by recycling to distillation column 94 (lines 209, 213, 214), the bottom residue of distillation column 94 (line 204), the bottom residue of distillation column 97 (line 218), and the bottom residue of distillation column 98 (line 224) are, for example, less than 10 vol% (e.g., 0.1 vol ppb to 10 vol%), preferably 0.2 vol ppb to 3.6 vol% (e.g., 1 vol ppb to 2 vol%), more preferably less than 1 vol% (e.g., 1 vol ppt to 1000 vol ppm), further preferably less than 700 vol ppm (e.g., 1 vol ppt to 500 vol ppm), and particularly preferably 10 vol ppt to 300 vol ppm (e.g., 100 vol ppt to 100 vol ppm).

[0137] The gas phases in the distillation processes of distillation column 94, distillation column 97, and distillation column 98 can each satisfy the gas phase described in (iii) above. The oxygen concentrations in the overhead stream from the top of distillation column 94 (line 203), the uncondensed gas component in condenser 94a (line 211), the overhead stream from the top of distillation column 97 (line 217), the uncondensed gas component in condenser 97a (line 221), the overhead stream from the top of distillation column 98 (line 223), and the uncondensed gas component in condenser 98a (line 227) are, for example, less than 10% by volume (e.g., 10% to 10% by volume), preferably 10% to 3.6% by volume (e.g., 20% to 2% by volume), more preferably 30% to 1% by volume (e.g., 100% to 0.1% by volume), and even more preferably 500% to 500% by volume (e.g., 1 to 100% by volume). In addition, when the above-mentioned processes satisfy (iii) above, the oxygen concentration is less than 7 volume% (e.g., 1 volume ppt to 5 volume%), preferably less than 3.6 volume% (e.g., 0.1 volume ppb to 2 volume%), more preferably 1 volume ppb to 1 volume% (e.g., 10 volume ppb to 0.5 volume%), further preferably 20 volume ppb to 0.3 volume%, and particularly preferably 50 volume ppb to 0.1 volume% (e.g., 100 volume ppb to 200 volume ppm).

[0138] Additionally, the organic and / or aqueous phases separated by decanter 4 can be introduced into the alkane separation process (illustration omitted). In the alkane separation process, alkanes contained in the organic and / or aqueous phases can be separated and removed using known methods, such as distillation. For example, the organic phase is fed to a distillation column (dealkation column) for the alkane separation process and distilled, separating into overhead and bottom streams from the top or upper part of the dealkation column. A portion of the bottom stream containing alkanes is heated and recycled to the dealkation column, while the remainder is fed to an incinerator unit for incineration. On the other hand, the overhead stream containing acetaldehyde and iodomethane is cooled and condensed in a condenser, stored in a tank as condensate, a portion of which is returned to the dealkation column, and the remainder is recycled to the reaction tank.

[0139] The aqueous or organic phase supplied to the aforementioned dealkane tower can satisfy the above (iv). The oxygen concentration in the aforementioned aqueous or organic phase is the same as the oxygen concentration in the aforementioned decanter 4.

[0140] The liquid phase in the above-described alkane separation process can be a liquid phase that satisfies (iv) above. The oxygen concentrations in the bottom residue of the above-described dealkane tower, the liquid phase from the overhead flow of the dealkane tower that is condensed in the condenser and refluxed, and the bottom residue of the above-described dealkane tower are, for example, less than 10 vol% (e.g., 0.1 vol ppb to 10 vol%), preferably 0.2 vol ppb to 3.6 vol% (e.g., 1 vol ppb to 2 vol%), more preferably less than 1 vol% (e.g., 1 vol ppt to 1000 vol ppm), further preferably less than 700 vol ppm (e.g., 1 vol ppt to 500 vol ppm), and particularly preferably 10 vol ppt to 300 vol ppm (e.g., 100 vol ppt to 100 vol ppm).

[0141] The gas phase in the above-described alkane separation process can be the gas phase that satisfies (iii) above. The gas composition of the overhead stream from the dealkane tower and the uncondensed gas in the condenser used to cool the overhead stream is, for example, less than 10% by volume (e.g., 10% to 10% by volume), preferably 10% to 3.6% by volume (e.g., 20% to 2% by volume), more preferably 30% to 1% by volume (e.g., 100% to 0.1% by volume), and even more preferably 500% to 500% by volume (e.g., 1 to 100% by volume). In addition, when the above-mentioned alkane separation process satisfies (iii) above, the oxygen concentration is less than 7 volume% (e.g., 1 volume ppt to 5 volume%), preferably less than 3.6 volume% (e.g., 0.1 volume ppb to 2 volume%), more preferably 1 volume ppb to 1 volume% (e.g., 10 volume ppb to 0.5 volume%), further preferably 20 volume ppb to 0.3 volume%, and particularly preferably 50 volume ppb to 0.1 volume% (e.g., 100 volume ppb to 200 volume ppm).

[0142] In the above Figure 1 In the process, the gaseous components generated in condenser 3a contain, for example, carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, iodomethane, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. These components are supplied from condenser 3a to scrubbing system 8 via lines 32 and 15. The iodomethane, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid in the gaseous components arriving at scrubbing system 8 are absorbed by the absorption solvent. Hydrogen iodide reacts with methanol or methyl acetate in the absorption solvent to generate iodomethane. Then, the liquid components containing these useful components, such as iodomethane, are recycled from scrubbing system 8 to reaction tank 1 via recirculation lines 48 and 23 for reuse.

[0143] The bottom liquid extracted from the bottom of distillation column 3 contains more components with boiling points higher than acetic acid (high-boiling-point components) compared to the overhead and side streams from distillation column 3. These include, for example, propionic acid, and the aforementioned catalyst and co-catalyst entrained in the mist. The bottom liquid also contains acetic acid, iodomethane, methyl acetate, and water. In this embodiment, a portion of this bottom liquid is continuously fed into evaporation tank 2 via lines 25 and 26 for recycling, while the remaining portion is continuously fed into reaction tank 1 via lines 25 and 23 for recycling.

[0144] The first acetic acid stream, continuously extracted from distillation column 3 in a sidestream form, is more enriched in acetic acid than the vapor stream continuously introduced into distillation column 3. That is, the acetic acid concentration of the first acetic acid stream is higher than that of the vapor stream. The acetic acid concentration of the first acetic acid stream is, for example, 90-99.9% by mass, preferably 93-99% by mass. In addition to acetic acid, the first acetic acid stream may also contain, for example, iodomethane, hydrogen iodide, water, methyl acetate, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid. It should be noted that, regarding the connection position of line 27 relative to distillation column 3, in the height direction of distillation column 3, it may be above the connection position of line 21 relative to distillation column 3, as shown in the figure, but it may also be below the connection position of line 21 relative to distillation column 3, or it may be at the same connection position as line 21 relative to distillation column 3. The first acetic acid stream from distillation column 3 is continuously introduced into subsequent distillation column 5 via line 27 at a given flow rate. It should be noted that the first acetic acid stream extracted as a side stream from distillation column 3, the bottom liquid of distillation column 3, or the condensate of the vapor at the bottom of distillation column 3 can be used directly as the product acetic acid, or it can be introduced directly and continuously into distillation column 6 without passing through distillation column 5. It should also be noted that a portion of the first acetic acid stream can be returned to distillation column 3 (illustration omitted).

[0145] In the first acetic acid stream flowing through line 27, potassium hydroxide can be supplied or added via line 55 (potassium hydroxide inlet line). Potassium hydroxide can be supplied or added in solution form, such as an aqueous solution. By supplying or adding potassium hydroxide to the first acetic acid stream, hydrogen iodide in the first acetic acid stream can be reduced. Specifically, hydrogen iodide reacts with potassium hydroxide to produce potassium iodide and water. This reduces corrosion of equipment such as distillation columns caused by hydrogen iodide. It should be noted that potassium hydroxide can be supplied or added to suitable locations where hydrogen iodide is present in this process. It should also be noted that the potassium hydroxide added to the process reacts with acetic acid to produce potassium acetate.

[0146] The above-described aqueous solution of potassium hydroxide supplied or added to the first acetic acid stream can also satisfy the above (iv). The oxygen concentration in the above-described aqueous solution of potassium hydroxide is, for example, less than 10 vol% (e.g., 0.1 vol ppb to 10 vol%), preferably 0.2 vol ppb to 3.6 vol% (e.g., 1 vol ppb to 2 vol%), more preferably less than 1 vol% (e.g., 1 vol ppt to 1000 vol ppm), further preferably less than 700 vol ppm (e.g., 1 vol ppt to 500 vol ppm), and particularly preferably 10 vol ppt to 300 vol ppm (e.g., 100 vol ppt to 100 vol ppm).

[0147] Distillation column 5 is a unit for performing the second distillation step, and in this embodiment can be considered as a so-called dehydration column. The second distillation step is a step for further purifying acetic acid by distilling the first acetic acid stream continuously introduced into distillation column 5. Distillation column 5 includes, for example, a tray column and a packed column. When a tray column is used as distillation column 5, its theoretical number of trays is, for example, 5 to 50, and the reflux ratio is, for example, 0.1 to 3000, depending on the number of theoretical trays. Inside distillation column 5, which is present in the second distillation step, the top pressure is set to, for example, 10 to 500 kPaG, preferably 150 to 250 kPaG, and the bottom pressure is set to be higher than the top pressure, for example, 130 to 310 kPaG, preferably 160 to 290 kPaG. Inside the distillation column 5 in the second distillation step, the top temperature is set to, for example, a temperature of 130-175°C, which is higher than the boiling point of water under the set top pressure and lower than the boiling point of acetic acid, and the bottom temperature is set to, for example, a temperature of 150-185°C, which is higher than the boiling point of acetic acid under the set bottom pressure.

[0148] From the top of distillation column 5, the vapor as the overhead stream is continuously extracted to line 33. From the bottom of distillation column 5, the bottom residue is continuously extracted to line 34. 5b is a reboiler. From a height position between the top and bottom of distillation column 5, the side stream (liquid or gas) can be continuously extracted to line 34.

[0149] The vapor extracted from the top of distillation column 5 contains more components with boiling points lower than acetic acid (low-boiling components) compared to the bottom residue from distillation column 5, including, for example, iodomethane, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. This vapor is continuously introduced into condenser 5a via line 33.

[0150] Condenser 5a partially condenses the vapor from distillation column 5, separating it into condensate and gaseous components. The condensate includes, for example, water and acetic acid. A portion of the condensate is continuously refluxed from condenser 5a to distillation column 5 via line 35. The remaining portion is continuously introduced from condenser 5a into reaction tank 1 via lines 35, 36, and 23 for recirculation. Additionally, the gaseous components generated in condenser 5a, including, for example, carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, iodomethane, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid, are supplied from condenser 5a to scrubbing system 8 via lines 37 and 15. Hydrogen iodide in the gaseous components arriving at the washing system 8 is absorbed by the absorption solvent in the washing system 8. The hydrogen iodide in the absorption solvent reacts with methanol or methyl acetate to generate iodomethane. Then, the liquid components containing the iodomethane and other useful components are recycled from the washing system 8 to the reaction tank 1 through the recycling lines 48 and 23 for reuse.

[0151] The gas phase in the dehydration process using distillation column 5 described above can be the gas phase that satisfies (iii) above. The oxygen concentrations in the overhead stream (line 33) from the top of distillation column 5 and the gas component that has not been condensed by condenser 6a (line 45) are, for example, less than 10 volume % (e.g., 10 volume ppb to 10 volume %), preferably 10 volume ppb to 3.6 volume % (e.g., 20 volume ppb to 2 volume %), more preferably 30 volume ppb to 1 volume % (e.g., 100 volume ppb to 0.1 volume ppm), and even more preferably 500 volume ppb to 500 volume ppm (e.g., 1 to 100 volume ppm). Furthermore, when the gas phase in the dehydration process satisfies condition (iii) above, the oxygen concentration is less than 7% by volume (e.g., 1 to 5 ppt), preferably less than 3.6% by volume (e.g., 0.1 to 2 ppt), more preferably 1 to 1% by volume (e.g., 10 to 0.5 ppt), even more preferably 20 to 0.3% by volume, and particularly preferably 50 to 0.1% by volume (e.g., 100 to 200 ppm).

[0152] The bottom liquid (or side stream) extracted from the bottom of distillation column 5 contains more components with boiling points higher than acetic acid (high-boiling components) compared to the overhead stream from distillation column 5. These include, for example, propionic acid, potassium acetate (in the case of potassium hydroxide being supplied to line 27), and the aforementioned catalyst and co-catalyst entrained in the mist. This bottom liquid may also contain acetic acid. This bottom liquid is then continuously fed into subsequent distillation column 6 via line 34 as the second acetic acid stream.

[0153] The second acetic acid stream (residue from the bottom of distillation column 5, line 34) described above can satisfy the above (iv). The oxygen concentration in the second acetic acid stream is, for example, less than 10 vol% (e.g., 0.1 vol ppb to 10 vol%), preferably 0.2 vol ppb to 3.6 vol% (e.g., 1 vol ppb to 2 vol%), more preferably less than 1 vol% (e.g., 1 vol ppt to 1000 vol ppm), further preferably less than 700 vol ppm (e.g., 1 vol ppt to 500 vol ppm), and particularly preferably 10 vol ppt to 300 vol ppm (e.g., 100 vol ppt to 100 vol ppm).

[0154] In distillation column 5, in order to convert the hydrogen iodide contained in the first acetic acid stream into iodomethane and extract it as the overhead stream from line 33, methanol (illustration omitted) may be added at one or more points in distillation column 5.

[0155] In this invention, the distillation process using the distillation column (dehydration column) 5 preferably meets the operating conditions described in (ii) above: a hydrogen partial pressure of 5 kPa (absolute pressure) or less, a carbon dioxide partial pressure of less than 20 kPa (absolute pressure), and an operating temperature exceeding 100°C. In this case, the hydrogen partial pressure (absolute pressure) is preferably 2 kPa or less, more preferably 1 kPa or less, and even more preferably 0.5 kPa or less. The lower limit of the hydrogen partial pressure (absolute pressure) is 0 kPa, but it may exceed 0.0001 kPa. The carbon dioxide partial pressure (absolute pressure) is preferably 5 kPa or less, more preferably 2 kPa or less, and even more preferably 1 kPa or less (e.g., 0.5 kPa or less). The lower limit of the carbon dioxide partial pressure (absolute pressure) is 0 kPa, but it may exceed 0.0001 kPa. The operating temperature is preferably 120°C or more, more preferably 130°C or more. The upper limit of the operating temperature is, for example, 170°C, preferably 165°C, more preferably 160°C, and even more preferably 155°C.

[0156] When the distillation process using the distillation column (dehydration column) 5 meets the above operating conditions (ii), the concentration of acetic acid in the feed liquid to the distillation column 5 may be 30% by mass or more (e.g., 30 to 99.999% by mass) and the concentration of formic acid may be 5 ppm by mass or more (e.g., 5 to 10,000 ppm by mass). Furthermore, in the feed liquid to distillation column 5, the concentration of acetic acid is preferably 80-99.9% by mass (e.g., 90-99.9% by mass, particularly 93-99% by mass), the concentration of iodomethane is preferably 0.01-16% by mass (e.g., 0.1-8% by mass, particularly 0.2-5% by mass), the concentration of water is preferably 0.05-18% by mass (e.g., 0.1-8% by mass, particularly 0.2-5% by mass), the concentration of methyl acetate is preferably 0.01-16% by mass (e.g., 0.1-8% by mass, particularly 0.2-5% by mass), and the concentration of formic acid is preferably 5-10000 ppm by mass (e.g., 10-1000 ppm by mass, more preferably 10-500 ppm by mass, further preferably 15-200 ppm by mass, and particularly preferably 20-100 ppm by mass). By ensuring that the distillation process using distillation column 5 meets the above operating conditions (ii), the formation of formic acid in distillation column 5 can be suppressed, and the formic acid can be effectively decomposed when a liquid containing formic acid is supplied to distillation column 5.

[0157] The second acetic acid stream is more enriched in acetic acid than the first acetic acid stream, which is continuously introduced into the distillation column 5. That is, the acetic acid concentration in the second acetic acid stream is higher than that in the first acetic acid stream. The acetic acid concentration in the second acetic acid stream is in the range of 99.1% to 99.99% by mass, which is higher than that in the first acetic acid stream. Furthermore, as described above, the second acetic acid stream also contains, in addition to acetic acid, substances such as propionic acid and hydrogen iodide. In this embodiment, when the side stream is extracted, the extraction position of the side stream from the distillation column 5 is lower than the introduction position of the first acetic acid stream introduced into the distillation column 5 in the height direction of the distillation column 5.

[0158] In the second acetic acid stream flowing through line 34, potassium hydroxide can be supplied or added via line 56 (potassium hydroxide inlet line). Potassium hydroxide can be supplied or added in solution form, such as an aqueous solution. By supplying or adding potassium hydroxide to the second acetic acid stream, hydrogen iodide in the stream can be reduced. Specifically, hydrogen iodide reacts with potassium hydroxide to produce potassium iodide and water. This reduces corrosion of equipment such as distillation columns caused by hydrogen iodide. It should be noted that the oxygen concentration in the second acetic acid stream after supplying or adding potassium hydroxide via the potassium hydroxide inlet line is the same as the oxygen concentration in the second acetic acid stream (the bottom liquid of distillation column 5) described above.

[0159] Distillation column 6 is a unit for performing the third distillation step, and in this embodiment, it is considered a so-called high-boiling-point removal column. The third distillation step is a step for further purifying acetic acid after purifying the second acetic acid stream continuously introduced into distillation column 6. Distillation column 6 includes, for example, tray columns and packed columns. When a tray column is used as distillation column 6, its theoretical number of trays is, for example, 5 to 50, and the reflux ratio is, for example, 0.2 to 3000, depending on the number of theoretical trays. Inside distillation column 6 in the third distillation step, the top pressure is set to, for example, -100 to 150 kPaG, and the bottom pressure is set to be higher than the top pressure, for example, -90 to 180 kPaG. Inside distillation column 6 in the third distillation step, the top temperature is set to, for example, a temperature 50 to 150°C higher than the boiling point of water at the set top pressure and lower than the boiling point of acetic acid, and the bottom temperature is set to, for example, a temperature 70 to 160°C higher than the boiling point of acetic acid at the set bottom pressure.

[0160] From the top of distillation column 6, the vapor as overhead stream is continuously extracted to line 38. From the bottom of distillation column 6, the bottom residue is continuously extracted to line 39. 6b is a reboiler. From a height position between the top and bottom of distillation column 6, the side stream (liquid or gas) is continuously extracted to line 46. In the height direction of distillation column 6, the connection position of line 46 relative to distillation column 6 can be above the connection position of line 34 relative to distillation column 6 as shown in the figure, but it can also be below the connection position of line 34 relative to distillation column 6, or it can be the same as the connection position of line 34 relative to distillation column 6.

[0161] The vapor extracted from the top of distillation column 6 contains more components with boiling points lower than acetic acid (low-boiling components) compared to the aforementioned bottom residue from distillation column 6. Besides acetic acid, these components include, for example, iodomethane, hydrogen iodide, water, methyl acetate, dimethyl ether, methanol, and formic acid. This vapor is continuously introduced into condenser 6a via line 38.

[0162] Condenser 6a partially condenses the vapor from distillation column 6, separating it into condensate and gaseous components. The condensate includes, in addition to acetic acid, substances such as iodomethane, hydrogen iodide, water, methyl acetate, dimethyl ether, methanol, and formic acid. At least a portion of the condensate is continuously refluxed from condenser 6a to distillation column 6 via line 40. A portion of the condensate (distillate) can be recycled from condenser 6a via lines 40, 41, and 42 to the first acetic acid stream in line 27 before it is introduced into distillation column 5. Simultaneously or alternatively, a portion of the condensate (distillate) can be recycled from condenser 6a via lines 40, 41, and 43 to the vapor stream in line 21 before it is introduced into distillation column 3. Furthermore, a portion of the condensate (distillate) can be recycled from condenser 6a via lines 40, 44, and 23 to reaction tank 1. Further, a portion of the distillate from condenser 6a can be supplied to washing system 8 as described above and used as an absorbent solvent within this system. In the washing system 8, the gaseous components that have absorbed the useful components are discharged outside the apparatus. Then, the liquid components containing the useful components are introduced from the washing system 8 through recirculation lines 48 and 23 to the reaction tank 1 for reuse. Furthermore, a portion of the distillate from the condenser 6a can be directed via lines not shown in the figure to various pumps operating within the apparatus (not shown) and used as a sealing fluid for these pumps. Further, a portion of the distillate from the condenser 6a can be routinely extracted outside the apparatus via an extraction line attached to line 40, or it can be extracted outside the apparatus in an irregular manner when necessary. When a portion of the condensate (distillate) is removed from the distillation process system in the distillation column 6, the amount of distillate (distillate quantity) is, for example, 0.01 to 30% by mass of the condensate produced in the condenser 6a, preferably 0.1 to 10% by mass, more preferably 0.3 to 5% by mass, and even more preferably 0.5 to 3% by mass. On the other hand, the gaseous components generated in condenser 6a include, for example, carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, iodomethane, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid, and are supplied from condenser 6a to scrubbing system 8 via lines 45 and 15.

[0163] The gas phase in the high-boiling point removal process using distillation column 6 described above can be the gas phase that satisfies (iii) above. The oxygen concentration in the overhead stream (line 38) from the top of distillation column 6 is, for example, less than 10% by volume (e.g., 10% to 10% by volume), preferably 10% to 3.6% by volume (e.g., 20% to 2% by volume), more preferably 30% to 1% by volume (e.g., 100% to 0.1% by volume), and even more preferably 500% to 500% by volume (e.g., 1 to 100% by volume). In addition, when the gas phase in the high-boiling point removal process satisfies condition (iii) above, the oxygen concentration is less than 7 vol% (e.g., 1 volpt to 5 vol%), preferably less than 3.6 vol% (e.g., 0.1 volpb to 2 vol%), more preferably 1 volpb to 1 vol% (e.g., 10 volpb to 0.5 vol%), further preferably 20 volpb to 0.3 vol%, and particularly preferably 50 volpb to 0.1 vol% (e.g., 100 volpb to 200 volp).

[0164] The bottom liquid extracted from the bottom of distillation column 6 via line 39 contains more components with boiling points higher than acetic acid (high-boiling-point components) compared to the overhead stream from distillation column 6, including, for example, propionic acid, acetic anhydride, and potassium acetate (in the case where potassium hydroxide is supplied to line 34, etc.). Furthermore, the bottom liquid extracted from the bottom of distillation column 6 via line 39 also contains corrosive metals such as metals generated and released from the inner walls of the components of the acetic acid preparation apparatus, as well as compounds of iodine and these corrosive metals derived from corrosive iodine. In this embodiment, such bottom liquid is discharged outside the acetic acid preparation apparatus.

[0165] The above-mentioned column bottom residue (line 39) can satisfy the above (iv). The oxygen concentration in the above-mentioned column bottom residue is, for example, less than 10 volume% (e.g., 0.1 volume ppb to 10 volume%), preferably 0.2 volume ppb to 3.6 volume% (e.g., 1 volume ppb to 2 volume%), more preferably less than 1 volume% (e.g., 1 volume ppt to 1000 volume ppm), further preferably less than 700 volume ppm (e.g., 1 volume ppt to 500 volume ppm), and particularly preferably 10 volume ppt to 300 volume ppm (e.g., 100 volume ppt to 100 volume ppm).

[0166] The side stream continuously extracted from distillation column 6 and fed to line 46 is continuously introduced as the third acetic acid stream into the subsequent ion exchange resin column 7. This third acetic acid stream is more enriched in acetic acid than the second acetic acid stream continuously introduced into distillation column 6. That is, the acetic acid concentration of the third acetic acid stream is higher than that of the second acetic acid stream. The acetic acid concentration of the third acetic acid stream is in a range higher than that of the second acetic acid stream, for example, 99.8 to 99.999% by mass. In this embodiment, the extraction position of the side stream from distillation column 6 is higher than the introduction position of the second acetic acid stream into distillation column 6 in the height direction of distillation column 6. In other embodiments, the extraction position of the side stream from distillation column 6 is lower than or the same as the introduction position of the second acetic acid stream into distillation column 6 in the height direction of distillation column 6. It should be noted that distillation column 6 can be replaced by a single-stage still (evaporator), and distillation column 6 can be omitted if impurities can be sufficiently removed using distillation column 5.

[0167] The third acetic acid stream (line 46) described above can satisfy the above (iv). The oxygen concentration in the third acetic acid stream is, for example, less than 10 vol% (e.g., 0.1 vol ppb to 10 vol%), preferably 0.2 vol ppb to 3.6 vol% (e.g., 1 vol ppb to 2 vol%), more preferably less than 1 vol% (e.g., 1 vol ppt to 1000 vol ppm), further preferably less than 700 vol ppm (e.g., 1 vol ppt to 500 vol ppm), and particularly preferably 10 vol ppt to 300 vol ppm (e.g., 100 vol ppt to 100 vol ppm).

[0168] In this invention, the distillation process using the distillation column (de-boiling column) 6 preferably satisfies the operating conditions described in (ii) above: a hydrogen partial pressure of 5 kPa (absolute pressure) or less, a carbon dioxide partial pressure of less than 20 kPa (absolute pressure), and an operating temperature exceeding 100°C. In this case, the hydrogen partial pressure (absolute pressure) is preferably 2 kPa or less, more preferably 1 kPa or less, and even more preferably 0.5 kPa or less. The lower limit of the hydrogen partial pressure (absolute pressure) is 0 kPa, but it may exceed 0.0001 kPa. The carbon dioxide partial pressure (absolute pressure) is preferably 5 kPa or less, more preferably 2 kPa or less, and even more preferably 1 kPa or less (e.g., 0.5 kPa or less). The lower limit of the carbon dioxide partial pressure (absolute pressure) is 0 kPa, but it may exceed 0.0001 kPa. The operating temperature is preferably 120°C or more, more preferably 130°C or more. The upper limit of the operating temperature is, for example, 165°C, preferably 160°C, and even more preferably 155°C.

[0169] When the distillation process using the distillation column (de-boiling column) 6 meets the above-described operating conditions (ii), the concentration of acetic acid in the feed liquid to the distillation column 6 is preferably 99.1 to 99.999% by mass, and the concentration of formic acid is preferably 5 to 9000 ppm by mass (e.g., 10 to 1000 ppm by mass, more preferably 10 to 500 ppm by mass, further preferably 15 to 200 ppm by mass, and particularly preferably 20 to 100 ppm by mass). By ensuring that the distillation process using the distillation column 6 meets the above-described operating conditions (ii), the formation of formic acid in the distillation column 6 can be suppressed, and the formic acid can be effectively decomposed when a liquid containing formic acid is supplied to the distillation column 6.

[0170] Ion exchange resin tower 7 is a purification unit for performing an adsorption removal process. This adsorption removal process further purifies acetic acid by adsorbing and removing trace amounts of iodoalkanes (iodohexane, iododecane, etc.) from the third acetic acid stream continuously introduced into the ion exchange resin tower 7. In the ion exchange resin tower 7, an ion exchange resin bed is formed by filling the tower with an ion exchange resin capable of adsorbing iodoalkanes. Examples of such ion exchange resins include, for instance, cation exchange resins in which some of the leaving protons of the exchange groups (sulfonic acid groups, carboxyl groups, phosphonic acid groups, etc.) are replaced by metals such as silver or copper. In the adsorption removal process, for example, the third acetic acid stream (liquid) is circulated inside the ion exchange resin tower 7 filled with such an ion exchange resin. During this circulation, impurities such as iodoalkanes in the third acetic acid stream are adsorbed onto the ion exchange resin and removed from the third acetic acid stream. In the ion exchange resin tower 7, which includes the adsorption removal process, the internal temperature is, for example, 18~100°C, and the flow rate of the acetic acid stream is [resin volume per m³]. 3 Acetic acid treatment capacity (m 3 / h)] is, for example, 3~15m 3 / h･m 3 (Resin volume).

[0171] A fourth acetic acid stream is continuously introduced from the lower end of the ion exchange resin column 7 into line 47. The acetic acid concentration of the fourth acetic acid stream is higher than that of the third acetic acid stream. That is, the fourth acetic acid stream is more enriched in acetic acid compared to the third acetic acid stream, which is continuously introduced into the ion exchange resin column 7. The acetic acid concentration of the fourth acetic acid stream is in a range higher than that of the third acetic acid stream, for example, 99.9 to 99.999% by mass or more. In this preparation method, this fourth acetic acid stream can be stored in a product tank not shown in the figure.

[0172] The fourth acetic acid stream described above satisfies (iv) above. The oxygen concentration in the fourth acetic acid stream described above is the same as the oxygen concentration in the third acetic acid stream described above.

[0173] In this acetic acid preparation apparatus, a purification unit, serving as a distillation column, can be provided as a product column or finished product column for further purification of the fourth acetic acid stream from ion exchange resin column 7. When such a product column is provided, it includes, for example, a tray column and a packed column, a distillation column. When a tray column is used as the product column, its theoretical number of trays is, for example, 5 to 50, and the reflux ratio is, for example, 0.5 to 3000, depending on the number of theoretical trays. Inside the product column in the purification process, the top pressure is set, for example, to -195 to 150 kPaG, and the bottom pressure is set higher than the top pressure, for example, -190 to 180 kPaG. Inside the product column, the top temperature is set, for example, to a temperature 50 to 150°C higher than the boiling point of water at the set top pressure and lower than the boiling point of acetic acid, and the bottom temperature is set, for example, to a temperature 70 to 160°C higher than the boiling point of acetic acid at the set bottom pressure. It should be noted that a product column or finished product column can also be replaced by a single-stage distillation apparatus (evaporator).

[0174] In the case of a product column, all or part of the fourth acetic acid stream (liquid) from ion exchange resin column 7 is continuously introduced into the product column. From the top of such a product column, vapor, as a top stream containing trace amounts of low-boiling-point components (e.g., iodomethane, water, methyl acetate, dimethyl ether, crotonaldehyde, acetaldehyde, and formic acid, etc.), is continuously extracted. This vapor is separated into condensate and gaseous components using a given condenser. A portion of the condensate is continuously refluxed back to the product column, and another portion of the condensate is recycled to reaction tank 1, or discarded outside the system, or both. The gaseous component is supplied to washing system 8. From the bottom of the product column, a bottom residue containing trace amounts of high-boiling-point components is continuously extracted and recycled to the second acetic acid stream, for example, in line 34 before being introduced into distillation column 6. From a height position between the top and bottom of the product column, a side stream (liquid) is continuously extracted as the fifth acetic acid stream. Regarding the extraction position from the side stream of the product column, the product is located at a lower position in the column's height direction, for example, than the introduction position of the fourth acetic acid stream into the product column. Regarding the fifth acetic acid stream, the acetic acid is more concentrated compared to the fourth acetic acid stream, which is continuously introduced into the product column. That is, the acetic acid concentration of the fifth acetic acid stream is higher than that of the fourth acetic acid stream. The acetic acid concentration of the fifth acetic acid stream is in a range higher than that of the fourth acetic acid stream, for example, 99.9 to 99.999% by mass or more. This fifth acetic acid stream can, for example, be stored in a product tank not shown in the figure. It should be noted that, regarding the ion exchange resin column 7, as an alternative to being located downstream of the distillation column 6 (or in addition to being located downstream of the distillation column 6), it can also be located downstream of the product column to treat the acetic acid stream exiting the product column.

[0175] The gas phase in the product process using the product tower described above can be the gas phase that satisfies (iii) above. The oxygen concentrations in the overhead stream from the top of the product tower and the condensate returned to the product tower are, for example, less than 10% by volume (e.g., 10% to 10% by volume), preferably 10% to 3.6% by volume (e.g., 20% to 2% by volume), more preferably 30% to 1% by volume (e.g., 100% to 0.1% by volume), and even more preferably 500% to 500% by volume (e.g., 1 to 100% by volume). Furthermore, when the gas phase in the product process satisfies (iii) above, the oxygen concentration is less than 7% by volume (e.g., 1 to 5 ppt), preferably less than 3.6% by volume (e.g., 0.1 to 2 ppt), more preferably 1 to 1% by volume (e.g., 10 to 0.5 ppt), even more preferably 20 to 0.3% by volume, and particularly preferably 50 to 0.1% by volume (e.g., 100 to 200 ppm).

[0176] The gas composition separated from the overhead stream from the top of the product tower can satisfy (iii) above. The oxygen concentration in the gas composition can be, for example, less than 10 vol% (e.g., 10 vol ppb to 10 vol%), preferably 10 vol ppb to 3.6 vol% (e.g., 20 vol ppb to 2 vol%), more preferably 30 vol ppb to 1 vol% (e.g., 100 vol ppb to 0.1 vol%), and even more preferably 500 vol ppb to 500 vol ppm (e.g., 1 to 100 vol ppm). Furthermore, when the gas composition satisfies (iii) above, the oxygen concentration is less than 7 vol% (e.g., 1 vol ppb to 5 vol%), preferably less than 3.6 vol% (e.g., 0.1 vol ppb to 2 vol%), more preferably 1 vol ppb to 1 vol% (e.g., 10 vol ppb to 0.5 vol%), even more preferably 20 vol ppb to 0.3 vol%, and particularly preferably 50 vol ppb to 0.1 vol% (e.g., 100 vol ppb to 200 vol ppm).

[0177] The aforementioned fifth acetic acid stream and the aforementioned bottom residue can satisfy the above (iv). The oxygen concentration in the aforementioned fifth acetic acid stream is, for example, less than 10 vol% (e.g., 0.1 vol ppb to 10 vol%), preferably 0.2 vol ppb to 3.6 vol% (e.g., 1 vol ppb to 2 vol%), more preferably less than 1 vol% (e.g., 1 vol ppt to 1000 vol ppm), further preferably less than 700 vol ppm (e.g., 1 vol ppt to 500 vol ppm), and particularly preferably 10 vol ppt to 300 vol ppm (e.g., 100 vol ppt to 100 vol ppm).

[0178] In this invention, the distillation process using a distillation column (product column) preferably meets the operating conditions described in (ii) above: a hydrogen partial pressure of 5 kPa (absolute pressure) or less, a carbon dioxide partial pressure of less than 20 kPa (absolute pressure), and an operating temperature exceeding 100°C. In this case, the hydrogen partial pressure (absolute pressure) is preferably 2 kPa or less, more preferably 1 kPa or less, and even more preferably 0.5 kPa or less. The lower limit of the hydrogen partial pressure (absolute pressure) is 0 kPa, but it may exceed 0.0001 kPa. The carbon dioxide partial pressure (absolute pressure) is preferably 5 kPa or less, more preferably 2 kPa or less, and even more preferably 1 kPa or less (e.g., 0.5 kPa or less). The lower limit of the carbon dioxide partial pressure (absolute pressure) is 0 kPa, but it may exceed 0.0001 kPa. The operating temperature is preferably 120°C or more, more preferably 130°C or more. The upper limit of the operating temperature is, for example, 165°C, preferably 160°C, and even more preferably 155°C.

[0179] When the distillation process using the distillation column (product column) meets the above-described operating conditions (ii), the concentration of acetic acid in the feed liquid to the distillation column (product column) is preferably 99.8 to 99.999% by mass, and the concentration of formic acid is preferably 5 to 2000 ppm by mass (e.g., 5 to 1000 ppm by mass, preferably 5 to 100 ppm by mass). By ensuring that the distillation process using the distillation column (product column) meets the above-described operating conditions (ii), the formation of formic acid in the distillation column (product column) can be suppressed, and the formic acid can be effectively decomposed when a liquid containing formic acid is supplied to the distillation column (product column).

[0180] In the scrubbing system 8, useful components (e.g., iodomethane, water, methyl acetate, acetic acid, etc.) are separated and recovered from the gaseous components generated in the acetic acid preparation process. In this embodiment, this separation and recovery can be performed using a wet scrubbing method, which is carried out using an absorbent solvent for capturing the useful components in the gaseous components. Preferably, the absorbent solvent contains at least acetic acid and / or methanol. Methyl acetate may also be contained in the absorbent solvent. For example, condensate from the vapor of distillation column 6 can be used as the absorbent solvent. The separation and recovery can be performed using pressure-fluctuation adsorption. The separated and recovered useful components (e.g., iodomethane, etc.) are recycled from the scrubbing system 8 to the reaction tank 1 via recirculation line 48. The gas after capturing the useful components is discarded via line 49. The same applies to the aforementioned gaseous components supplied to the scrubbing system 8 from other condensers, regarding the treatment in the scrubbing system 8 and the subsequent recycling and discarding to the reaction tank 1. In the preparation method of the present invention, a washing step is preferably included, in which exhaust gas from the process is absorbed and treated by an absorbent solvent containing at least acetic acid to separate a stream enriched with carbon monoxide and a stream enriched with acetic acid.

[0181] The washing system 8 may include, for example, a process of absorbing exhaust gas into an absorbent solvent under high pressure (high pressure absorption process), a process of absorbing exhaust gas into an absorbent solvent under low pressure, and a process of diffusing out the gas components absorbed in the above-mentioned high pressure absorption process and low pressure absorption process (diffusion process).

[0182] In the high-pressure absorption process, the gaseous components from reaction tank 1 (exhaust gas enriched with carbon monoxide and iodomethane) are washed in a high-pressure absorption tower by contacting acetic acid, which serves as the absorption solvent, separating into an overhead stream enriched with carbon monoxide and a bottom stream enriched with iodomethane, methyl acetate, and water. A portion of the overhead stream is fed to evaporation tank 2, while the remainder is fed to a boiler and utilized as a heat source for the process, or discharged into the atmosphere through a flare stack or vent stack. The remaining portion of the overhead stream can be incinerated or recovered. The bottom stream is fed to a diffusion tower.

[0183] In the low-pressure absorption process, the uncondensed gas components from the condenser 3a in the de-boiling process and the gas components from the evaporation tank 2 (exhaust gas enriched with acetic acid, iodomethane, and methyl acetate) merge to form a mixture. This mixture is then washed in the low-pressure absorption tower by contacting acetic acid, which serves as the absorption solvent, and separated into an overhead stream enriched with carbon monoxide, carbon dioxide, and nitrogen, and a bottom stream enriched with acetic acid, iodomethane, and methyl acetate. The overhead stream merges with the overhead stream from the high-pressure absorption tower to form a mixed gas, which is supplied to the boiling tank and used as a heat source for the process. A portion of the bottom stream merges with a portion of the bottom stream from the high-pressure absorption tower and is supplied to the evaporation tank 2. The remainder of the bottom stream merges with the bottom stream from the high-pressure absorption tower to form a mixed acetic acid stream, which is supplied to the diffusion tower.

[0184] In the diffusion process, the above-mentioned mixed acetic acid stream is distilled and stripped in a diffusion tower (stripping tower) to separate it into an overhead stream enriched with iodomethane and acetic acid (also containing methyl acetate, acetaldehyde, etc.) and a bottom stream enriched with acetic acid, methyl acetate, and water. The first portion of the bottom stream is heated using a heating unit and sent back to the lower part of the diffusion tower. Alternatively, the second portion (or the remainder) of the bottom stream can be combined with a portion of the condensate from the overhead stream of distillation tower 6 and mixed, with a portion of the mixture recycled to the upper part of the high-pressure absorption tower and the remainder recycled to the upper part of the low-pressure absorption tower. Alternatively, the overhead stream can be cooled and condensed in a condenser, and the gaseous components (a gaseous component enriched with iodomethane and carbon monoxide and also containing carbon dioxide, methane, ethyl acetate, acetaldehyde, etc.) can be combined with the gaseous components from the decanter 4 or the gaseous components from the vapor stream 17 from the evaporator 2 and cooled and condensed in a condenser. The condensate from the top of the column (a condensate enriched with iodomethane, acetic acid, and methyl acetate, and also containing water, acetaldehyde, etc.) can be recycled to reaction tank 1.

[0185] The gas phases in the aforementioned high-pressure absorption process, low-pressure absorption process, and diffusion process can respectively satisfy the above-mentioned (iii). The oxygen concentrations in the overhead stream from the high-pressure absorption tower, the overhead stream from the low-pressure absorption tower, the overhead stream from the diffusion tower, and the gas components in these overhead streams that have not been condensed due to cooling by the condenser are, for example, less than 10% by volume (e.g., 10% to 10% by volume), preferably 10% to 3.6% by volume (e.g., 20% to 2% by volume), more preferably 30% to 1% by volume (e.g., 100% to 0.1% by volume), and even more preferably 500% to 500% by volume (e.g., 1 to 100% by volume). Furthermore, when the gas phase in each of the above processes satisfies condition (iii) above, the oxygen concentration is less than 7 volume% (e.g., 1 volume ppt to 5 volume%), preferably less than 3.6 volume% (e.g., 0.1 volume ppb to 2 volume%), more preferably 1 volume ppb to 1 volume% (e.g., 10 volume ppb to 0.5 volume%), even more preferably 20 volume ppb to 0.3 volume%, and particularly preferably 50 volume ppb to 0.1 volume% (e.g., 100 volume ppb to 200 volume ppm).

[0186] The aforementioned high-pressure absorption process, low-pressure absorption process, and diffusion process can respectively satisfy the above (iv). The oxygen concentration in the condensate formed by the condenser in the bottom flow of the high-pressure absorption tower, the bottom flow of the low-pressure absorption tower, the bottom flow of the diffusion tower, and the top flow from the diffusion tower is, for example, less than 10% by volume (e.g., 0.1 ppb to 10% by volume), preferably 0.2 ppb to 3.6% by volume (e.g., 1 ppb to 2% by volume), more preferably less than 1% by volume (e.g., 1 ppt to 1000 ppm by volume), further preferably less than 700 ppm by volume (e.g., 1 ppt to 500 ppm by volume), and particularly preferably 10 ppt to 300 ppm by volume (e.g., 100 ppt to 100 ppm by volume).

[0187] In the above embodiments, as described above, the dwell time in the process that satisfies operating condition (i) and the process that satisfies operating condition (ii) is preferably 1 minute or more (e.g., 2 minutes or more, preferably 3 minutes or more, more preferably 5 minutes or more, and particularly 10 minutes or more). The upper limit of the dwell time is, for example, 2 hours, preferably 1 hour.

[0188] Additionally, a process solution with a formic acid concentration of 10 ppm or more (e.g., 10 to 10,000 ppm, preferably 15 to 1,000 ppm, more preferably 20 to 200 ppm) can be recycled to a process that meets the operating conditions (v) of a hydrogen partial pressure below 500 kPa (absolute pressure), a carbon dioxide partial pressure below 70 kPa (absolute pressure), and an operating temperature above 100°C. Examples of processes meeting operating conditions (v) include reaction processes, evaporation processes, and distillation processes (e.g., the aforementioned de-boiling point removal process, dehydration process, etc.). Processes meeting operating conditions (v) include processes meeting operating conditions (i) and processes meeting operating conditions (ii). By recycling a process solution with a formic acid concentration of 10 ppm or more to a process meeting operating conditions (v), the formic acid contained in the process solution can be effectively decomposed in that process.

[0189] Furthermore, the overhead liquid from at least one distillation step, such as the aforementioned low-boiling point removal step, dehydration step, high-boiling point removal step, or product step, can be recycled to a step that satisfies the aforementioned operating condition (i) or operating condition (ii). Examples of steps that satisfy operating condition (i) or operating condition (ii) include, for example, the aforementioned reaction step, evaporation step, low-boiling point removal step, and dehydration step. In this case, the preferred destination for recycling the overhead liquid from the distillation column is the reaction step, or an evaporation step or distillation step located upstream of the distillation step involving the distillation column (e.g., the aforementioned low-boiling point removal step, dehydration step, and high-boiling point removal step).

[0190] In the process liquid (e.g., the overhead liquid of a distillation column in at least one of the distillation steps, containing the aqueous and organic phases separated by the decanter) of the process recirculated to the operating conditions satisfying (v), the acetic acid concentration is preferably 5% by mass or more (e.g., 10% by mass or more), more preferably 20% by mass or more (e.g., 30% by mass or more), further preferably 40% by mass or more (e.g., 50% by mass or more), particularly preferably 60% by mass or more (e.g., 70% by mass or more), and especially 80% by mass or more (e.g., 90% by mass or more). The upper limit of the acetic acid concentration is preferably 99.999% by mass, or may be 99.99% by mass or 99.9% by mass. In addition, the process liquid recirculated may be the overhead liquid of a distillation column in which the acetic acid concentration in the feed liquid is within the above range.

[0191] Example The present invention will now be described in more detail with reference to the embodiments, but the present invention is not limited to these embodiments. It should be noted that "MeI" represents iodomethane, "MA" represents methyl acetate, "LiI" represents lithium iodide, and "Rh" represents rhodium. In the compositional analysis of the liquid phase, the water concentration is determined using the Karl Fischer method, the formic acid concentration is determined using liquid chromatography, and the rhodium concentration is determined using ICP analysis (or atomic absorption spectrometry). Regarding the lithium iodide concentration, Li is determined using ICP analysis, iodine is determined using electrotitration, and the concentrations of other components are determined using gas chromatography. The partial pressures of each gas component in the gas phase are calculated from the total pressure and the concentrations of each gas component determined using gas chromatography. "%" and "ppm" represent "mass %" and "mass ppm," respectively.

[0192] Comparative Example 1 In a 1000ml zirconium-based autoclave, MeI, MA, water, LiI, and rhodium iodide (in the experiment, a complex catalyst [Rh(CO)₂I₂]) were fed to achieve the initial feed composition shown in Table 1. - (The Rh concentration in the table is converted to metal concentrations.) After air replacement (maintaining atmospheric pressure), H2, CO2, CO, and air (nitrogen:oxygen (volume ratio) = 80:20) were fed into the autoclave to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, CO, and O2 concentration in the gas phase as shown in Table 1. The autoclave was maintained at 180°C for 30 minutes using an oil bath. The total pressure immediately after reaching 180°C was 5.5 MPaG, and decreased to 5.3 MPaG after 8 minutes. After cooling, samples were taken from the liquid for compositional analysis. The formic acid concentration was 55 ppm. The MA and water concentrations decreased to 2.3% and 1.6%, respectively. The reasons for this are a combination of the following: MA reacted with water to produce methanol and acetic acid; CO reacted with the equilibrium-produced methanol in a carbonylation reaction, consuming CO and methanol and producing acetic acid; some methanol dimerized, producing dimethyl ether and water; etc. It should be noted that water decreases due to the decomposition of MA, but increases due to the formation of dimethyl ether, resulting in minimal concentration changes. Other components remained largely unchanged. The compositional analysis results at the beginning and end of the experiment are shown in the table below. Additionally, acetic acid concentration was used as the balance, but other trace impurities were present at approximately 0.2%, primarily dimethyl ether and methanol.

[0193] Comparative Example 2 To achieve the initial feed composition shown in Table 1, the feed consisted of MeI, MA, water, LiI, and rhodium iodide (in the experiment, a complex catalyst [Rh(CO)₂I₂]). -(The Rh concentration in the table is converted to metal concentration), and acetic acid, were fed into the autoclave to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, CO, and O2 concentration in the gas phase as shown in Table 1. Otherwise, the same experiment as Comparative Example 1 was performed. After cooling, samples of the liquid were taken and analyzed. The formic acid concentration was 48 ppm. The MA and water concentrations decreased to 2.3% and 1.6%, respectively. The reasons are as explained in Comparative Example 1. It should be noted that the results of the composition analysis at the beginning and end of the experiment are shown in the table below. Furthermore, the acetic acid concentration was used as a margin, but other trace impurities were present at approximately 0.2%, mainly dimethyl ether and methanol.

[0194] Comparative Example 3 To achieve the initial feed composition shown in Table 1, the feed consisted of MeI, MA, water, LiI, and rhodium iodide (in the experiment, a complex catalyst [Rh(CO)₂I₂]). - (The Rh concentration in the table is converted to metal) and acetic acid were used to feed H2, CO2, CO, and air into the autoclave in a manner that brought the partial pressures of H2, CO2, CO, and O2 in the gas phase to the partial pressures (absolute pressure) or volume % shown in Table 1, respectively. Otherwise, the same experiment as Comparative Example 1 was performed. After cooling, samples of the liquid were taken and analyzed for composition. The formic acid concentration was 51 ppm. The MA and water concentrations decreased to 2.3% and 1.6%, respectively. The reasons for this are as explained in Comparative Example 1. The results of the composition analysis at the beginning and end of the experiment are shown in the table below. Furthermore, the acetic acid concentration was used as a margin, but other trace impurities were present at approximately 0.2%, mainly dimethyl ether and methanol.

[0195] Comparative Example 4 To achieve the initial feed composition shown in Table 1, the feed consisted of MeI, MA, water, LiI, and rhodium iodide (in the experiment, a complex catalyst [Rh(CO)₂I₂]). - (The Rh concentration in the table is converted to metal concentration), and acetic acid, to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, CO, and O2 concentration in the gas phase as shown in Table 1, respectively, were fed into the autoclave. The autoclave was maintained at 150°C for 30 minutes. Otherwise, the same experiment as Comparative Example 1 was performed. After cooling, samples of the liquid were taken and analyzed for composition. The formic acid concentration was 51 ppm. It should be noted that the composition analysis results at the beginning and end of the experiment are shown in the table below. Additionally, the acetic acid concentration was used as a margin, but other trace impurities were present at approximately 0.2%, mainly dimethyl ether and methanol.

[0196] Comparative Example 5 To achieve the initial feed composition shown in Table 1, MeI, MA, water, formic acid, and acetic acid were fed. H2, CO2, CO, and air were fed into the autoclave to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, CO, and O2 concentration in the gas phase as shown in Table 1, respectively. The autoclave was maintained at 110°C for 30 minutes. Otherwise, the same experiment as Comparative Example 1 was performed. After cooling, samples of the liquid were taken and analyzed. The formic acid concentration was 47 ppm. Since no catalyst was added, no carbonylation reaction occurred, and there were no changes in the basic components other than formic acid. The MA concentration decreased slightly, while the water concentration remained essentially unchanged. The decrease in MA concentration can be attributed to the reaction of MA with water to produce methanol and acetic acid, with some of the methanol being converted to dimethyl ether, resulting in water. Other components did not change significantly. It should be noted that the total pressure immediately after reaching 110°C was 0.9 MPaG, and the total pressure at 110°C at the end of the experiment was also 0.9 MPaG. It can be assumed that under the conditions of a rhodium complex catalyst (Comparative Examples 1-3), CO is consumed by the reaction, with slight amounts of H2 and CO2 generated. Consequently, there is a pressure decrease of approximately 0.1-0.5 MPa. Conversely, under conditions without a rhodium complex catalyst, since no gas is generated, the pressure does not decrease significantly. The results of compositional analysis at the beginning and end of the experiments are shown in the table below. Furthermore, acetic acid concentration was used as the balance, but other trace impurities were present at approximately 0.2%, mainly dimethyl ether and methanol.

[0197] Comparative Example 6 To achieve the initial feed composition as shown in Table 1, MeI, MA, water, formic acid, and acetic acid were fed. H2, CO2, CO, and air were fed into the autoclave to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, and O2 concentration in the gas phase as shown in Table 1, respectively. Otherwise, the same experiment as Comparative Example 5 was performed. After cooling, samples of the liquid were taken and analyzed. The formic acid concentration was 42 ppm. It should be noted that the results of the composition analysis at the beginning and end of the experiment are shown in the table below. Furthermore, the acetic acid concentration was used as a margin, but other trace impurities were present at approximately 0.2%, mainly dimethyl ether and methanol.

[0198] Comparative Example 7 To achieve the initial feed composition shown in Table 1, MeI, MA, water, formic acid, and acetic acid were fed. H2, CO2, CO, and air were fed into the autoclave to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, CO, and O2 concentration in the gas phase as shown in Table 1, respectively. Otherwise, the same experiment as Comparative Example 5 was performed. After cooling, samples of the liquid were taken and analyzed. The formic acid concentration was 45 ppm. It should be noted that the composition analysis results at the beginning and end of the experiment are shown in the table below. Furthermore, the acetic acid concentration was used as a margin, but other trace impurities were present at approximately 0.2%, mainly dimethyl ether and methanol.

[0199] Comparative Example 8 To achieve the initial feed composition as shown in Table 1, MeI, MA, water, formic acid, and acetic acid were fed. H2, CO2, CO, and air were fed into the autoclave to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, CO, and O2 concentration in the gas phase as shown in Table 1, respectively. The autoclave was maintained at 100°C for 30 minutes. Otherwise, the same experiment as Comparative Example 5 was performed. After cooling, samples of the liquid were taken and analyzed. The formic acid concentration was 48 ppm. It should be noted that the results of the composition analysis at the beginning and end of the experiment are shown in the table below. Additionally, the acetic acid concentration was used as a margin, but other trace impurities were present at approximately 0.2%, mainly dimethyl ether and methanol.

[0200] Example 1 To achieve the initial feed composition shown in Table 2, the feed consisted of MeI, MA, water, LiI, and rhodium iodide (in the experiment, a complex catalyst [Rh(CO)₂I₂]). - (The Rh concentration in the table is converted to metal concentrations), and acetic acid, were fed into the autoclave to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, CO, and O2 concentration in the gas phase as shown in Table 2. Otherwise, the same experiment as Comparative Example 1 was performed. After cooling, samples of the liquid were taken and analyzed. The formic acid concentration was 45 ppm. The MA and water concentrations decreased to 2.2% and 1.6%, respectively. The reasons are as explained in Comparative Example 1. Other components did not change significantly. The total pressure immediately after reaching 180°C was 3.9 MPaG, and the total pressure dropped to 3.4 MPaG at the end of the experiment. It should be noted that the results of the composition analysis at the beginning and end of the experiment are shown in the table below. Furthermore, the acetic acid concentration was used as a margin, but other trace impurities were present at approximately 0.2%, mainly dimethyl ether and methanol.

[0201] Example 2 To achieve the initial feed composition shown in Table 2, the feed consisted of MeI, MA, water, LiI, and rhodium iodide (in the experiment, a complex catalyst [Rh(CO)₂I₂]). - (The Rh concentration in the table is converted to metal concentration), and acetic acid, were fed into the autoclave to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, CO, and O2 concentration in the gas phase as shown in Table 2. Otherwise, the same experiment as in Example 1 was performed. After cooling, samples of the liquid were taken and analyzed. The formic acid concentration was 21 ppm. The MA and water concentrations decreased to 2.3% and 1.6%, respectively. The reasons are as explained in Comparative Example 1. Other components did not change significantly. It should be noted that the results of the composition analysis at the beginning and end of the experiment are shown in the table below. Additionally, the acetic acid concentration was used as a margin, but other trace impurities were present at approximately 0.2%, mainly dimethyl ether and methanol.

[0202] Example 3 To achieve the initial feed composition shown in Table 2, the feed consisted of MeI, MA, water, LiI, and rhodium iodide (in the experiment, a complex catalyst [Rh(CO)₂I₂]). - (The Rh concentration in the table is converted to metal concentration), and acetic acid, were fed into the autoclave to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, CO, and O2 concentration in the gas phase as shown in Table 2. Otherwise, the same experiment as in Example 1 was performed. After cooling, samples of the liquid were taken and analyzed. The formic acid concentration was 40 ppm. The MA and water concentrations decreased to 2.3% and 1.6%, respectively. The reasons are as explained in Comparative Example 1. Other components did not change significantly. It should be noted that the results of the composition analysis at the beginning and end of the experiment are shown in the table below. Additionally, the acetic acid concentration was used as a margin, but other trace impurities were present at approximately 0.2%, mainly dimethyl ether and methanol.

[0203] Example 4 Except for maintaining the temperature at 188°C for 30 minutes, the same experiment as in Example 1 was performed. After cooling, samples of the liquid were taken and analyzed for composition. The formic acid concentration was 37 ppm. The MA and water concentrations decreased to 2.0% and 1.4%, respectively. The reasons for this are as explained in Comparative Example 1. Other components did not change significantly. It should be noted that the results of composition analysis at the beginning and end of the experiment are shown in the table below. In addition, the acetic acid concentration was used as the balance, but other trace impurities were present at approximately 0.2%, mainly dimethyl ether and methanol.

[0204] Example 5 To achieve the initial feed composition shown in Table 2, MeI, MA, water, formic acid, and acetic acid were fed. H2, CO2, CO, and air were fed into the autoclave to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, and O2 concentration in the gas phase as shown in Table 2, respectively. The autoclave was maintained at 110°C for 30 minutes. Otherwise, the same experiment as in Example 1 was performed. After cooling, liquid samples were taken and compositional analysis was performed. The formic acid concentration was 38 ppm. Since no catalyst was added, no carbonylation reaction occurred, and there were no changes in the basic components other than formic acid. The total pressure immediately after reaching 110°C was 1.0 MPaG, and the total pressure at 110°C at the end of the experiment was also 1.0 MPaG. It can be assumed that under the conditions of the rhodium complex catalyst (Examples 1-4), CO is consumed by the reaction, with slight amounts of H2 and CO2 generated. Consequently, there is a pressure decrease of approximately 0.5-0.7 MPa. Conversely, under the condition without the rhodium complex catalyst, since no gas is generated, the pressure essentially does not decrease. It should be noted that the compositional analysis results at the beginning and end of the experiment are shown in the table below. Furthermore, the acetic acid concentration is used as the balance, but other trace impurities are present at approximately 0.2%, mainly dimethyl ether and methanol.

[0205] Example 6 To achieve the initial feed composition shown in Table 2, MeI, MA, water, formic acid, and acetic acid were fed. H2, CO2, CO, and air were fed into the autoclave to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, CO, and O2 concentration in the gas phase as shown in Table 2, respectively. Otherwise, the same experiment as in Example 5 was performed. After cooling, samples of the liquid were taken and analyzed. The formic acid concentration was 37 ppm. Since no catalyst was added, no carbonylation reaction occurred, and there were no changes in the basic components other than formic acid. It should be noted that the composition analysis results at the beginning and end of the experiment are shown in the table below. Additionally, the acetic acid concentration was used as a margin, but other trace impurities were present at approximately 0.2%, mainly dimethyl ether and methanol.

[0206] Example 7 To achieve the initial feed composition shown in Table 2, water, formic acid, and acetic acid were fed. H2, CO2, CO, and air were fed into the autoclave to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, CO, and O2 concentration in the gas phase as shown in Table 2, respectively. Otherwise, the same experiment as in Example 5 was performed. After cooling, samples of the liquid were taken and analyzed. The formic acid concentration was 35 ppm. Since no catalyst was added, no carbonylation reaction occurred, and there were no changes in the basic components other than formic acid. It should be noted that the results of the composition analysis at the beginning and end of the experiment are shown in the table below. Additionally, in Example 7, since MA was absent, the total amount of dimethyl ether and methanol at the end of the experiment was less than 0.1%.

[0207] Example 8 To achieve the initial feed composition shown in Table 2, MeI, MA, water, formic acid, and acetic acid were fed. H2, CO2, CO, and air were fed into the autoclave to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, and O2 concentration in the gas phase as shown in Table 2, respectively. Otherwise, the same experiment as in Example 5 was performed. After cooling, samples of the liquid were taken and analyzed. The formic acid concentration was 21 ppm. Since no catalyst was added, no carbonylation reaction occurred, and there were no changes in the basic components other than formic acid. It should be noted that the composition analysis results at the beginning and end of the experiment are shown in the table below. Additionally, the acetic acid concentration was used as a margin, but other trace impurities were present at approximately 0.2%, mainly dimethyl ether and methanol.

[0208] Example 9 To achieve the initial feed composition shown in Table 2, the feed consisted of MeI, MA, water, LiI, and rhodium iodide (in the experiment, a complex catalyst [Rh(CO)₂I₂]). - (The Rh concentration in the table is converted to metal), formic acid, and acetic acid were fed into the autoclave to achieve the partial pressures (absolute pressure) or volume % of H2, CO2, CO, and O2 concentration in the gas phase as shown in Table 2, respectively. The autoclave was maintained at 145°C for 10 minutes. Otherwise, the same experiment as in Example 5 was performed. After cooling, samples of the liquid were taken and analyzed for composition. The formic acid concentration was 29 ppm. It should be noted that the composition analysis results at the beginning and end of the experiment are shown in the table below. Additionally, in Example 9, due to the low MA content, the total dimethyl ether and methanol content at the end of the experiment was approximately 0.1%.

[0209] The conditions and results of the comparative examples and embodiments are shown in Tables 1 and 2. In Tables 1 and 2, "PH2" represents the partial pressure of hydrogen, "PCO2" represents the partial pressure of carbon dioxide, "PCO" represents the partial pressure of carbon monoxide, and "O2 in the gas phase" represents the oxygen concentration in the gas phase. In the tables, the acetic acid concentration is recorded as "balance," but in practice, the sample solution may sometimes contain impurities such as byproducts described in the section on reaction mixtures, totaling 1 ppm to 1%.

[0210] [Table 1] [Table 2] [Examination of Results] A comparison of Comparative Examples 1 and 2 shows that even when the hydrogen partial pressure is above 500 kPa and the carbon dioxide partial pressure is above 70 kPa, the amount of formic acid produced decreases if the oxygen concentration in the gas phase is below 7% by volume. A comparison of Comparative Examples 1 and 3 shows that even when the oxygen concentration in the gas phase is above 7% by volume, the amount of formic acid produced decreases if the hydrogen partial pressure is below 500 kPa and the carbon dioxide partial pressure is below 70 kPa. However, a comparison of Comparative Examples 1-4 with Example 1 shows that by setting the hydrogen partial pressure below 500 kPa, the carbon dioxide partial pressure below 70 kPa, the temperature above 150°C, and the oxygen concentration in the gas phase below 7% by volume, the amount of formic acid produced decreases further compared to both the case where only the hydrogen partial pressure is below 500 kPa and the carbon dioxide partial pressure is below 70 kPa, and the case where only the oxygen concentration in the gas phase is below 7% by volume. It should be noted that, as can be seen from the comparison between Comparative Example 3 and Comparative Example 4, even if the temperature is increased from 150°C to 180°C, the amount of formic acid generated cannot be suppressed when the oxygen concentration in the gas phase is above 7% by volume.

[0211] A comparison of Comparative Examples 5 and 6 shows that even when the hydrogen partial pressure exceeds 5 kPa and the carbon dioxide partial pressure is 20 kPa or more, the decomposition of formic acid can be promoted if the oxygen concentration in the gas phase is lower than 7% by volume. A comparison of Comparative Examples 5 and 7 shows that even when the oxygen concentration in the gas phase is 7% by volume or more, the decomposition of formic acid can be promoted if the hydrogen partial pressure is 5 kPa or less and the carbon dioxide partial pressure is lower than 20 kPa. However, a comparison of Comparative Examples 5-8 with Example 6 shows that by setting the hydrogen partial pressure to 5 kPa or less, the carbon dioxide partial pressure to 20 kPa or less, the temperature to over 100°C, and the oxygen concentration in the gas phase to 7% by volume, the decomposition of formic acid is further promoted, both compared to the case where only the hydrogen partial pressure is 5 kPa or less and the carbon dioxide partial pressure is lower than 20 kPa, and compared to the case where only the oxygen concentration in the gas phase is lower than 7% by volume. It should be noted that, as can be seen from the comparison between Comparative Example 7 and Comparative Example 8, even when the temperature is increased from 100°C to 110°C, the decomposition of formic acid is only slightly promoted.

[0212] A comparison of Examples 1 and 2 shows that when the oxygen concentration in the gas phase is below 7% by volume, the lower the partial pressure of hydrogen and carbon dioxide, the more formic acid formation is suppressed. Furthermore, a comparison of Examples 1 and 3 shows that when the partial pressure of hydrogen is below 500 kPa, the partial pressure of carbon dioxide is below 70 kPa, and the temperature exceeds 150°C, the lower the oxygen concentration in the gas phase, the more formic acid formation is suppressed. Additionally, a comparison of Examples 1 and 4 shows that when the partial pressure of hydrogen is below 500 kPa, the partial pressure of carbon dioxide is below 70 kPa, and the oxygen concentration in the gas phase is below 7% by volume, the higher the temperature, the more formic acid formation is suppressed.

[0213] As can be seen from Examples 5 to 9, when the hydrogen partial pressure is below 5 kPa, the carbon dioxide partial pressure is below 20 kPa, the temperature exceeds 100°C, and the oxygen concentration in the gas phase is below 7% by volume, the decomposition of formic acid can be promoted even when the composition, hydrogen partial pressure, carbon dioxide partial pressure, temperature, oxygen concentration in the gas phase, or residence time are different.

[0214] In summary, the technical solution of the present invention and its variations are noted below.

[0215] [1] A method for preparing acetic acid, wherein the acetic acid preparation process includes at least one step selected from the steps of operating conditions satisfying (i) below and the steps of operating conditions satisfying (ii) below, and for more than one process, the oxygen concentration is controlled in a manner satisfying at least one of (iii) below and (iv) below.

[0216] (i) Operating conditions with hydrogen partial pressure below 500 kPa (absolute pressure), carbon dioxide partial pressure below 70 kPa (absolute pressure), and operating temperature above 150°C (preferably 175°C). (ii) Operating conditions where the hydrogen partial pressure is below 5 kPa (absolute pressure), the carbon dioxide partial pressure is below 20 kPa (absolute pressure), and the operating temperature exceeds 100°C. (iii) Control the oxygen concentration in the gas phase to below 7% by volume. (iv) Control the oxygen concentration in the liquid phase to below 7 × 10⁻⁶. -5 g / g [2] The method for preparing acetic acid described in [1] above, wherein in (ii) above, the partial pressure of hydrogen is less than 1 kPa (absolute pressure) and the partial pressure of carbon dioxide is less than 2 kPa (absolute pressure).

[0217] [3] The method for preparing acetic acid described in [1] or [2] above has a reaction step that satisfies the operating conditions described in (i) above.

[0218] [4] The method for preparing acetic acid described in [3] above, wherein the concentration of acetic acid in the reaction mixture in the above reaction step is 30% by mass or more and the concentration of formic acid is 102 ppm by mass or less.

[0219] [5] The method for preparing acetic acid described in [3] or [4] above, wherein the concentration of acetic acid in the reaction mixture in the above reaction step is 50-90% by mass, the concentration of metal catalyst (in metal conversion) is 200-10000 ppm by mass, the concentration of iodomethane is 1-20% by mass, the concentration of ionic iodide is 1-25% by mass, the concentration of water is 0.1-15% by mass, the concentration of methyl acetate is 0.1-30% by mass, and the concentration of formic acid is below 102 ppm by mass.

[0220] [6] The method for preparing acetic acid described in any one of [1] to [5] above has an evaporation step or a distillation step that satisfies the operating conditions described in (ii) above.

[0221] [7] The method for preparing acetic acid described in [6] above, wherein in the above evaporation process, the concentration of acetic acid in the feed liquid to the evaporation tank is 50-90% by mass, the concentration of metal catalyst (in metal conversion) is 200-10000 ppm by mass, the concentration of iodomethane is 1-20% by mass, the concentration of ionic iodide is 1-25% by mass, the concentration of water is 0.1-15% by mass, the concentration of methyl acetate is 0.1-30% by mass, and the concentration of formic acid is less than 10000 ppm by mass.

[0222] [8] The method for preparing acetic acid described in [6] above, wherein in the distillation process described above, the concentration of acetic acid in the feed liquid to the distillation column is 30% by mass or more and the concentration of formic acid is 5 ppm by mass or more.

[0223] [9] The method for preparing acetic acid described in [6] above, wherein in the distillation process described above, the concentration of acetic acid in the feed liquid to the distillation column is 40-85% by mass, the concentration of iodomethane is 2-50% by mass, the concentration of water is 0.2-20% by mass, the concentration of methyl acetate is 0.2-50% by mass, and the concentration of formic acid is 5-10000 ppm by mass.

[0224]

[10] The method for preparing acetic acid described in [6] above, wherein in the distillation process described above, the concentration of acetic acid in the feed liquid to the distillation column is 80~99.9% by mass, the concentration of iodomethane is 0.01~16% by mass, the concentration of water is 0.05~18% by mass, the concentration of methyl acetate is 0.01~16% by mass, and the concentration of formic acid is 5~10000 ppm by mass.

[0225]

[11] The method for preparing acetic acid described in [6] above, wherein in the distillation process described above, the concentration of acetic acid in the feed liquid to the distillation column is 99.1~99.999% by mass and the concentration of formic acid is 5~9000 ppm by mass.

[0226]

[12] The method for preparing acetic acid according to any one of [1] to

[11] above, wherein the gas phase in (iii) above includes at least one selected from acetic acid, methyl acetate, methanol, water, acetaldehyde, byproducts derived from acetaldehyde, and dialkyl ethers, wherein the byproducts include at least one selected from iodoalkanes with 2 or more carbon atoms, alkanes with 4 or more carbon atoms, alkane carboxylic acids with 3 or more carbon atoms, alkanes, and ketones, and the dialkyl ethers include at least dimethyl ether.

[0227]

[13] The method for preparing acetic acid according to any one of [1] to

[12] above, wherein, for more than one process, the oxygen concentration is controlled in a manner selected from at least one of (iii-1) and (iv-1) below.

[0228] (iii-1) Control the oxygen concentration in the gas phase to below 5% by volume. (iv-1) Control the oxygen concentration in the liquid phase to below 2 × 10⁻⁶. -5 g / g

[14] The method for preparing acetic acid according to any one of [1] to

[13] above, wherein the ratio of oxygen to carbon monoxide in the gas phase in (iii) above is 2% by volume or less (preferably 1% by volume or less), and / or the ratio of oxygen to carbon monoxide in the liquid phase in (iv) above is 2% by volume or less (preferably 1% by volume or less).

[0229]

[15] The method for preparing acetic acid according to any one of [1] to

[14] above, wherein, in (iii) and / or (iv) above, at least one component selected from oxygen-containing gas, oxygen-containing compound and oxygen-generating agent is introduced, and the oxygen concentration in the gas phase in (iii) above is controlled to be 1 volume ppt or more (preferably 1 volume ppb or more), and / or the oxygen concentration in the liquid phase in (iv) above is controlled to be 0.1 × 10⁻⁶. -9 g / g or more.

[0230]

[16] The method for preparing acetic acid according to any one of claims [1] to

[15] , wherein, in (iii) and / or (iv) above, the oxygen concentration is controlled to be less than 0.25 mol relative to 1 mol of the total amount of hydrogen iodide and iodomethane.

[0231]

[17] The method for preparing acetic acid according to any one of [1] to

[16] above, wherein the gas phase in (iii) above and / or the liquid phase in (iv) above are the gas phase and / or liquid phase in the reaction step, evaporation step, or distillation step.

[0232]

[18] The method for preparing acetic acid according to any one of [1] to

[17] above, wherein the acetic acid preparation process includes a carbonylation reaction step of reacting methanol with carbon monoxide to generate acetic acid, an evaporation step of separating the reaction mixture obtained in the carbonylation reaction step into a vapor stream and a residual liquid stream, and a de-boiling step of distilling the vapor stream to separate it into a column overhead stream enriched with low-boiling components and a first acetic acid stream enriched with acetic acid, or in addition to these steps, it also includes at least one of the steps (a) to (d) below.

[0233] (a) A dehydration process in which the first acetic acid stream is distilled to separate a water-rich overhead stream and a second acetic acid stream that is more acetic acid-rich than the first acetic acid stream. (b) A de-boiling process that separates the first or second acetic acid stream into a bottoms stream enriched with high-boiling components by distillation, and a third acetic acid stream that is more enriched with acetic acid than the acetic acid stream before distillation. (c) An adsorption removal process in which the first, second, or third acetic acid stream is treated with an ion exchange resin to obtain a fourth acetic acid stream. (d) A process of distilling the first, second, third, or fourth acetic acid stream to obtain a fifth acetic acid stream that is more enriched in acetic acid than the acetic acid stream before distillation.

[19] The method for preparing acetic acid described in

[18] above, wherein the carbonylation reaction step meets the operating conditions of (i) above.

[0234]

[20] The method for preparing acetic acid described in

[18] or

[19] above, wherein at least one step selected from the above-mentioned evaporation step, low-boiling step, dehydration step, high-boiling step and product step satisfies the operating conditions of (ii) above.

[0235]

[21] The method for preparing acetic acid according to any one of

[18] to

[20] above, wherein the gas phase and / or liquid phase selected from at least one of the carbonylation reaction step, evaporation step, de-boiling step, dehydration step, de-boiling step and product step above are the gas phase and / or the liquid phase in (iii) above above.

[0236]

[22] The method for preparing acetic acid according to any one of [1] to

[21] above, wherein the residence time in the step that satisfies the operating conditions of (i) above or the step that satisfies the operating conditions of (ii) above is 1 minute or more (e.g., 10 minutes or more or 2 hours or less).

[0237]

[23] The method for preparing acetic acid according to any one of [1] to

[22] above, wherein the process liquid with a formic acid concentration of 10 ppm or more is recycled to a process that meets the operating conditions of hydrogen partial pressure below 500 kPa (absolute pressure), carbon dioxide partial pressure below 70 kPa (absolute pressure), and operating temperature above 100°C.

[0238]

[24] In the method for preparing acetic acid described in

[23] above, the concentration of acetic acid in the process liquid being recycled is 5% by mass or more (e.g., 5 to 99.999% by mass).

[0239]

[25] The method for preparing acetic acid according to any one of [1] to

[24] above, wherein the process for preparing acetic acid includes at least one distillation step, wherein the top liquid of the distillation column in the at least one distillation step is recycled to a step that satisfies the operating conditions of (i) above and / or a step that satisfies the operating conditions of (ii) above.

[0240]

[26] In the method for preparing acetic acid described above

[25] , the destination of the recirculated liquid at the top of the distillation column is the reaction process and / or the evaporation process or distillation process located upstream of the distillation process carried out in the distillation column.

[0241]

[27] In the method for preparing acetic acid described in

[25] or

[26] above, the concentration of acetic acid in the overhead liquid of the distillation column is 5% by mass or more (e.g., 80 to 99.999% by mass).

[0242]

[28] The method for preparing acetic acid according to any one of

[25] to

[27] above, wherein the overhead liquid of the distillation column is the overhead liquid of the distillation column in which the concentration of acetic acid in the feed liquid is 80% by mass or more (e.g., 80 to 99.999% by mass).

[0243]

[29] The method for preparing acetic acid according to any one of [1] to

[28] above, wherein, in (i) above, the partial pressure of hydrogen is 1 to 150 kPa (absolute pressure) or less, the partial pressure of carbon dioxide is less than 70 kPa (absolute pressure), and the operating temperature is more than 175°C and less than 250°C.

[0244]

[30] The method for preparing acetic acid according to any one of [1] to

[29] above, wherein, in (ii) above, the partial pressure of carbon dioxide is 12 kPa (absolute pressure) or less and the operating temperature is 106 to 250 °C.

[0245] Industrial applicability According to the present invention, the concentration of formic acid in acetic acid products can be reduced by simple means.

[0246] Symbol Explanation 1. Reaction tank 2 Evaporation tank Distillation towers 3, 5, and 6 4. Decanter 7 Ion exchange resin tower 8 Washing System 9. Acetaldehyde Separation and Removal System 16. Reaction mixture supply line 17. Steam discharge line 18, 19 Residual liquid recirculation lines 54. Carbon monoxide-containing gas introduction line 55, 56 Potassium hydroxide introduction circuit 57 Catalyst Circulation Pump 91 Distillation Column (First Acetaldehyde Removal Column) 92 Extraction Tower 93 Distillation Column (Second Acetaldehyde Removal Column) 94 Distillation column (extractive distillation column) 95 Decanter 96 Decanter 97. Distillation Column (Acetal Removal Column) 98. Distillation Column (Extractive Distillation Column) 99 Decanter 200 chimney-style tower tray

Claims

1. A method for preparing acetic acid, comprising a carbonylation reaction step in which methanol reacts with carbon monoxide to produce acetic acid, an evaporation step in which the reaction mixture obtained in the carbonylation reaction step is separated into a vapor stream and a residual liquid stream, and a de-low-boiling step in which the vapor stream is distilled to separate a column overhead stream enriched with low-boiling components and a first acetic acid stream enriched with acetic acid. The carbonylation reaction step meets the operating conditions described in (i), the evaporation step and the de-boiling point removal step meet the operating conditions described in (ii), and the oxygen concentration is controlled in a manner described in (iii): (i) Operating conditions where the partial pressure of hydrogen is less than 500 kPa in absolute pressure, the partial pressure of carbon dioxide is less than 70 kPa in absolute pressure, and the operating temperature exceeds 150°C. (ii) Operating conditions where the partial pressure of hydrogen is below 5 kPa in absolute pressure, the partial pressure of carbon dioxide is below 20 kPa in absolute pressure, and the operating temperature exceeds 100°C. (iii) Control the oxygen concentration in the gas phase to below 7% by volume. Furthermore, among them, The oxygen concentration in the process is controlled by one or more of the following methods (a) to (g): (a) In the carbon monoxide manufacturing process, the oxygen feed rate and / or water vapor feed rate relative to the carbon monoxide feedstock are controlled so as to utilize oxygen to completely and partially oxidize the carbon monoxide feedstock. (b) In the carbon monoxide manufacturing process, the suitability for use is determined based on the measured value of the oxygen concentration in the purified carbon monoxide; (c) In the carbon monoxide manufacturing process, the oxygen concentration in carbon monoxide is controlled by feedback control of the carbon monoxide preparation process based on the measured value of oxygen concentration in purified carbon monoxide. (d) In the carbon monoxide manufacturing process, the oxygen concentration in carbon monoxide is controlled by introducing a non-reactive gas based on the measured value of the oxygen concentration in the purified carbon monoxide. (e) For the gas fed into the process, determine whether the gas can be used based on the measured oxygen concentration, and / or use the gas in which the oxygen concentration is controlled or managed based on the measured value; (f) For liquids fed into the process, determine whether the liquid can be used, and / or whether the dissolved oxygen concentration is controlled or managed based on the measured value of dissolved oxygen concentration; (g) For purge gases in the process flow, reduce the purge volume when using nitrogen for purging, and / or switch the purge gas from nitrogen to carbon monoxide or other non-reactive gases.

2. The method for preparing acetic acid according to claim 1, wherein, In (i), the hydrogen partial pressure exceeds 1 kPa in absolute pressure.

3. The method for preparing acetic acid according to claim 1 or 2, wherein, In (i), the hydrogen partial pressure is below 150 kPa in absolute pressure.

4. The method for preparing acetic acid according to claim 1 or 2, wherein, In (i), the partial pressure of carbon dioxide is above 2 kPa in absolute pressure.

5. The method for preparing acetic acid according to claim 1 or 2, wherein, In (i), the operating temperature is below 250°C.

6. The method for preparing acetic acid according to claim 1 or 2, wherein, In (ii), the hydrogen partial pressure is above 0 kPa in absolute pressure.

7. The method for preparing acetic acid according to claim 1 or 2, wherein, In (ii), the partial pressure of carbon dioxide is above 0 kPa in absolute pressure.

8. The method for preparing acetic acid according to claim 1 or 2, wherein, In (ii), the partial pressure of carbon dioxide is less than 12 kPa in absolute pressure.

9. The method for preparing acetic acid according to claim 1 or 2, wherein, In (ii), the operating temperature is below 250°C.

10. The method for preparing acetic acid according to claim 1 or 2, wherein, In (ii), the operating temperature is above 106°C.

11. The method for preparing acetic acid according to claim 1 or 2, wherein, In (ii), the partial pressure of hydrogen is less than 1 kPa in absolute pressure and the partial pressure of carbon dioxide is less than 2 kPa in absolute pressure.

12. The method for preparing acetic acid according to claim 1 or 2, wherein, The acetic acid concentration in the reaction mixture during the carbonylation reaction process is above 30% by mass and the formic acid concentration is below 102 ppm by mass.

13. The method for preparing acetic acid according to claim 1 or 2, wherein, The reaction mixture in the carbonylation reaction process has the following concentrations: acetic acid concentration of 50-90% by mass, metal catalyst concentration of 200-10000 ppm by mass (converted to metal), iodomethane concentration of 1-20% by mass, ionic iodide concentration of 1-25% by mass, water concentration of 0.1-15% by mass, methyl acetate concentration of 0.1-30% by mass, and formic acid concentration of less than 102 ppm by mass.

14. The method for preparing acetic acid according to claim 1 or 2, wherein, In the evaporation process, the concentration of acetic acid in the feed liquid to the evaporation tank is 50-90% by mass, the concentration of metal catalyst is 200-10000 ppm by mass (converted to metal), the concentration of iodomethane is 1-20% by mass, the concentration of ionic iodide is 1-25% by mass, the concentration of water is 0.1-15% by mass, the concentration of methyl acetate is 0.1-30% by mass, and the concentration of formic acid is below 10000 ppm by mass.

15. The method for preparing acetic acid according to claim 1 or 2, wherein, The gas phase in (iii) comprises at least one selected from acetic acid, methyl acetate, methanol, water, acetaldehyde, byproducts derived from acetaldehyde, and dialkyl ethers, wherein the byproducts comprise at least one selected from iodoalkanes with 2 or more carbon atoms, alkanes with 4 or more carbon atoms, alkane carboxylic acids with 3 or more carbon atoms, alkanes, and ketones, and the dialkyl ethers comprise at least dimethyl ether.

16. The method for preparing acetic acid according to claim 1 or 2, wherein, The oxygen concentration is controlled in the manner described in (iii) for the carbonylation reaction step, the evaporation step, and the de-boiling step, and for more than one of the steps, the oxygen concentration is controlled in the manner described in (iii-1) below: (iii-1) Control the oxygen concentration in the gas phase to below 5% by volume.

17. The method for preparing acetic acid according to claim 1 or 2, wherein, In (iii), the ratio of oxygen to carbon monoxide in the gas phase is less than 2% by volume.

18. The method for preparing acetic acid according to claim 1 or 2, wherein, In step (iii), at least one component selected from oxygen-containing gas, oxygen-containing compound, and oxygen-generating agent is introduced to control the oxygen concentration in the gas phase in step (iii) to be above 1 volume ppt.

19. The method for preparing acetic acid according to claim 1 or 2, wherein, In (iii), the oxygen concentration is controlled to be less than 0.25 moles relative to the total amount of hydrogen iodide and iodomethane.

20. The method for preparing acetic acid according to claim 1, further comprising at least one of the following steps (a) to (d): (a) A dehydration process in which the first acetic acid stream is distilled to separate a water-rich overhead stream and a second acetic acid stream that is more acetic acid-rich than the first acetic acid stream. (b) A process of removing high-boiling components by distilling the first or second acetic acid stream into a bottoms stream enriched with high-boiling components and a third acetic acid stream that is more enriched with acetic acid than the acetic acid stream before distillation. (c) An adsorption removal process in which the first, second, or third acetic acid stream is treated with an ion exchange resin to obtain a fourth acetic acid stream; (d) A process of distilling the first, second, third, or fourth acetic acid stream to obtain a fifth acetic acid stream that is more acetic acid-rich than the acetic acid stream before distillation.

21. The method for preparing acetic acid according to claim 20, wherein, At least one of the dehydration process, the deboiling process, and the product process satisfies the operating conditions of (ii).

22. The method for preparing acetic acid according to claim 20, wherein, In the evaporation process, as satisfied in (ii), the hydrogen partial pressure is 4 kPa or less in absolute pressure.

23. The method for preparing acetic acid according to claim 20, wherein, In the evaporation process, as satisfied in (ii), the hydrogen partial pressure is 0 kPa or higher in absolute pressure.

24. The method for preparing acetic acid according to claim 20, wherein, In the evaporation process, as satisfied in (ii), the partial pressure of carbon dioxide is 12 kPa or less in absolute pressure.

25. The method for preparing acetic acid according to claim 20, wherein, In the evaporation process, as satisfied in (ii), the partial pressure of carbon dioxide is 0 kPa or higher in absolute pressure.

26. The method for preparing acetic acid according to claim 20, wherein, In the evaporation process, as described in (ii), the operating temperature is 112°C or higher.

27. The method for preparing acetic acid according to claim 20, wherein, In the evaporation process, as described in (ii), the operating temperature is below 200°C.

28. The method for preparing acetic acid according to claim 20, wherein, In the (ii) condition satisfied in the de-boiling step, the hydrogen partial pressure is 4 kPa or less in absolute pressure.

29. The method for preparing acetic acid according to claim 20, wherein, In the (ii) condition satisfied in the de-boiling step, the hydrogen partial pressure is 0 kPa or higher in absolute pressure.

30. The method for preparing acetic acid according to claim 20, wherein, In the (ii) condition satisfied in the de-boiling step, the partial pressure of carbon dioxide is 12 kPa or less in absolute pressure.

31. The method for preparing acetic acid according to claim 20, wherein, In the (ii) condition satisfied in the de-boiling step, the partial pressure of carbon dioxide is 0 kPa or higher in absolute pressure.

32. The method for preparing acetic acid according to claim 20, wherein, In the process of removing low-boiling point, as described in (ii), the operating temperature is 112°C or higher.

33. The method for preparing acetic acid according to claim 20, wherein, In the process of removing low-boiling point (ii), the operating temperature is below 165°C.

34. The method for preparing acetic acid according to claim 20, wherein, In the process of removing low-boiling point, the acetic acid concentration in the feed liquid to the distillation tower is 30% by mass or more, and the formic acid concentration is 5 ppm by mass or more.

35. The method for preparing acetic acid according to claim 20, wherein, In the process of removing low-boiling point, the concentration of acetic acid in the feed liquid to the distillation tower is 40-85% by mass, the concentration of iodomethane is 2-50% by mass, the concentration of water is 0.2-20% by mass, the concentration of methyl acetate is 0.2-50% by mass, and the concentration of formic acid is 5-10000 ppm by mass.

36. The method for preparing acetic acid according to claim 20, comprising the dehydration step that satisfies the operating conditions of (ii).

37. The method for preparing acetic acid according to claim 36, wherein, In the dehydration process, as satisfied in (ii), the hydrogen partial pressure is less than 2 kPa in absolute pressure.

38. The method for preparing acetic acid according to claim 36, wherein, In the dehydration process, as satisfied in (ii), the hydrogen partial pressure is 0 kPa or higher in absolute pressure.

39. The method for preparing acetic acid according to claim 36, wherein, In the dehydration process, as described in (ii), the partial pressure of carbon dioxide is less than 5 kPa in absolute pressure.

40. The method for preparing acetic acid according to claim 36, wherein, In the dehydration process, as satisfied in (ii), the partial pressure of carbon dioxide is 0 kPa or higher in absolute pressure.

41. The method for preparing acetic acid according to claim 36, wherein, In the dehydration process, as described in (ii), the operating temperature is 120°C or higher.

42. The method for preparing acetic acid according to claim 36, wherein, In the dehydration process, as described in (ii), the operating temperature is below 170°C.

43. The method for preparing acetic acid according to claim 36, wherein, In the dehydration process, the acetic acid concentration in the feed liquid to the distillation tower is 30% by mass or more, and the formic acid concentration is 5 ppm by mass or more.

44. The method for preparing acetic acid according to claim 36, wherein, In the dehydration process, the concentration of acetic acid in the feed liquid to the distillation tower is 80-99.9% by mass, the concentration of iodomethane is 0.01-16% by mass, the concentration of water is 0.05-18% by mass, the concentration of methyl acetate is 0.01-16% by mass, and the concentration of formic acid is 5-10000 ppm by mass.

45. The method for preparing acetic acid according to claim 20, comprising the high-boiling step that satisfies the operating conditions of (ii).

46. ​​The method for preparing acetic acid according to claim 45, wherein, In the high-boiling point removal process, as satisfied in (ii), the hydrogen partial pressure is 2 kPa or less in absolute pressure.

47. The method for preparing acetic acid according to claim 45, wherein, In the process of removing high-boiling point, as described in (ii), the hydrogen partial pressure is 0 kPa or higher in absolute pressure.

48. The method for preparing acetic acid according to claim 45, wherein, In the process of removing high-boiling point (ii), the partial pressure of carbon dioxide is less than 5 kPa in absolute pressure.

49. The method for preparing acetic acid according to claim 45, wherein, In the process of removing high-boiling point, as described in (ii), the partial pressure of carbon dioxide is 0 kPa or higher in absolute pressure.

50. The method for preparing acetic acid according to claim 45, wherein, In the high-boiling point removal process, as described in (ii), the operating temperature is 120°C or higher.

51. The method for preparing acetic acid according to claim 45, wherein, In the high-boiling point removal process, as described in (ii), the operating temperature is below 165°C.

52. The method for preparing acetic acid according to claim 45, wherein, In the process of removing high-boiling point, the concentration of acetic acid in the feed liquid to the distillation tower is 99.1-99.999% by mass or higher, and the concentration of formic acid is 5-9000 ppm by mass.

53. The method for preparing acetic acid according to claim 20, comprising the product step that satisfies the operating conditions described in (ii).

54. The method for preparing acetic acid according to claim 53, wherein, In the process described in (ii), the hydrogen partial pressure is 2 kPa or less in absolute pressure.

55. The method for preparing acetic acid according to claim 53, wherein, In the process of the product, as described in (ii), the hydrogen partial pressure is 0 kPa or higher in absolute pressure.

56. The method for preparing acetic acid according to claim 53, wherein, In the process described in (ii), the partial pressure of carbon dioxide is 5 kPa or less in absolute pressure.

57. The method for preparing acetic acid according to claim 53, wherein, In the process of the product, as described in (ii), the partial pressure of carbon dioxide is 0 kPa or higher in absolute pressure.

58. The method for preparing acetic acid according to claim 53, wherein, In the process described in (ii), the operating temperature is 120°C or higher.

59. The method for preparing acetic acid according to claim 53, wherein, In the process described in (ii), the operating temperature is below 165°C.

60. The method for preparing acetic acid according to claim 53, wherein, In the product manufacturing process, the acetic acid concentration in the feed liquid to the distillation tower is 99.8-99.999% by mass or higher, and the formic acid concentration is 5-2000 ppm by mass.

61. The method for preparing acetic acid according to claim 1 or 2, wherein, The dwell time in a process that meets the operating conditions of (i) or (ii) is more than 1 minute.

62. The method for preparing acetic acid according to claim 61, wherein, The dwell time in the process that meets the operating conditions of (i) or the process that meets the operating conditions of (ii) is less than 2 hours.

63. The method for preparing acetic acid according to claim 1 or 2, wherein, The process liquid with a formic acid concentration of 10 ppm or more is recycled to a process that meets the following operating conditions: (i) hydrogen partial pressure is less than 500 kPa in absolute pressure, carbon dioxide partial pressure is less than 70 kPa in absolute pressure, and operating temperature is more than 150°C.

64. The method for preparing acetic acid according to claim 63, wherein, The process that satisfies the operating conditions described in (i) is a carbonylation reaction process.

65. The method for preparing acetic acid according to claim 1, wherein, The acetic acid preparation process includes a distillation step selected from at least one of a low-boiling step, a dehydration step, a high-boiling step, and a product step, wherein the overhead liquid of the distillation column in the at least one distillation step is recycled to a step that satisfies the operating conditions of (i) and / or a step that satisfies the operating conditions of (ii).

66. The method for preparing acetic acid according to claim 65, wherein, The destination of the overhead liquid of the distillation column is the carbonylation reaction process and / or an evaporation or distillation process upstream of the distillation process selected from the low-boiling point removal process, dehydration process, high-boiling point removal process, and product process carried out in the distillation column.

67. The method for preparing acetic acid according to claim 65 or 66, wherein, The formic acid in the overhead liquid of the distillation column is decomposed through the recirculation.

68. The method for preparing acetic acid according to claim 65 or 66, wherein, The concentration of acetic acid in the recycled process solution is above 5% by mass.

69. The method for preparing acetic acid according to claim 65 or 66, wherein, The concentration of acetic acid in the overhead liquid of the distillation column is 5% by mass or more.

70. The method for preparing acetic acid according to claim 65 or 66, wherein, The overhead liquid of the distillation column is the overhead liquid of the distillation column in which the acetic acid concentration in the feed liquid is 80% or more by mass.

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