Process and apparatus for distilling a stream containing acetic acid and propionic acid in an acetic acid production unit

Abstract

This disclosure generally relates to methods and systems for distilling acetic acid products. In one aspect, this disclosure provides a method for separating a feed stream containing acetic acid and propionic acid in a distillation column. The method includes introducing the feed stream into the column through a feed inlet; withdrawing a first product stream containing acetic acid and propionic acid from a first extraction outlet located at a level above the feed inlet level; and withdrawing a second product stream containing acetic acid and propionic acid from a second extraction outlet located at a level between the feed inlet level and the first extraction outlet level. The volume ratio of the first product stream to the second product stream is in the range of 1:9 to 19:1; and the amount of propionic acid present in the second product stream is at least 50 ppmw greater than the amount of propionic acid present in the first product stream.

Description

Technical Field

[0001] This disclosure generally relates to methods and apparatus for distilling a feed stream containing acetic acid and propionic acid, which can be used in acetic acid production. Background Technology

[0002] Commercially, acetic acid has been produced for many years by carbonylating methanol with carbon monoxide in the presence of a Group VIII carbonylation catalyst. Generally, carbon monoxide and methanol are contacted in one or more reaction zones in the presence of a homogeneous or heterogeneous rhodium or iridium carbonylation catalyst, methyl iodine, and water. The effluent from the reaction zone is generally directed to a flash zone, where a significant portion of the volatile components evaporates from the Group VIII metal carbonylation catalyst; these substances can be recycled back to the reaction zone. The evaporated fraction is removed from the flash zone to provide a crude product stream, which includes not only the desired acetic acid product but also methyl iodine, methyl acetate, and water. Acetic anhydride is often provided as a byproduct. Notably, propionic acid is frequently a byproduct, and its amount is largely determined by the conditions in the reactor zone. Although less volatile than acetic acid, propionic acid is volatile enough to be included in the crude product stream.

[0003] Distillation is generally used to provide a purified acetic acid feed stream from the crude product stream. For example, some conventional methods use a three-column configuration, including a light fractionation column, a drying column, and a heavy fractionation column. The light fractionation column removes the “light” components, including water, methyl iodine, and methyl acetate overhead distillate. The “wet” acid feed stream is taken at the base and directed to the drying column, where more water is removed at the top to provide a “dry” acid feed stream at the base. In an alternative configuration, the light fractionation column is operated such that the acid feed stream is also dried by removing significantly more water at the top, along with methyl iodine and methyl acetate; in this configuration, the drying column can be omitted. The heavy fractionation column separates the effluent from the base of the drying column (or the light fractionation column, if the drying column is omitted) into an acetic acid product stream with a relatively low amount of propionic acid (generally taken from a side feed near the top of the heavy fractionation column) and a mixed propionic acid / acetic acid feed stream at the base of the heavy fractionation column.

[0004] However, distillation to provide an acetic acid product stream with low propionic acid content is an energy-intensive process. There is still a need to provide a separation method for acetic acid that improves energy efficiency while still meeting customer propionic acid specifications. Summary of the Invention

[0005] The scope of this disclosure is not affected in any way by the statements in the overview.

[0006] In one aspect, this disclosure provides a method for separating a feed stream containing acetic acid and propionic acid in a distillation column, the distillation column containing...

[0007] The bottom section that is in thermal connection with the heat source;

[0008] The feed inlet is located at a certain horizontal level above the bottom section;

[0009] The first extraction outlet is located at a certain horizontal level above the feed inlet;

[0010] A second extraction outlet is located at a certain level between the feed inlet and the first extraction outlet;

[0011] The steam outlet is located at a certain horizontal level above the first extraction outlet; and

[0012] The bottom exit is located at a certain horizontal level within the bottom section;

[0013] Methods include

[0014] Heat is supplied from a heat source at the bottom section of the distillation column;

[0015] The feed stream is introduced into the distillation column through the feed inlet;

[0016] A first product stream containing acetic acid and propionic acid is extracted from the first extraction outlet; and

[0017] A second product stream containing acetic acid and propionic acid is extracted from the second extraction outlet;

[0018] in

[0019] The volume ratio of the first product stream to the second product stream is in the range of 1:9 to 19:1; and

[0020] The amount of propionic acid present in the second product stream is at least 50 ppmw greater than the amount of propionic acid present in the first product stream.

[0021] Another aspect of this disclosure provides a distillation system for separating a feed stream containing acetic acid and propionic acid, the distillation system comprising...

[0022] Heat source; and

[0023] Includes the following distillation columns

[0024] The bottom section that is in thermal connection with the heat source;

[0025] The feed inlet is located at a certain level above the bottom section and is in communication with the source fluid of the feed stream containing acetic acid and propionic acid;

[0026] The first extraction outlet is located at a certain horizontal level above the feed inlet;

[0027] A second extraction outlet is located at a certain level between the feed inlet and the first extraction outlet;

[0028] The steam outlet is located at a certain horizontal level above the first extraction outlet; and

[0029] The bottom exit is located at a certain level within the bottom section.

[0030] Other aspects of this disclosure will be apparent to those skilled in the art from the following description. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the distillation system disclosed herein. Detailed Implementation

[0032] The inventors have noted that operators of the heavy distillation column can attempt to control the amount of propionic acid in the acetic acid product stream by adjusting the operation of the heavy distillation column. Increasing the column reflux ratio can reduce the concentration of propionic acid in the acetic acid product stream. However, this will also increase the column's energy consumption.

[0033] The inventors also noted that customer requirements regarding propionic acid content can vary. Some customers require strictly low propionic acid concentrations in their methods. However, others, such as those using acetic acid in methods for preparing purified terephthalic acid or vinyl acetate monomers, are much less sensitive to the presence of propionic acid.

[0034] One simple way to ensure that all customers' propionic acid requirements are met is to operate the heavy distillation column to the most stringent customer propionic acid specifications. However, this involves a variable cost penalty in the form of the reflux ratio required to meet these specifications and the energy required to operate the heavy distillation column.

[0035] It can also provide different acetic acid product streams with varying propionic acid concentrations to different customers. This reduces the total energy required for the operation, specifically due to the reduced energy used in the distillation of lower purity products. To achieve this, the equipment can operate on a competitive basis, producing acetic acid of different propionic acid grades over specific time periods. However, this requires very careful planning, introduces additional complexity, and necessitates additional storage to isolate products of different purities.

[0036] The inventors hereby provide a method for operating a heavy distillation column to simultaneously provide two different acetic acid product streams with different propionic acid concentrations, while operating with significant energy savings. Regarding... Figure 1One embodiment of this method is described. A distillation column 120 is used to separate a feed stream 110 comprising acetic acid and propionic acid. The distillation column 120 includes a bottom section 122 in thermal communication with a heat source 150, which may be, for example, a reboiler. The distillation column 120 further includes a feed inlet 132 located at a certain level above the bottom section 122; a first extraction outlet 136 (in the form of a first side outlet) located at a certain level above the feed inlet 132; a second extraction outlet 134 (in the form of a second side outlet) located at a certain level between the feed inlet 132 and the first extraction outlet 136; a vapor outlet 138 located at a certain level above the first extraction outlet 136; and a bottom outlet 140 located at a certain level within the bottom section 122. The method includes providing heat from the heat source 150 at the bottom section 122 of the distillation column 120; and guiding the feed stream 110 into the distillation column 120 through the feed inlet 132. A first product stream 146, containing acetic acid (e.g., at least 99% purity) and propionic acid, is withdrawn from a first extraction outlet 136, and a second product stream 144, also containing acetic acid (e.g., at least 99% purity) and propionic acid, is withdrawn from a second extraction outlet 134. The volume ratio of the first product stream to the second product stream is in the range of 1:9 to 19:1. Notably, the amount of propionic acid present in the second product stream is at least 50 ppm (ppmw) greater by weight than the amount of propionic acid present in the first product stream. In this way, the method provides two different product streams with different propionic acid concentrations. Based on the disclosure herein, those skilled in the art can adjust process conditions, such as the reflux ratio, the relative positioning of the extraction outlets, and the ratio of the product streams, to provide a product stream with a desired propionic acid concentration.

[0037] Based on the disclosure herein, those skilled in the art can provide the first and second product streams at various volume ratios. For example, in some embodiments further described herein, the volume ratio of the first product stream to the second product stream is in the range of 1:2 to 5:1. In some such embodiments, the volume ratio of the first product stream to the second product stream is in the range of 1.5:1 to 4:1, or in the range of 1:1 to 3:1. In this embodiment, a relatively larger portion of the higher purity first product stream is extracted. In some embodiments further described herein, the volume ratio of the first product stream to the second product stream is in the range of 2:1 to 1:5. In some such embodiments, the volume ratio of the first product stream to the second product stream is in the range of 1:4 to 1:1.5, or in the range of 1:3 to 1:1; in this embodiment, a relatively larger portion of the lower purity second product stream is extracted.

[0038] Similarly, based on the disclosure herein, those skilled in the art can provide different first and second product streams having various propionic acid concentrations. For example, in some embodiments as further described herein, the first product stream comprises no more than 500 ppmw of propionic acid (e.g., no more than 350 ppmw, or no more than 250 ppmw, or no more than 150 ppmw). For example, in some embodiments as further described herein, the first product stream has propionic acid in the range of 50-350 ppmw or 75-250 ppmw. In other embodiments as further described herein, the first product stream has propionic acid in the range of 50-500 ppmw, or 50-250 ppmw, or 50-150 ppmw, or 75-500 ppmw, or 75-350 ppmw, or 75-150 ppmw, or 100-500 ppmw, or 100-350 ppmw, or 100-250 ppmw.

[0039] In some embodiments as further described herein, the second product stream (i.e., also having at least 50 ppmw more propionic acid than the first product stream) has at least 200 ppmw propionic acid (e.g., at least 300 ppmw, or at least 400 ppmw, or at least 500 ppmw). In some embodiments as further described herein, the second product stream has 200-1200 ppmw of propionic acid, or 200-1000 ppmw of propionic acid, or 200-800 ppmw of propionic acid, or 200-600 ppmw of propionic acid, or 300-1200 ppmw of propionic acid, or 300-1000 ppmw of propionic acid, or 300-800 ppmw of propionic acid, or 400-1200 ppmw of propionic acid, or 400-1000 ppmw of propionic acid, or 400-800 ppmw of propionic acid, or 400-600 ppmw of propionic acid, or 500-1200 ppmw of propionic acid, or 500-1000 ppmw of propionic acid, or 500-800 ppmw of propionic acid.

[0040] As will be appreciated by those skilled in the art, the product stream is otherwise substantially acetic acid. For example, in some embodiments, the product stream comprises at least 99 vol% acetic acid. Of course, one or more of the first and second product streams may include small amounts of impurities such as water, aldehydes, and formic acid. For example, in some embodiments as further described herein, one or more of the product streams independently comprise up to 2,500 ppmw of water (e.g., 500-1,500 ppmw of water).

[0041] Those skilled in the art will recognize that one of the parameters used to provide product streams of varying propionic acid concentrations, based on the disclosure herein, is the number of theoretical stages separating the first and second extract outlets. For example, in some embodiments further described herein, the first and second extract outlets separate at least one theoretical stage. In some embodiments further described herein, the first and second extract outlets separate at least two, three, or four theoretical stages. For example, in some embodiments further described herein, the first and second extract outlets separate 1-10 theoretical stages, such as 2-10, 3-10, 1-7, 2-7, 3-7, 1-5, 2-5, or 3-5 theoretical stages. Those skilled in the art will recognize that the phrase "theoretical stage" will be broadly understood within distillation terminology and can refer to an equilibrium stage, ideal stage, theoretical tray, or theoretical plate. In the case of a tray column, the number of theoretical stages is related to the actual number of physical trays based on the tray or plate efficiency.

[0042] In some embodiments further described herein, the feed inlet and the first extraction outlet are separated by at least five theoretical stages. In some embodiments further described herein, the feed inlet and the first extraction outlet are separated by at least six, or at least ten, or at least twelve theoretical stages. For example, in some embodiments further described herein, the feed inlet and the first extraction outlet are separated by 10-20 theoretical stages, such as 10-18, or 10-16, or 12-20, or 12-18, or 12-16 theoretical stages.

[0043] In certain desirable embodiments as otherwise described herein, the first extraction outlet and the second extraction outlet are separated by 2-10 (e.g., 2-7 or 2-5) theoretical stages, and the feed inlet and the first extraction outlet are separated by 10-18 (e.g., 12-18 or 12-16) theoretical stages.

[0044] For example, in some embodiments as further described herein, the first and second extraction outlets separate 1-4 (e.g., 1-3) theoretical stages, the first product stream contains up to 400 ppmw (e.g., up to 300 ppmw or up to 200 ppmw) of propionic acid, and the second product stream contains 250-1,000 ppmw (e.g., 350-1,000 ppmw or 450-1,000 ppmw) of propionic acid. In some embodiments as further described herein, the first and second extraction outlets separate 3-7 (e.g., 4-7) theoretical stages, the first product stream contains up to 400 ppmw (e.g., up to 300 ppmw or up to 200 ppmw) of propionic acid, and the second product stream contains 350-1,200 ppmw (e.g., 450-1,200 ppmw or 550-1,200 ppmw) of propionic acid.

[0045] Based on the disclosure herein, those skilled in the art can select the locations of the first and second extraction outlets to provide acetic acid products with propionic acid concentrations that meet downstream customer requirements while achieving lower energy consumption. Furthermore, using a second extraction outlet (e.g., instead of simply bypassing the column with a portion of the feed) offers additional advantages. Removing the product from a second extraction outlet at a level above the feed inlet helps reduce the concentration of corrosive metals in the product, as any ingress with the feed is likely to remain dissolved in the liquid phase and should not migrate upwards with the gas phase. As described below, alkali metals can be mixed with the feed to the heavy fraction column to prevent iodide contamination of the product. Similarly, subjecting all product to distillation and collecting it at a level above the feed point similarly limits any contamination of the product by alkali metals.

[0046] Similarly, those skilled in the art can select the location of the feed inlet in the distillation column based on the disclosure herein, such as based on the overall composition distribution of the column and the composition of the feed stream, to provide a desired combination of energy efficiency and desired product purity.

[0047] For example, modeling using process simulation software can be used to determine the appropriate locations of the feed inlet, the first extraction outlet, and the second extraction outlet. Commercially available software such as ASPEN PLUS is suitable for use by those skilled in the art or by general technical personnel.

[0048] To provide operational flexibility, the distillation column used in the methods and systems described herein may, in some embodiments, include multiple feed inlets located at different vertical positions on the column. Those skilled in the art can select one of the desired feed inlets for use in the methods described herein to provide the desired product purity.

[0049] Similarly, the distillation column used in the methods and systems described herein may include three or more extraction outlets, each located at a different vertical position on the column, in some embodiments. Those skilled in the art can select one of the desired extraction outlets as a first extraction outlet and another as a second extraction outlet in the methods described herein to provide the desired product purity.

[0050] The feed stream is generally the feed stream of an acetic acid production unit, such as the carbonylation of methanol to produce acetic acid, as described in more detail below. For example, in some embodiments, the feed stream includes at least a portion of the effluent from the light fraction recovery zone of the acetic acid production unit. Figure 1 In the implementation scheme, the feed stream 110 is the effluent from the light fraction recovery zone 170.

[0051] The feed stream can be provided at various inlet pressures, depending on its source. For example, in some embodiments as further described herein, the inlet pressure of the feed stream is in the range of 1-10 bara, such as 2-8 bara or 2-6 bara.

[0052] Distillation columns can operate at a variety of pressures. For example, in some embodiments as further described herein, the top pressure of the distillation column is in the range of 0.5-2.5 bara, such as 0.75-2.25 bara or 1-2 bara.

[0053] The feed stream may include various amounts of propionic acid. For example, in some embodiments as further described herein, propionic acid is present in the feed stream in an amount ranging from 250 to 1,200 ppmw. In some such embodiments, propionic acid is present in the feed stream in an amount ranging from 300 to 1,200 ppmw or 350 to 1,200 ppmw. Those skilled in the art will understand that the amount of propionic acid in the feed stream will depend on a variety of factors, including the conditions in the reaction zone where acetic acid is prepared and the purity of the feed entering the reaction zone.

[0054] Similarly, the feed stream may include various amounts of metal. In some embodiments, the feed stream may contain up to 5 ppmw, for example, up to 2.5 ppmw or up to 1 ppmw of a combined amount comprising one or more metals. In some other embodiments, the feed stream may contain 1-100 ppmw, for example, 1-75 ppmw, or 1-50 ppmw, or 1-25 ppmw, or 5-100 ppmw, or 5-75 ppmw, or 5-50 ppmw, or 5-25 ppmw of a combined amount comprising one or more alkali metals (e.g., sodium and / or potassium, in ionic form). The inventors note that the presence of alkali metals can help prevent iodide contamination of the product stream. However, the distillation method described herein can separate the metal from the product stream. Therefore, in some embodiments as further described herein, the first product stream and the second product stream each individually contain less than 1 ppmw, for example, less than 0.5 ppmw or less than 0.25 ppmw of a combined amount of metal.

[0055] The product streams from this distillation process are conventionally cooled (using a suitable heat exchanger) before entering the storage tank. For example, in some embodiments further described herein, one or more of the product streams are cooled to below 50°C (e.g., below 40°C) before entering the storage tank. However, in other embodiments further described herein, one or more of the product streams can be supplied with heat to downstream users. This is particularly advantageous when the downstream process is partially integrated with the acetic acid production process described herein. This reduces energy consumption in downstream facilities due to the reduced need for preheating the acid before further processing. Therefore, those skilled in the art can avoid adequately cooling one or more of the product streams before supplying them to users. For example, in some embodiments further described herein, one or more of the product streams are supplied to downstream users at a temperature of 90-160°C (e.g., 100-150°C).

[0056] In some embodiments, as further described herein, the overhead feed stream is removed from the distillation column via a vapor outlet and transferred to a heat exchanger. In the heat exchanger, the overhead feed stream is cooled by generating steam (e.g., low-pressure steam), which can advantageously be used for various process operations. For example, a portion of the condensed overhead feed stream can be returned to the distillation column as a reflux stream. For example, in… Figure 1 In one embodiment, the overhead feed stream 148 is transferred to a heat exchanger 160, where it is cooled by heat exchange with a condensate (or boiler feedwater) stream 164 to provide low-pressure steam 162. In other embodiments, cooling water may be used for condensation of the overhead feed stream. Similarly... Figure 1 As shown in the implementation scheme, the condensed overhead stream is separated to provide distillate stream 114 and reflux stream 112, which are then recycled to reflux inlet 130 on distillation column 120.

[0057] In some embodiments, as further described herein, the bottom feed stream containing propionic acid is removed from the distillation column through a bottom outlet. Figure 1In some embodiments, the bottom stream 142 is withdrawn through the bottom outlet 140. In some embodiments further described herein, the bottom stream substantially comprises (e.g., at least 90 wt.%, or at least 95 wt.%, or at least 97.5 wt.%, or at least 99 wt.%) propionic acid and acetic acid. For example, in some embodiments further described herein, the bottom stream comprises 5-50 wt.% acetic acid and 50-95 wt.% propionic acid. In some embodiments further described herein, the bottom stream comprises up to 10 wt.% (e.g., up to 5 wt.% or up to 2.5 wt.%) a combined amount of acetic anhydride, butyric acid, and one or more of metal salts and acetates (e.g., alkali metal salts and acetates). In some embodiments further described herein, the bottom stream is withdrawn from the distillation column and transferred to a storage tank (e.g., after cooling the bottom stream). In some such embodiments, the withdrawn bottom stream is purified (e.g., by distillation) and then transferred to a storage tank.

[0058] Another aspect of this disclosure is a distillation system for separating a feed stream comprising acetic acid and propionic acid. The distillation system includes a heat source and a distillation column. The distillation column includes a bottom section in thermal communication with the heat source; a feed inlet located at a certain level above the bottom section, the feed inlet being in fluid communication with a source fluid of the feed stream comprising acetic acid and propionic acid; a first extraction outlet (e.g., as a side outlet) located at a certain level above the feed inlet; a second extraction outlet (e.g., as a side outlet) located at a certain level between the feed inlet and the first extraction outlet; a vapor outlet located at a certain level above the first extraction outlet; and a bottom outlet located at a certain level within the bottom section. One embodiment of this distillation system relates to... Figure 1 The distillation system of this disclosure can be arranged as otherwise described herein with respect to the distillation method.

[0059] For example, in some embodiments as further described herein, the first extraction outlet and the second extraction outlet of the system are separated by at least one theoretical level. In some embodiments as further described herein, the first extraction outlet and the second extraction outlet of the system are separated by at least two, at least three, or at least four theoretical levels. For example, in some embodiments as further described herein, the first extraction outlet and the second extraction outlet of the system are separated by 1-10 theoretical levels, such as 2-10, 3-10, 1-7, 2-7, 3-7, 1-5, 2-5, or 3-5 theoretical levels.

[0060] In some embodiments further described herein, the system's feed inlet and first extraction outlet are separated by at least five theoretical stages. In some embodiments further described herein, the system's feed inlet and first extraction outlet are separated by at least six, at least ten, or at least twelve theoretical stages. For example, in some embodiments further described herein, the system's feed inlet and first extraction outlet are separated by 10-20 theoretical stages, such as 10-18, 10-16, 12-20, 12-18, or 12-16 theoretical stages.

[0061] In certain desirable embodiments as otherwise described herein, the first extraction outlet and the second extraction outlet of the system are separated by 2-10 (e.g., 2-7 or 2-5) theoretical stages, and the feed inlet of the system and the first extraction outlet are separated by 10-18 (e.g., 12-18 or 12-16) theoretical stages.

[0062] In this invention, a vapor outlet is provided at a certain level above the first extraction outlet. The vapor outlet is typically located at the top of the column. The first extraction outlet is located at a certain level below the vapor outlet and is preferably a side extraction outlet located at least two theoretical stages from the top of the column, such as two to eight theoretical stages from the top. The location can also be defined relative to the total number of theoretical stages present in the column. Thus, the first extraction outlet may be located between two and eight theoretical stages from the top of the column and / or between two and a certain number of theoretical stages representing 25% of the total number of theoretical stages in the column (from the top). (For example, if there are a total of 25 theoretical stages, 25% would represent at most five theoretical stages from the top.)

[0063] The second extraction outlet is located at a certain level between the feed inlet and the first extraction outlet, i.e., below the first extraction outlet and above the feed inlet. It is generally located at least three theoretical stages from the top of the column, for example, four to twelve theoretical stages from the top. The location can also be defined relative to the total number of theoretical stages. Therefore, the second extraction outlet could be located between four to twelve theoretical stages from the top of the column and / or between a number of theoretical stages representing 20% ​​and 40% of the total number of theoretical stages in the column (in each case, from the top of the column). (For example, if there are a total of 25 theoretical stages, then 20%–40% would correspond to five to ten theoretical stages from the top of the column.)

[0064] The feed inlet is preferably located at least 7 theoretical stages from the top of the column, for example, 7-15 theoretical stages from the top. In terms of location relative to the total number of theoretical stages, the feed inlet is typically located in the central portion of the column. Preferably, it is located between 25% and 75% of the total number of theoretical stages in the column (in each case, from the top of the column). (Of course, these values ​​are subject to other requirements in the claims regarding the location relative to the first and second extraction outlets.)

[0065] This invention provides a bottom outlet located at a certain level within the bottom section. The bottom outlet is typically located at the bottom of the bottom section (and is also the bottom section of the tower).

[0066] The acetic acid production unit described herein is a unit that produces at least acetic acid, and includes units that co-produce acetic acid and acetic anhydride. Those skilled in the art will recognize that various acetic acid production units can be configured to implement the separation methods described herein. Generally, an acetic acid production unit includes a reaction zone (e.g., including a reactor) and a light fraction recovery zone (e.g., including a light fractionation column) configured to recover acetic acid. Other reaction zones and recovery zones may be present. For example, a flash evaporation zone is generally used between the reaction zone and the light fraction recovery zone. Exhaust streams from any part of the acetic acid production unit may be scrubbed as described herein. Production equipment and operation for the production of acetic acid (e.g., by carbonylation of methanol and / or its reactive derivatives with carbon monoxide in the presence of a Group VIII metal catalyst) are familiar to those skilled in the art.

[0067] In the acetic acid production methods described herein, methanol and / or its reactive derivatives may be introduced into the reaction zone as liquid reactants (i.e., to form a liquid reaction composition in the reaction zone). For example, in certain desirable embodiments of the methods further described herein, methanol is introduced into the reaction zone as a reactant. In other methods further described herein, one or more reactive derivatives are introduced into the reaction zone as reactants, or a combination of methanol and one or more methanol reactive derivatives is introduced into the reaction zone as a reactant. The “reactive derivatives” of methanol as used herein are methyl acetate, dimethyl ether, and methyl iodide. In certain embodiments further described herein, methanol and / or methyl acetate are used as liquid reactants. In one embodiment further described herein, methanol is used as a reactant; in another embodiment further described herein, methyl acetate is used as a reactant; and in yet another embodiment further described herein, a mixture of methanol and methyl acetate is used as a reactant.

[0068] The methods described herein can employ various carbonylation catalysts, such as Group VIII noble metal carbonylation catalysts. The catalyst may include Group VIII species supported on an inert support, such as a carbon support. In certain desirable embodiments further described herein, the carbonylation catalyst comprises rhodium, iridium, or mixtures thereof. In one particular embodiment of the invention, the carbonylation catalyst is iridium. In another particular embodiment further described herein, the carbonylation catalyst is a rhodium catalyst. Catalyst promoters may optionally be present, for example selected from alkali metal iodides (e.g., lithium iodide), alkaline earth metal iodides, aluminum group metal iodides, organic iodide salts, ruthenium, osmium, rhenium, and mixtures thereof. When the catalyst is a rhodium catalyst, the optional carbonylation catalyst promoter may be desirablely selected from alkali metal iodides (e.g., lithium iodide), alkaline earth metal iodides, aluminum group metal iodides, and / or organic iodide salts and mixtures thereof. When the catalyst is an iridium catalyst, the optional carbonylation catalyst promoter may be desirablely selected from ruthenium, osmium, rhenium, and mixtures thereof.

[0069] When the carbonylation catalyst is an iridium catalyst, the iridium catalyst may comprise any iridium-containing compound substantially soluble in the liquid reaction composition. The iridium catalyst may be added to the liquid reaction composition in any suitable form that is substantially soluble in the liquid reaction composition or can be converted to a soluble form. The iridium catalyst is desirablely used as a chlorine-free compound (e.g., an acetate) soluble in one or more components of the liquid reaction composition (e.g., water and / or acetic acid), and thus can be added to the reaction as a solution therein. Examples of suitable iridium-containing compounds that may be added to the liquid reaction composition include IrCl3, IrI3, IrBr3, [Ir(CO)2I]2, [Ir(CO)2Cl]2, [Ir(CO)2Br]2, and [Ir(CO)4I2]. - H + [Ir(CO)2Br2] - H + ,

[0070] [Ir(CO)2I2] - H + [Ir(CH3)I3(CO)2] - H + Ir4(CO) 12 IrCl 3. 4H2O, IrBr 3. 4H₂O, Ir₃(CO) 12Iridium metal, Ir2O3, IrO2, Ir(acac)(CO)2, Ir(acac)3, iridium acetate, [Ir3O(OAc)6(H2O)3][OAc] and hexachloroiridium acid [H2IrCl6], and desirable chlorine-free iridium complexes such as acetate, oxalate and acetoacetate.

[0071] When present, the concentration of the iridium catalyst in the liquid reaction composition in the reaction zone can, for example, be independently in the range of 100-6000 ppmw iridium.

[0072] When the carbonylation catalyst is an iridium catalyst, the carbonylation catalyst promoter is preferably ruthenium. The promoter may include any ruthenium-containing compound that is substantially soluble in the liquid reaction composition. The ruthenium promoter may be added to the liquid reaction composition in any suitable form that is substantially soluble in the liquid reaction composition or can be converted to a soluble form. The ruthenium promoter compound is preferably used as a chlorine-free compound (e.g., an acetate) soluble in one or more components of the liquid reaction composition (e.g., water and / or acetic acid), and therefore can be added to the reaction as a solution thereof.

[0073] Examples of suitable ruthenium-containing compounds that can be used include ruthenium(III) chloride, ruthenium(III) chloride trihydrate, ruthenium(IV) chloride, ruthenium(III) bromide, ruthenium(III) iodide, ruthenium metal, ruthenium oxide, ruthenium(III) formate, and [Ru(CO)3I3]. - H + Ruthenium tetrachloro(II, III), ruthenium acetate(III), ruthenium propionate(III), ruthenium butyrate(III), pentacarbonylruthenium, dodecacarbonyltriruthenium, and mixed halogenated carbonylruthenium (e.g., dichlorotricarbonylruthenium(II) dimer, dibromotricarbonylruthenium(II) dimer) and other organorruthenium complexes (e.g., tetrachlorobis(4-methylisopropylphenyl)diruthenium(II), tetrachlorobis(phenyl)diruthenium(II), dichloro(cyclooctyl-1,5-diene)ruthenium(II) polymers, and tri(acetylated pyruvate)ruthenium(III)). Desiredly, the ruthenium-containing compounds do not contain impurities that provide or generate in situ ionic iodides that inhibit the reaction, such as alkali or alkaline earth metals or other metal salts.

[0074] The ruthenium auxiliaries may be present in at most any effective amount within the solubility limit of the liquid reaction composition, liquid fraction, and / or any liquid process stream recycled to the carbonylation reaction zone. For example, the ruthenium auxiliaries suitably exist in the liquid reaction composition at a molar ratio of each ruthenium auxiliaries to iridium in the range of [0.1-100]:1, preferably [greater than 0.5]:1, more preferably [greater than 1]:1, and preferably [at most 20]:1, more preferably [at most 15]:1, and still more preferably [at most 10]:1. ​​The concentration of the ruthenium auxiliaries in the liquid reaction composition in the reaction zone is generally less than 6000 ppmw. Suitable auxiliary concentration ranges are, for example, 400-5000 ppmw, such as 2000-4000 ppmw.

[0075] Suitable rhodium carbonylation catalysts are described, for example, in EP-A-0 161 874, US 6,211,405, and EP-A-0728727, each of which is hereby incorporated herein by reference in its entirety. When the carbonylation catalyst is a rhodium catalyst, the concentration of the rhodium catalyst in the liquid reaction composition is preferably in the range of 50-5000 ppmw, preferably 100-1500 ppmw rhodium. When using rhodium as a catalyst, an alkali metal iodide, such as lithium iodide, is preferably used as an auxiliary agent, as described, for example, in the references cited above.

[0076] Therefore, in some embodiments, methanol and / or its reactive derivatives are catalyzed by a soluble Group VIII metal carbonylation catalyst (e.g., containing rhodium and / or iridium) in a liquid reaction composition comprising methanol and / or its reactive derivatives via homogeneous carbonylation with carbon monoxide. In some embodiments, the liquid reaction composition comprises water and one or more of methyl iodide and methyl acetate. In some such embodiments, the liquid reaction composition further comprises a propionic acid byproduct.

[0077] As described above, water can be present in the liquid reaction composition. Those skilled in the art will recognize that water is formed in situ in the liquid reaction composition via an esterification reaction between methanol and / or its reactive derivatives with an acetic acid product. In some embodiments, water may also be introduced into the reaction zone (e.g., together with or separately from other components of the liquid reaction composition). In some desirable embodiments, water is present in the liquid reaction composition in an amount ranging from 0.1 wt.% to 15 wt.% (e.g., from 1 wt.% to 15 wt.% or from 1 wt.% to 8 wt.%).

[0078] Methyl acetate can be formed in situ in a liquid reaction composition by reacting methanol and / or its reactive derivatives with acetic acid products or solvents. In some embodiments further described herein, the concentration of methyl acetate in the liquid reaction composition in the reaction zone is in the range of 2-50 wt%, for example, 3-35 wt%.

[0079] As described above, propionic acid byproducts may also be present in the liquid reaction composition. In some embodiments, propionic acid is present in the liquid reaction composition in an amount ranging from 200 ppmw to 2,500 ppmw, for example, from 400 ppmw to 2,000 ppmw, or from 600 ppmw to 1,400 ppmw.

[0080] In some desirable embodiments, methyl iodine is present in the liquid reaction composition in an amount ranging from 1 wt.% to 20 wt.%. For example, in some such embodiments, methyl iodine is present in the liquid reaction composition in an amount ranging from 2 wt.% to 16 wt.%.

[0081] In some embodiments, as further described herein, the liquid reaction composition comprises a solvent. For example, in some such embodiments, the liquid reaction composition comprises an acetic acid solvent (e.g., recycled from the separation zone of an acetic acid production unit).

[0082] As described above, acetic acid can be produced in the reaction zone by carbonylating methanol and / or its reactive derivatives with carbon monoxide. In some embodiments further described herein, the carbon monoxide supplied to the reaction zone is substantially pure. In other embodiments, the carbon monoxide supplied to the reaction zone contains one or more impurities such as carbon dioxide, methane, nitrogen, hydrogen, or an inert gas. In some embodiments further described herein, the partial pressure of carbon monoxide (e.g., in the reactor of the reaction zone) is in the range of 1 bar to 70 bar, for example, in the range of 1 bar to 35 bar.

[0083] In some embodiments, as further described herein, the carbonylation reaction in the reaction zone is carried out at a total pressure in the range of 10 barg to 100 barg (e.g., in the reactor of the reaction zone).

[0084] The effluent from the reaction zone can be directed to a flash separation zone (e.g., via a flash valve), where it is separated into a vapor stream and a liquid stream containing the carbonylation catalyst. The liquid stream can be introduced (i.e., as recirculation) back to the reaction zone. For example, in some embodiments as further described herein, the effluent from the reaction zone is directed to a flash separation zone to form a vapor fraction and a liquid fraction, and at least a portion of the liquid fraction is recirculated back to the reaction zone.

[0085] Flash separation zones are known in the art. In some embodiments, the flash separation zone may include an adiabatic flash vessel. Alternatively or additionally, the flash separation zone may be heated, for example, by a heater. The flash separation zone is generally operated at pressures in the range of 0-10 barg, preferably 0-3 barg.

[0086] The carbonylation methods described herein can be performed as batch or continuous processes. In some preferred embodiments, the carbonylation methods are performed as continuous processes.

[0087] In some embodiments, at least a portion of the vapor fraction from the flash separation zone is directed to the light fraction recovery zone of the acetic acid production unit. In some embodiments, the light fraction recovery zone of the acetic acid production unit is configured to separate at least components more volatile than acetic acid from the acetic acid. For example, in some embodiments, acetic acid is produced in the reaction zone by carbonylating methanol and / or its reactive derivatives with carbon monoxide in the presence of a Group VIII metal catalyst system, and the light fraction recovery zone of the acetic acid production unit is configured to separate acetic acid and further separate methyl iodide and methyl acetate, which can be recycled back to the reaction zone.

[0088] In some embodiments as further described herein, the light fraction recovery zone comprises a distillation column that separates the crude acetic acid product containing acetic acid and propionic acid from a light fraction comprising methyl iodide and methyl acetate. In some such embodiments, the light fraction recovery zone further comprises a drying column. For example, in some embodiments as further described herein, the light fraction recovery zone comprises a combination of a light fraction and a drying column, wherein water is removed from the crude acetic acid product to form a dried acetic acid product containing acetic acid and propionic acid. The “dry” or “dried” feed containing acetic acid (e.g., and optionally propionic acid) as used herein contains an amount of water of up to 1,500 ppmw.

[0089] Suitable columns and their configurations for use in the light fraction recovery zone are generally known in the art. Typically, a first fraction comprising at least acetic acid and propionic acid, and an overhead vapor fraction comprising methyl acetate, water, acetic acid, carbon monoxide, and methyl iodine are formed in the light fraction recovery zone. In some embodiments, the separated water may be recycled to the reaction zone or removed from the acetic acid production unit.

[0090] In some embodiments as further described herein, the light fraction recovery zone further includes one or more condensers and / or coolers to condense the overhead vapor fraction and form a liquid fraction. Those skilled in the art will recognize that any suitable method known in the art can be used to condense the overhead vapor fraction into a liquid phase. For example, in some embodiments, at least one heat exchanger (e.g., supplied with water as a cooling medium) is used to condense the fraction. Components of the uncondensed overhead fraction (e.g., carbon monoxide, carbon dioxide, inert gases, reaction byproduct gases) are removed from the light fraction recovery zone as a waste gas stream. In some embodiments, acetic acid is produced in the reaction zone by carbonylating methanol and / or its reactive derivatives with carbon monoxide in the presence of a Group VIII metal catalyst system, and the waste gas stream taken from the light fraction recovery zone further includes methyl iodine (e.g., present as entrained and / or evaporated methyl iodine), methyl acetate, and water.

[0091] In some embodiments, the liquid fraction formed in the light fraction recovery zone comprises methyl acetate, water, and acetic acid. In some embodiments, acetic acid is produced in the reaction zone by carbonylating methanol and / or its reactive derivatives with carbon monoxide in the presence of a Group VIII metal catalyst system, and the liquid fraction further comprises methyl iodine. In some embodiments, the liquid fraction further comprises entrained or dissolved gaseous components (e.g., carbon monoxide, carbon dioxide, inert gases).

[0092] In some embodiments, as further described herein, the light fraction recovery zone includes a decanter, wherein the liquid fraction is separated into two layers: a lower (e.g., organic) layer containing methyl acetate and an upper (e.g., aqueous) layer containing water. In some embodiments, acetic acid is produced in the reaction zone by carbonylating methanol and / or its reactive derivatives with carbon monoxide in the presence of a Group VIII metal catalyst system, and the lower layer further contains methyl iodine. In some embodiments, at least a portion (e.g., all) of the upper layer from the decanter is returned as a reflux stream to the distillation column of the light fraction recovery zone. In some embodiments, at least a portion (e.g., all) of the upper layer from the decanter is recycled to the reaction zone. In some embodiments, exhaust gas is removed from the decanter and transferred to an exhaust gas scrubbing unit (e.g., prior to disposal).

[0093] Generally, the feed stream containing acetic acid and propionic acid from the light fraction recovery section (e.g., the first fraction formed in the light fraction recovery section) is used as the feed stream for the distillation process described herein. That is, the distillation process described herein can be operated as a so-called "heavy fraction" process to provide purified acetic acid product.

[0094] Certain aspects of this disclosure are further described through the following non-limiting embodiments.

[0095] A set of distillation processes were modeled using ASPEN software. The table below provides a series of seven examples. In all examples, the distillation column has 25 theoretical stages, with the feed stream fed to the inlet above stage 9, and the first extract stream exiting from the first extract outlet on stage 2 (both measured from the top of the column). In examples 2 and 3, the second extract stream exits from the second extract outlet on stage 4, while in examples 4 and 5, the second extract stream exits from the second extract outlet on stage 7. In all examples, the feed propionic acid concentration is 500 ppm (ppmw) by weight, the feed inlet pressure is 4 bara, and the column top pressure is 1.5 bara. In all examples, "Product 1" is an acetic acid product with 150 ppmw acetic acid as a purer product stream. While this value will meet the requirements of most customers, those skilled in the art or of ordinary skill will recognize that cleaner product streams can provide propionic acid with higher or lower concentrations.

[0096]

[0097] Example 1 illustrates the standard operating mode of the heavy distillation column in an acetic acid production unit. Here, all product is supplied as a relatively pure product stream with a propionic acid concentration of 150 ppmw from the first side outlet. This results in significant energy costs in the form of a reboiler load of 13.733 MW.

[0098] Example 2 illustrates an embodiment of the heavy fraction distillation method according to this disclosure, wherein the second side outlet is located below the first side outlet but above the feed inlet. In this example, 75% of the product, as Product 1, is taken out through the first side outlet at a propionic acid concentration of 150 ppmw, while 25% of the product, as Product 2, is taken out through the second side outlet at a propionic acid concentration of 300 ppmw. Notably, compared to Example 1, the total energy requirement of the column reboiler is reduced by 13.5%.

[0099] Example 3 is essentially an extension of Example 2. Here, 50% of the product is taken out as Product 1 through the first side outlet, and 50% is taken out as Product 2 through the second side outlet at a propionic acid concentration of 310 ppmw. Here, the total energy consumption is reduced by 27%.

[0100] When a customer can accept a product with an even higher concentration of propionic acid, the second side can be positioned relatively closer to the feed inlet. This is illustrated in Examples 4 and 5, which produce a second product with a propionic acid concentration closer to 500 ppmw. Additional energy is saved compared to Examples 2 and 3.

[0101] If the customer can accept a product with a propionic acid concentration equal to the propionic acid concentration in the feed stream entering the column, an alternative is to simply separate a portion of the feed entering the column, and completely bypass the heavy fractionation column with that portion of the material. This is illustrated in Examples 6 and 7, compared with Examples 4 and 5, respectively. Surprisingly, modeling shows that bypassing the column with a portion of the feed results in a greater net energy consumption than feeding the entire stream into the column and removing the second product near the feed point. Without intending to be bound by theory, the inventors note that the column immediately upstream (depending on the configuration, a drying column or a light fractionation column) generally operates at a higher pressure than the heavy fractionation column used to provide the acetic acid product. The stream leaving this upstream column is generally at or near saturation temperature and therefore flashes upon entering the downstream heavy fractionation column. The resulting vapors generate some of the vapor load within the column required to achieve the desired separation and thus reduce the need for a column reboiler. Without intending to be bound by theory, this is believed to explain the unexpected results that feeding the entire feed stream into the tower (Examples 4 and 5) results in lower energy consumption than feeding around a portion of the tower (Examples 6 and 7).

[0102] The foregoing detailed description and accompanying drawings have been provided through explanation and illustration, and are not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments described herein will be apparent to those skilled in the art and remain within the scope of the appended claims and their equivalents.

[0103] It should be understood that the elements and features listed in the appended claims can be combined in different ways to produce new claims that also fall within the scope of this disclosure. Therefore, given that the dependent claims appended below are subordinate to only a single independent or dependent claim, it should be understood, or that such dependent claims may be alternatively subordinate to any of the foregoing claims (whether independent or dependent), and such new combinations should be understood as forming part of this specification.

Claims

1. A method for separating a feed stream containing acetic acid and propionic acid in a distillation column and improving energy efficiency, said distillation column comprising... The bottom section that is in thermal connection with the heat source; The feed inlet is located at a certain horizontal position above the bottom section; A first extraction outlet located at a certain horizontal position above the feed inlet; A second extraction outlet is located at a certain horizontal level between the feed inlet and the first extraction outlet; A steam outlet located at a certain horizontal level above the first extraction outlet; and The bottom exit is located at a certain horizontal position within the bottom section; The method includes Heat is supplied from a heat source at the bottom section of the distillation column; The feed stream is introduced into the distillation column through the feed inlet. The feed stream contains 350-1200 ppmw of propionic acid and contains one or more alkali metals in a combined amount of 1-100 ppmw. A first product stream containing acetic acid and propionic acid is taken out from the first extraction outlet. The first product stream contains up to 400 ppmw of propionic acid and less than 1 ppmw of combined metal. and A second product stream containing acetic acid and propionic acid is taken out from the second extraction outlet. The second product stream contains 400-1200 ppmw of propionic acid and less than 1 ppmw of combined metal. in The volume ratio of the first product stream to the second product stream is in the range of 1:9 to 19:1; and The amount of propionic acid present in the second product stream is at least 50 ppmw greater than the amount of propionic acid present in the first product stream.

2. The method according to claim 1, wherein the volume ratio of the first product stream to the second product stream is in the range of 1:2 to 5:

1.

3. The method according to claim 1, wherein the volume ratio of the first product stream to the second product stream is in the range of 2:1 to 1:

5.

4. The method of claim 1, wherein the first product stream comprises 50-350 ppmw of propionic acid.

5. The method of claim 1, wherein the second product stream comprises 400-1000 ppmw of propionic acid.

6. The method of claim 1, wherein the first extraction outlet and the second extraction outlet are separated by at least one theoretical stage.

7. The method according to claim 1, wherein the first extraction outlet and the second extraction outlet are separated by 3-7 theoretical stages.

8. The method according to claim 7, wherein The first product stream contains up to 300 ppmw of propionic acid; and The second product stream contains 450-1200 ppmw of propionic acid.

9. The method of claim 1, wherein the feed inlet and the first extraction outlet are separated by at least 5 theoretical stages.

10. The method according to any one of claims 1-9, wherein the feed stream comprises at least a portion of the effluent from the light fraction recovery zone of the acetic acid production unit.

11. The method according to any one of claims 1-9, wherein the feed stream comprises one or more alkali metals present in the feed stream in a combined amount of 5-75 ppmw.

12. The method according to any one of claims 1-9, further comprising: The overhead feed stream taken from the distillation column through the steam outlet is transferred to a heat exchanger, and In the heat exchanger, steam is generated to cool the overhead feed stream.

13. The method according to any one of claims 1-9, wherein the inlet pressure of the feed stream is 1-10 bara.

14. The method according to any one of claims 1-9, wherein the top pressure of the distillation column is 0.5-2.5 bara.

15. The method according to claim 1, wherein the volume ratio of the first product stream to the second product stream is in the range of 1.5:1 to 4:

1.

16. The method according to claim 1, wherein the volume ratio of the first product stream to the second product stream is in the range of 1:1 to 3:

1.

17. The method according to claim 1, wherein the volume ratio of the first product stream to the second product stream is in the range of 1:4 to 1:1.

5.

18. The method according to claim 1, wherein the volume ratio of the first product stream to the second product stream is in the range of 1:3 to 1:

1.

19. The method of claim 1, wherein the first extraction outlet and the second extraction outlet are separated by 1-10 theoretical stages.

20. The method of claim 1, wherein the first extraction outlet and the second extraction outlet are separated by 1-4 theoretical stages.

21. The method according to claim 1, wherein the first extraction outlet and the second extraction outlet are separated by 1-3 theoretical stages.

22. The method according to claim 1, wherein the first extraction outlet and the second extraction outlet are separated by 4-7 theoretical stages.

23. The method according to claim 1, wherein the feed inlet and the first extraction outlet are separated by 5-20 theoretical stages.

24. The method of claim 1, wherein the feed inlet and the first extraction outlet are separated by 10-20 theoretical stages.

25. The method according to any one of claims 1-9, wherein the inlet pressure of the feed stream is 2-8 bara.

26. The method according to any one of claims 1-9, wherein the inlet pressure of the feed stream is 2-6 bara.

27. The method according to any one of claims 1-9, wherein the top pressure of the distillation column is 0.75-2.25 bara.

28. The method according to any one of claims 1-9, wherein the top pressure of the distillation column is 1-2 bara.

29. The method according to any one of claims 1-9, wherein the feed stream comprises one or more alkali metals present in the feed stream in a combined amount of 5-50 ppmw.

30. A distillation system for separating a feed stream containing acetic acid and propionic acid, said distillation system comprising... Heat source; and Includes the following distillation columns The bottom section that is in thermal communication with the heat source; A feed inlet located at a certain horizontal level above the bottom section, the feed inlet being in fluid communication with the source fluid of the feed stream containing acetic acid and propionic acid; The first extraction outlet is located at a level at least 5 theoretical stages above the feed inlet and at a side extraction outlet at least 2 theoretical stages from the top of the tower. and A second extraction outlet is located between the feed inlet and the first extraction outlet, and is at least three theoretical levels away from the first extraction outlet; A steam outlet located at a certain horizontal level above the first extraction outlet; and The bottom exit is located at a certain horizontal level within the bottom section.

31. The distillation system according to claim 30, wherein the distillation column includes a plurality of feed inlets.

32. The distillation system of claim 30, wherein the distillation column comprises at least three extraction outlets.

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