Production and purification of acetic acid

Abstract

A method for producing and purifying acetic acid is provided herein. Specifically, the present disclosure relates to an improved method for producing and purifying acetic acid by carbonylating methanol in the presence of a Group VIII metal carbonylation catalyst. The acetic acid production and purification method described herein controls the formation and quality composition of acetal impurities such as 1,1-dimethoxyethane.

Description

[0001] priority

[0002] This application claims priority to U.S. Provisional Application No. 63 / 034,078, filed on June 3, 2020, which is incorporated herein by reference for all purposes. Technical Field

[0003] The present disclosure generally relates to a process for producing and purifying acetic acid. Specifically, the present disclosure relates to an improved process for producing and purifying acetic acid by carbonylating methanol in the presence of a Group VIII metal carbonylation catalyst. More specifically, the present disclosure relates to production and purification of acetal impurities such as 1,1-dimethoxyethane in which the acetal impurity is controlled. Background Art

[0004] The carbonylation process of methanol (production of acetic acid by carbonylation of methanol) is a known suitable industrial process for producing (synthesizing) acetic acid. In this process, for example, methanol and carbon monoxide are reacted continuously in a reaction vessel (reactor) in the presence of a metal catalyst (catalyst system). The reaction mixture is evaporated in an evaporator (e.g., a flasher), and the flashed vapor portion is purified in a distillation column (called a light ends column or splitter column) to remove low-boiling components. The reaction mixture can then be further purified or treated in one or more subsequent distillation columns to obtain the product acetic acid.

[0005] In the carbonylation process for producing acetic acid, other by-products (e.g., carbonyl impurities) are also formed during the reaction and / or subsequent purification steps. For example, acetaldehyde can be formed as a by-product (impurity). In addition, these by-products can undergo subsequent reactions to produce other impurities. For example, crotonaldehyde can be produced by the aldol condensation of acetaldehyde, and 2-ethyl crotonaldehyde can be produced by the aldol condensation of crotonaldehyde. These by-products (impurities) can reduce the quality of the product acetic acid, particularly by affecting the potassium permanganate test value of the product acetic acid. Therefore, it is desirable to reduce or remove the presence of these impurities in the logistics during the acetic acid carbonylation process.

[0006] Systems for reducing the concentration of acetaldehyde in an acetic acid carbonylation process are known and widely used in the industrial production of acetic acid. These systems typically include treating at least a portion of the overhead of a first distillation column, such as by distillation, absorption, or a combination thereof, to separate the acetaldehyde. In some cases, the overhead can be collected in a receiver and separated into multiple phases (phase separation). In particular, the overhead can be separated into multiple phases, such as an upper phase and a lower phase. A portion of the upper phase and / or the lower phase can then be further processed to separate and remove the acetaldehyde.

[0007]

[0006] Although existing carbonylation processes are efficient, there remains a desire to further improve the recovery of acetic acid in a safe and efficient manner. Summary of the invention

[0008] Embodiments of the present disclosure provide an industrial process for effectively and efficiently producing acetic acid having a good potassium permanganate test value.

[0009] Another embodiment of the present disclosure is to provide a method for effectively reducing the mass composition of 1,1-dimethoxyethane in the process of producing acetic acid. Unless otherwise specified, as used herein, the term "mass composition" or "concentration" refers to the mass fraction of a substance relative to the total mass and is generally expressed in wt%.

[0010] Still another embodiment of the present disclosure is to provide a method for effectively preventing oxidation of 1,1-dimethoxyethane in a process for producing acetic acid.

[0011] The inventors have conducted careful and intensive studies to achieve the above-mentioned objects; as a result of these studies, the following methods have been developed.

[0012] In one embodiment, a method for producing acetic acid is provided, which includes (a) a reaction step of continuously reacting methanol with carbon monoxide in a reactor in the presence of a reactive system comprising a metal catalyst, an ionic metal iodide and methyl iodide, acetic acid, methyl acetate and water; (b) an evaporation step of separating the reaction mixture obtained in the reaction step into a vapor stream and a liquid residue stream with or without heating; (c) a first distillation step of distilling at least a portion of the vapor stream in a first distillation column to form a first overhead stream and a side stream, wherein the first overhead stream comprises water, methyl iodide, and / or acetaldehyde, and the side stream comprises acetic acid; (d) a phase separation step of condensing the first overhead stream in one or more condensers, collecting the condensate in a receiver, and separating the condensate into an upper phase and a lower phase; (e) a second distillation step of distilling at least a portion of the lower phase in a second distillation column to form a second mixture comprising acetaldehyde; and (f) an acetaldehyde separation step of separating the second mixture into an acetaldehyde stream and a first return stream. The mass composition of 1,1-dimethoxyethane in the liquid stream of the process is controlled to be no more than 0.2 wt.%. The process may also include at least one step selected from the group consisting of (g) to (k): (g) a purification step of purifying the side stream to obtain an acetic acid product stream; (h) an off-gas treatment step of absorbing one or more off-gas streams from the process with an absorption solvent and forming a carbon monoxide stream and a second return stream; (i) a high boiling point compound removal step of removing high boiling point components from the acetic acid product stream; (j) an iodine removal step of contacting the acetic acid product stream with an ion exchange resin and separating iodine compounds from the acetic acid product stream; and (k) an extraction step of extracting acetaldehyde from the acetaldehyde stream to form an extract and a residual liquid, wherein the extract contains acetaldehyde and the residual liquid contains methyl iodide; wherein the mass composition of 1,1-dimethoxyethane in the liquid stream of the process is controlled to be no more than 0.2 wt.%. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The invention will be better understood in view of the attached non-limiting drawings, in which:

[0014] Figure 1 A schematic diagram of a process for producing and purifying acetic acid according to an embodiment of the present disclosure is shown.

[0015] Figure 2 A schematic diagram of an acetaldehyde or PRC removal system according to an embodiment of the present disclosure is shown.

[0016] Figure 3 A schematic diagram of an acetaldehyde or PRC removal system according to an embodiment of the present disclosure is shown.

[0017] At the outset, it should be noted that in the development of any such actual embodiment, many implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. In addition, the methods disclosed herein may also include components other than those cited or specifically mentioned, which will be apparent to one of ordinary or reasonable skill in the art.

[0018] As will be apparent from the figures and text presented herein, a variety of embodiments are contemplated. DETAILED DESCRIPTION

[0019] As discussed herein, these methods relate to continuous acetic acid production, and in particular, acetic acid production methods that include one or more steps for removing and / or reducing acetaldehyde and other carbonyl impurities from the system. The inventors have conducted detailed research and have determined that other impurities may be present in the system. In particular, the inventors have discovered that acetals (including 1,1-dimethoxyethane) may be present in various liquid streams in acetic acid production and purification systems. The inventors have discovered that 1,1-dimethoxyethane can enter the liquid stream by various factors, including by introducing the stream into the process from the outside and / or by in situ formation, for example, by the reaction of acetaldehyde and methanol. In one embodiment, the liquid stream may contain methyl iodide and have a mass ratio of methyl iodide to water greater than 1 (e.g., greater than 3 or greater than 5).

[0020] The presence of 1,1-dimethoxyethane in the system presents a number of problems. As an impurity, 1,1-dimethoxyethane reduces the quality (purity) of the acetic acid product. In addition, the inventors have discovered that 1,1-dimethoxyethane can react to form acetaldehyde. Increased amounts of acetaldehyde reduce the quality (purity) of the acetic acid product, and acetaldehyde may undergo side reactions to produce other carbonyl impurities. Thus, the in situ formation of acetaldehyde negatively affects the quality of the acetic acid product and reduces the efficiency of the system used to reduce and / or remove acetaldehyde from the system (e.g., by an acetaldehyde removal system).

[0021] The method for producing acetic acid (producing acetic acid by carbonylation of methanol) according to one embodiment of the present disclosure includes: a reaction step of continuously reacting methanol and / or a methanol derivative (such as methyl acetate, or dimethyl ether) with carbon monoxide in a reactor (reaction vessel) in the presence of a reactive system (catalyst system) comprising a metal catalyst (e.g., a rhodium or iridium catalyst), an ionic metal iodide and methyl iodide, acetic acid, methyl acetate and water; an evaporation step of separating the reaction mixture obtained in the reaction step into a vapor stream and a liquid residue stream with or without heating; and a first distillation column (which may also be referred to as a splitter column or a light ends column) to distill at least a portion of the vapor stream to form a first overhead stream and a side stream. The invention relates to a process for producing a first distillation column comprising: a first distillation step of a first overhead stream, wherein the first overhead stream comprises water, methyl iodide, and / or acetaldehyde, and the side stream comprises acetic acid; a phase separation step of condensing the first overhead stream in one or more condensers, collecting the condensate in a receiver, and separating the condensate into an upper phase (e.g., an aqueous phase, or otherwise referred to as a light phase) and a lower phase (e.g., an organic phase, or otherwise referred to as a heavy phase); a second distillation step of distilling at least a portion of the lower phase in a second distillation column to form a second mixture comprising acetaldehyde; an acetaldehyde separation step of separating the second mixture into an acetaldehyde stream and a first return stream; and wherein the mass composition of 1,1-dimethoxyethane in the liquid stream of the process is controlled and / or maintained at no more than 0.2 wt.%. In addition to these steps, the process may include at least one step selected from the group consisting of: a purification step of purifying the side stream to obtain an acetic acid product stream; an off-gas treatment step of absorbing one or more off-gas streams from the process with an absorption solvent and forming a carbon monoxide stream and a second return stream; a high boiling point compound removal step of removing high boiling point components from the acetic acid product stream; an iodine removal step of contacting the acetic acid product stream with an ion exchange resin and separating iodine compounds from the acetic acid product stream; an extraction step of extracting acetaldehyde from the acetaldehyde stream to form an extract and a residual liquid, wherein the extract comprises acetaldehyde and the residual liquid comprises methyl iodide. It should be understood that the order of the purification step, the high boiling point compound removal step, the iodine removal step, and / or the ion exchange resin may vary depending on the process, and in some embodiments, these steps may be skipped.

[0022] According to one embodiment, the mass composition of 1,1-dimethoxyethane in the liquid stream of the method is controlled and / or maintained by the methanol mass composition. The inventors have found that the formation of acetal, especially 1,1-dimethoxyethane, is due to the reversible reaction of methanol and acetaldehyde. Since the reaction can be controlled with an excess (large amount) of reagents to drive the reaction to completion, the formation of 1,1-dimethoxyethane is driven by excess (large amount) of methanol. Therefore, the mass composition of 1,1-dimethoxyethane in the liquid stream can be limited or reduced by controlling the amount of methanol. In particular, because acetaldehyde can form 1,1-dimethoxyethane in the first distillation column, the mass composition of 1,1-dimethoxyethane can be reduced by controlling the amount of methanol in the vapor stream produced in the evaporation step. In addition, because acetaldehyde can form 1,1-dimethoxyethane in the second distillation column, the mass composition of 1,1-dimethoxyethane can be reduced by controlling the amount of methanol in the lower phase of the phase separation step.

[0023] According to another embodiment, the mass composition of 1,1-dimethoxyethane in the liquid stream of the process is controlled by adjusting the residence time in the first distillation column and / or the second distillation column. As described above, the inventors have found that 1,1-dimethoxyethane tends to form in one or both of these columns. In particular, the inventors have found that the distillation time contributes to the formation of 1,1-dimethoxyethane. Therefore, the mass composition of 1,1-dimethoxyethane in the liquid stream can be reduced by controlling the residence time. In particular, the mass composition of 1,1-dimethoxyethane can be reduced by controlling the residence time in the first distillation column (light ends column; splitter column) and / or the second distillation column. In these embodiments, the residence time in the first distillation column (light ends column, splitter column) and / or the second distillation column is, for example, not less than 1 minute, preferably not less than 2 minutes, or more preferably not less than 5 minutes. Because one or more distillation steps are important aspects of purification, it is important not to strictly limit the residence time so as not to negatively affect the final purity of the product acetic acid.

[0024] In an embodiment of the present disclosure, the mass composition of 1,1-dimethoxyethane in the liquid stream is less than or equal to 0.2 wt.%, e.g., less than or equal to 0.15 wt.%, less than or equal to 0.1 wt.%, or less than or equal to 0.05 wt.%. In terms of ranges, the mass composition of 1,1-dimethoxyethane can be from 0.001 wt.% to 0.2 wt.%, e.g., from 0.01 to 0.2 wt.%, from 0.01 to 0.15 wt.%, from 0.01 to 0.1 wt.%, or from 0.01 to 0.05 wt.%.

[0025] Several embodiments of the present disclosure will be described below. Figure 11 is a flow chart of a methanol carbonylation process (producing acetic acid by methanol carbonylation) 100 according to one embodiment. The device is configured to be able to continuously produce acetic acid. The acetic acid production device associated with the carbonylation process flow chart includes a reactor (reaction vessel) 106, an evaporator 122, a first distillation column 140, a receiver 160, a second distillation column (202) and a third column 180. As shown and described herein, various liquid streams are produced by the process described herein. Due to the higher methyl iodide content, most of these liquid streams are recycled to the reactor 106, and therefore any impurities in these liquid streams tend to accumulate in the process. In particular, returning the liquid stream obtained from the residue (bottom stream) 206 of the second distillation column to the reactor 106 may have an accumulation of impurities, and therefore it is desirable to control or maintain the mass composition of 1,1-dimethoxyethane in the residue 206 at a value less than or equal to 0.2 wt.%. In one embodiment, the liquid stream returned to the reactor has a mass ratio of methyl iodide to water greater than 1 (e.g., greater than 3 or greater than 5). To further prevent the formation of peroxides, the liquid stream is shielded from contact with air.

[0026] As shown and described herein, the liquid flow may be Figure 1 Any of the liquid streams shown in , including a stream selected from a side stream, a condensate, a bottom stream, an upper phase, a lower phase, a second mixture, and / or an acetic acid product stream. In one embodiment, the liquid stream is the bottom or residue from a distillation column.

[0027] Those skilled in the art should understand that Figure 1 Various processing equipment including heat exchangers, receivers, pumps, controllers, valves, etc. are not shown in detail. Unless otherwise specified, those skilled in the art will understand that such processing equipment will be appropriately used without such processing equipment.

[0028] Hereinafter, the embodiments will be explained in detail with reference to the accompanying drawings as needed. Each step described in various processes has at least one corresponding unit and can be interchanged.

[0029] Reaction steps

[0030] In the reaction step, methanol (methanol derivative) and carbon monoxide are continuously reacted in a reactor in the presence of a reactive system. Figure 1In the embodiment shown in, a feed (first reagent) stream 102 containing methanol and a feed (second reagent) stream 104 containing carbon monoxide are fed into a reactor (e.g., a liquid phase carbonylation reactor) 106, where a carbonylation reaction occurs to form acetic acid. Although not shown, flow transmitters may be present on both feed streams to control and / or monitor the flow of each respective stream. In particular, controlling and / or monitoring the mass flow of the feed stream containing methanol is useful for determining the efficiency of the process.

[0031] The methanol-containing feed stream 102 comprises methanol and / or at least one reactive derivative selected from the group consisting of methanol, dimethyl ether and methyl acetate. The methanol-containing feed stream 102 may be partially from fresh feed, a recycle stream from a system, or some combination thereof. At least some of the methanol and / or its reactive derivatives will be converted to methyl acetate in the liquid medium by an esterification reaction with acetic acid and thus exist as it. The amount of unreacted methanol in the reactor 106 may be less than or equal to 1 wt.%, for example, less than or equal to 0.8 wt.%, less than 0.5 wt.%, or less than 0.3 wt.%, based on the total weight of the reaction mixture.

[0032] The methanol-containing feed stream 102 may contain 1,1-dimethoxyethane and may be a source of impurities in the system. Preferably, the methanol-containing feed stream is substantially free of 1,1-dimethoxyethane. Depending on the source of the raw material, the methanol-containing feed stream may contain a very small amount of acetal, for example, less than or equal to 0.2wt.%, for example, less than or equal to 0.0001wt.% or less than or equal to 0.00005wt.%. Because the methanol-containing feed stream may contain 1,1-dimethoxyethane, and even if the content is very low, it is preferred not to contact the methanol-containing feed stream with an oxygen-containing stream (such as air) to reduce the risk of oxidation.

[0033] In one embodiment, a stream comprising 1,1-dimethoxyethane may be contacted with a sufficient amount of ozone to form methyl acetate.Methyl acetate may be used as a reactant to increase the production of acetic acid.

[0034] The feed stream 104 containing carbon monoxide can mainly contain greater than or equal to 95 volume %, for example, greater than or equal to 97 volume % or greater than or equal to 99 volume % carbon monoxide. In some embodiments, small amounts of impurities such as hydrogen, carbon dioxide, oxygen, and / or nitrogen can be present in an amount less than 5 volume %, for example, less than 3 volume % or less than 1 volume %. These small amounts of impurities may also be produced by various side reactions under operating conditions.

[0035] Typical reaction temperatures for carbonylation are from 150°C to 250°C, with a temperature range of 180°C to 225°C being a preferred range. The reaction pressure for carbonylation may be from 15 atm to 40 atm (absolute), with a pressure range of 20 to 35 atm (absolute) being preferred. The carbon monoxide partial pressure in the reactor may vary widely, but is typically from 2 to 30 atm, e.g., from 3 to 10 atm. The hydrogen partial pressure in the reactor is typically from 0.3 to 2 atm, e.g., from 0.3 to 1.5 atm, or from 0.4 to 1.5 atm. These partial pressures may be maintained by known methods, including introducing a stream containing carbon monoxide or a stream containing hydrogen into the reactor, and / or by, for example, discharging the reactor via line 108. A portion of the discharged stream line 108 may be condensed and returned to the reactor via line 110. The productivity of acetic acid may be from 5 to 50 mol / L·h, for example from 10 to 40 mol / L·h, and preferably 15 to 35 mol / L·h."Higher productivity" generally refers to operation above 20 mol / L·h.

[0036] Reactor (liquid phase carbonylation reactor) 106 is preferably self-stirring (able to stir), for example, a mechanically stirred container, a container with separation or pump circulation mixing, or a bubble tower type container with or without a stirrer, wherein the reaction liquid or slurry content is preferably automatically maintained at a predetermined level, and the level is preferably kept substantially constant during normal operation. Fresh methanol, carbon monoxide and enough water are continuously introduced into carbonylation reactor 106 as required to maintain a suitable mass composition in the reaction mixture. Carbon monoxide is introduced at a rate sufficient to maintain a desired internal reactor pressure. In certain embodiments, carbon monoxide is continuously introduced into the carbonylation reactor, desirably below a stirrer, which can be used to stir the contents and fully disperse carbon monoxide in the entire liquid reaction medium.

[0037] The material of the carbonylation reactor and its internal components is not particularly limited and may be metal, ceramic, glass, or a combination thereof. For example, the material may include zirconium-based materials and alloys that tend to have high corrosion resistance, but may also include iron-based alloys (stainless steel), nickel-based alloys (HASTELLOY TM or INCONEL TM ), titanium-based materials and alloys, or aluminum-based materials or alloys.

[0038] As described above, the reaction system of the reaction step includes a metal catalyst. The metal catalyst (carbonylation catalyst) may include a Group VIII metal. Suitable Group VIII catalysts include cobalt, rhodium and / or iridium catalysts. When a rhodium catalyst is used, the rhodium catalyst may be added in any suitable form such that rhodium is present in the catalyst solution as a catalyst comprising [Rh(CO)2I2]- In one embodiment, the metal catalyst can be a homogeneous catalyst. For example, other embodiments can support the catalyst on a fixed bed.

[0039] The reactive system also includes a catalyst co-promoter, such as an ionic metal iodide (iodide salt; salt co-promoter). These can be in the form of a soluble salt of an alkali metal or alkaline earth metal, a quaternary ammonium, a phosphonium salt, or a mixture thereof. In certain embodiments, the catalyst co-promoter is lithium iodide, lithium acetate, and / or a mixture thereof. The ionic metal iodide can be added as a non-iodide salt that will produce an iodide salt. The iodide catalyst stabilizer can be directly introduced into the reaction system. Alternatively, the iodide salt can be generated in situ because a wide range of non-iodide salt precursors will react with methyl iodide or hydroiodic acid in the reaction mixture to produce the corresponding co-promoter iodide salt stabilizer under the operating conditions of the reaction system. For additional details on rhodium catalysis and iodide salt generation, see U.S. Patent Nos. 5,001,259; 5,026,908; 5,144,068 and 7,005,541, which are incorporated herein by reference in their entirety. The carbonylation of methanol using an iridium catalyst is well known and generally described in U.S. Patent Nos. 5,942,460, 5,932,764, 5,883,295, 5,877,348, 5,877,347 and 5,696,284, which are incorporated herein by reference in their entireties.

[0040] The reactive system also includes a halogen-containing co-catalyst (co-promoter) of the catalyst system, such as a halogen compound comprising an organic halide. Therefore, alkyl, aryl and substituted alkyl or aryl halides can be used. Preferably, the halogen-containing co-catalyst exists in the form of an alkyl halide. Even more preferably, the halogen-containing co-catalyst exists in the form of an alkyl halide, wherein the alkyl group corresponds to the alkyl group of the carbonylated feed alcohol. Therefore, in the carbonylation of methanol to acetic acid, the halide promoter will include methyl halide and more preferably methyl iodide.

[0041] Carbonylation reaction is an exothermic reaction with heat generation, and the reaction temperature can be controlled (or regulated) to maintain production levels. In view of the nature of carbonylation reaction, the temperature of the reactor can be regulated by a variety of methods. For the purpose of this disclosure, any suitable cooling can be used to regulate the temperature of the reactor. In one embodiment, the temperature can be controlled by using a heat removal (or heat removal) unit or a cooling unit (such as a jacket), using a pump circulation loop for cooling, recycling the condensate that has been cooled or removed heat, or other devices. The available method of removing part of the reaction heat cools the steam (exhaust gas) from the reactor in a condenser, a heat exchanger, or other devices to separate the steam into a liquid component and a gaseous component, and the liquid component and / or the gaseous component can be recycled to the reactor. U.S. Patent No. 5,374,774 describes a cooling unit in the recirculation line of the reactor. A pump circulation loop can be used to generate additional heat to generate steam while regulating the temperature of the carbonylation reactor, which is further described in U.S. Patent No. 8,530,696. In some embodiments, the temperature of the reactor can be controlled by condensing a portion of the flash column overhead that is returned to the reactor, as further described in US Pat. No. 8,957,248.

[0042] Returning to reactor 106, the catalyst in the reaction medium acts to promote the carbonylation reaction of methanol. In commercial production, metal catalysts do not directly activate methanol, so a more reactive methyl substrate (reactant) must be generated in situ. An iodide promoter (such as hydrogen iodide) converts methanol into methyl iodide. However, since most of the reaction medium is acetic acid, methanol is esterified to methyl acetate, which is activated to methyl iodide by hydrogen iodide.

[0043] The components of the reaction mixture are maintained within defined limits to ensure sufficient acetic acid production. The reaction mixture comprises a mass composition of a metal catalyst (e.g., a rhodium catalyst) in an amount of from 100 to 3000 wppm, e.g., from 400 to 2000 wppm, or from 400 to 1500 wppm, based on metal (e.g., rhodium). The mass composition of water in the reaction mixture is maintained to be less than 14 wt.%, e.g., from 0.1 wt.% to 14 wt.%, from 0.2 wt.% to 10 wt.%, or from 0.25 wt.% to 5 wt.%. Preferably, the reaction is carried out under low water conditions and the reaction mixture contains less than 4 wt.% of water, e.g., less than 3.5 wt.%, less than 3 wt.%, or less than 2 wt.%. In terms of range, the reaction mixture contains 0.1 to 3.5 wt.% of water, e.g., from 0.1 to 3 wt.% or from 0.5 to 2.8 wt.%. The mass composition of methyl iodide in the reaction mixture is maintained at from 1 to 25wt.%, for example, from 5 to 20wt.%, from 4 to 13.9wt.%. The mass composition of iodide salt (e.g., lithium iodide) in the reaction mixture is maintained at from 1 to 25wt.%, for example, from 2 to 20wt.%, from 3 to 20wt.%. The mass composition of methyl acetate in the reaction mixture is maintained at from 0.5 to 30wt.%, for example, from 0.3 to 20wt.%, from 0.6 to 4.1wt.%. The following amounts are based on the gross weight of the reaction mixture. The range disclosed in the present application includes endpoints, sub-ranges, and single values. In a continuous process, the amount of the component is maintained within the provided range, and it is expected to fluctuate within these ranges. One of ordinary skill will readily understand how to control the method to maintain the amount of the component in the reaction medium.

[0044] The mass composition of acetic acid in the reaction mixture is generally greater than 30 wt.%, such as greater than 40 wt.% or greater than 50 wt.%.

[0045] In some embodiments, by maintaining in the reaction mixture an ester of a desired carboxylic acid and an alcohol (desirably an alcohol for carbonylation), and additional iodide ions in excess and above the iodide ions present as hydrogen iodide, a desired reaction rate can be obtained even at low water mass compositions. The desired ester is methyl acetate. The additional iodide ions are desirably iodide salts, with lithium iodide (LiI) being preferred. As described in U.S. Pat. No. 5,001,259, it has been found that under low water conditions, methyl acetate and lithium iodide act as rate promoters only when each of these components is present in a higher mass composition, and the promotion effect is higher when two of these components are present simultaneously.

[0046] In one embodiment, the carbonylation reaction of methanol to acetic acid product is carried out by contacting the methanol feed with gaseous carbon monoxide (second reagent) bubbled through an acetic acid solvent reaction mixture containing a metal (rhodium) catalyst, a MeI promoter, a methanol ester (methyl acetate) and an additional soluble iodide salt under temperature and pressure conditions suitable for forming the carbonylation product. It is generally believed that the mass composition of iodide ions in the catalyst system is important, rather than the cations associated with the iodide, and at a given iodide molar mass composition, the nature of the cation is not as significant as the influence of the iodide mass composition. Any metal iodide salt, or any iodide salt of any organic cation, or other cations such as those based on amine or phosphine compounds (optionally quaternary cations), can be maintained in the reaction mixture, provided that the salt can be fully dissolved in the reaction mixture to provide the desired level of iodide. When the iodide is a metal salt, it is preferably an iodide salt of a member of the group consisting of Group IA and Group IIA metals of the periodic table, as described in the "Handbook of Chemistry and Physics" published by CRC Press, Cleveland, Ohio, 2002-03 (83rd edition). In particular, alkali metal iodides are useful, with lithium iodide being particularly suitable. In the low water carbonylation process, the additional iodide ions exceeding and above the iodide ions present as hydrogen iodide are generally present in the catalyst solution in an amount such that the total iodide mass composition is from 1 to 25 wt.%, and methyl acetate is generally present in an amount from 0.5 to 30 wt.%, and methyl iodide is generally present in an amount from 1 to 25 wt.%. The rhodium catalyst is generally present in an amount from 200 to 3000 wppm.

[0047] The reaction mixture (crude liquid mixture) may also contain impurities, which should be controlled to avoid the formation of by-products. Examples of these components include iodoethane, formic acid, propionic acid, acetaldehyde, crotonaldehyde, 2-ethyl crotonaldehyde, and / or butyl acetate. The impurities present in the reaction mixture can be classified as low-boiling impurities with a boiling point lower than acetic acid and high-boiling impurities with a boiling point higher than acetic acid. Examples of low-boiling impurities include acetaldehyde and crotonaldehyde; Examples of high-boiling impurities include propionic acid, butyl acetate, 2-ethyl crotonaldehyde and mixtures thereof. There may also be gas components (including but not limited to hydrogen, methane, and / or carbon dioxide) released during various reactions.

[0048] These impurities have a negative impact on the quality (purity) of the acetic acid product. In addition, by-product and / or impurity formation is not limited to the reactor, and it may occur in downstream processing systems such as distillation columns or receiver vessels. As described above, for the purposes of the present disclosure, the presence of 1,1-dimethoxyethane is particularly problematic and can accumulate when the liquid stream has an unacceptably high content. Although 1,1-dimethoxyethane tends to benefit downstream processing systems to a greater extent than the reactor, in some embodiments, the mass composition of 1,1-dimethoxyethane in the reaction mixture (e.g., liquid stream) can be maintained below 0.2 wt.% or lower, for example, less than or equal to 0.15 wt.%, less than or equal to 0.1 wt.%, or less than or equal to 0.05 wt.%.

[0049] Byproducts can be controlled by adjusting the reaction medium, and in addition, byproducts can be removed by separation methods as further described herein. For example, as described in U.S. Patent No. 8,017,802, formic acid can be controlled by the water content in the reactor and / or the temperature of the reactor, resulting in a formic acid content in the acetic acid product of less than 160wppm, for example, less than 140wppm, or less than 100wppm. The separation of byproducts may be limited by related costs. When byproducts, especially higher boiling point components, are not removed, these components can be concentrated in the acetic acid product. Therefore, it is useful to limit the generation of byproducts in the reactor to reduce the need for separation. For example, in some embodiments, by maintaining the mass composition of iodoethane in the reaction medium at less than or equal to 750wppm, for example, less than or equal to 400wppm, less than or equal to 350wppm, or less than or equal to 300wppm, the mass composition of propionic acid in the acetic acid product can be further maintained below 250wppm without removing propionic acid from the acetic acid product.

[0050] In one embodiment, carbon monoxide is continuously introduced into the reactor (liquid phase carbonylation reactor), desirably below a stirrer that can be used to stir the contents (reaction liquid). Through the stirring device, the gaseous feed of carbon monoxide can be fully dispersed in the contents (reaction liquid). Exhaust gas (gas purge stream) 108 can be discharged from the reactor 106 to prevent the accumulation of gaseous byproducts and to maintain or control the carbon monoxide partial pressure at a given total reactor pressure. In one embodiment, the gas purge stream 108 contains a small amount of hydrogen iodide less than or equal to 1 wt.%, e.g., less than or equal to 0.9 wt.%, less than or equal to 0.8 wt.%, less than or equal to 0.7 wt.%, less than or equal to 0.6 wt.%, less than or equal to 0.5 wt.%, and / or the gas stream contains hydrogen iodide in an amount greater than or equal to 0.001 wt.%, e.g., greater than or equal to 0.005 wt.%, greater than or equal to 0.01 wt.%, greater than or equal to 0.05 wt.%, greater than or equal to 0.1 wt.%. The temperature of the reactor can be controlled and the carbon monoxide feed introduced at a rate sufficient to maintain the desired total reactor pressure.

[0051] Evaporation step

[0052] In the evaporation step (e.g., flash separation step), at least a portion of the reaction mixture is continuously withdrawn from reactor 106 via line 120 and introduced into or fed to an evaporator and / or flasher 122. The reaction mixture is withdrawn from reactor 106 at a rate sufficient to maintain a constant level therein, and may be withdrawn continuously. The reaction medium may be separated in the flash separation step to obtain a vapor stream (flash overhead) 126 comprising acetic acid and a liquid residue stream 124 comprising a solution containing the catalyst. In some embodiments, the liquid residue stream is recycled to reactor 106 via line 124.

[0053] In some embodiments, a converter reactor (not shown) or a tubular reactor (not shown) may be used in the flow path between the reactor and the evaporator. Tubular reactors are described in U.S. Patent No. 5,672,744 and are used to react dissolved carbon monoxide in the reaction medium. Chinese Patent No. CN 1043525 C describes a converter reactor that allows the reaction to proceed to a greater extent before subsequent flash evaporation.

[0054] The catalyst-containing solution collected in the lower portion of the flash vessel 122 can be primarily acetic acid containing rhodium and iodide salts, and lesser amounts of methyl acetate, methyl iodide, lithium acetate, and water, and is recycled to the reactor. Before returning the liquid residue to the reactor, the slip stream can be passed through a corrosion metal removal bed, such as an ion exchange bed, to remove any entrained corrosion metals, such as nickel, iron, chromium, zinc, and molybdenum, as described in U.S. Pat. No. 5,731,252, which is incorporated herein by reference in its entirety. In addition, a corrosion metal removal bed can be used to remove nitrogen compounds, such as amines, as described in U.S. Pat. No. 8,697,908, which is incorporated herein by reference in its entirety.

[0055] The evaporation step may include isothermal flash, adiabatic flash, or a combination of these conditions. In some embodiments, the flash evaporator 122 and / or the reaction mixture may be heated during the evaporation step. In some embodiments, for example, the evaporation step may be carried out at a temperature of about 100°C to 250°C, for example, from 110°C to 200°C, from 120°C to 180°C, from 125°C to 170°C, or from 130°C to 160°C. In some embodiments, for example, the pressure (gauge pressure) of the evaporator may be from 0.1 to 10 atm, for example, from 0.3 to 10 atm, from 0.5 to 5 atm, from 0.8 to 3 atm, or from 1 to 2 atm.

[0056] Flasher 122 can be a vertical evaporator with a quasi-spherical, elliptical, or hemispherical head. In order to maintain or enter, flasher 122 can have one or more manholes (manway). The nozzle for the reaction medium in pipeline 120 can be at the top of flasher 122, for example, above the liquid level in flasher 122. One or more nozzles (not shown) can be present, which tangentially introduce the reaction medium to further deviate from the vapor portion. In some embodiments, the top of flasher 122 can have a larger barrel diameter than the bottom. Flasher 122 should have a large volume so that the reaction medium fed therein is maintained in flasher 122, so that the desired carbonylation product is evaporated into a vapor stream (flash tower overhead product) 126, and before the recirculation liquid residual stream 124. In one embodiment, a residence time of about one minute or more in flasher 122 is desirable, and in some embodiments, a residence time of at least about two minutes or longer can be used.

[0057] In addition to acetic acid, vapor stream 126 also includes methyl iodide, methyl acetate, water, PRC and other iodide compounds. The dissolved gas leaving reactor 106 and entering flasher 122 includes a portion of carbon monoxide and may also contain gaseous byproducts such as methane, hydrogen and carbon dioxide. This dissolved gas, unless consumed, leaves flasher 122 as a portion of vapor stream 126. In one embodiment, the carbon monoxide in gas purge stream 108 may be fed to the bottom of flasher 122 to enhance rhodium stability.

[0058] In one embodiment, the vapor stream 126 comprises acetic acid, methyl iodide, methyl acetate, water, acetaldehyde, and hydrogen iodide. In one embodiment, the vapor stream 126 comprises acetic acid in an amount from 45 to 75 wt.%, methyl iodide in an amount from 20 to 50 wt.%, methyl acetate in an amount less than or equal to 9 wt.%, and water in an amount less than or equal to 15 wt.%, based on the total weight of the vapor stream. In another embodiment, the vapor stream 126 comprises acetic acid in an amount from 45 to 75 wt.%, methyl iodide in an amount from 24 to less than or equal to 36 wt.%, methyl acetate in an amount less than or equal to 9 wt.%, and water in an amount less than or equal to 15 wt.%, based on the total weight of the vapor stream. More preferably, the vapor stream 126 comprises acetic acid in an amount from 55 to 75 wt.%, methyl iodide in an amount from 24 to 35 wt.%, methyl acetate in an amount from 0.5 to 8 wt.%, and water in an amount from 0.5 to 14 wt.%. In yet another preferred embodiment, vapor stream 126 comprises acetic acid in an amount from 60 to 70 wt.%, methyl iodide in an amount from 25 to 35 wt.%, methyl acetate in an amount from 0.5 to 6.5 wt.%, and water in an amount from 1 to 8 wt.%. The acetaldehyde mass composition in the vapor stream may be in an amount from 0.005 to 1 wt.%, for example, from 0.01 to 0.8 wt.%, or from 0.01 to 0.7 wt.%, based on the total weight of the vapor stream. In some embodiments, acetaldehyde may be present in an amount less than or equal to 0.01 wt.%. Vapor stream 126 may comprise hydrogen iodide in an amount less than or equal to 1 wt.%, for example, less than or equal to 0.5 wt.%, or less than or equal to 0.1 wt.%, based on the total weight of the vapor stream.

[0059] The vapor stream 126 may contain methanol, and in particular a small amount of methanol. As described above, methanol reacts with acetaldehyde to form the acetal 1,1-dimethoxyethane. As also described above, the inventors have discovered that although the acetal formation reaction is in equilibrium, it can be driven to completion with an excess (high mass composition) of methanol. Therefore, in some embodiments, the mass composition of methanol in the vapor stream 126 may be controlled so as to control the mass composition of 1,1-dimethoxyethane in the liquid stream to no more than 0.2 wt.%.

[0060] In some embodiments, an optional demister may be used near the vapor outlet to coalesce droplets. An optional wash section (not shown) may be further employed in the vapor outlet of the flasher to prevent and / or reduce the entrainment of metal catalysts or other metal components into the vapor stream 126. Washing liquid may be introduced into the optional wash section. In another embodiment, an online separator may be used in the pipeline of the vapor stream 126 to impart swirling motion and coalesce any entrained liquid. Liquid may be discharged back to the flasher 122 to reduce entrainment in the vapor stream 126.

[0061] The liquid residue stream 124 comprises acetic acid, metal catalyst, corrosion metals, and various other compounds, including but not limited to formic acid, butyl acetate, and / or 1,1-dimethoxyethane. In one embodiment, the liquid residue stream comprises acetic acid in an amount of from 60 to 90 wt.%, metal catalyst in an amount of from 0.01 to 0.5 wt.%, corrosion metals (e.g., nickel, iron, chromium) in a total amount of from 10 to 2500 wppm, lithium iodide in an amount of from 5 to 20 wt.%, methyl iodide in an amount of from 0.5 to 5 wt.%, methyl acetate in an amount of from 0.1 to 5 wt.%, water in an amount of from 0.1 to 8 wt.%, acetaldehyde in an amount less than or equal to 1 wt.% (e.g., from 0.0001 to 1 wt.%), and hydrogen iodide in an amount less than or equal to 0.5 wt.% (e.g., from 0.0001 to 0.5 wt.%).

[0062] In order to handle the liquid residue stream 124 in a manner that maintains flow, prevents equipment damage, and provides adequate control, a vortex breaker (not shown) may be used near the liquid outlet of the flash vessel 122 .

[0063] In some embodiments, the evaporation rate of the evaporation step (with or without heating) can be from 20 to 80% by mass, for example, from 20 to 70%, from 25 to 75%, from 25 to 60%, from 25% to 50%, or from 30% to 40%.

[0064] First distillation step

[0065] In the first distillation step, at least a portion of the vapor stream 126 is fed (continuously fed) to a first distillation column (referred to as a light ends column or splitter column) 140. In order to separate acetic acid in the vapor stream 126, the first distillation column 140 may include a plate column, a packed column, or a combination thereof. The vapor stream 126 is separated into a first overhead stream 142, a side stream 144, and a bottom stream 146. The first overhead stream 142 is taken out from the top of the first distillation column and / or an upper position of the first distillation column. The side stream 144 is a side fraction (liquid stream) mainly containing acetic acid, which is taken from a position below the upper position of the overhead stream and above the inlet position of the vapor stream 126. The bottom stream 146 is taken out from the bottom of the first distillation column and / or a position below the inlet position of the vapor stream 126. The proportion of the first overhead stream 142 may be about 20% to 60%, for example, about 35% to 50%, of the vapor stream 126. Side stream 144 may be present in an amount of about 30% to 80% of vapor stream 126, e.g., about 40% to 70%, and is typically greater than first overhead stream 142. Bottom stream 146 may be present in an amount of about 0% to 10% of vapor stream 126, e.g., from about 0% to 3%.

[0066] The first overhead stream preferably contains low boiling impurities, such as water, methyl acetate, methyl iodide and carbonyl impurities. The amount of water in the first overhead stream is generally greater than or equal to 5 wt.%. Carbonyl impurities present in the first overhead stream include acetaldehyde and various by-products derived from acetaldehyde, such as crotonaldehyde and 2-ethyl crotonaldehyde.

[0067] Side stream 144 preferably contains a higher mass composition of acetic acid than vapor stream 126. The amount of acetic acid may be greater than 80 wt.%, e.g., greater than 85 wt.% or greater than 90 wt.%. However, side stream 144 may contain a small amount of water and methyl iodide and / or methyl acetate. The mass composition of water in the side cut may vary depending on the reflux ratio, but is typically from 0.5 wt.% to 5 wt.%, e.g., from 0.6 to 3 wt.%, or from 0.7 to 2.8 wt.%. Although preferably concentrated in the overhead, methyl iodide may also be present in the side cut 144 in a smaller amount of no more than 6 wt.%, e.g., no more than 3 wt.% or no more than 1.7 wt.%. Similarly, the mass composition of methyl acetate in the side cut 144 is lower than that of the overhead 142, and may be no more than 3 wt.%, e.g., no more than 1.5 wt.% or no more than 1.2 wt.%. In order to maintain high quality acetic acid, side stream 144 is substantially free of 1,1-dimethoxyethane.

[0068] The acetic acid removed as side stream 144 is preferably subjected to further purification, such as in a purification step, a high boiling point compound removal step, an iodine removal step, or a combination thereof. Such further purification can remove water, higher boiling point impurities, or other impurities from the stream, thereby producing a high quality (purity) acetic acid product stream. These steps are discussed in more detail below.

[0069] In one embodiment, in order to isolate the entrained catalyst (such as rhodium) into further purification steps (distillation columns), a resin bed (not shown) may be present downstream of the first distillation column 140. The amount of entrained catalyst may be as high as 100 ppb. The resin bed may be operated to treat the side stream 144 during steady-state operation and may be offline for recovery. Due to the corrosive nature of the side stream 144, the resin bed downstream of the first distillation column 140 benefits from nickel-based or zirconium-based metallurgy. The resin bed may be a polymer substrate comprising a polymer having a nitrogen-containing heterocyclic repeating unit, including but not limited to vinyl pyrrolidone or vinyl pyridine resins. Such suitable resins are further described in U.S. Pat. No. 7,902,398, the entire contents and disclosure of which are hereby incorporated by reference.

[0070] The bottom stream 146 of the first distillation column typically contains at least water and acetic acid. The water in the bottom stream 146 can be in the range of from 0.5 wt.% to 5 wt.%, for example, from 0.6 to 3 wt.%. The bottom stream 146 can also contain methanol, propionic acid and other compounds. The bottom stream 146 can also contain entrained metal catalyst (rhodium catalyst). The bottom stream 146 can be discharged, or a portion or all of the bottom stream can be recycled to the reaction step (reactor). The bottom stream 146 can be a liquid stream and can contain low levels of 1,1-dimethoxyethane.

[0071] The internal temperature of the first distillation column (light fraction column, splitter column) 140 varies with the internal pressure. An operating pressure of from 0.6 to 3 atm (gauge pressure), for example, from 0.7 to 1.5 atm, may be employed. For example, at an internal pressure of about 1 atm, the first distillation column may have a top temperature (top distillate temperature) of from 50° C. to 180° C., for example, from 70° C. to 170° C., from 80° C. to 160° C., or from 90° C. to 150° C. In an embodiment using a plate column, the number of theoretical plates may be in the range of from 5 to 80 plates, for example, from 10 to 60 plates or from 15 to 50 plates.

[0072] A portion of the vapor stream 126 fed to the first distillation column and separated in the first distillation column is distilled at a sufficient residence time to enrich the acetic acid in the side stream 124. Therefore, in some embodiments, the residence time of the vapor stream 126 in the first distillation can be controlled so as to control the mass composition of 1,1-dimethoxyethane in the liquid stream to no more than 0.2 wt.%. In particular, the residence time in the first distillation column can be controlled to be not less than 30 seconds, for example, not less than 1 minute, or not less than 5 minutes.

[0073] Phase separation step

[0074] As described above, the first overhead stream 142 separated from the first distillation column 140 contains at least one reactive component, such as methyl iodide, methyl acetate, and water, and it is preferred that these reactive components be retained in the process. The first overhead stream (light ends overhead) 142 is condensed by one or more condensers (heat exchangers) 148 into stream 150, which can be recycled to the reactor 106 and / or refluxed to the first distillation column 140. Waste gas components can be discharged from the condensed first overhead stream 142 and subjected to subsequent treatment, as described below.

[0075] In some embodiments, the first overhead stream 142 may preferably be condensed, for example, in a condenser 148 and directed to a receiver (overhead phase separation unit or decanter) 160 via line 150. It is desirable to maintain conditions such that the condensed first overhead stream 142 may be separated in the receiver to form an upper phase (light phase, aqueous phase) 162 and a lower phase (heavy phase, organic phase) 164. Typically, the upper phase is rich in water and the lower phase is rich in methyl iodide. The phase separation should maintain two separate phases without forming a third phase or emulsion between the phases. The waste gas component 170 may be discharged from the decanter and subjected to subsequent processing. In some embodiments, the average residence time of the condensed first overhead stream 142 in the overhead decanter is greater than or equal to 1 minute, e.g., greater than or equal to 3 minutes, greater than or equal to 5 minutes, greater than or equal to 10 minutes, and / or the average residence time is less than or equal to 60 minutes, e.g., less than or equal to 45 minutes, or less than or equal to 30 minutes, or less than or equal to 25 minutes. In addition, the overhead decanter can be arranged and constructed to maintain a low interface level to prevent excessive retention of methyl iodide.

[0076] The upper phase 162 mainly includes water, typically with methyl acetate, acetic acid, hydrogen iodide, acetaldehyde, dimethyl ether, methanol and a smaller amount of methyl iodide. Methyl iodide is concentrated in the lower phase, which further includes methyl acetate, acetaldehyde, dimethyl ether, hydrogen iodide, methanol and a smaller amount of water and acetic acid. Other by-products and impurities, such as but not limited to hydrogen iodide, formic acid, dimethyl ether and crotonaldehyde, may be present in an insignificant amount. The mass composition of 1,1-dimethoxyethane in the upper and lower phases can be less than or equal to 2wt.%, for example, less than or equal to 1wt.% or less than or equal to 0.5wt.%. In some embodiments, based on wt.%, the mass composition of acetaldehyde in the upper phase is greater than that of the lower phase. For example, the upper phase can have the following composition shown in Table 1.

[0077]

[0078]

[0079] Sufficient reflux flow may be obtained from the upper phase or the lower phase. In some embodiments, a portion of the upper phase 162 may be refluxed to the first distillation column 140 via line 162a. The reflux ratio of the upper phase to the first distillation column 140 (the amount of the upper phase reflux / the amount of the distillate of the upper phase) is from 0.5 to 20, e.g., from 0.5 to 15, from 1 to 15, or from 1.5 to 12. A portion of the upper phase 162 may be returned (recycled) to, for example, the reactor 106 via line 166.

[0080] The lower phase 164 (which is mainly methyl iodide) is returned (recirculated) to the reactor 106 via line 168. This makes the weight ratio of methyl iodide to water in the lower phase 164 greater than 1, such as greater than 3 or greater than 5. In some embodiments, a portion of the lower phase can be refluxed to the first distillation column 140 alone or with the upper phase. The specific gravity of the lower phase can be from 1.3 to 2, such as from 1.5 to 1.8, from 1.5 to 1.75 or from 1.55 to 1.7. As described in U.S. Pat. No. 6,677,480, the specific gravity measured in the lower phase can be related to the mass composition of methyl acetate in the reaction medium. As the specific gravity decreases, the mass composition of methyl acetate in the reaction medium increases. In some embodiments, the receiver can be arranged and constructed to maintain a low interface level to prevent excessive retention of methyl iodide. For example, the lower phase can have the following composition shown in Table 2.

[0081]

[0082]

[0083] The lower phase may contain 1,1-dimethoxyethane. The mass composition of 1,1-dimethoxyethane in the lower phase may be less than 0.2 wt.%, for example, less than 0.15 wt.%, less than 0.1 wt.%, or less than 0.05 wt.%. The upper phase may also contain 1,1-dimethoxyethane. The mass composition of 1,1-dimethoxyethane in the upper phase may be less than 0.2 wt.%, for example, less than 0.15 wt.%, less than 0.1 wt.%, or less than 0.05 wt.%. Because the upper and / or lower phases may contain 1,1-dimethoxyethane, it is preferred that a stream containing air or oxygen is not contacted with the upper and / or lower phases to reduce the risk of oxidation.

[0084] The ratio of the flow rate of the upper phase removed from the receiver relative to the flow rate of the lower phase removed from the receiver can be, for example, about 0.1 / 1 to 10 / 1 (e.g., about 0.3 / 1 to 3 / 1), and preferably about 0.5 / 1 to 2 / 1 (e.g., about 0.7 / 1 to 1.5 / 1), based on weight.

[0085] In a continuous process, flow rates may vary, which, if not regulated, can cause interruptions and processing difficulties. To account for these variations, the process may be configured with a storage tank to buffer the flow between the first distillation column 30 and the receiver, or for the upper or lower phase after the receiver. When used, the storage tank should be sized to account for variations in flow rates entering and leaving the receiver of up to 20%.

[0086] Second distillation step; acetaldehyde separation step

[0087] At least a portion of the upper phase and / or the lower phase may be separated and directed to an acetaldehyde or PRC removal system via line 162' or 164', respectively, to recover methyl iodide and methyl acetate in the acetaldehyde removal process. The upper phase and the lower phase each contain carbonyl impurities (byproducts), such as acetaldehyde, crotonaldehyde, and 2-ethylcrotonaldehyde, and the process may include removing these carbonyl impurities that deteriorate the quality (purity) of the acetic acid product.

[0088] Figure 1 One embodiment of an acetaldehyde or PRC removal system 200 for treating upper phase 162' and / or lower phase 164' (which is a portion of first overhead 142) is shown.

[0089] In one embodiment, the upper phase or the lower phase, or a combination thereof, may be introduced into the second distillation column 202 via line 201, wherein acetaldehyde is removed in the second overhead 204 or the side fraction 204', and 1,1-dimethoxyethane (including 1,1-dimethoxyethane formed in the column) falls from the column 202 and enters the bottom stream (liquid) 206. The distillation may be performed as a batch distillation or a continuous distillation. In order to separate, the second distillation column 202 may include a plate column, a packed column, or a combination thereof. In an embodiment using a plate column, the number of theoretical plates thereof may be in the range of from 1 to 100 plates, for example, from 2 to 80 plates or from 5 to 75 plates.

[0090] The upper phase 162' and / or the lower phase 164' are fed to a position between the first tray from the top of the column and the first tray above the bottom of the column. To prevent excessive formation of 1,1-dimethoxyethane, the phases may be separated without introducing methanol into the distillation column 202. The feed to the distillation column 202 may contain a relatively small amount of methanol.

[0091] Any 1,1-dimethoxyethane within the distillation column 202 (fed to the distillation column or formed within the column) falls into a bottom stream (liquid) 206. This liquid stream is withdrawn at a lower portion of the distillation column 202, such as from the bottom of the column or near the bottom. The mass composition of 1,1-dimethoxyethane in the bottom stream (liquid) 206 may be less than 0.2 wt.%, e.g., from 0.001 to 0.2 wt.%, from 0.005 to 0.18 wt.%, or from 0.01 to 0.15 wt.%. In one embodiment, the bottom stream (liquid) 206 has a mass ratio of methyl iodide to water greater than 1 (e.g., greater than 3 or greater than 5).

[0092] Without being bound by theory, it is believed that 1,1-dimethoxyethane is formed in the presence of acetaldehyde and methanol. Although methanol is fed to the reactor, the methanol is converted rapidly and the mass composition of the unreacted methanol in the reactor is too low for the formation of 1,1-dimethoxyethane. However, during the purification process, there is a region where the presence of acetaldehyde and methanol is sufficient to result in the formation of 1,1-dimethoxyethane.

[0093] In one embodiment, it has been found that 1,1-dimethoxyethane is formed in sufficient quantities in a column that concentrates acetaldehyde into a stream. When concentrating acetaldehyde, the conditions in the column are sufficient to produce 1,1-dimethoxyethane, which is concentrated and collected in a liquid stream.

[0094] In the feed composition (first mixture) of the distillation column 202, the total mass composition of the permanganate reducing compound (PRC) (where acetaldehyde is a representative compound) may vary from 0.05 to 50 wt.%, from 0.05 to 10 wt.%, from 0.1 to 5 wt.%, or from 0.1 to 1 wt.%. Thus, a target amount of the permanganate reducing compound may be separated. As used herein, the term "mass composition of permanganate reducing compounds" or "mass composition of permanganate reducing compounds" may be the total mass composition of all permanganate reducing compounds or may be the mass composition of each permanganate reducing compound. Representative permanganate reducing compounds include acetaldehyde.

[0095] In addition to the permanganate reducing compound (PRC), the feed also contains a mass composition of methyl iodide from 2.5 wt.% to 90 wt.%, e.g., from 10 wt.% to 85 wt.%, or from 20 to 70 wt.%, and a mass composition of water from 0.5 wt.% to 90 wt.%, e.g., from 1 wt.% to 90 wt.%, or from 1.5 wt.% to 85 wt.%. The feed composition can also include methyl acetate in an amount up to 30 wt.%, e.g., from 0.1 to 28 wt.%, or from 1 to 20 wt.%, acetic acid in an amount up to 25 wt.%, e.g., from 0.01 to 12 wt.%, or from 0.5 to 7.5 wt.%, dimethyl ether in an amount up to 1 wt.%, e.g., from 0.001 to 1 wt.%, or from 0.004 to 0.8 wt.%, and methanol in an amount less than or equal to 2 wt.%, e.g., less than or equal to 1.8 wt.%, less than or equal to 1.5 wt.%, less than or equal to 1.1 wt.%, less than or equal to 1.0 wt.%, or less than or equal to 0.5 wt.%.

[0096] The feed composition may be a homogeneous liquid or a mixture of upper and lower phases.

[0097] The second distillation column 202 operates to separate a second mixture, which can be taken out as an overhead stream 204 or a side draw 204', and a bottom stream (liquid) 206. In some embodiments, the second mixture 204 / 204' comprises an overhead stream (or a portion thereof) and / or a side draw (or a portion thereof). The extraction in the second distillation column 202 produces a second mixture containing significantly more acetaldehyde than the feed. Therefore, the ratio of acetaldehyde to methyl iodide in the second mixture 204 / 204' (based on weight) is greater than the ratio of acetaldehyde to methyl iodide in the feed. The second mixture contains acetaldehyde (which is the main PRC), which has been distilled or processed in the second distillation column 202 and can be subsequently processed to remove acetaldehyde from the process or reduce the mass composition of acetaldehyde, as described in further detail. The second mixture can be taken out higher and closer to the top of the second distillation column 202 than the first position. In order to provide rectification, in one embodiment, there can be at least one or more actual trays between the first position and the withdrawal position of the second mixture 204 / 204'.

[0098] In one embodiment, acetaldehyde can be separated from methyl iodide in an efficient method that reduces the loss of methyl iodide, and can function or operate even when the first mixture contains methyl acetate, methanol, acetic acid, or a combination thereof. In particular, acetaldehyde is separated from the first mixture to reduce the mass composition of acetaldehyde in the bottom stream (liquid) 206. Due to the relative amounts of methyl iodide and methyl acetate, the bottom stream 206 can be returned to the reactor 102 as a recycle (directly or indirectly). Indirect recycle refers to a process in which the stream passes through another vessel before returning to the reactor 102. Direct recycle can include combining the recycle stream with one or more other recycle streams.

[0099] Even if the acetaldehyde mass composition in the bottom stream (liquid) 206 is reduced or suppressed, it is useful to control the reaction byproducts of acetaldehyde, which can be converted into acetaldehyde when recycled. In particular, one byproduct of acetaldehyde that tends to be formed in the extraction step of the second distillation column 202 may be acetal, which includes but is not limited to 1,1-dimethoxyethane. Acetaldehyde can be converted into 1,1-dimethoxyethane through various secondary or side reactions with methanol. Due to the relatively high boiling point of acetal compared to aldehyde, acetal falls into the bottom stream (liquid) 206, which is then recycled. For example, 1,1-dimethoxyethane has a boiling point of 64°C. The formation of acetal reduces the efficiency of the second distillation column because the second distillation column 202 does not effectively remove acetaldehyde, but accumulates acetal in the lower part and returns to the reactor. Acetaldehyde can be generated from the recycled acetal by reversible reactions and reduce the efficiency of the second distillation column 202. There are many factors that lead to an increase in acetal, including when excess methanol is introduced into the second distillation column 202 in the first mixture or alone. The temperature and reflux ratio of the second distillation column 202 may also affect the acetal mass composition.

[0100] In one embodiment, bottom stream (liquid) 206 comprises no more than 0.2 wt.%, e.g., no more than 0.18 wt.%, no more than 0.15 wt.%, or no more than 0.1 wt.% of 1,1-dimethoxyethane by mass composition. Maintaining such low levels helps to increase the robustness of the process to air (oxygen) intrusion that may lead to peroxide formation. As discussed herein, the formation of peroxides causes many problems.

[0101] The feed composition to the second distillation column 202 may include a portion of the upper phase 162' or the lower phase 164'. This allows the feed to be introduced into the second distillation column 202 at a position above the first tray from the bottom.

[0102] In addition, an extractant having a suitable composition may be fed to the second distillation column 202 for extracting acetaldehyde from the mixture. Figure 1-3203. The extractant may be a suitable composition for extracting acetaldehyde from the first mixture. For ease of handling, the extractant may also be separated from methyl iodide using low energy techniques including liquid-liquid separation and / or membrane separation. The extractant may include extraction water, a mixed solvent, or a water-soluble organic solvent (glycol, glycerol, acetone, ether, and / or ester). The introduction of water is beneficial for maintaining the extraction mixture in a liquid-liquid separation state, and therefore, the extractant includes not less than 80 wt.% of water, for example, not less than 90 wt.% of water, or not less than 95 wt.% of water. In one embodiment, in order to prevent excessive formation, the extractant does not actually include methanol or other monohydric alcohols. Therefore, additional methanol is not fed into the second distillation tower 202. Although water or aqueous extractants are preferred extraction mixtures, in some embodiments, the second distillation tower 202 may distill the upper phase and / or lower phase without additional water.

[0103] In one embodiment, the water supply can be a fresh water source external to the process. In another embodiment, the water supply for the extractant can be an internal stream containing water that is redirected to the top of the second distillation column 202. As further described herein, any suitable internal stream can be used. In one embodiment, it is preferred to use an internal stream lacking methanol as an extractant, including an internal stream having a methanol mass composition of less than or equal to 0.1 wt.%. When the internal streams contain higher amounts of methanol, these internal streams can be treated in a suitable manner to reduce the methanol mass composition. Any acetaldehyde or other PRC contained in these internal streams can be recovered in the second mixture 204 / 204'.

[0104] like Figure 2As shown, the second distillation column 202 separates the components introduced therein by extractive distillation. In one embodiment, the feed line can be extracted with an extractant 203, which is introduced in the upper region of the second distillation column 204 above the first position. In one embodiment, the extractant 203 can be introduced into the top of the second distillation column 202 separated from the first position. Therefore, in the embodiment in which the extractant 203 is made to include a portion of the upper phase 162', the upper phase 162' is fed above the feed and separated from the feed. The mass composition of PRC is higher in the upper region, and the introduction of the extractant 203 into this region produces an extractant mixture, which is taken out as a second mixture (tower top distillate 204 or side fraction 204') rich in acetaldehyde and other PRCs. In one embodiment, the second mixture 204 / 204' contains less than or equal to 2 wt.% of methanol, for example, less than or equal to 1 wt.% of methanol or less than or equal to 0.5 wt.% of methanol. When extracting PRC (including acetaldehyde) into the second mixture (overhead 204 or side fraction 204'), a relatively small amount of extractant can be used as opposed to the lower stream. For example, the flow ratio of extractant 203 relative to the feed composition (based on weight) can be in the range of from 0.0001 / 100 to 100 / 100, such as 0.001 / 100 to 50 / 100, 0.0001 / 100 to 20 / 100, 0.001 / 100 to 10 / 100, 0.01 / 100 to 8 / 100, or 0.1 / 100 to 5 / 100.

[0105] In the case where the feed composition fed to the second distillation column 202 contains the lower phase 164' after liquid-liquid separation, the first mixture may have the composition shown in the above Table 2. Although not necessarily preferred, in the case where the feed composition contains the upper phase 162' from the liquid-liquid separation, the feed may have the composition shown in the above Table 1.

[0106] When extractive distillation is used in the second distillation step, PRC and / or methyl iodide can be processed in an efficient manner with reduced energy requirements.

[0107] By using extractive distillation, the number of stages of the second distillation column 202 can be reduced. In one embodiment, the second distillation column has less than or equal to 100 stages, for example, less than or equal to 80 stages or less than or equal to 45 stages. To illustrate the feed position and the position of the withdrawn second mixture, the following example using 50 stages is provided, wherein the bottom 52 is the 0th tray and the top 51 is the 50th tray. These illustrative examples can be applied to distillation columns with different numbers of stages or trays. The first position for introducing the feed line 201 can be selected in the range of from the 1st tray to the 35th tray, for example, from the 3rd tray to the 25th tray or the 5th tray to the 20th tray. The second position for withdrawing the side draw can be selected in the range of from the 20th to the 49th tray, for example, from the 25th to the 48th tray or from the 35th to the 48th tray, provided that the second position is withdrawn from a tray (such as a collector tray) above the first position. In one embodiment, there may be at least 1 tray separating the second position from the first position, e.g., at least 5 trays. Although extractant 203 may be added at the top (e.g., the 50th tray), in some embodiments, the extractant may be added at a position selected from the 45th to the 50th (e.g., from the 45th to the 49th) tray.

[0108] The extraction efficiency can be improved by adding extractant 203 countercurrently to the upper region of second distillation column 202. In addition, extractant 203 can be added by spraying or spraying or any other suitable method, including adding in the form of droplets. In one embodiment, extractant 203 can have a temperature from 0°C to 60°C, such as 10°C to 50°C and 20°C to 40°C. In other embodiments, the method can involve preheating or raising the temperature of the extractant to a temperature from 30°C to 150°C, such as 50°C to 110°C or from 60°C to 110°C.

[0109] Figure 2 The second distillation column 202 includes a collector tray (tray) 205, which may be referred to as a cap tray or a chimney tray, to enable good vapor distribution from the feed line 201 to the upper region and to accommodate the entire amount of the extract mixture to be withdrawn as the side draw 204'. Any suitable design of the collector tray 205 may be used with the embodiments described herein. The collector tray 205 is actually located at the position where the side draw 204' is withdrawn, and therefore the extractant 203 is added above the collector tray 205. This allows the liquid that descends from the upper portion of the second distillation column 202 to be received on the collector tray 205. Although in Figure 2 Not shown, but in the case where the collector tray 205 is disposed at a lower position than the feed position of the mixed composition, the collector tray 205 is located above the bottom stream 206 .

[0110] In one embodiment, second distillation column 202 is provided with at least one collector tray 205, for example, two or more collector trays 205. For distillation column 202 having multiple collector trays, extractant 203 may be added above the uppermost collector tray.

[0111] In one embodiment, the second distillation column 202 is provided with a collector tray 205, and the extractant (or extraction water) 203 is added to the upper region above the collector tray 205. The upper region is formed in the space between the first position and the top of the distillation column 202. Acetaldehyde has a relatively low boiling point as a PRC, and methyl iodide has a relatively low boiling point as a C1-C 12 Alkyl iodide has a relatively low boiling point, which is formed in the upper region, and the second mixture is taken out from the upper region as stream 204 and / or 204'. In one embodiment, the collector tray 205 may be disposed at an upper position of the second distillation column 202. PRC is effectively extracted by discharging the side cut 204' from the upper region, and thus the position of the collector tray 205 is substantially higher than the first position of the feed line 201 (mixed composition). The collector tray 205 is not limited to a specific position, and may be disposed at the same height level as the height level of the first position, or may be disposed in a lower region below the first position.

[0112] The height level of the collector tray 205 in the second distillation column 202 is higher than the first position of the feed line 201. In the case when the collector tray 205 is used, the collector tray may replace the tray in the distillation column. Depending on the number of trays of the second distillation column, the height level of the collector tray 205 is between the uppermost tray of the column (the 1st tray from the top of the column) and the tray that is at least one tray higher than the feed line 201, or is located at or near the top of the second distillation column. In the case where the total number of trays of the distillation column is 40, the height level of the collector tray 205 may be selected from a range corresponding to the 1st to 35th trays from the top of the distillation column, for example, from the 2nd to the 30th tray, or from the 2nd to the 15th tray. These illustrative examples may be applied to distillation columns having different numbers of stages or trays.

[0113] The operating conditions of the second distillation column 202 are configured to maintain an extraction environment for recovering the second mixture 204 / 204'. As understood, the internal temperature of the second distillation column 202 is related to the top pressure (absolute). The pressure is generally controlled or maintained within a pressure of from 100 kPa to 500 kPa, such as from 100 kPa to 250 kPa or from 100 kPa to 200 kPa. At atmospheric pressure (about 101 kPa), the second distillation column is operated at a top temperature of from 15°C to 120°C, such as from 15°C to 100°C or from 15°C to 80°C, and a bottom temperature of from 35°C to 165°C, such as from 202°C to 150°C or from 202°C to 115°C. In one embodiment, the bottom temperature is maintained above a temperature at which acetaldehyde remains in a gaseous state below the collector tray 205 to drive acetaldehyde toward the upper region to be extracted into the second mixture 204 / 204'.

[0114] Depending on the composition of the mixture in feed line 201, acetaldehyde can be efficiently extracted into the second mixture 204 / 204', although there is a tendency to react with PRC (including acetaldehyde) and C1-C 12 Alkyl iodides (including methyl iodide) have an affinity for both methyl acetate and / or acetic acid. In one embodiment, the mass composition of acetaldehyde in the second mixture (including overhead 204 and / or side fraction 204') is 5 to 1000 times greater than the amount in the feed composition (based on weight), such as from 10 to 500 times or from 20 to 300 times.

[0115] In one embodiment, the second mixture 204 / 204' is taken from the side draw 204' and the overhead 204 is condensed and refluxed at the top of the second distillation column 202. The reflux ratio of the second distillation column (which is the amount of the overhead refluxed / the amount of the overhead distillate distilled) is from 0.2 to 15, for example, from 0.5 to 15, from 1 to 15, or from 2 to 10. In some embodiments, there may be a distillate removed from the top of the second distillation column 202, but the condensed portion of the overhead 204 is generally refluxed. The overhead 204 leaves the second distillation column 202 at a temperature of from 15°C to 120°C. One condenser (or multiple condensers as needed) can condense the overhead 204 to a temperature below the boiling point of methyl iodide. The condensed liquid 204a is accumulated in the overhead receiver 212 and refluxed via line 207. To maintain extractive conditions, line 207 may enter second distillation column 202 between the location of extractant 203 and the withdrawal location of side draw 204' (e.g., above collector tray 205). This reflux may be used to prevent excess extractant, i.e., water, from being present in overhead 204.

[0116] In the lower part of the second distillation column 202, a miscible solvent may be fed directly or indirectly. The solvent may be miscible with the process stream containing methyl iodide. The miscible solvent may be at least one component selected from the group consisting of water, acetic acid, methyl iodide and methanol. When added, the miscible solvent may not exceed 30 wt.%, such as not more than 15 wt.% or not more than 10 wt.%, relative to the amount of the side draw 204' taken out from the collector tray 55.

[0117] The composition of the overhead product 204 generally has a PRC and C1-C2 content greater than the total mass composition of the remaining organic components. 12 In one embodiment, the composition of the overhead distillate 204 can have a total PRC mass composition of from 1 to 75 wt.%, for example, from 5 to 65 wt.%, or from 10 to 50 wt.%, and from 1 to 85 wt.%, for example, from 10 to 80 wt.%, or from 20 to 60 wt.% of C1-C 12 Alkyl iodide mass composition. The remaining organic components may include methyl acetate, acetic acid, methanol, and dimethyl ether, and are present in different amounts, which may include an amount less than or equal to 10 wt.%, for example, less than or equal to 5 wt.%. The water mass composition in the overhead distillate 204 may be less than the water in the feed composition in the feed line 201. The overhead distillate 204 has a water mass composition of no more than 20 wt.%, for example, no more than 15 wt.%, no more than 10 wt.%, or no more than 5 wt.%. The overhead distillate 204 may have a methyl acetate mass composition of up to 10 wt.%, for example, up to 5 wt.%, or up to 1 wt.%. The overhead distillate 204 may have an acetic acid mass composition of up to 10 wt.%, for example, up to 5 wt.%, or up to 1 wt.%. It is generally desirable to have an acetic acid mass composition as low as possible by reducing the acetic acid in the feed line 201. Likewise, the mass composition of dimethyl ether and methanol in the overhead can also be maintained at a low level. The mass composition of dimethyl ether in the overhead can be no greater than 1.8 wt.%, for example, no greater than 1.5 wt.%, or no greater than 1 wt.%, and the mass composition of methanol in the overhead can be no greater than 0.55 wt.%, for example, no greater than 0.35 wt.%, or no greater than 0.25 wt.%.

[0118] In some embodiments, the second mixture may be overhead 204 or may include a portion of overhead 204. In some embodiments, overhead 204 is condensed and the distillate portion is used as the second mixture to remove PRC. Figure 2In the illustrated embodiment, the composition of overhead 204 may be phase separated in overhead receiver 212 into aqueous phase 208 and organic phase 207. Organic phase 207 may be enriched in methyl iodide and depleted in water, while aqueous phase 208 may contain useful amounts of PRC and water.

[0119] The side cut 204' (second mixture) is taken out of the distillation column 202 as a liquid stream. Due to the extractive nature of the distillation column 202, the side cut 204' can have a weight-based ratio of PRC to alkyl iodide (e.g., a ratio of acetaldehyde to methyl iodide) that is greater than the ratio of PRC to alkyl iodide in the feed composition. This further concentrates the PRC for removal. The method according to the present invention concentrates the PRC without accumulating a large amount of acetal. In one embodiment, the composition of the side cut 204' can have a PRC mass composition of from 0.1 to 90 wt.%, such as from 0.2 to 65 wt.% or from 0.5 to 50 wt.%, and a C1-C2-H2O mass composition of from 0.5 to 95 wt.%, such as from 1 to 95 wt.%, from 5 to 90 wt.%, or from 10 to 60 wt.%. 12 % by weight of alkyl iodides, a mass composition of methyl acetate from 0.1 to 25 wt.%, for example, from 0.5 to 20 wt.% or from 0.5 to 10 wt.%, a mass composition of acetic acid from 0 to 10 wt.%, for example, from 0.01 to 5 wt.% or from 0.05 to 1 wt.%, a mass composition of water from 0.1 to 20 wt.%, for example, from 0.5 to 15 wt.% or from 0.5 to 8 wt.%, a mass composition of methanol from 0 to 2.5 wt.%, for example, from 0.01 to 2.1 wt.% or from 0.05 to 2 wt.%, a mass composition of acetal from 0 to 2.5 wt.%, for example, from 0.01 to 1.7 wt.% or from 0.05 to 1.5 wt.%, and a mass composition of dimethyl ether from 0 to 1.2 wt.%, for example, from 0.01 to 0.8 wt.% or from 0.05 to 0.5 wt.%.

[0120] like Figure 2As shown, the condensed overhead distillate 204a is collected in a container 214. The container 214 may be a buffer tank or may be a liquid-liquid separation container capable of receiving the second mixture and separating the condensed overhead distillate 204a into each phase. In some embodiments, the container 214 separates the liquid-liquid separable condensed overhead distillate 204a into an aqueous phase 215 and an organic phase 216. PRC (including acetaldehyde) is more advantageously distributed in the aqueous phase 215 than the organic phase 216. In addition, the extractant is more advantageously separated into the aqueous phase 215, and the extractant can be recovered by subsequent treatment of the aqueous phase, although it is not necessary to recover the extractant. The organic phase can be returned to the second distillation column 202 or combined with the bottom stream 206 and returned to the reactor 106. In addition, it is desirable to have a reduced amount of methyl iodide in the aqueous phase 215 so that the acetaldehyde can be discharged without further treatment.

[0121] In some embodiments, vessel 214 separates second mixture 204' into aqueous phase 215 and organic phase 216. The mass flow ratio of aqueous phase 215 and organic phase 216 can be from 1:1000 to 1:1 (aqueous phase to organic phase), for example from 1:900 to 1:10 or from 1:650 to 1:100. At equilibrium, the aqueous phase can be a smaller flow than the organic phase based on mass flow. Organic phase 216 lacks extractant and can be returned to second distillation column 202.

[0122] The aqueous phase 215 has a higher PRC (acetaldehyde) mass composition than the organic phase 216, and the aqueous phase 215 may have a higher PRC (acetaldehyde) mass composition than the organic phase 216. 12 Using acetaldehyde and methyl iodide as representatives, the aqueous phase 215 can have a ratio (based on weight) of the former to the latter of from 2:1 to 60:1, such as from 3:1 to 45:1, from 3:1 to 30:1, or from 4:1 to 20:1. The composition of the aqueous phase 215 comprises a PRC (acetaldehyde) mass composition of from 1 to 50 wt.%, such as from 5 to 45 wt.% or from 10 to 35 wt.%, a water mass composition of from 40 to 95 wt.%, such as from 50 to 90 wt.% or from 60 to 75 wt.%, and a C1-C2 mass composition of from 0.01 to 15 wt.%, such as from 0.1 to 10 wt.% or from 0.5 to 6 wt.%. 12% by mass of alkyl iodide (methyl iodide), a mass composition of methyl acetate from 0.1 to 25 wt.%, for example, from 0.5 to 20 wt.% or from 0.5 to 10 wt.%, a mass composition of acetic acid from 0 to 5 wt.%, for example, from 0.01 to 2.5 wt.% or from 0.05 to 1 wt.%, a mass composition of methanol from 0 to 2.5 wt.%, for example, from 0.01 to 2.1 wt.% or from 0.05 to 2 wt.%, a mass composition of acetal from 0 to 2.5 wt.%, for example, from 0.01 to 1.7 wt.% or from 0.05 to 1.5 wt.%, and a mass composition of dimethyl ether from 0 to 1.2 wt.%, for example, from 0.01 to 0.8 wt.% or from 0.05 to 0.5 wt.%.

[0123] The organic phase 216 can be returned to the second distillation column 202 below the collector tray 205. In one embodiment, the composition of the organic phase 216 comprises from 0.1 to 90 wt.%, for example, from 5 to 85 wt.%, or from 10 to 80 wt.% of C1-C 12 Alkyl iodide (methyl iodide) mass composition, from 0.1 to 30wt.%, for example, from 0.5 to 20wt.% or from 0.5 to 10wt.% Methyl acetate mass composition, from 0.01 to 15wt.%, for example, from 0.5 to 10wt.% or from 0.5 to 5wt.% PRC (acetaldehyde) mass composition, from 0 to 5wt.%, for example, from 0.01 to 2.5wt.% or from 0.05 to 1wt.% Acetic acid mass composition, from 0.01 to 5wt.%, for example, from 0.0 % or from 0.05 to 0.5 wt.% of dimethyl ether.

[0124] For efficient operation, in one embodiment, lower stream 207 contains a majority of the methyl iodide from the feed composition, particularly when the feed composition is made to include a portion of lower stream 164'. Lower stream 207 of distillation column 202 contains useful methyl iodide that is returned to reactor 106. To achieve production, the distillation column removes 60% to 99.9% (e.g., from 75% to 99.5% or from 80% to 99.1%) of the methyl iodide in the feed composition into lower stream 207. Successful removal of methyl iodide provides a C1-C2O2 having a C1-C2O2 content of from 10 to 90 wt.%, e.g., from 15 to 85 wt.%, or from 20 to 80 wt.%. 12The mass composition of the lower stream 207 of alkyl iodide (methyl iodide) is maintained at a level sufficient to convert or transform 1,1-dimethoxyethane. However, this will increase the 1,1-dimethoxyethane returned to the reactor. In order to overcome these disadvantages and effectively use the lower stream 207, the water mass composition is maintained at a level sufficient to convert or transform 1,1-dimethoxyethane.

[0125] The bottom stream 206 also contains insufficient water to convert 1,1-dimethoxyethane to any great extent. This makes the mass ratio of methyl iodide to water greater than 1, e.g., greater than 3 or greater than 5. In one embodiment, the water mass composition of the bottom stream 206 is less than 1.5 wt.%, e.g., less than 1.1 wt.%, less than 1.0 wt.%, less than 0.7 wt.%, or less than 0.5 wt.%. It is expected that the acetaldehyde content in the bottom stream 206 is very small and can be less than 0.05 wt.%, e.g., less than 0.04 wt.%, less than 0.02 wt.%, or less than 0.005 wt.%. The methanol mass composition in the liquid stream is also relatively small and is typically less than 1 wt.%, e.g., less than 0.8 wt.%, less than 0.5 wt.%, or less than 0.1 wt.%. Methyl acetate and acetic acid may accumulate in liquid stream 42, and the respective mass compositions of methyl acetate are from 5 to 60 wt.%, e.g., 10 to 50 wt.%, or 10 to 35 wt.%, and acetic acid is from 1 to 20 wt.%, e.g., from 1.5 to 15 wt.%, or from 2 to 10 wt.%.

[0126] Lower stream 207 may be withdrawn at a temperature from 30°C to 1214°C, eg, 35°C to 120°C or 202°C to 100°C.

[0127] Supplemental acetaldehyde removal

[0128] Although acetaldehyde (including other PRCs) is removed in the second mixture 204 / 204' from the second distillation column 202, it may be desirable to remove or reduce acetaldehyde and recover useful organic components and / or extractants through supplemental treatment. There are several methods available to achieve this supplemental removal of acetaldehyde. For the purposes of the present invention, these supplemental removal methods (if used) can vary depending on the requirements of the processing facility. One aspect of supplemental acetaldehyde removal is that the method should not increase the 1,1-dimethoxyethane mass composition in the lower stream 207.

[0129] In one embodiment, acetaldehyde can be removed or reduced by purging the second mixture 204 / 204' from the process. This can be done with a second mixture 204 / 204' containing very small amounts of methyl iodide, particularly less than 1 wt.%, for example, less than 0.5 wt.%. When the second mixture 204 / 204' contains higher amounts of methyl iodide, it may be desirable to avoid purging the second mixture 204 / 204' by employing a supplemental acetaldehyde removal method.

[0130] In another embodiment, the second extraction step of the second mixture 204 / 204' can be carried out in an extractor without a stage or in a distillation tower with a stage. For this supplementary acetaldehyde removal method, the second extraction uses a second extractant (additional water) and can produce an extractant containing acetaldehyde and a residual liquid containing methyl iodide. This allows the recovery of the residual liquid and further treatment or purging of the extractant. In this arrangement, the second extraction can be positioned as a stage continuous with the second distillation tower 202. There can be a condenser / cooler between the extraction stages (i.e., the second distillation tower 202 and the extractor). The temperature of the second mixture 204 / 204' using the condenser / cooler can be from 10°C to 80°C, for example, from 12°C to 65°C or from 13°C to 45°C.

[0131] In one embodiment, the acetaldehyde separation step includes separating the second mixture 204 / 204' in one or more extractors and / or one or more additional distillation columns.

[0132] Figure 2 Another embodiment of an acetaldehyde or PRC removal system 200 is shown. Figure 2 As shown, the feed composition is represented as the overhead 201, but it should be understood that the feed composition may include a portion of the overhead 201, including the upper and lower streams of the above-mentioned liquid-liquid separation. After the feed composition is distilled and / or extracted in the distillation column 202, the second mixture is taken out from the second distillation column 202 as a side stream 204' and introduced into the container 214. The overhead 204 is condensed, and the condensed portion 204a is refluxed via a pipeline 207. Water is used as an extractant 203 and introduced into the top of the second distillation column 202. The lower stream 206 is removed from the bottom of the second distillation column 202 and recycled to a container containing at least 1.5 wt.% or more of water to convert the 1,1-dimethoxyethane contained therein. The organic phase 216 from the container 214 is recycled to the lower part of the second distillation column 202. Organic phase 216, which is richer in methyl iodide than the second mixture, may be recycled to a location below where side draw 204' was taken, such as below collector tray 205. Organic phase 216 from vessel 214 may also be recycled to a reactor (not shown).

[0133] A portion of side draw 204' contains relatively more acetaldehyde in aqueous phase 215. Aqueous phase 215 may be present in second distillation column 202 as a suitable extractant mixture due to its water content and a portion thereof in line 215' may be recycled as extractant 203. This recycled portion in line 215' may contain all of extractant 203, or may be combined with another water source to contain a portion of extractant 203. In one embodiment, aqueous phase 215 is not used as an extractant mixture, and line 215' may have a closable valve or line 215 may be removed from the process.

[0134] Similar to the higher temperature of the side draw 204' (second mixture), the aqueous phase 215 from the vessel 214 can be cooled by a condenser (cooler) before being collected in the decanter 220. Cooling water can be used as a coolant. The temperature of the aqueous phase 215 can be from -5°C to 60°C, for example from 0°C to 30°C or from 3°C to 20°C.

[0135] In the decanter 220, there may be a residual amount of methyl iodide that can be separated into a residual stream 221 by liquid-liquid separation. The residual stream 221 contains more methyl iodide than the aqueous phase 215. The residual stream 221 (heavy phase or lower phase rich in methyl iodide) formed in the decanter 220 is recycled to the second distillation column 202 by combining with the organic phase 216 of the vessel 214 or adding to the second distillation column 202 below the collecting plate 205. Although the residual stream 221 can bypass the second distillation column 202 and return to the reactor 106 together with the lower stream 206, it is preferred to first reduce impurities in the residual stream 221 before returning to the reactor 106. In order to prevent phase problems, it is not recommended to introduce the residual stream 221 back to the vessel 214.

[0136] Decanter 220 also produces liquid stream 222. Liquid stream 222 contains the target acetaldehyde to be removed. The mass flow ratio of liquid stream 222 and residual stream 221 can be from 1:500 to 1:0.5 (liquid to residue), for example, from 1:400 to 1:1 or from 1:375 to 1:10. Although liquid stream 222 is relatively small, it contains a useful amount of acetaldehyde. The acetaldehyde mass composition (based on amount) in liquid stream 222 can be 2x (twice) greater than the amount in residual stream 221, for example, 3x or 4x greater.

[0137] Although the liquid stream 222 can be treated to reduce the acetaldehyde mass composition, there may be a method of seeking to further retain methyl iodide and / or the extractant (water) used to extract the mixture. Therefore, the third distillation column 230 can be used to further separate the liquid stream 222 or a portion thereof. In such a distillation, the third distillation column 230 produces an overhead distillate stream 231 containing acetaldehyde in an amount of from 1 to 99 wt.% and methyl iodide in an amount of from 0.1 to 30 wt.%, and a bottom stream 232 containing an extractant in an amount of not less than 10 wt.% as a main component and methyl iodide in an amount not exceeding 1 wt.% (provided that each stream (including impurities) has a total amount of 100% by weight). A portion of the bottom stream 232 can be used as an extractant via a pipeline 233 and returned to the second distillation column 202. In other embodiments, the bottom stream 232 can be removed or discharged from the process.

[0138] The third distillation column 230 may have a top pressure (absolute) of from 100 to 500 kPa, for example, from 115 to 375 kPa and from 125 to 250 kPa. In order to effectively separate the top distillate, the third distillation column 230 has a top temperature of from 10°C to 90°C, for example, from 15°C to 80°C or from 20°C to 60°C, and / or a bottom temperature of from 70°C to 170°C, for example, from 80°C to 160°C or from 90°C to 150°C at atmospheric pressure. The number of stages (plates) in the third distillation column 230 may be a sufficient number for separation, for example, from 1 to 50 plates, for example, from 2 to 45 plates or from 3 to 30 plates. The reflux ratio (reflux: distillate) of the third distillation column 230 is from 1:20 to 20:1, for example, from 1:15 to 15:1 or from 5:1 to 10:1.

[0139] Compared to the second mixture, the overhead stream 231 or its distillate contains more acetaldehyde and has a lower mass composition of methyl iodide. In one embodiment, the composition of the overhead stream 231 comprises a mass composition of PRC (acetaldehyde) of from 45 to 99 wt.%, such as from 50 to 99 wt.% or from 60 to 98 wt.%, a mass composition of C1-C2+ of from 0.1 to 30 wt.%, such as from 0.5 to 25 wt.% or from 1 to 20 wt.%. 12% by mass of alkyl iodide (methyl iodide), a mass composition of methyl acetate from 0.1 to 25 wt.%, for example, from 0.5 to 20 wt.% or from 0.5 to 12 wt.%, a mass composition of acetic acid from 0 to 5 wt.%, for example, from 0 to 1.5 wt.% or from 0 to 1 wt.%, a mass composition of water from 0 to 5 wt.%, for example, from 0 to 2.5 wt.% or from 0.01 to 2 wt.%, a mass composition of methanol from 0 to 2.5 wt.%, for example, from 0.01 to 2.1 wt.% or from 0.05 to 2 wt.%, a mass composition of acetal from 0 to 2.5 wt.%, for example, from 0.01 to 1.7 wt.% or from 0.05 to 1.5 wt.%, and a mass composition of dimethyl ether from 0 to 1.2 wt.%, for example, from 0.01 to 0.8 wt.% or from 0.05 to 0.5 wt.%. In one embodiment, the ratio of methyl iodide to acetic acid (on a weight basis) in the overhead stream 231 can be higher than that in the feed to the third distillation column 230. Additionally or separately, the ratio of methyl iodide to methyl acetate (on a weight basis) in the overhead stream 231 can be higher than that in the feed to the third distillation column 230.

[0140] The overhead stream 231 has a temperature of from 15° C. to 100° C., from 20° C. to 90° C., or from 35° C. to 75° C. at atmospheric pressure. The overhead stream 231 may be condensed using a conventional condenser / cooler to cool the overhead stream 231 to a temperature of no more than 60° C., e.g., no more than 45° C. or no more than 30° C.

[0141] In one embodiment, when the extractant 203 is water, the liquid in the bottom stream 232 contains water as a main component. In addition to the main component, the bottom stream 232 may contain methyl acetate and lesser amounts of acetic acid, methanol, dimethyl ether, methyl iodide, and / or acetaldehyde. This allows a portion of the bottom stream 232 or all of the bottom stream 232 to be used as the extractant 203 by recycling to the second distillation column 202 in the pipeline 233. The bottom stream 232 may have a water mass composition of from 85 to 99.99 wt.%, such as from 90 to 99.98 wt.% or from 92 to 99 wt.%. Methyl acetate may be retained in the lower portion of the third distillation column 230 and taken out in the lower stream 232. The mass composition of methyl acetate in the bottom stream 232 may be from 0.1 to 15 wt.%, such as from 0.5 to 10 wt.% or from 0.7 to 7 wt.%. When present, other components are typically present in lower individual amounts of no more than 5 wt.%. In one embodiment, bottom stream 232 may have a mass composition of acetaldehyde of no more than 1 wt.%, such as no more than 0.5 wt.%, or no more than 0.3 wt.%, a mass composition of methyl iodide of no more than 1.5 wt.%, such as no more than 1 wt.%, or no more than 0.5 wt.%, a mass composition of acetic acid of no more than 5 wt.%, such as no more than 1 wt.%, or no more than 0.5 wt.%, a mass composition of methanol of no more than 1 wt.%, such as no more than 0.5 wt.%, or no more than 0.1 wt.%, and / or a mass composition of dimethyl ether of no more than 0.1 wt.%, such as no more than 0.01 wt.%, or no more than 0.001 wt.%. Bottom stream 232 has a temperature of from 65° C. to 165° C., such as from 230° C. to 120° C. or from 85° C. to 105° C. at atmospheric pressure.

[0142] although Figure 2 Liquid stream 222 is shown being distilled, but in other embodiments, second mixture 204 / 204 ′ and / or aqueous stream 215 may be distilled in third distillation column 230 without passing through vessel 214 and / or decanter 220 .

[0143] Separating methyl iodide from acetaldehyde by distillation alone has proven incapable of completely recovering methyl iodide, even though the methyl iodide mass composition in the overhead stream 231 is low. Further simple distillation can produce marginal or incremental improvements in recovering methyl iodide, so more efficient processing offers an attractive benefit for supplemental processing. Extraction with or without distillation can be used as an effective method to increase methyl iodide recovery. In one embodiment, a second extractive distillation column can be used to increase methyl iodide recovery. As shown in FIG. Figure 2As shown, the overhead stream 231 or its distillate portion is introduced into the fourth distillation tower 240, which is subjected to an extractive distillation operation using an aqueous extracting mixture. The fourth distillation tower 240 is operated in a manner to obtain an overhead stream 241 rich in methyl iodide and an aqueous bottom stream 242 rich in acetaldehyde and an extracting agent (water). At least a portion (including the entire portion) of the aqueous bottom stream 242 can be recycled or returned to the second distillation tower 202 as an extracting mixture via a pipeline 243.

[0144] In one embodiment, the fourth distillation column 240 separates an upper stream 241 in which the ratio of methyl iodide to acetaldehyde (on a weight basis) is greater than the ratio fed in the overhead (distillate) stream 231. The upper stream 241 can be taken out as an overhead or a stream near the top of the fourth distillation column 240. To maintain recovery, it may be useful to direct the upper stream 241 directly or indirectly to the reactor 106. In some embodiments, a portion of the upper stream 241 can be introduced into the second distillation column 202, preferably at the bottom.

[0145] For extraction, it is sufficient to add a water-extraction mixture in a countercurrent direction at the top of the fourth distillation column 240 via pipeline 244. As described in U.S. Pat. No. 8,859,810 (the entire content and disclosure of which are incorporated by reference), the water-extraction mixture may contain water, glycols, glycerol, high boiling alcohols, including mixtures thereof. For water extractive distillation, the water may have the same temperature as the extractant. The water may be added as warm water or heated water having the same temperature as the extractant or as vaporized water (or steam). In one embodiment, the temperature of the water-extraction mixture 84 is controlled or maintained in the range of 0°C to 60°C, such as 10°C to 50°C or 20°C to 40°C. The weight ratio [former / latter] of the flow rate of the water-extraction mixture 244 relative to the flow rate of the overhead distillate stream 231 or its distillate portion may be in the range of from 1:1000 to 10:1, such as from 1:500 to 5:1, 1:100 to 5:1 or 1:4 to 4:1.

[0146] In the fourth distillation column 240, the overhead stream 241 is cooled and / or condensed, for example, by a condenser (indirect condenser), and a first portion of the condensate is returned or refluxed to the distillation column 240, while a second portion of the condensate is recycled to the distillation column 240. Figure 1 The reactor 106 in the second distillation column 202 and / or the fourth distillation column 240. The bottom stream 242 is a liquid stream and can be taken out at the lower part (including the bottom or near the bottom) of the distillation column 240, and contains acetaldehyde and an extractant. Due to the enriched mass composition of acetaldehyde, the liquid stream 242 is purged or discharged to the outside of the system. A portion of the liquid stream 242 can be used as an extractant in the second distillation column 202 and / or the fourth distillation column 240. The weight ratio of methyl iodide to acetaldehyde of the overhead stream 241 is greater than the weight ratio of methyl iodide to acetaldehyde in the liquid stream 242.

[0147] The fourth distillation column 240 may have a top pressure (absolute) of from 100 to 500 kPa, for example, from 100 to 400 kPa and from 105 to 350 kPa. In order to effectively separate the top distillate, the fourth distillation column 240 has a top temperature of from 10° C. to 90° C., for example, from 15° C. to 80° C. or from 20° C. to 60° C., and / or a bottom temperature of from 70° C. to 170° C., for example, from 80° C. to 160° C. or from 90° C. to 150° C. at atmospheric pressure. The number of stages (plates) in the fourth distillation column 240 may be a sufficient number for separation, for example, from 1 to 50 plates, for example, from 2 to 45 plates or from 3 to 30 plates. The reflux ratio (reflux: distillate) of the fourth distillation column 240 is from 1:20 to 20:1, for example, from 1:15 to 15:1 or from 5:1 to 10:1.

[0148] In one embodiment, the fourth distillation tower 240 may have, for example, a theoretical stage (or tray) of less than 50 trays, and the overhead stream 241 or a condensed portion thereof may have a mass composition of methyl iodide from 20 to 80 wt.%, for example, from 30 to 75 wt.% or 40 to 65 wt.%, a mass composition of PRC from 0.1 to 70 wt.%, for example, from 0.5 to 65 wt.% or from 1 to 20 wt.%, a mass composition of methyl acetate from 0.01 to 15 wt.%, for example, from 0.05 to 10 wt.% or from 0.1 to 10 wt.%, a mass composition of acetic acid from 0 to 5 wt.%, for example, from 0 to 3 wt.% or from 0 to 1 wt.%, and a mass composition of water from 0 to 10 wt.%, from 0 to 8 wt.% or from 0.01 to 5 wt.%. The mass composition of other organic substances (such as dimethyl ether and / or methanol) in the mass composition of the overhead distillate stream 241 may be a small portion, for example, not more than 1 wt. % or not more than 0.5 wt. %. In addition, when the fourth distillation column 240 contains less than 50 trays, the bottom stream 242 can have a PRC mass composition of from 1 to 90 wt.%, for example, from 5 to 80 wt.% or from 10 to 50 wt.%, a water mass composition of from 10 to 95 wt.%, from 15 to 90 wt.% or from 20 to 85 wt.%, a methyl iodide mass composition of from 0 to 2 wt.%, for example, from 0.01 to 1.5 wt.% or from 0.05 to 1 wt.%, a methyl acetate mass composition of from 0.01 to 15 wt.%, for example, from 0.05 to 10 wt.% or from 0.1 to 10 wt.%, a acetic acid mass composition of from 0 to 5 wt.%, for example, from 0 to 3 wt.% or from 0 to 1 wt.%, and a mass composition of organic matter (dimethyl ether and / or methanol) of not more than 3 wt.%, for example, not more than 1 wt.% or not more than 0.5 wt.%. When bottoms liquid 242 is discharged and / or purged from the process, the mass ratio of acetaldehyde to methyl iodide can be from 20:1 to 2000:1, such as from 35:1 to 1800:1 or from 50:1 to 1000:1.

[0149] In a continuous process for producing acetic acid, the process streams (vapor streams or liquid streams) may contain various impurity components, although not described in detail above. These impurities may form in the reactor through side reactions. In order to avoid such impurities, it is desirable to suppress the formation of impurities or to purge the impurities to prevent accumulation. The various process streams may contain various amounts of formic acid, higher acids, and / or hydrogen iodide.

[0150] Can exist Figure 2Various configurations of the separation method shown in . This includes additional units that supplement or replace the third and / or fourth distillation columns. This allows the liquid stream 222 from the decanter 220 to bypass the third distillation column 230 and be fed into the fourth distillation column 240, or can be fed into one or more extraction containers. Therefore, if necessary, acetaldehyde can be extracted from the liquid stream 222 with water by one or more water extraction containers equipped with mixers and settlers or by the fourth distillation column 240. In other embodiments, it may not be necessary to use the third and / or fourth distillation columns to purify the liquid stream 222.

[0151] Figure 3 Another embodiment of an acetaldehyde or PRC removal system 200 is shown. In one embodiment, the feed composition in line 201 introduced into the second distillation column 202 has a composition from Figure 1 This allows the feed composition in line 201 to contain C1-C2 in an amount of from 60 to 98 wt.%, e.g., from 60 to 95 wt.% or from 75 to 93 wt.%. 12 Alkyl iodide (mainly represented by methyl iodide), PRC (acetaldehyde) in an amount of up to 5 wt.%, such as up to 3 wt.% or up to 0.5 wt.%, and water in an amount of up to 3 wt.%, such as up to 1 wt.% or up to 0.8 wt.%. In addition, the feed line also contains a small amount of methanol, and if the methanol needs to be adjusted, the feed line can be made to include a portion of the upper phase. As described above, the extractant is added above the collector tray 205 via line 203. Any vapor at the top is collected, condensed and refluxed to the second distillation column 202.

[0152] In this example, the side stream 204' is condensed or cooled from -5°C to 60°C for direct feeding to the decanter 220, thus skipping Figure 2 The vessel 214 in the side stream 204 is used for liquid-liquid separation to obtain a residual stream 221 (containing methyl iodide) and a liquid stream 222 (containing acetaldehyde). The mass flow ratio of the liquid stream 222 and the residual stream 221 can be from 1:500 to 1:0.5 (liquid to residual), such as from 1:400 to 1:1 or from 1:375 to 1:10. The side stream can have a composition suitable for phase separation, and in one embodiment, the composition of the side stream 204' can have a PRC mass composition of from 0.1 to 90 wt.%, such as from 0.2 to 65 wt.% or from 0.5 to 50 wt.%, from 0.5 to 95 wt.%, such as from 1 to 95 wt.%, from 5 to 90 wt.% or from 10 to 60 wt.% of C1-C 12alkyl iodide (methyl iodide) mass composition, methyl acetate mass composition from 0.1 to 25 wt.%, such as from 0.5 to 20 wt.% or from 0.5 to 10 wt.%, acetic acid mass composition from 0 to 10 wt.%, such as from 0.01 to 5 wt.% or from 0.05 to 1 wt.%, water mass composition from 0.1 to 20 wt.%, such as from 0.5 to 15 wt.% or from 0.5 to 8 wt.%, methanol mass composition from 0 to 2.5 wt.%, such as from 0.01 to 2.1 wt.% or from 0.05 to 2 wt.%, acetal mass composition from 0 to 2.5 wt.%, such as from 0.01 to 1.7 wt.% or from 0.05 to 1.5 wt.%, and dimethyl ether mass composition from 0 to 1.2 wt.%, such as from 0.01 to 0.8 wt.% or from 0.05 to 0.5 wt.%. Figure 3 The method shown further concentrates the PRC without accumulating large amounts of acetal.

[0153] like Figure 3 As shown, the residual stream 221 is combined with the lower stream 206 from the second distillation column 202. In some embodiments, the residual stream 221 can be fed to the lower portion of the second distillation column 202. As a miscible solvent, acetic acid is fed to the lower portion of the second distillation column 202 via feed line 203', and in some embodiments can be fed below the feed position of stream 203. Although Figure 3 Not shown, but a miscible solvent fed to the second distillation column 202 may be present.

[0154] Once removed from decanter 220, liquid stream 222 is fed to third distillation column 230. Although liquid stream 222 is relatively small, it contains a useful amount of acetaldehyde. The acetaldehyde mass composition (based on amount) in liquid stream 222 may be 2x greater than that in residual stream 221, such as 3x greater or 4x greater. As described above, third distillation column 230 operates to produce an overhead stream 231 containing acetaldehyde in an amount from 1 to 99 wt.% and methyl iodide in an amount from 0.1 to 30 wt.%, and a bottom stream 232 containing an extractant in an amount of not less than 10 wt.% as a main component and methyl iodide in an amount not exceeding 1 wt.% (provided that each stream (including impurities) has a total amount of 100% by weight). A portion of bottom stream 232 may be used as an extractant and returned to second distillation column 202. In other embodiments, bottom stream 232 may be removed or discharged from the process.

[0155] Compared to the second mixture, the overhead stream 231 or its distillate contains more acetaldehyde and has a lower mass composition of methyl iodide. In one embodiment, the composition of the overhead stream 231 comprises a mass composition of PRC (acetaldehyde) of from 45 to 99 wt.%, such as from 50 to 99 wt.% or from 60 to 98 wt.%, a mass composition of C1-C2+ of from 0.1 to 30 wt.%, such as from 0.5 to 25 wt.% or from 1 to 20 wt.%. 12 % by mass of alkyl iodide (methyl iodide), a mass composition of methyl acetate from 0.1 to 25 wt.%, for example, from 0.5 to 20 wt.% or from 0.5 to 12 wt.%, a mass composition of acetic acid from 0 to 5 wt.%, for example, from 0 to 1.5 wt.% or from 0 to 1 wt.%, a mass composition of water from 0 to 5 wt.%, for example, from 0 to 2.5 wt.% or from 0.01 to 2 wt.%, a mass composition of methanol from 0 to 2.5 wt.%, for example, from 0.01 to 2.1 wt.% or from 0.05 to 2 wt.%, a mass composition of acetal from 0 to 2.5 wt.%, for example, from 0.01 to 1.7 wt.% or from 0.05 to 1.5 wt.%, and a mass composition of dimethyl ether from 0 to 1.2 wt.%, for example, from 0.01 to 0.8 wt.% or from 0.05 to 0.5 wt.%. In one embodiment, the ratio of methyl iodide to acetic acid (on a weight basis) in the overhead stream 231 can be higher than that in the feed to the third distillation column 230. Additionally or separately, the ratio of methyl iodide to methyl acetate (on a weight basis) in the overhead stream 231 can be higher than that in the feed to the third distillation column 230. The bottoms stream 232 can have a mass composition of water from 85 to 99.99 wt.%, e.g., from 90 to 99.98 wt.%, or from 92 to 99 wt.%. In one embodiment, the bottoms stream 232 is removed from the process, or at least a portion thereof can be returned to the second distillation column 202 as an extractant.

[0156] Similar to the previous figure, Figure 3 The overhead stream 231 or its distillate portion is treated by introducing them into the fourth distillation column 240, which performs an extractive distillation operation using an aqueous extract mixture. As described above, the fourth distillation column 240 is operated in a water-extract mixture via the pipeline 244 to obtain an overhead stream 241 rich in methyl iodide and an aqueous bottom stream 242 rich in acetaldehyde and an extractant (water). At least a portion (including the entire portion) of the aqueous bottom stream 242 can be recycled or returned to the second distillation column 202 as an extract mixture via the pipeline 243.

[0157] Subsequent processing steps

[0158] As described above, the methods described herein include one or more subsequent purification steps for purifying or improving the quality (purity) of the acetic acid. These are

[0159] In some embodiments, the side stream 144 of the first distillation column 140 is subjected to further purification. For example, the acetic acid removed via the side stream 144 can be further distilled, such as in a dehydration column 180. The dehydration column 180 separates the side stream 144 to form an overhead stream 182 consisting primarily of water and an acetic acid product stream 184, 184' consisting primarily of acetic acid. The overhead stream 182 may contain water in an amount of 25 wt.% or more, such as 30 wt.% or more or 50 wt.% or more. In one embodiment, the overhead stream 182 may contain water in an amount from 50 to 75 wt.%. Methyl acetate and methyl iodide are also removed from the side stream and concentrated in the overhead stream 182. The acetic acid product streams 184, 184' preferably contain acetic acid or consist essentially of acetic acid. In a preferred embodiment, the acetic acid product streams 184, 184' contain acetic acid in an amount greater than or equal to 90 wt.%, such as greater than or equal to 95 wt.% or greater than or equal to 98 wt.%. Moreover, in preferred embodiments, the acetic acid product stream 184, 184' comprises less than or equal to 0.2 wt.% water, such as less than or equal to 0.15 wt.% water.

[0160] The overhead stream 182 from the dehydration tower 180 may contain reactive components, such as methyl iodide, methyl acetate, and water, and it is preferred that these reactive components be retained in the process. To do this, the overhead stream 182 may be condensed, for example, by a heat exchanger, and recycled to the reactor 106 and / or refluxed to the dehydration tower 180. In one embodiment, the overhead stream 182 (or a condensed portion thereof) is refluxed to the dehydration tower 180 at a reflux ratio of from 0.2 to 15, wherein the reflux ratio is the amount of the refluxed overhead stream / the amount of the distilled overhead stream. Waste gas components may be discharged from the condensed overhead stream 182. Similar to the condensed low-boiling overhead vapor stream, the condensed overhead stream of the dehydration tower 180 may also be separated to form an aqueous phase and an organic phase, and these phases may be recycled or refluxed as needed to maintain the amount in the reaction medium.

[0161] The acetic acid product stream is preferably a lower stream from a dehydration column. In some embodiments of the process, the acetic acid product stream is a bottom stream 184 of a dehydration column, a lower side stream of a dehydration column, or a combination thereof.

[0162] As described above, acetic acid product streams 184, 184' contain product acetic acid. Preferably, acetic acid product streams 184, 184' have a higher amount of acetic acid than side stream (crude acetic acid stream) 144. However, the acetic acid product stream may contain a small amount of impurities, such as 1,1-dimethoxyethane. The mass composition of 1,1-dimethoxyethane in the acetic acid product stream may be less than 0.2 wt.%, such as less than 0.15 wt.%, less than 0.1 wt.%, or less than 0.05 wt.%. Because the acetic acid product stream may contain 1,1-dimethoxyethane, it is preferred that a stream containing air or oxygen is not contacted with the acetic acid product stream in order to reduce the risk of oxidation.

[0163] The acetic acid product stream 184, 184' can be further processed, for example, by removing high boiling components. In some embodiments, the high boiling components are removed by subjecting the acetic acid product stream to further distillation, for example, in a heavies removal column (refining column) 190. The heavies removal column separates the acetic acid product stream 184 into a heavies stream 194 as a bottoms stream. Exhaust gas 192 can be removed as a waste gas and subjected to further processing, for example, in a scrubber system. Purified acetic acid product stream 196 can be removed as a side stream of the heavies removal column 190 and subjected to further processing.

[0164] The acetic acid product stream 184' and / or the purified acetic acid product stream 196 can be further treated, for example, by an ion exchange resin 198, before storage or transportation for commercial use. Carboxylic acid streams, such as acetic acid streams, contaminated with halides and / or corrosion metals can be contacted with an ion exchange resin composition under a wide range of operating conditions. Preferably, the ion exchange resin composition is provided in a guard bed. The use of guard beds to purify contaminated carboxylic acid streams is well documented in the art, for example, U.S. Pat. Nos. 4,615,806; 5,653,853; 5,731,252; and 6,225,498, which are hereby incorporated by reference in their entirety. Typically, a contaminated liquid carboxylic acid stream is contacted with an ion exchange resin composition, preferably disposed in a guard bed. Halide contaminants (e.g., iodide contaminants) are reacted with metals to form metal iodides. In some embodiments, hydrocarbon moieties (e.g., methyl groups) that can be associated with iodides can esterify carboxylic acids. For example, in the case where acetic acid is contaminated with methyl iodide, methyl acetate will be produced as a byproduct of iodide removal. The formation of this esterification product generally has no deleterious effect on the treated carboxylic acid stream.

[0165] In one embodiment, there may be some undesirable lithium carryover due to the presence of lithium acetate in the reaction medium. Without being bound by theory, lithium derived from lithium acetate may be carried over into the acetic acid product during the reaction. Lithium may cause displacement of strong acid cation sites with metal exchange. By using a cation exchanger to remove lithium prior to using a resin with strong acid cation sites with metal exchange, displacement of silver, mercury, palladium and / or rhodium from the metal exchange sites by lithium is reduced or eliminated. According to one embodiment, lithium, and preferably lithium cations, are removed prior to removing iodide to prevent displacement in the metal ion exchange resin.

[0166] It is also found that in the absence of heavy fractions and other refining equipment, lithium can be entrained in the crude acid product. Even a very small amount of 10wppb of lithium in the crude acid product may also cause the problem of replacing the metal exchange resin. The amount of lithium in the acid-containing crude acid product leaving the dehydration tower (such as the last tower in the primary purification series) of the acetic acid process can be up to or equal to 10wppm of lithium, such as up to or equal to 5wppm, up to or equal to 1wppm, up to or equal to 500wppb, up to or equal to 300wppb or up to or equal to 100wppb. In terms of scope, the crude acid product can include the lithium of the amount from 0.01wppm to 10wppm, such as from 0.05wppm to 5wppm or from 0.05wppm to 1wppm. By using a cation exchanger in the acid form before the crude acid product is introduced into the metal exchange resin, a large amount of lithium can be removed. For example, in the stream, more than or equal to 90wt.%, for example, 95wt.% or 99wt.% of lithium may be removed by a cation exchanger. Thus, the stream exiting the acid form cation exchanger can contain less than or equal to 50 wppb, such as less than or equal to 10 wppb or less than or equal to 5 wppb lithium. This removal of lithium can greatly extend the life of the metal exchange resin.

[0167] The configuration of the guard bed in the acetic acid purification train can vary widely. For example, the guard bed can be configured after the dehydration column. Additionally or alternatively, the guard bed can be configured after the heavy fraction removal column (refining column). Figure 1In the illustrated embodiment, for example, acetic acid product stream 184' and purified acetic acid product stream 196 from heavy ends removal column 190 are withdrawn and processed in guard bed 198. Preferably, the guard bed is configured at a location where the temperature of the acetic acid product stream is relatively low, such as less than or equal to 120°C or less than or equal to 100°C. In addition to the advantages discussed above, low temperature operation provides less corrosion compared to high temperature operation. Low temperature operation provides less formation of corrosion metal contaminants, which, as discussed above, may reduce overall resin life. In addition, because the lower operating temperature results in less corrosion, the vessel advantageously does not need to be made of expensive corrosion-resistant metals, and lower grade metals, such as standard stainless steel, can be used. The final product stream 199 is then removed from guard bed 198.

[0168] In one embodiment, the flow rate through the guard bed is in the range of from 0.1 bed volume / hour ("BV / hr") to 50 BV / hr, e.g., 1 BV / hr to 20 BV / hr or from 6 BV / hr to 10 BV / hr. The bed volume of the organic medium is the volume of the medium equal to the volume occupied by the resin bed. A flow rate of 1 BV / hr means that an amount of organic liquid equal to the volume occupied by the resin bed passes through the resin bed over a period of one hour.

[0169] To avoid exhausting the resin with purified acetic acid product having a high total iodide mass composition, in one embodiment, the purified acetic acid product in bottoms stream 127 is contacted with a guard bed when the total iodide mass composition of the purified acetic acid product is less than or equal to 1 wppm. The total iodide mass composition includes iodide from an organic source (C1 to C 14 In one embodiment, the purified acetic acid composition comprises iodides of both alkyl iodides (such as iodides) and inorganic sources (such as hydrogen iodide). As a result of the guard bed treatment, a purified acetic acid composition is obtained. In one embodiment, the purified acetic acid composition comprises iodides less than or equal to 100wppb, such as less than or equal to 90wppb, less than or equal to 50wppb, or less than or equal to 25wppb. In one embodiment, the purified acetic acid composition comprises corrosion metals less than or equal to 1000wppb, such as less than or equal to 750wppb, less than or equal to 500wppb, or less than or equal to 250wppb. For purposes of the present invention, corrosion metals include metals selected from the group consisting of nickel, iron, chromium, molybdenum and combinations thereof. In terms of scope, the purified acetic acid composition may comprise iodides from 0 to 100wppb, such as from 1 to 50wppb, and / or corrosion metals from 0 to 1000wppb, such as from 1 to 500wppb. In other embodiments, the guard bed removes at least 25 wt.%, e.g., at least 50 wt.%, or at least 75 wt.% of iodide from the crude acetic acid product. In one embodiment, the guard bed removes at least 25 wt.%, e.g., at least 50 wt.%, or at least 75 wt.% of corrosion metals from the crude acetic acid product.

[0170] As mentioned above, waste gas streams can be discharged during various steps of the process, such as reaction steps, phase separation steps and / or purification steps. These discharge streams, in particular Figure 1 The pipelines 108, 112, 170, 182 and 192 may contain residual organic matter and iodide. In order to recover these valuable components, these pipelines can be fed to a scrubber 114, which is operated, for example, with an absorption solvent (scrubbing solvent) 115 (such as acetic acid, methanol, methyl acetate, and combinations thereof) to remove methyl acetate and methyl iodide. Suitable scrubbers are described in U.S. Patent No. 8,318,977, which is incorporated herein by reference in its entirety. As described herein, the exhaust gas treatment step may include multiple absorption steps, for example, with different absorption solvents and / or different pressures. For example, the effluent stream may be introduced into an absorption tower to recover any remaining useful components in the pipeline, such as carbon monoxide, methyl acetate and / or methyl iodide. The tower may be operated at a pressure greater than or equal to 0.7 atm (gauge pressure), for example, greater than or equal to 1 atm or greater than or equal to 2 atm. Separation and recovery in the absorption tower may be carried out according to a wet process using an absorption liquid (such as water, acetic acid, methyl acetate or methanol). The crude acetic acid product can be used as an absorption solvent 115. This wet process removes substantially all of the remaining methyl iodide present in the gas portion, and the methyl iodide is recovered by absorption into an absorption liquid. The effluent gas (gas component) 116 can be removed (diffused) from the top of the absorption tower. At the same time, the second effluent stream can continue to the same or another pressure absorption tower (not shown) to recover any remaining useful components in the pipeline, such as carbon monoxide, methyl acetate and / or methyl iodide. The methyl iodide, hydrogen iodide and other condensable gas components in the second effluent stream can be absorbed by the absorption liquid (absorbent) in the same or another absorption tower. The second effluent gas (gas component) can be removed (diffused) from the absorption tower with the first absorption solvent at a first pressure and / or from the absorption tower with the second absorption solvent at a second pressure lower than the first pressure.

[0171] The distillation tower described herein can be a conventional distillation tower, such as a plate tower, a packed tower, etc., and a combination thereof. The plate tower can include a porous plate tower, a bubble tower, a Kittel tray tower, a single-flow tray tower, or a corrugated tray tower. For the plate tower, the theoretical plate number is not particularly limited, and depending on the type of the component to be separated, up to 80 plates can be included, for example, from 2 to 80, from 5 to 60, from 5 to 50, or more preferably from 7 to 35. The distillation tower can include a combination of different distillation units. Unless excluded, a combination of a bubble tower and a porous plate tower, and a combination of a porous plate tower and a packed tower can be used.

[0172] The material of each component or unit related to the distillation system, including towers, valves, condensers, receivers, pumps, reboilers and internal components, and various pipelines each connected to the distillation system, can be made of suitable materials such as glass, metal, ceramic, or a combination thereof, and is not particularly limited to a specific one. According to the present invention, the material of the aforementioned distillation system and various pipelines is a transition metal or an alloy based on a transition metal such as an iron alloy, for example, stainless steel, nickel or a nickel alloy, zirconium or a zirconium alloy thereof, titanium or a titanium alloy thereof, or an aluminum alloy. Suitable iron-based alloys include alloys containing iron as a main component, such as stainless steel also containing chromium, nickel, molybdenum, etc. Suitable nickel-based alloys include alloys containing nickel as a main component and one or more of chromium, iron, cobalt, molybdenum, tungsten, manganese, etc., for example, HASTELLOY TM and INCONEL TM The corrosion-resistant metal may be particularly suitable as a material for distillation systems and various pipelines.

[0173] At the outset, it should be noted that in the development of any such actual embodiment, many implementation-specific decisions must be made to achieve the developer's specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. In addition, the methods disclosed herein may also include components other than those cited or specifically mentioned, which will be apparent to one of ordinary or reasonable skill in the art.

[0174] As is apparent from the figures and text presented above, a variety of embodiments are contemplated.

[0175] The present invention will be better understood in light of the following non-limiting examples.

[0176] Examples

[0177] The present invention will be better understood in light of the following non-limiting examples.

[0178] In Example 1, a semi-empirical simulator was used to study the effect of methanol mass composition on the formation of 1,1-dimethoxyethane. The test evaluated the effect of methanol mass composition on the mass composition of 1,1-dimethoxyethane in the lower stream (residue stream) of the second distillation tower of the experiment. In these tests, three different samples of the lower phase from the receiver were fed into the second distillation tower of the second distillation step. The second distillation tower had a top temperature of 37°C, a bottom temperature of 44°C, and a top pressure of 1.01 atm. The extractive distillation had 45 trays. The extractant was an aqueous stream, and the extractant was fed to the top tray. The extractant did not contain methanol, and no other source of methanol was fed to the extractive distillation tower except the first mixture. The temperature of the extractant was 20°C. The results of these examples are shown in Table 3.

[0179]

[0180] Each run was operated using a first mixture in the inlet having a mass ratio of methyl iodide to water greater than 1. In addition, the liquid stream in the residue also had a mass ratio of methyl iodide to water greater than 1, such as 424 (Example 1), 175 (Example 2), and 782 (Example 3).

[0181] It is apparent from the results of Example 1 that, since the feed mixture has a lower methanol mass composition, the mass composition of 1,1-dimethoxyethane in the bottom stream (residue stream) of the second distillation column decreases.

[0182] Although the present invention has been described in detail, modifications within the spirit and scope of the present invention will be apparent to those skilled in the art. In view of the above discussion, the relevant knowledge in the art, and the references discussed above in conjunction with the background technology and specific embodiments, the disclosures thereof are incorporated herein by reference. In addition, it should be understood that the various aspects of the present invention and the various embodiments and portions of the various features described below and / or in the appended claims may be combined or interchanged in whole or in part. In the above description of the various embodiments, those embodiments that mention another embodiment may be appropriately combined with other embodiments, as understood by those skilled in the art. In addition, it will be understood by those of ordinary skill in the art that the above description is only by way of example and is not intended to limit the present invention.

[0183] Example

[0184] As used hereinafter, any reference to a series of embodiments should be understood as a separate reference to each of those embodiments (eg, "Embodiments 1-4" should be understood as "Embodiments 1, 2, 3, or 4").

[0185] Embodiment 1 is a method for producing acetic acid, which comprises: (a) a reaction step of continuously reacting methanol with carbon monoxide in a reactor in the presence of a reactive system comprising a metal catalyst, an ionic metal iodide and methyl iodide, acetic acid, methyl acetate and water; (b) an evaporation step of separating the reaction mixture obtained in the reaction step into a vapor stream and a liquid residue stream with or without heating; (c) a first distillation step of distilling at least a portion of the vapor stream in a first distillation column to form a first overhead stream and a side stream, wherein the first overhead stream comprises water, methyl iodide, and / or acetaldehyde, and the side stream comprises acetic acid; (d) a phase separation step of condensing the first overhead stream in one or more condensers, collecting the condensate in a receiver, and separating the condensate into an upper phase and a lower phase; (e) distilling at least a portion of the lower phase in a second distillation column to form a liquid phase comprising acetic acid; (i) a high boiling point compound removal step for removing high boiling point components from the acetic acid product stream; (j) an iodine removal step for contacting the acetic acid product stream with an ion exchange resin and separating iodine compounds from the acetic acid product stream; and (k) an extraction step for extracting acetaldehyde from the acetaldehyde stream to form an extract and a residual liquid, wherein the extract contains acetaldehyde and the residual liquid contains methyl iodide; wherein the mass composition of 1,1-dimethoxyethane in the liquid stream of the process is controlled to be not more than 0.2 wt.%.

[0186] Embodiment 2 is the method of embodiment 1, wherein the liquid stream contains at least one member selected from the group consisting of methyl iodide, methyl acetate, and water.

[0187] Embodiment 3 is the method of embodiment 1-2, wherein the liquid stream has a mass ratio of methyl iodide to water greater than 1.

[0188] Embodiment 4 is the method of embodiments 1-3, wherein the stream comprising air or oxygen is not contacted with the liquid stream.

[0189] Example 5 is the method of Example 1-4, wherein the mass composition of 1,1-dimethoxyethane in the liquid stream is controlled to be no greater than 0.2 wt.% by controlling the mass composition of methanol in the first overhead distillate stream.

[0190] Example 6 is the method of Example 1-5, wherein the mass composition of 1,1-dimethoxyethane in the liquid stream is controlled to be no more than 0.2 wt.% by controlling the residence time in the first distillation column and / or the second distillation column to be no less than 1 minute.

[0191] Example 7 is the method of Example 1-6, wherein the mass composition of 1,1-dimethoxyethane in the liquid stream is controlled from 0.01 wt.% to 0.2 wt.%.

[0192] Embodiment 8 is the method of embodiment 1-7, wherein the mass composition of 1,1-dimethoxyethane in the liquid stream is controlled to be no greater than 0.1 wt.%.

[0193] Embodiment 9 is the method of embodiments 1-8, wherein the liquid stream is selected from a side stream, a bottom stream, a condensate, an upper phase, a lower phase, a second mixture, and / or an acetic acid product stream.

[0194] Embodiment 10 is the method of embodiments 1-9, wherein the weight ratio of methyl iodide to water in the lower phase is greater than 1.

[0195] Embodiment 11 is the method of embodiments 1-10, wherein the second mixture is an overhead stream and / or a side draw stream of the second distillation column.

[0196] Embodiment 12 is the method of Embodiment 1-11, wherein the upper phase is refluxed to the first distillation column at a reflux ratio of from 0.5 to 20, wherein the reflux ratio is the amount of the refluxed upper phase / the amount of the distilled upper phase.

[0197] Embodiment 13 is the method of embodiment 1-12, wherein the overhead of the second distillation column is refluxed to the second distillation column at a reflux ratio of from 0.2 to 15, wherein the reflux ratio is the amount of overhead refluxed / the amount of overhead distilled.

[0198] Embodiment 14 is the method of embodiments 1-13, wherein the second mixture comprises less than or equal to 2 wt. % methanol.

[0199] Embodiment 15 is the method of embodiment 1-14, wherein the purification step comprises at least one step selected from the group consisting of steps (l)-(n): (l) feeding the side stream to a dehydration column to produce an overhead stream and a bottom stream comprising the acetic acid product; (m) feeding the side stream to a dehydration column to produce an overhead stream and a bottom stream comprising the acetic acid product; (n) feeding the side stream to a dehydration column to produce an overhead stream comprising water in an amount of 25 wt.% or more, and a bottom stream comprising water in an amount of less than or equal to 0.2 wt.%.

[0200] Embodiment 16 is the method of embodiment 15, wherein the overhead stream is refluxed to the dehydration tower at a reflux ratio of from 0.2 to 15, wherein the reflux ratio is the amount of the overhead stream refluxed / the amount of the overhead stream distilled.

[0201] Embodiment 17 is a method according to claims 1-16, wherein the waste gas treatment step includes at least one step selected from the group consisting of steps (o)-(q): (o) absorbing the waste gas stream at a first pressure with a first absorption solvent selected from the group consisting of acetic acid, methanol, methyl acetate and combinations thereof; (p) absorbing the waste gas stream at a second pressure with a second absorption solvent selected from the group consisting of acetic acid, methanol, methyl acetate and combinations thereof, wherein the second pressure is less than the first pressure; and (q) diffusing the gas components absorbed in steps (o) and / or (p).

[0202] Embodiment 18 is the method of embodiments 1-17, wherein the higher boiling point component comprises propionic acid, butyl acetate, 2-ethylcrotonaldehyde, or a mixture thereof.

[0203] Embodiment 19 is the method of embodiments 1-18, further comprising maintaining a hydrogen partial pressure in the reactor from 0.3 to 2 atm by introducing a stream containing hydrogen into the reactor.

[0204] Embodiment 20 is the method of embodiments 1-19, wherein the acetaldehyde separation step comprises separating the second mixture in one or more extractors and / or one or more additional distillation columns.

Claims

1. A method for producing acetic acid, comprising: (a) a reaction step of continuously reacting methanol with carbon monoxide in a reactor in the presence of a reactive system comprising a metal catalyst, an ionic metal iodide and methyl iodide, acetic acid, methyl acetate and water; (b) an evaporation step of separating the reaction mixture obtained in the reaction step into a vapor stream and a liquid residue stream with or without heating; (c) a first distillation step of distilling at least a portion of the vapor stream in a first distillation column to form a first overhead stream and a side stream, wherein the first overhead stream comprises water, methyl iodide, and / or acetaldehyde, and the side stream comprises acetic acid; (d) a phase separation step of condensing the first overhead stream in one or more condensers, collecting the condensate in a receiver, and separating the condensate into an upper phase and a lower phase; (e) a second distillation step of distilling at least a portion of the lower phase in a second distillation column to form a second mixture comprising acetaldehyde; (f) an acetaldehyde separation step of separating the second mixture into an acetaldehyde stream and a first return stream; and at least one step selected from the group consisting of (g) to (k): (g) a purification step of purifying the side stream to obtain an acetic acid product stream; (h) an off-gas treatment step of absorbing one or more off-gas streams from the process with an absorption solvent and forming a carbon monoxide stream and a second return stream; (i) a high boiling point compound removal step of removing high boiling point components from the acetic acid product stream; (j) an iodine removal step of contacting the acetic acid product stream with an ion exchange resin and separating iodine compounds from the acetic acid product stream; as well as (k) an extraction step of extracting acetaldehyde from the acetaldehyde stream to form an extract and a residual liquor, wherein the extract comprises acetaldehyde and the residual liquor comprises methyl iodide; wherein the mass composition of 1,1-dimethoxyethane in the liquid stream of the process is controlled to be no greater than 0.2 wt.% by maintaining the amount of unreacted methanol in the reaction mixture at less than 1 wt.% based on the total weight of the reaction mixture; The residence time in the first distillation tower or the second distillation tower is not less than 1 minute.

2. The method of claim 1, wherein: The liquid stream contains at least one member selected from the group consisting of methyl iodide, methyl acetate and water.

3. The method of claim 1, wherein: The liquid stream has a mass ratio of methyl iodide to water greater than 1.

4. The method of claim 1, wherein: No stream comprising air or oxygen is brought into contact with the liquid stream.

5. The method of claim 1, wherein: By controlling the mass composition of methanol in the first overhead distillate stream, the mass composition of 1,1-dimethoxyethane in the liquid stream is controlled to be no more than 0.2 wt.%.

6. The method of claim 1, wherein: By controlling the residence time in the first distillation column and / or the second distillation column to be not less than 1 minute, the mass composition of 1,1-dimethoxyethane in the liquid stream is controlled to be not more than 0.2 wt.%.

7. The method of claim 1, wherein: The mass composition of the 1,1-dimethoxyethane in the liquid stream is controlled to be 0.01 to 0.2 wt.%.

8. The method of claim 1, wherein: The mass composition of the 1,1-dimethoxyethane in the liquid stream is controlled to be no greater than 0.1 wt.%.

9. The method of claim 1, wherein: The liquid stream is selected from the group consisting of the side stream, the bottoms stream, the condensate, the upper phase, the lower phase, the second mixture, and / or the acetic acid product stream.

10. The method of claim 1, wherein: The weight ratio of methyl iodide to water in the lower phase is greater than 1.

11. The method of claim 1, wherein: The second mixture is an overhead stream and / or a side draw stream of the second distillation column.

12. The method of claim 1, wherein: The upper phase is refluxed to the first distillation column at a reflux ratio of 0.5 to 20, wherein the reflux ratio is the amount of the refluxed upper phase / the amount of the distilled upper phase.

13. The method of claim 1, wherein: The overhead product of the second distillation column is refluxed to the second distillation column at a reflux ratio of 0.2 to 15, wherein the reflux ratio is the amount of the overhead product refluxed / the amount of the overhead product distilled.

14. The method of claim 1, wherein: The second mixture comprises less than or equal to 2 wt.% methanol.

15. The method of claim 1, wherein: The purification step comprises at least one step selected from the group consisting of steps (1)-(n): (1) feeding the side stream to a dehydration column to produce an overhead stream and a bottoms stream comprising the acetic acid product; (m) feeding the side stream to a dehydration column to produce an overhead stream and a bottom side draw stream comprising the acetic acid product; (n) feeding the side stream to a dehydration column to produce an overhead stream comprising water in an amount of 25 wt.% or more, and a bottoms stream comprising water in an amount of less than or equal to 0.2 wt.%.

16. The method of claim 15, wherein: The overhead stream is refluxed to the dehydration tower at a reflux ratio of 0.2 to 15, wherein the reflux ratio is the amount of the overhead stream refluxed / the amount of the overhead stream distilled.

17. The method of claim 1, wherein: The exhaust gas treatment step comprises at least one step selected from the group consisting of steps (o)-(q): (o) absorbing the off-gas stream at a first pressure with a first absorption solvent selected from the group consisting of acetic acid, methanol, methyl acetate, and combinations thereof; (p) absorbing the off-gas stream with a second absorption solvent selected from the group consisting of acetic acid, methanol, methyl acetate, and combinations thereof at a second pressure, wherein the second pressure is less than the first pressure; and (q) diffusing the gas components absorbed in the steps (o) and / or (p).

18. The method of claim 1, wherein: The high boiling point component includes propionic acid, butyl acetate, 2-ethylcrotonaldehyde, or a mixture thereof.

19. The method of claim 1, further comprising maintaining a hydrogen partial pressure in the reactor of 0.3 to 2 atm by introducing a stream comprising hydrogen into the reactor.

20. The method of claim 1, wherein: The acetaldehyde separation step comprises separating the second mixture in one or more extractors and / or one or more additional distillation columns.

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