Method for producing acetic acid

Abstract

To provide a production method of acetic acid which can separate acetaldehyde and methyl iodide efficiently and has reduced loss of methyl iodide.SOLUTION: A production method of acetic acid includes a membrane separation step of acquiring separately a flow rich in acetaldehyde and a flow rich in methyl iodide by using a silica film having a center pore diameter of 0.3-5.0 nm and a water contact angle of a surface of 0-70° from a process flow containing acetaldehyde and methyl iodide in a production process of acetic acid. Preferably, the acetaldehyde concentration in the process flow is 0.1-99.9 mass%, and the methyl iodide concentration is 0.01-99 mass%.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to a method for producing acetic acid. [Background technology]

[0002] The methanol carbonylation process is known as an industrial method for producing acetic acid. In this process, for example, in a reaction tank, methanol and carbon monoxide are reacted in the presence of a catalyst to produce acetic acid, the resulting reaction mixture is separated in an evaporation tank into a vapor phase containing acetic acid and low boiling components and a residual liquid phase containing acetic acid and the catalyst, the vapor phase is distilled in a distillation column (low boiling removal column) to separate it into an overhead stream containing low boiling components and an acetic acid stream, and the acetic acid stream is further purified to obtain the product acetic acid. In this process, acetaldehyde is by-produced during the reaction, and this acetaldehyde causes a decrease in the quality of the product acetic acid.

[0003] Therefore, the condensate of the overhead stream from the low boiling removal tower is separated into an aqueous phase and an organic phase in a decanter, and the aqueous phase or the organic phase is distilled in an acetaldehyde removal tower. The condensate of the overhead stream (containing acetaldehyde and methyl iodide) is extracted with water, and the aqueous phase obtained by the extraction is further distilled to separate and remove the acetaldehyde.

[0004] In addition, methyl iodide is a useful component that acts as a co-catalyst for metal catalysts such as rhodium catalysts in the methanol carbonylation process, and is also expensive. For this reason, methyl iodide separated from acetaldehyde is recycled to the reactor and reused in the reaction process. However, since acetaldehyde and methyl iodide have similar boiling points, it is difficult to completely separate acetaldehyde and methyl iodide. For this reason, methyl iodide that could not be separated was discarded together with acetaldehyde. In addition, in order to compensate for the loss of methyl iodide, it is necessary to produce methyl iodide as necessary and add new methyl iodide to the reactor, which causes problems such as a high workload and high costs.

[0005] As a method for efficiently separating acetaldehyde and methyl iodide, for example, the methods disclosed in Patent Documents 1 to 4 are known. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2017 / 057142 [Patent Document 2] JP 2006-94764 A [Patent Document 3] Japanese Patent Application Publication No. 9-40590 [Patent Document 4] Japanese Patent Application Publication No. 9-77697 Summary of the Invention [Problem to be solved by the invention]

[0007] However, even the methods disclosed in Patent Documents 1 to 4 cannot completely separate acetaldehyde and methyl iodide, and a method capable of separating them more efficiently has been desired.

[0008] Therefore, an object of the present disclosure is to provide a method for producing acetic acid that can efficiently separate acetaldehyde and methyl iodide and reduces the loss of methyl iodide. [Means for solving the problem]

[0009] As a result of intensive research to achieve the above object, the inventors of the present disclosure have found that by separating a mixture of acetaldehyde and methyl iodide using a specific separation membrane, acetaldehyde and methyl iodide can be efficiently separated. The present disclosure relates to a product that has been completed based on these findings and further research.

[0010] That is, the present disclosure provides a method for producing acetic acid, comprising a membrane separation step in which a process stream containing acetaldehyde and methyl iodide is separated into an acetaldehyde-rich stream and a methyl iodide-rich stream by using a silica membrane having a median pore size of 0.3 to 5.0 nm and a surface water contact angle of 0 to 70°.

[0011] The acetaldehyde concentration in the process stream is preferably 0.1 to 99.9 mass %, and the methyl iodide concentration is preferably 0.01 to 99 mass %.

[0012] The total concentration of acetaldehyde and methyl iodide in the process stream is preferably 1 mass % or more.

[0013] The method for producing acetic acid includes a carbonylation reaction step of reacting methanol with carbon monoxide to produce acetic acid in the presence of a catalyst system including a metal catalyst and methyl iodide, as well as acetic acid, methyl acetate, and water; A separation step of subjecting the reaction mixture obtained in the carbonylation reaction step to one or more evaporation and / or distillation processes to separate the reaction mixture into a stream containing a metal catalyst, an acetic acid stream rich in acetic acid, and a stream richer in lower boiling components than the acetic acid stream; and an acetaldehyde separation and removal system for separating acetaldehyde from at least a portion of a condensate obtained by condensing the stream rich in low boiling components by one or more distillation processes; It is preferable that the acetaldehyde-rich stream separated in the acetaldehyde separation and removal system is subjected to the membrane separation step as the process stream.

[0014] It is preferred to recycle a portion of the methyl iodide-rich stream separated and obtained by the membrane separation step to the process in the acetaldehyde separation and removal system.

[0015] The process stream preferably includes an overhead stream of a distillation column that performs a distillation process in the acetaldehyde separation and removal system.

[0016] It is preferred to reflux a portion of the methyl iodide-rich stream separated and obtained by the membrane separation step to the distillation column.

[0017] The acetaldehyde separation and removal step includes: a distillation step (A1) of distilling at least a portion of the condensate obtained by condensing the stream rich in low boiling components to separate and obtain a first overhead stream rich in acetaldehyde and a residual stream; an extraction step (B1) for subjecting at least a portion of the condensate obtained by condensing the first overhead stream to extraction and separating it into an aqueous phase rich in acetaldehyde and an organic phase rich in methyl iodide; a distillation step (C1) for subjecting the acetaldehyde-rich aqueous phase to distillation to separate and obtain a second acetaldehyde-rich overhead stream and a water-rich bottoms stream, The second overhead stream and / or a condensate obtained by condensing the second overhead stream may be subjected to the membrane separation step as the process stream.

[0018] The process stream preferably comprises a condensate obtained by condensing the second overhead stream.

[0019] The methyl iodide-rich stream separated in the membrane separation step may be recycled to the distillation step (A1) and / or the extraction step (B1).

[0020] The acetaldehyde separation and removal step includes: a distillation step (A2) of distilling at least a portion of the condensate obtained by condensing the stream rich in low boiling components, and withdrawing a liquid descending from a concentration zone in the distillation column where methyl iodide and acetaldehyde are concentrated as a side stream; A separation step (B2) of separating the side stream into an aqueous phase and an organic phase; and a distillation step (C2) for subjecting the aqueous phase to distillation to separate and obtain a third overhead stream rich in acetaldehyde and a bottoms stream rich in water, The process may further include an extractive distillation step (D2) for subjecting the third overhead stream rich in acetaldehyde to extractive distillation to separate and obtain a fourth overhead stream rich in acetaldehyde and a water-rich bottoms product, One or more streams selected from the group consisting of the third overhead stream, the condensate obtained by condensing the third overhead stream, the fourth overhead stream, and the condensate obtained by condensing the fourth overhead stream may be subjected to the membrane separation step as the process stream.

[0021] Preferably, the process stream comprises a condensate obtained by condensing the fourth overhead stream.

[0022] The methyl iodide-rich stream separated in the membrane separation step may be recycled to the distillation step (A2).

[0023] Preferably, the process stream is a liquid.

[0024] The recovery rate of methyl iodide in the membrane separation step is preferably 1 mass % or more.

[0025] The concentration of methyl iodide in the methyl iodide-rich stream separated and obtained by the membrane separation step is higher than the concentration of methyl iodide in the process stream, and is preferably 1 mass % or more.

[0026] The acetaldehyde concentration in the acetaldehyde-rich stream separated and obtained by the membrane separation step is higher than the acetaldehyde concentration in the process stream, and is preferably 0.1 mass % or more.

[0027] The silica membrane is preferably provided on at least one surface of the porous substrate.

[0028] The present disclosure also provides a method for producing an acetaldehyde-rich stream and a methyl iodide-rich stream, comprising separating and obtaining an acetaldehyde-rich stream and a methyl iodide-rich stream from a mixture containing acetaldehyde and methyl iodide using a silica membrane having a median pore size of 0.3 to 5.0 nm and a surface water contact angle of 0 to 70°.

[0029] The mixture preferably has an acetaldehyde concentration of 0.1 to 99.9% by mass and a methyl iodide concentration of 0.01 to 99% by mass. Effect of the Invention

[0030] According to the above-mentioned method for producing acetic acid, acetaldehyde and methyl iodide can be efficiently separated, so that the loss of methyl iodide can be reduced, and therefore the workload of adding methyl iodide to the reaction process can be reduced. [Brief description of the drawings]

[0031] [Figure 1] FIG. 2 is a schematic flow diagram showing an example of a membrane separation process. [Diagram 2] FIG. 4 is a schematic flow diagram showing another example of a membrane separation process. [Diagram 3] FIG. 4 is a schematic flow diagram showing another example of a membrane separation process. [Figure 4] FIG. 1 is a production flow diagram showing one embodiment of an acetic acid production system. [Diagram 5] FIG. 1 is a schematic flow diagram showing an example of an acetaldehyde separation and removal system. [Figure 6] FIG. 4 is a schematic flow diagram showing another example of an acetaldehyde separation and removal system. [Figure 7] FIG. 4 is a schematic flow diagram showing yet another example of an acetaldehyde separation and removal system. [Figure 8] FIG. 4 is a schematic flow diagram showing yet another example of an acetaldehyde separation and removal system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032] A method for producing acetic acid according to an embodiment of the present disclosure includes a membrane separation step in which a process stream containing acetaldehyde and methyl iodide is separated into an acetaldehyde-rich stream and a methyl iodide-rich stream using a silica membrane having a median pore size of 0.3 to 5.0 nm and a surface water contact angle of 0 to 70°.

[0033] The silica membrane is a porous membrane formed mainly from an inorganic compound containing silica, with a central pore size of 0.3 to 5.0 nm and a water contact angle of 0 to 70° on the surface. The silica membrane mainly allows acetaldehyde to pass through and concentrates methyl iodide without allowing it to pass through. That is, the permeate stream separated and obtained in the membrane separation step is a stream richer in acetaldehyde than the process stream (feed stream), and the concentrated stream separated and obtained is a stream richer in methyl iodide than the process stream (feed stream). This is because the silica membrane basically allows acetaldehyde to pass through due to the molecular sieve function, since the central pore size is within the above range. On the other hand, a lot of methyl iodide should pass through only due to the molecular sieve function, but it is presumed that the silica membrane does not allow methyl iodide to pass through because the surface is moderately hydrophilic due to the water contact angle being within the above range. Due to the performance of the silica membrane, acetaldehyde and methyl iodide can be efficiently separated.

[0034] The silica membrane may have an amino group and / or an ammonium group. That is, the silica membrane may have a median pore size of 0.3 to 5.0 nm, a surface water contact angle of 0 to 70°, and an amino group and / or an ammonium group (hereinafter, sometimes referred to as "amino group, etc."). The silica membrane has a median pore size within the above range, and therefore allows acetaldehyde to pass through due to its molecular sieve function. On the other hand, methyl iodide would also pass through if it only had the molecular sieve function, but the silica membrane has a water contact angle within the above range, and therefore the surface of the membrane exhibits moderate hydrophilicity, and furthermore, the silica membrane has amino groups, etc., and therefore the surface of the membrane is moderately hydrophilic, making it difficult for methyl iodide to pass through. Due to the performance of the silica membrane, acetaldehyde and methyl iodide can be separated more efficiently.

[0035] When the silica membrane has amino groups or the like, its performance (separation ability) tends not to decrease even when it is used repeatedly, so that in the acetic acid production process, it becomes possible to more efficiently separate and obtain a stream rich in acetaldehyde and a stream rich in methyl iodide from a process stream containing acetaldehyde and methyl iodide. Although the reason is not clear, it will be explained below by comparing with a conventional silica membrane (a silica membrane having no amino groups or the like on its surface and having hydroxyl groups).

[0036] Conventional silica membranes, for example, silica membranes having hydroxyl groups on the membrane surface and no amino groups, etc., have molecular sieve function and hydrophilic surface properties like the above silica membrane, but tend to reduce acetaldehyde permeability when used repeatedly. On the other hand, the above silica membrane has a feature that its performance is highly stable and its acetaldehyde permeability is not easily reduced even when used repeatedly. The reason is unclear, but the process stream containing acetaldehyde and methyl iodide in the acetic acid production process may contain components that are highly reactive with hydroxyl groups, and in the conventional silica membrane, the hydroxyl groups are reduced by reaction with the components, which may result in a reduction in acetaldehyde permeability. On the other hand, the amino groups, etc. in the above silica membrane have a low reactivity with the components, so it is considered that its performance is highly stable and its acetaldehyde permeability is not reduced even when used repeatedly.

[0037] When the silica membrane has an amino group or the like, the silica membrane preferably has a monovalent organic group having an amino group or an ammonium group in the chain and / or at the end. In the monovalent organic group having an amino group or an ammonium group, the monovalent organic group is not particularly limited, but examples thereof include alkyl groups such as cyclic alkyl groups, linear alkyl groups, or branched alkyl groups, groups in which two or more alkyl groups are bonded via heteroatoms such as oxygen atoms or sulfur atoms, and other polymer chains. Among these, an alkyl group is preferred, more preferably a linear or branched alkyl group, and even more preferably a linear alkyl group. The number of carbon atoms in the monovalent organic group is not particularly limited, but is preferably 1 to 20, more preferably 1 to 10, and even more preferably 1 to 7. The above-mentioned term "having an amino group or an ammonium group in the chain and / or at the end" means, more specifically, for example, that a secondary amino group or an ammonium group, a tertiary amino group or an ammonium group, or a quaternary ammonium group is present in the chain of the monovalent organic group, that an amino group or an ammonium group is bonded to a secondary or tertiary carbon atom in the chain of the monovalent organic group, or that an amino group (-NH 2; primary amino group) or ammonium group (-NH 3 + ; primary ammonium group) is bonded.

[0038] The amino group may be any of a primary amino group, a secondary amino group, and a tertiary amino group. The ammonium group may be any of a primary ammonium group, a secondary ammonium group, a tertiary ammonium group, and a quaternary ammonium group. The amino group may be a substituted amino group substituted with an organic group. The amino group may be an unsubstituted amino group, and may be a substituted amino group by subjecting a silica membrane having an unsubstituted amino group to organic matter treatment or by using the silica membrane for separating organic matter. The amino group may be one type or two or more types.

[0039] The substituent of the substituted amino group etc. is a monovalent organic group, and examples thereof include an alkyl group (preferably an alkyl group having 1 to 8 carbon atoms, more preferably an alkyl group having 1 to 6 carbon atoms, and even more preferably an alkyl group having 1 to 4 carbon atoms), an aryl group (preferably an aryl group having 6 to 14 carbon atoms, and more preferably an aryl group having 6 to 10 carbon atoms), an oxyalkyl group, an oxyaryl group, or a group formed by combining these. When the substituted amino group etc. has multiple substituents, the multiple substituents may be the same or different.

[0040] The substituent (monovalent organic group) of the substituted amino group etc. may have a substituent other than the organic group. Examples of the substituent other than the organic group include hydrophilic groups such as sulfoxy group, carboxyl group, hydroxyl group, phosphoryl group, etc. When the substituted amino group etc. has a plurality of substituents other than the organic group, the plurality of substituents may be the same or different.

[0041] The silica membrane may contain a monovalent organic group that does not have an amino group or the like. Examples of the monovalent organic group that does not have an amino group or the like include an alkyl group; a group having a carboxy group, a hydroxyl group, or a mercapto group at the end of the alkyl group; and an ether bond or a thioether bond in the alkyl chain. The alkyl group is not particularly limited, but is, for example, a cyclic alkyl group, a straight chain alkyl group, or a branched chain alkyl group. The number of carbon atoms of the alkyl group is not particularly limited, but is preferably 1 to 20, more preferably 1 to 10, and even more preferably 1 to 7. Hereinafter, the amino group or the like contained in the silica membrane and the monovalent organic group that does not have an amino group or the like may be collectively referred to as a monovalent organic group.

[0042] The silica film preferably has a structure derived from a hydrophilic polymer having an amino group or a structure derived from a silane coupling agent having an amino group. That is, the silica film is preferably one in which an amino group or the like is introduced by the hydrophilic polymer or the silane coupling agent. In other words, the silica film preferably has an amino group or the like derived from the hydrophilic polymer or the silane coupling agent. The hydrophilic polymer may be used in one type or in two or more types. The silane coupling agent may be used in one type or in two or more types.

[0043] When the silica membrane has a structure derived from the hydrophilic polymer, when the hydrophilic polymer is introduced into the silica membrane, the hydroxyl group present on the surface of the silica membrane reacts with the hydrophilic polymer. Therefore, the amount of hydroxyl groups on the surface of the silica membrane can be reduced, and the reaction between the hydroxyl group and the component highly reactive with the hydroxyl group in the mixed solution containing acetaldehyde and methyl iodide is reduced. On the other hand, the amino group derived from the hydrophilic polymer increases, so that the surface of the silica membrane can maintain a moderate hydrophilicity, and acetaldehyde and methyl iodide can be efficiently separated. In addition, the performance is highly stable, and the permeability of acetaldehyde is not easily reduced even when used repeatedly.

[0044] The hydrophilic polymer is not particularly limited as long as it is a polymer having an amino group or the like and a group reactive with the silica film in the polymer molecule, and examples thereof include polymers of a monomer mixture containing a betaine monomer and an alkoxysilyl group-containing compound. Examples of the betaine monomer include sulfoxybetaine monomer, carboxybetaine monomer, and phosphorylbetaine monomer. Examples of such polymers include polymers having a silanol group at the molecular end and having a plurality of (2 or more, for example, 5 or more) molecular chains having hydrophilic groups such as amino groups extending from the main skeleton. In addition, the hydrophilic polymer may contain a hydroxyl group, a carboxyl group, a sulfo group, and the like as hydrophilic groups other than the amino group. The above-mentioned polymers can react with hydroxyl groups present on the surface of the silica film by the silanol group at the molecular end. Examples of the hydrophilic polymers include "LAMBIC-771W" and "LAMBIC-1000W" manufactured by Osaka Organic Chemical Industry Co., Ltd.

[0045] When the silica membrane has a structure derived from the hydrophilic polymer, the silica membrane can be prepared by dehydration condensation reaction of a hydrophilic polymer having an amino group and / or an ammonium group with a silica membrane having a silanol group. The reaction can be carried out, for example, by applying a liquid containing a hydrophilic polymer to the surface of the silica membrane in the presence of a solvent as necessary, and reacting at 80°C for 0.1 to 5 hours. In addition, the silica membrane may be treated to increase the number of hydroxyl groups by introducing water vapor or irradiating the silica membrane with plasma before the reaction with the hydrophilic polymer.

[0046] When the silica membrane has a structure derived from the silane coupling agent, when the silane coupling agent is introduced into the silica membrane, the hydroxyl group present on the surface of the silica membrane is condensed by dehydration reaction with the alkoxy group of the silane coupling agent. Therefore, the amount of hydroxyl groups on the surface of the silica membrane can be reduced, and the reaction between the component with high reactivity with hydroxyl groups and the hydroxyl groups in the mixed solution containing acetaldehyde and methyl iodide is reduced. On the other hand, the amino groups derived from the silane coupling agent are increased, so that the surface of the silica membrane can maintain a moderate hydrophilicity, and acetaldehyde and methyl iodide can be efficiently separated. In addition, the performance is highly stable, and the permeability of acetaldehyde is not easily reduced even when used repeatedly.

[0047] The silane coupling agent contains an alkoxy group and a monovalent organic group each bonded to a silicon atom, and at least one of the monovalent organic groups has an amino group and / or an ammonium group in the chain and / or at the end. That is, the silane coupling agent contains an alkoxy group and a monovalent organic group having an amino group or the like bonded to a silicon atom, and may further contain a monovalent organic group not having an amino group or the like.

[0048] In the monovalent organic group having an amino group or the like, the monovalent organic group is not particularly limited, but examples thereof include alkyl groups such as cyclic alkyl groups, linear alkyl groups, and branched alkyl groups, and groups in which two or more alkyl groups are bonded via heteroatoms such as oxygen atoms and sulfur atoms. Among these, alkyl groups are preferred, more preferably linear or branched alkyl groups, and even more preferably linear alkyl groups. The number of carbon atoms in the monovalent organic group is not particularly limited, but is preferably 1 to 20, more preferably 1 to 10, and even more preferably 1 to 7.

[0049] In the silane coupling agent, the number of atoms in the main chain between the silicon atom in the silane coupling agent and the nitrogen atom in the amino group and / or the ammonium group is preferably 1 to 8, more preferably 2 to 4. When the number of atoms in the main chain is 8 or less (particularly 4 or less), the pores are prevented from becoming small, and the hydrophilicity of the silica membrane surface is made appropriate, resulting in better permeability. For example, when the main chain is a divalent linear hydrocarbon group, the number of atoms in the main chain is the same as the number of carbon atoms.

[0050] In the monovalent organic group having an amino group or the like, the amino group may be any of a primary amino group, a secondary amino group, and a tertiary amino group. The ammonium group may be any of a primary ammonium group, a secondary ammonium group, a tertiary ammonium group, and a quaternary ammonium group. The amino group or the like may be a substituted amino group or the like substituted with an organic group. The amino group or the like may be an unsubstituted amino group or the like, and may be made into a substituted amino group or the like by subjecting a silica membrane having an unsubstituted amino group or the like to an organic matter treatment or by using it for separating an organic matter. For example, when a methyl iodide (MeI) solution is passed through a monovalent organic group having a primary amino group, a secondary amino group or a primary ammonium group may be generated from the primary amino group, and other amino groups or ammonium groups may also be generated. The amino group may be only one type, or may be two or more types.

[0051] The silane coupling agent may contain a monovalent organic group that does not have an amino group or the like. Examples of the monovalent organic group that does not have an amino group or the like include an alkyl group; a group having a carboxy group, a hydroxyl group, or a mercapto group at the end of the alkyl group; and an alkyl group having an ether bond or a thioether bond in the chain. The alkyl group is not particularly limited, but is, for example, a cyclic alkyl group, a straight chain alkyl group, or a branched chain alkyl group. The number of carbon atoms of the alkyl group is not particularly limited, but is preferably 1 to 20, more preferably 1 to 10, and even more preferably 1 to 7.

[0052] The alkoxy group of the silane coupling agent is represented by the general formula RO- (R represents an alkyl group). The alkyl group as R is preferably a straight-chain or branched-chain alkyl group, more preferably a straight-chain alkyl group. The number of carbon atoms of the alkyl group as R (i.e., the number of carbon atoms of the alkoxy group) is not particularly limited, but is preferably 1 to 10, more preferably 1 to 5, even more preferably 1 to 4, and particularly preferably 1 or 2. Specific examples of the alkoxy group of the silane coupling agent include a methoxy group, an ethoxy group, a propoxy group, an isopropoxy group, a butoxy group, a sec-butoxy group, and a t-butoxy group.

[0053] The silane coupling agent is preferably represented by the following formula: R x -Si-R 1 m (OR 2 ) 3-m (R 1 R represents an alkyl group having 1 to 3 carbon atoms which may have a substituent. 2 R represents an alkyl group having 1 to 3 carbon atoms which may have a substituent. X represents an alkyl group having an amino group and / or the above ammonium group in the chain and / or at the end, and m represents an integer of 1 to 3.

[0054] R 1 and R 2 In the formula (I), the substituent that the alkyl group may have is not particularly limited as long as it does not interfere with the hydrolysis and dehydration condensation of the silane coupling agent, and examples thereof include an alkoxy group having 1 to 6 carbon atoms, an alkenyloxy group having 2 to 6 carbon atoms, an aliphatic acyl group having 2 to 6 carbon atoms, a benzoyl group, a nitro group, a nitroso group, a hydroxy group, a cyano group, a sulfonic acid group, an amino group such as a primary amino group, a carboxy group, a hydroxyl group, a mercapto group, and a halogen atom.

[0055] R XIn the alkyl group having an amino group or the like, the alkyl group is not particularly limited, but is preferably, for example, a cyclic alkyl group, a straight chain alkyl group, or a branched chain alkyl group, more preferably a straight chain or branched chain alkyl group, and further preferably a straight chain alkyl group. The number of carbon atoms in the alkyl group is not particularly limited, but is preferably 1 to 20, more preferably 1 to 10, and further preferably 1 to 7.

[0056] Above R X In the alkyl group having an amino group or the like, the number of atoms in the main chain between the nitrogen atom in the amino group or the like and Si is preferably 1 to 8, more preferably 2 to 4. When the number of atoms in the main chain is 8 or less (particularly 4 or less), the pores are prevented from becoming small, and the hydrophilicity of the silica membrane surface is made appropriate, resulting in better permeability.

[0057] Specific examples of the silane coupling agent include 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, trimethoxy[3-(methylamino)propyl]silane, [3-(N,N-dimethylamino)propyl]trimethoxysilane, trimethoxy[3-(phenylamino)propyl]silane, and N-[2-(N-vinylbenzylamino)ethyl]-3-aminopropyl. These silane coupling agents include N-[8-(trimethoxysilyl)octyl]ethane-1,2-diamine, N-[3-(trimethoxysilyl)propyl]-1-butanamine, [3-(diethylamino)propyl]trimethoxysilane, 3-[(1,3-dimethylbutylidene)amino]propyltriethoxysilane, (3-mercaptopropyl)trimethoxysilane, (3-mercaptopropyl)triethoxysilane, and N,N-bis[(diphenylphosphino)methyl]-3-(triethoxysilyl)propylamine. These silane coupling agents may be used alone or in combination.

[0058] In the silica film, the amino group and / or the ammonium group are bonded to silicon atoms constituting the silica film, for example, via a group represented by the following formula (1). -X-Si(-O-) 3 (1) [In formula (1), X represents a divalent organic group, the bond extending to the left of X is bonded to a nitrogen atom in the amino group or the ammonium group, and the bond extending to the right of O is bonded to a silicon atom constituting the silica film and / or a silicon atom in another group represented by formula (1)]

[0059] The number of atoms in the main chain in X is preferably 1 to 8, and more preferably 2 to 4. When the number of atoms in the main chain is 8 or less (particularly 4 or less), the pores are prevented from becoming small, and the hydrophilicity of the silica membrane surface is made appropriate, resulting in better permeability.

[0060] The silica film can be prepared by dehydration condensation reaction of a silane coupling agent having an amino group and / or an ammonium group with a silica film having a silanol group. The dehydration condensation reaction can be carried out, for example, by impregnating a cloth with a liquid containing a silane coupling agent, applying the liquid to a silica film having a silanol group, and reacting the liquid at 110°C for 0.5 to 1 hour. The silica film having a silanol group may be treated to increase the number of hydroxyl groups by introducing water vapor or irradiating the silica film with plasma before the reaction with the silane coupling agent.

[0061] The silica membrane may contain a metal oxide as an inorganic compound forming the membrane. Examples of the metal oxide include alumina, zirconia, titania, and magnesia. The silica membrane can adjust the membrane pore size, the hydrophilicity or hydrophobicity of the surface, the water contact angle, and other surface properties of the silica membrane by adjusting the type and ratio of the metal oxide used in combination with silica, the plasma treatment of the silica membrane, the introduction of organic functional groups to the surface, and the type and amount of organic functional groups in the organosilica.

[0062] The central pore diameter of the silica membrane is 0.3 to 5.0 nm, preferably 0.3 to 2.0 nm, more preferably 0.5 to 1.5 nm, and further preferably more than 0.5 nm and 1.0 nm or less. When the silica membrane has an amino group or the like, the central pore diameter is 0.3 to 5.0 nm, preferably 0.5 to 5.0 nm, more preferably 0.8 to 4.0 nm, and further preferably 1.0 to 3.0 nm. The central pore diameter is a median diameter (D50) and is measured by a single component gas permeation test (H 2 , CO 2 , N 2 , C.H. 4 , C.F. 4 ,SCIENCE FICTION 6 ) or the value measured by the nanoperm porometry method.

[0063] The water contact angle of the silica membrane surface is 0 to 70°, preferably 0 to 50°, and more preferably 0 to 40°. When the silica membrane has an amino group or the like, the water contact angle is 0 to 70°, preferably 15 to 70°, and more preferably 20 to 70°. When the water contact angle is within the above range, the surface exhibits appropriate hydrophilicity, making it difficult for hydrophobic low molecules to permeate, and allowing efficient separation of a mixed solution containing components with different molecular sizes and physical properties (hydrophilicity).

[0064] The silica membrane is preferably provided on at least one surface of a porous substrate. The porous substrate acts as a substrate for supporting the silica membrane. The porous substrate has pores that allow acetaldehyde to pass through. The thickness of the silica membrane is, for example, 0.1 to 10 μm, preferably 0.1 to 1 μm.

[0065] The porous substrate may be any known or commonly used material. Examples of the material constituting the porous substrate include inorganic materials such as silica, alumina, mullite, zirconia, cordierite, titania, silicon nitride, silicon carbide, stainless steel, copper, aluminum, titanium, and ceramics.

[0066] The process stream to be subjected to the membrane separation step may be a mixture containing at least acetaldehyde and methyl iodide, and may be one or more process streams selected from all process streams in all devices and piping in the acetic acid production process. The process stream may be liquid or gas.

[0067] The acetaldehyde concentration in the process stream may be, for example, 0.1% by mass or more, 1% by mass or more, 2% by mass or more, 5% by mass or more, 10% by mass or more, 20% by mass or more, 30% by mass or more, 40% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, 75% by mass or more, or 80% by mass or more. The acetaldehyde concentration in the process stream may be, for example, 99.9% by mass or less, 99% by mass or less, 95% by mass or less, 90% by mass or less, 85% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, 10% by mass or less, or 5% by mass or less.

[0068] The methyl iodide concentration in the process stream may be, for example, 0.01% by mass or more, 0.1% by mass or more, 0.5% by mass or more, 1% by mass or more, 2% by mass or more, 3% by mass or more, 5% by mass or more, 6% by mass or more, 7% by mass or more, 10% by mass or more, 12% by mass or more, 15% by mass or more, 20% by mass or more, or 30% by mass or more. The methyl iodide concentration in the process stream may be, for example, 99% by mass or less, 95% by mass or less, 90% by mass or less, 80% by mass or less, 70% by mass or less, 50% by mass or less, 20% by mass or less, 10% by mass or less, 5% by mass or less, 3% by mass or less, 1% by mass or less, or 0.1% by mass or less.

[0069] The total concentration of acetaldehyde and methyl iodide in the process stream is not particularly limited, but is preferably 1% by mass or more, and may be 10% by mass or more, 20% by mass or more, 30% by mass or more, 40% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 85% by mass or more. The total concentration may be, for example, 99% by mass or less, 95% by mass or less, 90% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less.

[0070] The process stream may contain components other than acetaldehyde and methyl iodide, such as components that may be generated in the process for producing acetic acid, such as carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, formic acid, propionic acid, crotonaldehyde, 2-ethylcrotonaldehyde, alkanes, and alkyl iodides such as hexyl iodide and decyl iodide.

[0071] The water concentration in the process stream may be, for example, 0.1% by mass or more, 0.5% by mass or more, 1% by mass or more, 2% by mass or more, 5% by mass or more, 7% by mass or more, 10% by mass or more, 20% by mass or more, 30% by mass or more, 40% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more. The water concentration in the process stream may be, for example, 95% by mass or less, 90% by mass or less, 80% by mass or less, 70% by mass or less, 50% by mass or less, 20% by mass or less, 15% by mass or less, 10% by mass or less, 5% by mass or less, or 1% by mass or less.

[0072] The methyl iodide concentration in the methyl iodide-rich stream obtained by the membrane separation step may be higher than the methyl iodide concentration in the process stream, for example, 1% by mass or more, 5% by mass or more, 10% by mass or more, 15% by mass or more, 20% by mass or more, 25% by mass or more, 30% by mass or more, 40% by mass or more, or 50% by mass or more. The methyl iodide concentration in the methyl iodide-rich stream may be, for example, 100% by mass or less, 90% by mass or less, 70% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, 10% by mass or less, or 5% by mass or less.

[0073] The recovery rate of methyl iodide in the membrane separation step ([methyl iodide flow rate in the concentrated stream / methyl iodide flow rate in the feed stream]×100) may be, for example, 1% by mass or more, 5% by mass or more, 10% by mass or more, 20% by mass or more, 30% by mass or more, 40% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, 95% by mass or more, or 99% by mass or more. The recovery rate of methyl iodide may be, for example, 100% by mass or less, 99.5% by mass or less, 95% by mass or less, 93% by mass or less, 90% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less.

[0074] The acetaldehyde concentration in the acetaldehyde-rich stream obtained by the membrane separation step may be higher than the acetaldehyde concentration in the process stream, for example, 0.1% by mass or more, 1% by mass or more, 5% by mass or more, 10% by mass or more, 20% by mass or more, 30% by mass or more, 50% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more. The acetaldehyde concentration in the acetaldehyde-rich stream may be, for example, 100% by mass or less, 99.5% by mass or less, 95% by mass or less, 93% by mass or less, 90% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less.

[0075] The recovery rate of acetaldehyde in the membrane separation step ([acetaldehyde flow rate in the permeate stream / acetaldehyde flow rate in the feed stream]×100) may be 1% by mass or more, 5% by mass or more, 10% by mass or more, 15% by mass or more, 20% by mass or more, 30% by mass or more, 50% by mass or more, 60% by mass or more, 80% by mass or more, or 90% by mass or more. The recovery rate of acetaldehyde may be, for example, 100% by mass or less, 99.5% by mass or less, 95% by mass or less, 93% by mass or less, 90% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less.

[0076] An embodiment of the membrane separation step will be described below. Figure 1 is a schematic flow diagram showing an embodiment of the membrane separation step in the present disclosure, in which the process stream to be subjected to the membrane separation step is a gas (process gas). According to this flow, for example, when the process gas is subjected to the membrane separation step, the process gas is supplied to a membrane module 58 through a line 60.

[0077] The membrane separation step is carried out in a membrane module 58. An inert gas such as nitrogen or helium may be blown into the permeation side of the membrane module 58 equipped with the silica membrane as a sweep gas 70. The permeation side may also be depressurized using a vacuum pump or an ejector. This increases the partial pressure difference between acetaldehyde and methyl iodide, improving the recovery rate of methyl iodide. In addition, the retention of the permeate flow passing through the silica membrane can be prevented. The temperature and pressure of the process gas charged in the membrane separation step are not particularly limited as long as they are within a range in which the process flow can be maintained in a gaseous state.

[0078] The membrane separation step is preferably carried out under a condition in which the temperature of the process gas is 50 to 130°C (for example, 60 to 120°C, preferably 70 to 110°C, more preferably 80 to 100°C). The membrane separation step is preferably carried out under pressure in order to increase the partial pressure difference between acetaldehyde and methyl iodide to improve the recovery rate of methyl iodide, and the pressure is preferably 140 to 300 kPa, more preferably 200 to 290 kPa, and even more preferably 260 to 280 kPa in absolute pressure. By carrying out the membrane separation step in the above temperature range and under pressure, the partial pressure difference of acetaldehyde between the concentrated stream and the permeate stream can be increased to increase the permeability of acetaldehyde, so that acetaldehyde can easily permeate the silica membrane, and acetaldehyde and methyl iodide can be separated more efficiently.

[0079] A methyl iodide-rich stream (line 61) obtained by concentration without permeating the silica membrane in the membrane separation step is cooled and condensed in a condenser 61a and supplied as a condensate to a receiver 59. When a liquid level controller 59b detects that the liquid level in the receiver 59 exceeds a predetermined liquid level, a valve 65b is opened to obtain a methyl iodide-rich stream (line 66) through lines 63, 64, and 65 by a pump 63a. The recovered methyl iodide-rich stream (line 66) may be treated as a distillate from a unit (e.g., a distillation column such as a distillation column 93 or 98 described later) that discharges the process gas sent through line 60. 59a is a temperature controller for the condenser 61a. 65a is a flow meter for the liquid passing through line 65.

[0080] The methyl iodide-rich stream that does not permeate the silica membrane and is concentrated can be directly or indirectly recycled to the reaction step and reused as a cocatalyst in the reaction step. The methyl iodide-rich stream (line 66) is preferably recycled to the process in the acetaldehyde separation and removal system described below, and more preferably recycled to the feed stream (line 117) of the distillation tower 91, the feed stream (line 118) of the extraction tower 92, or the feed stream (line 228) of the distillation tower 94 described below.

[0081] Also, the methyl iodide-rich stream (line 63) from receiver 59 may be recovered through line 67 by opening valve 67b. The recovered methyl iodide-rich stream (line 67) may be refluxed to a unit (e.g., distillation column 93, distillation column 98, etc. described below) that discharges the process gas sent through line 60. 67a is a flow meter for the liquid passing through line 67.

[0082] The gas generated in the receiver 59 is merged with line 71 through line 68. Valve 69a is opened according to the value of pressure gauge 60a, and the gas is discharged from line 69 through lines 68 and 71. In membrane module 58, the gas that has not permeated the silica membrane may be directly discharged from line 69 through line 71. The gas discharged from line 69 contains a large amount of methyl iodide, and is therefore recycled and reused directly or indirectly to the reaction step or an appropriate point in the process. For example, it can be recycled to exhaust gas lines 32, 37, and 45 of distillation columns 3, 5, and 6 described below.

[0083] On the other hand, the acetaldehyde-rich stream that permeates the silica membrane in the membrane separation step is subjected to incineration via line 62. 62a is a flow meter.

[0084] In this manner, the process gas (line 60) is separated by the membrane separation step into an acetaldehyde-rich stream (line 62) and a methyl iodide-rich stream (lines 66, 67, 69).

[0085] Another embodiment of the membrane separation step will be described. FIG. 2 is a schematic flow diagram showing one embodiment of the membrane separation step in the present disclosure in the case where the process stream to be subjected to the membrane separation step is a liquid (process liquid). According to this flow, for example, when the process liquid is subjected to the membrane separation step, the process liquid is supplied to the receiver 72 through a line 60. When the liquid level controller 72a detects that the liquid level in the receiver 72 exceeds a predetermined liquid level, the valve 74b is opened, and the process liquid is supplied to the receiver 73 through lines 74 and 75 from the bottom of the receiver 72 by the pump 74a. The gas generated in the receiver 72 is connected to a line 116 or a line 227, etc., described later, through a line 76, and is disposed of.

[0086] The process liquid in the receiver 73 is supplied from the bottom of the receiver 73 to the membrane module 58 through lines 77 and 78 by a pump 77a. At this time, the process liquid is heated by a heater 77b whose steam amount is adjusted by a temperature controller 78a. At this time, in order to maintain a pressure in the receiver 73 at which the circulating liquid supplied through line 81 does not vaporize, a valve 79a is opened by the pressure controller 73a, and nitrogen is supplied to the receiver 73 through line 79. If the pressure rises more than necessary due to operational modulation, the gas in the receiver 73 is connected to a line 116 or line 227, which will be described later, through line 85 by opening a valve 85a, and is disposed of.

[0087] The membrane separation step is carried out in a membrane module 58. An inert gas such as nitrogen or helium may be blown into the permeation side of the membrane module 58 equipped with the silica membrane as a sweep gas 70. The permeation side may also be depressurized using a vacuum pump, an ejector, or the like. This increases the partial pressure difference between acetaldehyde and methyl iodide, improving the recovery rate of methyl iodide. In addition, the retention of the permeate flow passing through the silica membrane can be prevented. The temperature and pressure of the process gas charged in the membrane separation step are not particularly limited as long as they are within a range in which the process flow can be maintained in a liquid state.

[0088] The membrane separation step is preferably carried out under a condition in which the temperature of the process liquid is 10 to 180° C. (for example, 20 to 150° C., preferably 30 to 130° C., more preferably 50 to 110° C., and even more preferably 70 to 90° C.). The membrane separation step is preferably carried out under pressure in order to increase the partial pressure difference between acetaldehyde and methyl iodide and improve the recovery rate of methyl iodide, and the pressure is preferably 110 to 2000 kPa, more preferably 200 to 1800 kPa, even more preferably 300 to 1500 kPa, even more preferably 400 to 1200 kPa, and particularly preferably 600 to 1000 kPa, in absolute pressure. By carrying out the membrane separation process within the above temperature range and under pressure, the temperature of the concentrated stream can be increased and the partial pressure difference of acetaldehyde between the concentrated stream and the permeate stream can be increased, so that the permeability and permeation amount of acetaldehyde can be increased, making it easier for acetaldehyde to permeate through the silica membrane, and allowing acetaldehyde and methyl iodide to be separated more efficiently.

[0089] When the liquid level controller 73b detects that the liquid level in the receiver 73 exceeds a predetermined level, a part of the methyl iodide-rich stream (line 80) that does not pass through the silica membrane is recovered through lines 82 and 83 by opening the valve 82c while adjusting the flow rate with the flow rate controller 82a. When passing through lines 82 and 83, the methyl iodide-rich stream is cooled by the condenser 82b. In order to increase the amount flowing through the membrane module 58 and increase the efficiency (permeability) of membrane separation, the stream from line 80 is returned to the receiver 73 through line 81. 82d is a temperature controller for the cooler 82b, which adjusts the flow rate of cooling water.

[0090] The methyl iodide-rich stream that does not permeate the silica membrane and is concentrated can be directly or indirectly recycled to the reaction step and reused as a cocatalyst in the reaction step. The methyl iodide-rich stream (line 83) is preferably recycled to the process in the acetaldehyde separation and removal system described below, and more preferably recycled to the feed stream (line 117) of the distillation tower 91, the feed stream (line 118) of the extraction tower 92, or the feed stream (line 228) of the distillation tower 94 described below. The methyl iodide-rich stream (line 83) may also be refluxed to a unit that discharges the process gas sent through line 60 (for example, a distillation tower such as the distillation tower 93 or 98 described below).

[0091] On the other hand, the acetaldehyde-rich stream separated by permeation through the silica membrane is directly sent to incineration via line 84.

[0092] As described above, the process liquid (line 60) is separated into an acetaldehyde-rich stream (line 84) and a methyl iodide-rich stream (line 83) by the membrane separation step.

[0093] Further another embodiment of the membrane separation step will be described. FIG. 3 is a schematic flow diagram showing one embodiment of the membrane separation step in the present disclosure in the case where the process stream to be subjected to the membrane separation step is a liquid (process liquid). According to this flow, for example, when the process liquid is subjected to the membrane separation step, the process liquid is supplied to the receiver 73 through a line 60. The process liquid in the receiver 73 is supplied to the membrane module 58 through lines 77 and 78 from the bottom of the receiver 73 by a pump 77a. At that time, when the liquid level controller 73b detects that the liquid level in the receiver 73 exceeds a predetermined liquid level, the supply amount to the membrane module 58 is adjusted by adjusting the inverter of the pump 77a. In addition, the process liquid is heated by a heater 77b whose steam amount is adjusted by a temperature controller 78a. The gas generated in the receiver 73 is connected to a line 116 or a line 227, etc., described later, through a line 76, and is disposed of.

[0094] The membrane separation step is carried out in a membrane module 58. An inert gas such as nitrogen or helium may be blown into the permeation side of the membrane module 58 equipped with the silica membrane as a sweep gas 70. The permeation side may also be depressurized using a vacuum pump or an ejector. This increases the partial pressure difference of acetaldehyde, improving the recovery rate of methyl iodide. In addition, the retention of the permeate flow passing through the silica membrane can be prevented. The temperature and pressure of the process gas charged in the membrane separation step are not particularly limited as long as they are within a range in which the process flow can be maintained in a liquid state.

[0095] In the methyl iodide-rich stream (line 80) obtained by concentration without permeating the silica membrane, a valve 82c is adjusted by a pressure controller 82a to maintain a pressure at which the methyl iodide-rich stream does not vaporize, and the methyl iodide-rich stream is recovered through lines 82 and 83. When passing through lines 82 and 83, the methyl iodide-rich stream is cooled by a condenser 82b. 82d is a temperature controller for the cooler 82b, which adjusts the flow rate of cooling water.

[0096] The membrane separation step is preferably carried out under a condition in which the temperature of the process liquid is 10 to 180°C (for example, 20 to 150°C, preferably 30 to 130°C, more preferably 50 to 110°C, and even more preferably 70 to 90°C). The membrane separation step is preferably carried out under pressure in order to increase the partial pressure difference between acetaldehyde and methyl iodide to improve the recovery rate of methyl iodide, and the pressure is preferably 110 to 2000 kPa, more preferably 200 to 1800 kPa, even more preferably 300 to 1500 kPa, even more preferably 400 to 1200 kPa, and particularly preferably 600 to 1000 kPa in absolute pressure. By carrying out the membrane separation step in the above temperature range and under pressure, the partial pressure difference of acetaldehyde between the concentrated stream and the permeated stream can be increased to increase the permeability of acetaldehyde, and acetaldehyde can easily permeate the silica membrane, so that acetaldehyde and methyl iodide can be separated more efficiently.

[0097] The methyl iodide-rich stream that does not permeate the silica membrane and is concentrated can be directly or indirectly recycled to the reaction step and reused as a cocatalyst in the reaction step. The methyl iodide-rich stream (line 83) is preferably recycled to the process in the acetaldehyde separation and removal system described below, and more preferably recycled to the feed stream (line 117) of the distillation tower 91, the feed stream (line 118) of the extraction tower 92, or the feed stream (line 228) of the distillation tower 94 described below. The methyl iodide-rich stream (line 83) may also be refluxed to a unit that discharges the process gas sent through line 60 (for example, a distillation tower such as the distillation tower 93 or 98 described below).

[0098] On the other hand, the acetaldehyde-rich stream separated by permeation through the silica membrane is directly sent to incineration via line 84.

[0099] As described above, the process liquid (line 60) is separated into an acetaldehyde-rich stream (line 84) and a methyl iodide-rich stream (line 83) by the membrane separation step.

[0100] The membrane separation step is described as one step in the process of producing acetic acid, but is not limited to this embodiment, and can be performed as a method for separating acetaldehyde and methyl iodide, which separates and obtains a stream rich in acetaldehyde and a stream rich in methyl iodide from a mixture containing acetaldehyde and methyl iodide using the silica membrane, or as a method for producing a stream rich in acetaldehyde and a stream rich in methyl iodide.Preferred embodiments of this separation method and production method are the same as those described in the membrane separation step.

[0101] In each embodiment, even if the membrane performance is deteriorated, the membrane performance can be restored by passing a solvent such as methanol, methyl acetate, acetic acid, or water through the membrane module 58 or by immersing the membrane module 58 in the solvent. The solvent may be passed through the membrane module 58 by external circulation using a pump or the like, or may be run-through cleaning. The membrane module 58 may be immersed in the solvent by filling the membrane module with the solvent and leaving it to stand. Furthermore, clogging can be removed more efficiently by heating the solvent.

[0102] In the above-mentioned method for producing acetic acid, the process for producing acetic acid may include a carbonylation reaction step in which methanol and carbon monoxide are reacted to produce acetic acid, and a separation step in which the reaction mixture obtained in the carbonylation reaction step is subjected to one or more evaporation and / or distillation treatments to separate the reaction mixture into at least a stream containing a metal catalyst, an acetic acid stream rich in acetic acid, and a stream richer in lower boiling components than the acetic acid stream. The carbonylation reaction step is carried out, for example, in the presence of a catalyst system containing a metal catalyst and methyl iodide. The carbonylation reaction step may further be carried out in the presence of acetic acid, methyl acetate, and water. The separation step preferably includes, for example, an evaporation step in which the reaction mixture obtained in the carbonylation reaction step is separated into at least a vapor stream and a residual liquid stream by evaporation, and a low boiling removal step in which the vapor stream is subjected to distillation to separate and obtain at least an overhead stream rich in low boiling components and a first acetic acid stream rich in acetic acid. The first acetic acid stream may be a side cut stream, a bottom stream, or both. In the low boiling point removal step, the vapor stream is preferably subjected to distillation to separate into an overhead stream rich in low boiling components, a first acetic acid stream (side cut stream) rich in acetic acid, and a bottom stream. The separation step may also include a dehydration step in which the first acetic acid stream is subjected to distillation to separate and obtain at least an overhead stream rich in water and a second acetic acid stream enriched in acetic acid more than the first acetic acid stream.

[0103] The separation step may include, instead of the evaporation step and the dehydration step, a step of separating the reaction mixture obtained in the carbonylation reaction step into a stream containing the catalyst, an overhead stream rich in low boiling components, and a first acetic acid stream rich in acetic acid (evaporative dehydration step). The separation step may include, instead of the dehydration step and the dehydration step, a dehydration step also having the function of the dehydration step (so-called dehydration step), that is, a step of subjecting the vapor stream to distillation to separate into an overhead stream rich in low boiling components and an acetic acid stream dehydrated to a water concentration equivalent to that of the second acetic acid stream. Therefore, the evaporative dehydration step may also be a step (evaporative dehydration step) having the function of the dehydration step. The acetic acid stream rich in acetic acid obtained from the dehydration step and the dehydration step corresponds to the second acetic acid stream.

[0104] The above-mentioned method for producing acetic acid may further include at least one of the following steps (a) to (c): (a) A high boiling point removal step in which the acetic acid stream rich in acetic acid obtained in the above separation step (for example, the first or second acetic acid stream) is distilled to separate it into a bottom stream rich in high boiling components and a third acetic acid stream that is richer in acetic acid than the acetic acid stream before being subjected to distillation. (b) an adsorption removal step of treating the acetic acid-rich acetic acid stream (e.g., the first or second acetic acid stream) or the third acetic acid stream obtained in the separation step with an ion exchange resin to obtain a fourth acetic acid stream; (c) A product step of distilling the acetic acid-rich acetic acid stream (e.g., the first or second acetic acid stream) or the third or fourth acetic acid stream obtained in the above separation step to obtain a fifth acetic acid stream that is more enriched in acetic acid than the acetic acid stream before being subjected to the distillation.

[0105] The acetic acid production method may further include an acetaldehyde separation and removal system for separating acetaldehyde from at least a portion of a condensate obtained by condensing the low boiling point component-rich stream by one or more distillation processes. The acetaldehyde separation and removal system may further include an extraction step for separating an aqueous phase containing acetaldehyde by one or more extraction processes from at least a portion of the acetaldehyde separated by the one or more distillation processes.

[0106] In the above-mentioned acetic acid production method, it is preferable to subject the acetaldehyde-rich stream separated in the distillation treatment in the above-mentioned acetaldehyde separation and removal system and / or the aqueous layer containing acetaldehyde separated in the extraction treatment of the above-mentioned acetaldehyde-rich stream to the above-mentioned membrane separation step as the above-mentioned process stream. In particular, it is preferable that the above-mentioned process stream is an overhead stream of a distillation tower performing the above-mentioned distillation treatment. And, it is preferable that at least a part of the methyl iodide-rich stream separated and obtained by the above-mentioned membrane separation step is recycled to the process in the above-mentioned acetaldehyde separation and removal system, and it is particularly preferable that it is recycled to the above-mentioned distillation tower (i.e., the distillation tower that discharges the above-mentioned process stream).

[0107] Hereinafter, an embodiment of the acetic acid production method including the membrane separation step will be described. Figure 4 is an example of a production flow diagram (methanol carbonylation process) showing an embodiment of the acetic acid production system. The acetic acid production apparatus according to this acetic acid production flow includes a reaction tank 1, an evaporation tank 2, a distillation tower 3, a decanter 4, a distillation tower 5, a distillation tower 6, an ion exchange resin tower 7, a scrubber system 8, an acetaldehyde separation and removal system 9, condensers 1a, 2a, 3a, 5a, 6a, a heat exchanger 2b, reboilers 3b, 5b, 6b, lines 11 to 56, and a pump 57, and is configured to be able to continuously produce acetic acid.

[0108] In the method for producing acetic acid according to the present embodiment, a reaction step, an evaporation step (flash step), a first distillation step, a second distillation step, a third distillation step, and an adsorption removal step are performed in the reaction tank 1, the evaporation tank 2, the distillation tower 3, the distillation tower 5, the distillation tower 6, and the ion exchange resin tower 7, respectively. The first distillation step is also called a low boiling point removal step, the second distillation step is also called a dehydration step, and the third distillation step is also called a high boiling point removal step. In this embodiment, the steps are not limited to those described above, and in particular, the distillation tower 5, the distillation tower (high boiling point removal tower) 6, the ion exchange resin tower 7, and the acetaldehyde separation and removal system 9 (such as an acetaldehyde removal tower) may not be provided. In addition, as described later, a product tower may be provided downstream of the ion exchange resin tower 7.

[0109] The reaction tank 1 is a unit for carrying out a reaction process. This reaction process is a process for continuously producing acetic acid by a reaction (methanol carbonylation reaction) represented by the following chemical formula (1). In a steady operation state of the acetic acid production apparatus, a reaction mixture stirred by, for example, a stirrer is present in the reaction tank 1. The reaction mixture contains the raw materials methanol and carbon monoxide, a metal catalyst, a co-catalyst, water, acetic acid to be produced, and various by-products, and the liquid phase and the gas phase are in equilibrium. CH 3 OH + CO → CH 3 COOH (1)

[0110] The raw materials in the reaction mixture are liquid methanol and gaseous carbon monoxide. Methanol is continuously supplied from a methanol storage section (not shown) to the reaction tank 1 through a line 11. Carbon monoxide is continuously supplied from a carbon monoxide storage section (not shown) to the reaction tank 1 through a line 12. The carbon monoxide does not necessarily have to be pure carbon monoxide, and may contain small amounts (for example, 5 mass % or less, preferably 1 mass % or less) of other gases such as nitrogen, hydrogen, carbon dioxide, and oxygen.

[0111] The metal catalyst in the reaction mixture is for promoting the carbonylation reaction of methanol, and can be, for example, a rhodium catalyst or an iridium catalyst. The rhodium catalyst can be, for example, a catalyst represented by the chemical formula [Rh(CO) 2 I 2 ] - The iridium catalyst may be, for example, a rhodium complex represented by the chemical formula [Ir(CO) 2 I 2 ] - The iridium complex represented by the following formula can be used. The metal catalyst is preferably a metal complex catalyst. The concentration of the catalyst (metal equivalent) in the reaction mixture is, for example, 200 to 10,000 ppm by mass, preferably 300 to 5,000 ppm by mass, and more preferably 400 to 2,500 ppm by mass, based on the entire liquid phase (reaction mixture) of the reaction mixture.

[0112] The co-catalyst is an iodide for supporting the action of the above-mentioned catalyst, and for example, methyl iodide or an ionic iodide is used. Methyl iodide can act to promote the catalytic action of the above-mentioned catalyst. The concentration of methyl iodide is, for example, 1 to 20 mass% based on the entire liquid phase of the reaction mixture, and preferably 5 to 15 mass%. The ionic iodide is an iodide (particularly, an ionic metal iodide) that generates iodide ions in the reaction liquid, and can act to stabilize the above-mentioned catalyst and inhibit side reactions. Examples of the ionic iodide include alkali metal iodides such as lithium iodide, sodium iodide, and potassium iodide. The concentration of the ionic iodide in the reaction mixture is, for example, 1 to 25 mass% based on the entire liquid phase of the reaction mixture, and preferably 5 to 20 mass%. In addition, when an iridium catalyst or the like is used, a ruthenium compound or an osmium compound can also be used as the co-catalyst. The total amount of these compounds used is, for example, 0.1 to 30 mol (metal equivalent), preferably 0.5 to 15 mol (metal equivalent) per mol of iridium (metal equivalent).

[0113] Water in the reaction mixture is a component necessary for producing acetic acid in the reaction mechanism of the carbonylation reaction of methanol, and is also a component necessary for solubilizing water-soluble components in the reaction system. The concentration of water in the reaction mixture is, for example, 0.1 to 15 mass%, preferably 0.5 to 10 mass%, more preferably 1 to 6 mass%, and even more preferably 1.5 to 4 mass%, based on the entire liquid phase of the reaction mixture. The water concentration is preferably 15 mass% or less from the viewpoint of suppressing the energy required for removing water in the purification process of acetic acid and promoting the efficiency of acetic acid production. Water may be continuously supplied to the reaction tank 1 in order to control the water concentration.

[0114] The acetic acid in the reaction mixture includes acetic acid charged in advance in the reaction tank 1 before the operation of the acetic acid production apparatus, and acetic acid generated as a main product of the methanol carbonylation reaction. Such acetic acid can function as a solvent in the reaction system. The concentration of acetic acid in the reaction mixture is, for example, 50 to 90 mass%, preferably 60 to 80 mass%, based on the entire liquid phase of the reaction mixture.

[0115] The main by-product contained in the reaction mixture is, for example, methyl acetate. This methyl acetate can be produced by the reaction of acetic acid with methanol. The concentration of methyl acetate in the reaction mixture is, for example, 0.1 to 30 mass %, preferably 1 to 10 mass %, based on the entire liquid phase of the reaction mixture.

[0116] The by-product contained in the reaction mixture also includes hydrogen iodide. When the above-mentioned catalyst or co-catalyst is used, this hydrogen iodide is inevitably generated due to the reaction mechanism of the methanol carbonylation reaction. The concentration of hydrogen iodide in the reaction mixture is, for example, 0.01 to 2 mass% based on the entire liquid phase of the reaction mixture.

[0117] Examples of by-products include hydrogen, methane, carbon dioxide, acetaldehyde, crotonaldehyde, 2-ethylcrotonaldehyde, dimethyl ether, alkanes, formic acid, and propionic acid, as well as alkyl iodides such as hexyl iodide and decyl iodide. The reaction mixture may also contain metals such as iron, nickel, chromium, manganese, and molybdenum (hereinafter sometimes referred to as "corrosive metals") that are generated by corrosion of the equipment, and other metals such as cobalt, zinc, and copper. The above-mentioned corrosive metals and other metals may be collectively referred to as "corroded metals, etc."

[0118] In the reaction tank 1 in which the above-described reaction mixture exists, the reaction temperature is set to, for example, 150 to 250°C, the reaction pressure as the total pressure is set to, for example, 1.5 to 3.5 MPa (absolute pressure), and the carbon monoxide partial pressure is set to, for example, 0.4 to 1.8 MPa (absolute pressure), preferably 0.6 to 1.6 MPa (absolute pressure), and more preferably 0.9 to 1.4 MPa (absolute pressure).

[0119] The vapor in the gas phase in the reaction vessel 1 during operation of the apparatus includes, for example, carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid. This vapor can be extracted from the reaction vessel 1 through a line 13. By adjusting the amount of vapor extracted, the pressure in the reaction vessel 1 can be controlled, and for example, the pressure in the reaction vessel 1 is maintained constant. The vapor extracted from the reaction vessel 1 is introduced into a condenser 1a.

[0120] The condenser 1a separates the vapor from the reaction vessel 1 into a condensed fraction and a gas fraction by cooling and partially condensing the vapor. The condensed fraction includes, for example, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid, and is introduced from the condenser 1a through a line 14 to the reaction vessel 1 for recycling. The gas fraction includes, for example, carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid, and is supplied from the condenser 1a through a line 15 to the scrubber system 8.

[0121] In FIG. 4, the gas portions from condenser 2a (line 20), condenser 3a (line 32), condenser 5a (line 37), and condenser 6a (line 45) all join line 15 and are supplied to scrubber system 8, but only the gas portion from condenser 1a may be supplied to scrubber system 8 through line 15, and the gas portions from condensers 3a, 5a, and 6a (lines 32, 37, and 45) may all join line 20 and be supplied to scrubber system 8.

[0122] As described above, acetic acid is continuously produced in the reaction tank 1 during operation of the apparatus. The reaction mixture containing such acetic acid is continuously withdrawn from the reaction tank 1 and introduced into the subsequent evaporation tank 2 through a line 16.

[0123] The evaporation tank 2 is a unit for carrying out an evaporation step (flash step) in which the reaction mixture, which is continuously introduced into the evaporation tank 2 through a line 16 (reaction mixture feed line), is partially evaporated and separated into a vapor stream (volatile phase) and a residual liquid stream (less volatile phase).

[0124] Evaporation may be caused by reducing the pressure without heating the reaction mixture, or by reducing the pressure while heating the reaction mixture. In the evaporation step, the temperature of the vapor stream is, for example, 100 to 260°C, preferably 120 to 200°C, the temperature of the residual liquid stream is, for example, 80 to 200°C, preferably 100 to 180°C, and the pressure in the tank is, for example, 50 to 1000 kPa (absolute pressure).

[0125] The ratio of the vapor flow and the residual liquid flow separated in the evaporation step is, for example, 10 / 90 to 50 / 50 (vapor flow / residual liquid flow) in mass ratio. The vapor generated in this step contains, for example, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid, as well as alkyl iodides such as ethyl iodide, propyl iodide, butyl iodide, hexyl iodide, and decyl iodide, and is continuously extracted from the evaporation tank 2 to a line 17 (vapor flow discharge line).

[0126] A part of the vapor stream extracted from the evaporation tank 2 is continuously introduced into the condenser 2a, and another part of the vapor stream is continuously introduced into the next distillation column 3 through a line 21. The acetic acid concentration of the vapor stream is, for example, 40 to 85 mass% (preferably 50 to 85 mass%), more preferably 50 to 75 mass% (for example, 55 to 75 mass%), the methyl iodide concentration is, for example, 2 to 50 mass% (preferably 5 to 30 mass%), the water concentration is, for example, 0.2 to 20 mass% (preferably 1 to 15 mass%), and the methyl acetate concentration is, for example, 0.2 to 50 mass% (preferably 2 to 30 mass%). The hexyl iodide concentration of the vapor stream is, for example, 0.1 to 10,000 ppb by mass, usually 0.5 to 1,000 ppb by mass, and often 1 to 100 ppb by mass (for example, 2 to 50 ppb by mass).

[0127] The residual liquid stream generated in this step contains the catalyst and co-catalyst (methyl iodide, lithium iodide, etc.) contained in the reaction mixture, as well as water, methyl acetate, acetic acid, formic acid, and propionic acid remaining in this step without volatilization, and is continuously introduced from the evaporation tank 2 to the heat exchanger 2b through a line 18 using a pump 57. The heat exchanger 2b cools the residual liquid stream from the evaporation tank 2. The cooled residual liquid stream is continuously introduced from the heat exchanger 2b through a line 19 to the reaction tank 1 and recycled. The lines 18 and 19 are collectively referred to as a residual liquid stream recycle line. The acetic acid concentration of the residual liquid stream is, for example, 55 to 90 mass%, preferably 60 to 85 mass%.

[0128] The condenser 2a separates the vapor stream from the evaporation tank 2 into a condensed fraction and a gas fraction by cooling and partially condensing it. The condensed fraction includes, for example, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid, and is introduced from the condenser 2a through lines 22 and 23 into the reaction tank 1 and recycled. The gas fraction includes, for example, carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid, and is supplied from the condenser 2a to the scrubber system 8 through lines 20 and 15. The reaction for producing acetic acid in the above-mentioned reaction step is an exothermic reaction, and part of the heat accumulated in the reaction mixture is transferred to the vapor generated from the reaction mixture in the evaporation step (flash step). The condensed fraction generated by cooling this vapor in the condenser 2a is recycled to the reaction tank 1. That is, in this acetic acid production apparatus, the heat generated by the carbonylation reaction of methanol is efficiently removed by the condenser 2a.

[0129] The distillation column 3 is a unit for performing the first distillation step, and in this embodiment, it is positioned as a so-called low boiling component removal column. The first distillation step is a step of distilling a vapor stream continuously introduced into the distillation column 3 to separate and remove low boiling components. More specifically, in the first distillation step, the vapor stream is distilled to separate it into an overhead stream rich in at least one low boiling component selected from methyl iodide and acetaldehyde, and an acetic acid stream rich in acetic acid.

[0130] The distillation column 3 is, for example, a fractionation column such as a plate column or a packed column. When a plate column is used as the distillation column 3, the theoretical plate is, for example, 5 to 50 plates, and the reflux ratio is, for example, 0.5 to 3000 depending on the number of theoretical plates. Inside the distillation column 3, the column top pressure is set to, for example, 80 to 160 kPaG, and the column bottom pressure is set to, for example, 85 to 180 kPaG, higher than the column top pressure. Inside the distillation column 3, the column top temperature is set to, for example, 90 to 130°C, which is a temperature lower than the boiling point of acetic acid at the set column top pressure, and the column bottom temperature is set to, for example, 120 to 165°C (preferably 125 to 160°C), which is a temperature higher than the boiling point of acetic acid at the set column bottom pressure.

[0131] A vapor stream from the evaporation tank 2 is continuously introduced into the distillation column 3 through a line 21, and vapor as an overhead stream is continuously withdrawn from the top of the distillation column 3 through a line 24. A bottoms stream is continuously withdrawn from the bottom of the distillation column 3 through a line 25. 3b is a reboiler. An acetic acid stream (first acetic acid stream; liquid) is continuously withdrawn from a height position between the top and bottom of the distillation column 3 through a line 27 as a side stream.

[0132] The vapor withdrawn from the top of the distillation column 3 contains a larger amount of components (low boiling point components) having a boiling point lower than that of acetic acid, as compared with the bottoms and side stream from the distillation column 3, such as methyl iodide, hydrogen iodide, water, methyl acetate, dimethyl ether, methanol, acetaldehyde, and formic acid. The vapor also contains acetic acid. Such vapor is continuously introduced into the condenser 3a through a line 24.

[0133] The condenser 3a separates the vapor from the distillation column 3 into a condensed fraction and a gas fraction by cooling and partially condensing the vapor. The condensed fraction contains, for example, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid, and is continuously introduced from the condenser 3a through a line 28 into the decanter 4. The condensed fraction introduced into the decanter 4 is separated into an aqueous phase (upper phase) and an organic phase (methyl iodide phase; lower phase).

[0134] The aqueous phase includes water and, for example, methyl iodide, hydrogen iodide, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. The organic phase includes, for example, methyl iodide and, for example, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid.

[0135] In this embodiment, a portion of the aqueous phase is refluxed to the distillation column 3 through line 29, and another portion of the aqueous phase is recycled by being introduced into the reaction tank 1 through lines 29, 30, and 23. A portion of the organic phase is recycled by being introduced into the reaction tank 1 through lines 31 and 23. Another portion of the organic phase and / or another portion of the aqueous phase is introduced into the acetaldehyde separation and removal system 9 through lines 31, 50, and / or lines 30, 51.

[0136] In the acetaldehyde separation and removal step using the acetaldehyde separation and removal system 9, acetaldehyde contained in at least a portion (e.g., organic phase and / or aqueous phase) of the condensate obtained by condensing the overhead stream rich in low boiling components is separated and removed by a known method, for example, distillation, extraction, or a combination of these. The separated acetaldehyde is discharged outside the apparatus through line 53. In addition, useful components (e.g., methyl iodide, etc.) contained in the organic phase and / or aqueous phase are recycled to the reaction tank 1 through lines 52 and 23 for reuse.

[0137] FIG. 5 is a schematic flow diagram showing an example of an acetaldehyde separation and removal system. According to this flow, for example, when the organic phase is treated in the acetaldehyde separation and removal step, the organic phase (feed stream) is supplied to a distillation tower (first deacetaldehyde tower) 91 through a line 101 and distilled to separate and obtain a first overhead stream (line 102) rich in acetaldehyde and a residual stream (line 103) rich in methyl iodide (distillation step (A1)). The first overhead stream is condensed in a condenser 91a, a part of the condensate is refluxed to the top of the distillation tower 91 (line 104), and the other part of the condensate is supplied to an extraction tower 92 (line 105). The feed stream (line 101) may be joined with a stream rich in methyl iodide separated and obtained by the membrane separation step through a line 117. This allows the stream rich in methyl iodide to be recycled to the distillation step (A1).

[0138] The condensate (feed stream) supplied to the extraction tower 92 is extracted with water introduced from a line 109 (extraction step (B1)). The extraction separates the acetaldehyde-rich aqueous phase (line 107) from the methyl iodide-rich organic phase (line 108). The acetaldehyde-rich aqueous phase (extract) obtained by the extraction is supplied to the distillation tower (second deacetaldehyde tower) 93 via a line 107 and distilled to separate and obtain a second overhead stream (line 112) rich in acetaldehyde and a water-rich residual stream (line 113) (distillation step (C1)). The second overhead stream rich in acetaldehyde is then condensed in a condenser 93a, a portion of the condensate is refluxed to the top of the distillation tower 93 (line 114), and the other portion of the condensate is discharged outside the system (line 115). The feed stream (line 105) to the extraction tower 92 may be joined via line 118 with a stream rich in methyl iodide separated and obtained in the membrane separation step. This allows the stream rich in methyl iodide to be recycled to the extraction step (B1). The stream rich in methyl iodide may also be refluxed to the distillation tower 93 as a reflux liquid.

[0139] In addition, the bottoms of the first deacetaldehyde column 91, i.e., a methyl iodide-rich residual stream, the bottoms of the second deacetaldehyde column 93, i.e., a methyl iodide-rich raffinate (line 108) obtained in the extraction column 92, and a water-rich residual stream, the bottoms of the second deacetaldehyde column 93, are recycled to the reaction tank 1 via lines 103, 111, and 113, respectively, or are recycled to an appropriate point in the process and reused. For example, the methyl iodide-rich raffinate obtained in the extraction column 92 can be recycled to the distillation column 91 via line 110. The liquid in 113 is usually discharged to the outside as wastewater. The gases (lines 106, 116) not condensed in the condensers 91a and 93a are absorbed in the scrubber system 8 or are disposed of.

[0140] In addition, when the aqueous phase is treated in the acetaldehyde separation and removal step according to the flow chart of FIG. 5, for example, the aqueous phase (feed stream) is supplied to a distillation tower (first deacetaldehyde tower) 91 through a line 101 and distilled to separate and obtain a first overhead stream (line 102) rich in acetaldehyde and a residual stream (line 103) rich in water (distillation step (A1)). The first overhead stream is condensed in a condenser 91a, and a part of the condensate is refluxed to the top of the distillation tower 91 (line 104), and the other part of the condensate is supplied to an extraction tower 92 (line 105). The feed stream (line 101) may be joined with a stream rich in methyl iodide separated and obtained in the membrane separation step through a line 117. This allows the stream rich in methyl iodide to be recycled to the distillation step (A1).

[0141] The condensate (feed stream) supplied to the extraction tower 92 is extracted with water introduced from line 109 (extraction step (B1)). The extraction separates the acetaldehyde-rich aqueous phase (line 107) from the methyl iodide-rich organic phase (line 108). The acetaldehyde-rich aqueous phase (extract) obtained by the extraction is supplied to the distillation tower (second deacetaldehyde tower) 93 via line 107 and distilled to separate and obtain an acetaldehyde-rich second overhead stream (line 112) and a water-rich residual stream (line 113). The acetaldehyde-rich second overhead stream is then condensed in the condenser 93a, a portion of the condensate is refluxed to the top of the distillation tower 93 (line 114), and the other portion of the condensate is discharged outside the system (line 115). A methyl iodide-rich stream separated and obtained in the membrane separation step may be combined with the feed stream (line 105) to the extraction tower 92 via a line 118. This allows the methyl iodide-rich stream to be recycled to the extraction step (B1).

[0142] In addition, the water-rich residual liquid stream which is the bottom product of the first deacetaldehyde column 91, the methyl iodide-rich raffinate (line 108) obtained in the extraction column 92, and the water-rich residual liquid stream which is the bottom product of the second deacetaldehyde column 93 are recycled to the reaction tank 1 via lines 103, 111, and 113, respectively, or are recycled and reused at appropriate points in the process. For example, the methyl iodide-rich raffinate obtained in the extraction column 92 can be recycled to the distillation column 91 via line 110. The liquid in 113 is usually discharged to the outside as wastewater. The gases (lines 106 and 116) which are not condensed in the condensers 91a and 93a are absorbed and treated in the scrubber system 8, or are disposed of.

[0143] One or more of the first overhead stream (line 102) rich in acetaldehyde, the condensate obtained by condensing the first overhead stream (lines 104, 105), the extract (107), the second overhead stream (line 112) rich in acetaldehyde, and the condensate obtained by condensing the second overhead stream (lines 114, 115) are acetaldehyde-rich streams and may be subjected to the membrane separation step as the process stream. Among them, since methyl iodide has been sufficiently separated and recovered and it is difficult to separate and obtain methyl iodide further by conventional purification using a distillation column, the process stream to be subjected to the membrane separation step is preferably the second overhead stream and / or the condensate obtained by condensing the second overhead stream, and from the viewpoint that the silica membrane has a better recovery rate of methyl iodide and the number of facilities can be relatively reduced, the second overhead stream (particularly line 112) which is a gas is preferred. Line 112 is an overhead stream from the distillation step (C1). From the viewpoint of increasing the partial pressure difference of acetaldehyde, a condensate (particularly, line 115) obtained by condensing the second overhead stream, which is a liquid, is preferred.

[0144] In addition to the above-mentioned method, acetaldehyde contained in at least a part (e.g., organic phase and / or aqueous phase) of the condensate obtained by condensing the overhead stream rich in low boiling components can also be separated and removed by a distillation step (distillation step (A2)) using extractive distillation, azeotropic distillation, or the like, in which the acetaldehyde is subjected to distillation and the liquid descending from the concentration zone in the distillation tower where methyl iodide and acetaldehyde are concentrated is withdrawn as a side stream. In the case of extractive distillation, specifically, for example, the organic phase and / or aqueous phase (charge liquid) obtained by separating the condensate of the steam withdrawn from the top of the distillation tower 3 is subjected to distillation (extractive distillation), an extractant (usually water) is introduced into the concentration zone in the distillation tower where methyl iodide and acetaldehyde are concentrated (for example, the space from the top to the charge liquid supply position), and the liquid descending from the concentration zone (extract liquid) is withdrawn as a side stream (side cut stream). Next, this side stream is separated into an aqueous phase and an organic phase (liquid separation step (B2)), the aqueous phase is subjected to distillation to separate and obtain a third overhead stream rich in acetaldehyde and a bottom product rich in water (distillation step (C2)), and the acetaldehyde can be discharged outside the system. Furthermore, the third overhead stream rich in acetaldehyde can be subjected to extractive distillation to separate and obtain a fourth overhead stream rich in acetaldehyde and a bottom product rich in water (extractive distillation step (D2)), and the acetaldehyde can be further concentrated and discharged outside the system.

[0145] In addition, when a relatively large amount of water is present in the distillation column, the liquid descending from the concentration zone may be withdrawn as a side stream without introducing the extraction solvent into the distillation column. For example, the distillation column may be provided with a unit (such as a chimney tray) capable of receiving the liquid descending from the concentration zone, and the liquid received by this unit may be withdrawn as a side stream.

[0146] The position of introduction of the extraction solvent is preferably above the position of supply of the charge liquid, more preferably near the top of the tower. The position of withdrawal of the side stream is preferably below the position of introduction of the extraction solvent in the height direction of the tower and above the position of supply of the charge liquid. According to this method, acetaldehyde can be extracted at a high concentration from the concentrate of methyl iodide and acetaldehyde by the extraction solvent (usually water), and since the area between the introduction part of the extraction solvent and the side cut part is used as the extraction area, acetaldehyde can be efficiently extracted with a small amount of extraction solvent. Therefore, for example, compared with a method in which the extract by the extractive distillation is withdrawn from the bottom of the distillation tower (extractive distillation tower), the number of stages of the distillation tower can be significantly reduced and the steam load can also be reduced. In addition, since the ratio of methyl iodide to acetaldehyde in the water extract (MeI / AD ratio) can be reduced using a small amount of extraction solvent compared to the method of combining the dealdehydication distillation and water extraction in the above-mentioned FIG. 5, acetaldehyde can be removed under conditions in which the loss of methyl iodide to the outside of the system can be suppressed.

[0147] The acetaldehyde concentration in the side stream is much higher than that in the feed liquid and the bottoms (column bottom liquid). Also, the ratio of acetaldehyde to methyl iodide in the side stream is higher than that of methyl iodide in the feed liquid and the bottoms.

[0148] The organic phase (methyl iodide phase) obtained by separating the side stream may be recycled to the distillation column. In this case, the recycling position of the organic phase obtained by separating the side stream is preferably below the side stream withdrawal position in the height direction of the column and preferably above the feed liquid supply position.

[0149] In addition, a miscible solvent for the components (e.g., methyl acetate, etc.) constituting the organic phase obtained by separating the condensate of the steam extracted from the top of the distillation column 3 may be introduced into this distillation column. Examples of the miscible solvent include acetic acid and ethyl acetate. The introduction position of the miscible solvent is preferably below the side stream extraction position in the height direction of the column, and preferably above the feed liquid supply position. In addition, when the organic phase obtained by separating the side stream is recycled to this distillation column, the introduction position of the miscible solvent is preferably below the recycle position.

[0150] By recycling the organic phase obtained by separating the side stream to the distillation column or by introducing the miscible solvent into the distillation column, the methyl acetate concentration in the liquid withdrawn as the side stream can be reduced, and the methyl acetate concentration in the aqueous phase obtained by separating the liquid can be reduced, thereby making it possible to suppress the incorporation of methyl iodide into the aqueous phase.

[0151] The theoretical plates of the above distillation column are, for example, 1 to 100 plates, preferably 2 to 50 plates, more preferably 3 to 30 plates, and even more preferably 5 to 20 plates. Compared with the 80 to 100 plates of conventional distillation columns and extractive distillation columns used in deacetaldehyde removal, the number of theoretical plates is smaller, allowing acetaldehyde to be separated and removed efficiently.

[0152] The mass ratio (former / latter) of the flow rate of the extraction solvent to the flow rate of the feed liquid (the organic phase and / or the aqueous phase obtained by separating the process stream) may be selected from the range of 0.0001 / 100 to 100 / 100, but is usually 0.0001 / 100 to 20 / 100, preferably 0.001 / 100 to 10 / 100, more preferably 0.01 / 100 to 8 / 100, and even more preferably 0.1 / 100 to 5 / 100.

[0153] The temperature at the top of the distillation column is, for example, 15 to 120° C., preferably 20 to 90° C., more preferably 20 to 80° C., and further preferably 25 to 70° C. The pressure at the top of the column is, for example, about 0.1 to 0.5 MPa in absolute pressure. Other conditions of the distillation column may be the same as those of a distillation column or an extractive distillation column used in conventional deacetaldehyde removal.

[0154] FIG. 6 is a schematic flow diagram showing an example of an acetaldehyde separation and removal system using the extractive distillation. In this example, the organic phase and / or aqueous phase (feed stream) obtained by separating the condensate of the steam extracted from the top of the distillation tower 3 is supplied to the middle stage (position between the top and bottom) of the distillation tower 94 through a supply line 201, and water is introduced from near the top of the tower through a line 202 as necessary, and distillation is performed in the distillation tower 94 (distillation step (A2)). A stream rich in methyl iodide separated and obtained in the membrane separation step may be merged with the feed stream (line 201) through a line 228. This allows the stream rich in methyl iodide to be recycled to the distillation step (A2).

[0155] A chimney tray 200 is disposed above the feed position of the above-mentioned feed stream in the distillation column 94 for receiving the liquid (extract liquid) descending from the concentration zone in the column where methyl iodide and acetaldehyde are concentrated. In this extractive distillation, the liquid on the chimney tray 200 is preferably withdrawn in its entirety and introduced into the decanter 95 through a line 208 for liquid separation.

[0156] The aqueous phase (containing acetaldehyde) in the decanter 95 is introduced through a line 212 into a cooling cooler 95a for cooling, and the methyl iodide dissolved in the aqueous phase is separated into two phases and separated in a decanter 96. Both the separation in the decanter 95 and the separation in the decanter 96 correspond to a separation step (B2).

[0157] The aqueous phase in the decanter 96 is supplied to a distillation column 97 (deacetaldehyde column) via a line 216 and distilled, and separated into a third overhead stream (line 217) rich in acetaldehyde and a bottom stream (line 218) rich in water (distillation step (C2)). The third overhead stream is condensed in a condenser 97a via a line 217, and a part of the condensate (mainly acetaldehyde and methyl iodide) is refluxed to the top of the distillation column 97 (line 219), and the remainder is either discarded or supplied to a distillation column 98 (extractive distillation column) via a line 220.

[0158] Water is introduced from near the top of the distillation column 98 through line 222 and subjected to extractive distillation, and separated into a fourth overhead stream (line 223) rich in acetaldehyde and a bottom stream (line 224) rich in water (extractive distillation step (D2)). The fourth overhead stream is introduced through line 223 to the condenser 98a for condensation, and a part of the condensate (mainly methyl iodide) is refluxed to the top of the column (line 225), and the rest is recycled to the reaction system through line 226, but may be removed from the system. The organic phase (methyl iodide phase) in the decanter 95 is preferably recycled in its entirety through lines 209 and 210 to the distillation column 94 below the position of the chimney tray 200. A part of the aqueous phase of the decanter 95 and the organic phase of the decanter 96 are recycled to the distillation column 94 through lines 213, 210 and lines 214, 210, respectively, but may not be recycled. A portion of the aqueous phase from decanter 95 may be used as the extraction solvent (water) in distillation column 94. A portion of the aqueous phase from decanter 96 may be recycled to distillation column 94 via line 210. Alternatively, the methyl iodide-rich stream may be refluxed to distillation column 98.

[0159] In some cases (for example, when the feed stream contains methyl acetate), a miscible solvent (acetic acid, ethyl acetate, etc.) for the components (for example, methyl acetate, etc.) constituting the organic phase obtained by separating the condensate of the steam extracted from the top of distillation column 3 can be charged into distillation column 94 through line 215 to improve distillation efficiency. The miscible solvent is supplied to distillation column 94 at a position above the feed stream supply section (connection section of line 201) and below the connection section of recycle line 210. The bottoms of distillation column 94 are recycled to the reaction system.

[0160] The vapor from the top of distillation column 94 is conducted through line 203 to condenser 94a where it is condensed, and the condensate is separated in decanter 99. The organic phase is returned to the top of distillation column 94 through line 206, and the aqueous phase is conducted through line 207 to decanter 95.

[0161] The bottoms of distillation column 97 (mainly water) and distillation column 98 (extractive distillation column) (water containing a small amount of acetaldehyde) are removed from the system via lines 218 and 224, respectively, or recycled to the reaction system. Gases (lines 211, 221, 227) that are not condensed in condensers 94a, 97a, 98a are absorbed in scrubber system 8 or disposed of.

[0162] Figure 7 is a schematic flow diagram showing another example of the acetaldehyde separation and removal system using the extractive distillation described above. In this example, the vapor condensate at the top of the distillation column 94 is led to a hold tank 100, and the entire amount is refluxed to the top of the distillation column 94 through a line 206. The rest is the same as the example in Figure 6.

[0163] Figure 8 is a schematic flow diagram showing yet another example of the acetaldehyde separation and removal system using the extractive distillation described above. In this example, the entire amount of liquid on the chimney tray 200 is extracted, and is introduced directly into the cooling cooler 95a through the line 208 without passing through the decanter 95, where it is cooled and supplied to the decanter 96. The rest of the system is the same as the example in Figure 7.

[0164] One or more of the extract containing methyl iodide and acetaldehyde (line 208), the aqueous phase containing acetaldehyde (line 212), the aqueous phase (line 216), the vapor at the top of distillation column 97 (line 217), the condensate thereof (lines 219, 220), the fourth overhead stream (line 223), and the condensate obtained by condensing the fourth overhead stream (lines 225, 226) are acetaldehyde-rich streams and may be subjected to the membrane separation step as the process stream. Among them, since methyl iodide has been sufficiently separated and recovered and further separation of methyl iodide is difficult by conventional purification using a distillation column, the process stream to be subjected to the membrane separation step is preferably one or more streams selected from the group consisting of the third overhead stream, the condensate obtained by condensing the third overhead stream, the fourth overhead stream, and the condensate obtained by condensing the fourth overhead stream, and from the viewpoint that the silica membrane has a better recovery rate of methyl iodide and the number of facilities can be relatively reduced, the third overhead stream (particularly, line 217) and / or the fourth overhead stream (particularly, line 223) which are gases are preferred. In addition, from the viewpoint that the partial pressure difference between acetaldehyde and methyl iodide can be increased and a sweep gas is not required, the condensate obtained by condensing the third overhead stream which is a liquid (particularly, line 220) and / or the condensate obtained by condensing the fourth overhead stream (particularly, line 226) are preferred. Line 217 is the overhead stream of the distillation step (C2), line 223 is the overhead stream of the extractive distillation step (D2), line 220 is the distillate stream from the distillation step (C2), and line 226 is the distillate stream from the extractive distillation step (D2).

[0165] 4, the gas components generated in the condenser 3a include, for example, carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid, and are supplied from the condenser 3a to the scrubber system 8 through lines 32 and 15. The methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid in the gas components that reach the scrubber system 8 are absorbed by an absorbing liquid in the scrubber system 8.

[0166] The bottoms withdrawn from the bottom of the distillation column 3 contain a larger amount of components (high boiling point components) with a boiling point higher than that of acetic acid compared to the overhead stream and the side stream from the distillation column 3, such as propionic acid, and the above-mentioned entrained catalyst and co-catalyst. This bottoms also contains acetic acid, methyl iodide, methyl acetate, water, and the like. In this embodiment, a part of such bottoms is continuously introduced into the evaporation tank 2 through lines 25 and 26 for recycling, and another part of the bottoms is continuously introduced into the reaction tank 1 through lines 25 and 23 for recycling.

[0167] The first acetic acid stream continuously extracted as a side stream from the distillation column 3 is enriched in acetic acid compared to the vapor stream continuously introduced into the distillation column 3. That is, the acetic acid concentration of the first acetic acid stream is higher than that of the vapor stream. The acetic acid concentration of the first acetic acid stream is, for example, 90 to 99.9% by mass, preferably 93 to 99% by mass. In addition to acetic acid, the first acetic acid stream contains, for example, methyl iodide, hydrogen iodide, water, methyl acetate, dimethyl ether, methanol, acetaldehyde, formic acid, and propionic acid, as well as alkyl iodides such as ethyl iodide, propyl iodide, butyl iodide, hexyl iodide, and decyl iodide.

[0168] The connection position of the line 27 to the distillation tower 3 may be above the connection position of the line 21 to the distillation tower 3 in the height direction of the distillation tower 3 as shown in the figure, but may be below the connection position of the line 21 to the distillation tower 3, or may be the same as the connection position of the line 21 to the distillation tower 3. The first acetic acid stream from the distillation tower 3 is continuously introduced into the next distillation tower 5 through the line 27. The first acetic acid stream withdrawn as a side stream of the distillation tower 3, the bottom liquid of the distillation tower 3, or the condensate of the steam at the bottom of the distillation tower 3 can also be continuously introduced into the distillation tower 6 described later without passing through the distillation tower 5 (dehydration step).

[0169] Potassium hydroxide can be supplied or added to the first acetic acid stream flowing through line 27 via line 55 (potassium hydroxide introduction line). Potassium hydroxide can be supplied or added as a solution such as an aqueous solution. Supplying or adding potassium hydroxide to the first acetic acid stream can reduce hydrogen iodide in the first acetic acid stream. Specifically, hydrogen iodide reacts with potassium hydroxide to produce potassium iodide and water. This can reduce corrosion of equipment such as a distillation column caused by hydrogen iodide. Potassium hydroxide can be supplied or added to an appropriate location where hydrogen iodide is present in this process. Potassium hydroxide added during the process also reacts with acetic acid to produce potassium acetate.

[0170] The distillation column 5 is a unit for performing the second distillation step, and in this embodiment, is positioned as a so-called dehydration column. The second distillation step is a step for further purifying acetic acid by distilling the first acetic acid stream continuously introduced into the distillation column 5.

[0171] The distillation column 5 is, for example, a rectification column such as a plate column or a packed column. When a plate column is used as the distillation column 5, the theoretical plate is, for example, 5 to 50 plates, and the reflux ratio is, for example, 0.2 to 3000 depending on the number of theoretical plates. Inside the distillation column 5 in the second distillation step, the column top pressure is set to, for example, 150 to 250 kPaG, and the column bottom pressure is set to, for example, 160 to 290 kPaG, higher than the column top pressure. Inside the distillation column 5 in the second distillation step, the column top temperature is set to, for example, 130 to 160°C, which is a temperature higher than the boiling point of water at the set column top pressure and lower than the boiling point of acetic acid, and the column bottom temperature is set to, for example, 150 to 175°C, which is a temperature higher than the boiling point of acetic acid at the set column bottom pressure.

[0172] From the top of the distillation column 5, vapor is continuously withdrawn as an overhead stream into a line 33. From the bottom of the distillation column 5, a bottoms liquid is continuously withdrawn into a line 34. 5b is a reboiler. A side stream (liquid or gas) may be continuously withdrawn into a line 34 from a height position between the top and bottom of the distillation column 5.

[0173] The vapor withdrawn from the top of distillation column 5 contains a larger amount of components having a boiling point lower than that of acetic acid (low boiling point components) compared to the bottoms from distillation column 5, such as methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid. Such vapor is continuously introduced into condenser 5a through line 33.

[0174] The condenser 5a separates the vapor from the distillation column 5 into a condensed fraction and a gas fraction by cooling and partially condensing it. The condensed fraction includes, for example, water and acetic acid. A part of the condensed fraction is continuously refluxed from the condenser 5a to the distillation column 5 through a line 35. Another part of the condensed fraction is continuously introduced from the condenser 5a through lines 35, 36, and 23 into the reaction vessel 1 and recycled. The gas fraction generated in the condenser 5a includes, for example, carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid, and is supplied from the condenser 5a to the scrubber system 8 through lines 37 and 15. As described above, the gas fraction from the condenser 5a may be supplied to the scrubber system 8 without merging with the line 15. The hydrogen iodide in the gas fraction that reaches the scrubber system 8 is absorbed into an absorbing liquid in the scrubber system 8, and methyl iodide is produced by a reaction between the hydrogen iodide in the absorbing liquid and methanol or methyl acetate. The liquid fraction containing useful components such as methyl iodide is then recycled from the scrubber system 8 through recycle lines 48 and 23 to the reaction tank 1 for reuse.

[0175] The bottoms (or side stream) withdrawn from the bottom of the distillation column 5 contains, in addition to acetic acid, a larger amount of components (high boiling point components) having a higher boiling point than acetic acid compared to the overhead stream from the distillation column 5, such as acetic anhydride, propionic acid, acetates, and iodide salts such as potassium iodide and metal iodide salts derived from corrosion metals, as well as the above-mentioned catalysts and co-catalysts entrained in the water. Examples of the acetates include metal acetates such as potassium acetate formed when an alkali such as potassium hydroxide is supplied to the line 27 or the like. Examples of the acetates include metal acetates formed from acetic acid and corrosion metals such as metals liberated from the inner walls of the components of the acetic acid production apparatus. Examples of the iodide salts include potassium iodide formed when an alkali such as potassium hydroxide is supplied to the line 27 or the like. This bottoms may also contain acetic acid. Such bottoms are continuously introduced into the next distillation column 6 as a second acetic acid stream through the line 34. The bottoms (or side stream) withdrawn from the bottom of the distillation column 5 also contains the above-mentioned corroded metals, etc., and compounds (iodide salts) of iodine derived from corrosive iodine and the corroded metals, etc.

[0176] The second acetic acid stream is enriched in acetic acid compared to the first acetic acid stream continuously introduced into the distillation tower 5. That is, the acetic acid concentration of the second acetic acid stream is higher than that of the first acetic acid stream. The acetic acid concentration of the second acetic acid stream is, for example, 99.1 to 99.99 mass%, as long as it is higher than that of the first acetic acid stream. In addition, as described above, the second acetic acid stream may contain, in addition to acetic acid, for example, propionic acid, hydrogen iodide, etc. In this embodiment, when a side stream is extracted, the extraction position of the side stream from the distillation tower 5 is lower in the height direction of the distillation tower 5 than the introduction position of the first acetic acid stream into the distillation tower 5.

[0177] Potassium hydroxide can be supplied or added to the second acetic acid stream flowing through line 34 via line 56 (potassium hydroxide introduction line). Potassium hydroxide can be supplied or added as a solution, such as an aqueous solution. Supplying or adding potassium hydroxide to the second acetic acid stream can reduce hydrogen iodide in the second acetic acid stream. Specifically, hydrogen iodide reacts with potassium hydroxide to produce potassium iodide and water. This can reduce corrosion of equipment, such as a distillation column, caused by hydrogen iodide.

[0178] The distillation column 6 is a unit for performing the third distillation step, and in this embodiment, it is positioned as a so-called high boiler removal column. The third distillation step is a step for purifying the second acetic acid stream continuously introduced into the distillation column 6 to further purify acetic acid.

[0179] The distillation column 6 is, for example, a rectification column such as a plate column or a packed column. When a plate column is used as the distillation column 6, the theoretical plate is, for example, 5 to 50 plates, and the reflux ratio is, for example, 0.2 to 3000 depending on the number of theoretical plates. Inside the distillation column 6 in the third distillation step, the column top pressure is set to, for example, -100 to 150 kPaG, and the column bottom pressure is set to, for example, -90 to 180 kPaG, higher than the column top pressure. Inside the distillation column 6 in the third distillation step, the column top temperature is set to, for example, 50 to 150°C, which is higher than the boiling point of water at the set column top pressure and lower than the boiling point of acetic acid, and the column bottom temperature is set to, for example, 70 to 160°C, which is higher than the boiling point of acetic acid at the set column bottom pressure.

[0180] From the top of the distillation column 6, vapor is continuously withdrawn as an overhead stream into line 38. From the bottom of the distillation column 6, bottoms are continuously withdrawn into line 39. 6b is a reboiler. From a height position between the top and bottom of the distillation column 6, a side stream (liquid or gas) is continuously withdrawn into line 46. In the height direction of the distillation column 6, the connection position of line 46 to the distillation column 6 may be above the connection position of line 34 to the distillation column 6 as shown in the figure, but it may also be below the connection position of line 34 to the distillation column 6 or may be the same as the connection position of line 34 to the distillation column 6.

[0181] The vapor withdrawn from the top of the distillation column 6 contains a larger amount of components having a boiling point lower than that of acetic acid (low boiling point components) compared to the bottoms from the distillation column 6, and contains, in addition to acetic acid, for example, methyl iodide, hydrogen iodide, water, methyl acetate, dimethyl ether, methanol, formic acid, etc. Such vapor is continuously introduced into the condenser 6a through a line 38.

[0182] Condenser 6a separates the vapor from distillation column 6 into a condensate and a gaseous fraction by cooling and partially condensing it. The condensate includes, in addition to acetic acid, for example, methyl iodide, hydrogen iodide, water, methyl acetate, dimethyl ether, methanol, and formic acid. At least a portion of the condensate is continuously refluxed from condenser 6a through line 40 to distillation column 6. A portion of the condensate (distillate) can be recycled from condenser 6a through lines 40, 41, and 42 to the first acetic acid stream in line 27 before being introduced into distillation column 5. Additionally or alternatively, a portion of the condensate (distillate) can be recycled from condenser 6a through lines 40, 41, and 43 to the vapor stream in line 21 before being introduced into distillation column 3.

[0183] Also, a part of the condensed fraction (distillate) may be recycled to the reaction tank 1 from the condenser 6a through lines 40, 44, and 23. Furthermore, a part of the distillate from the condenser 6a may be supplied to the scrubber system 8 and used as an absorption liquid in the system. In the scrubber system 8, the gas fraction after absorbing the useful fraction is discharged outside the apparatus, and the liquid fraction containing the useful components is introduced or recycled from the scrubber system 8 through recycle lines 48 and 23 to the reaction tank 1 for reuse. In addition, a part of the distillate from the condenser 6a may be led through lines not shown to various pumps (not shown) operating in the apparatus and used as a seal liquid for the pumps. Furthermore, a part of the distillate from the condenser 6a may be steadily withdrawn outside the apparatus through a withdrawal line attached to the line 40, or may be non-steadily withdrawn outside the apparatus when necessary.

[0184] When a part of the condensed fraction (distillate fraction) is removed from the distillation process system in the distillation column 6, the amount of the distillate fraction (distillate amount) is, for example, 0.01 to 30 mass% of the condensate generated in the condenser 6a, preferably 0.1 to 10 mass%, more preferably 0.3 to 5 mass%, more preferably 0.5 to 3 mass%. On the other hand, the gas fraction generated in the condenser 6a includes, for example, carbon monoxide, hydrogen, methane, carbon dioxide, nitrogen, oxygen, methyl iodide, hydrogen iodide, water, methyl acetate, acetic acid, dimethyl ether, methanol, acetaldehyde, and formic acid, and is supplied from the condenser 6a to the scrubber system 8 through lines 45 and 15. As described above, the gas fraction from the condenser 6a may be supplied to the scrubber system 8 without merging with the line 15.

[0185] The bottoms withdrawn from the bottom of the distillation column 6 through the line 39 contain a larger amount of components (high boiling point components) with a higher boiling point than acetic acid compared to the overhead stream from the distillation column 6, and include, for example, acetate, acetic anhydride, propionic acid, etc. Examples of the acetate include potassium acetate formed when an alkali such as potassium hydroxide is supplied to the line 34, etc. Examples of the acetate include metal acetate salts formed of acetic acid and corrosion metals, such as metals generated and liberated on the inner walls of the components of the acetic acid production apparatus. The bottoms withdrawn from the bottom of the distillation column 6 through the line 39 further contain the corrosion metals, etc., and compounds of iodine derived from corrosive iodine and the corrosion metals, etc. In this embodiment, such bottoms are discharged to the outside of the acetic acid production apparatus.

[0186] The side stream continuously extracted from the distillation tower 6 to the line 46 is continuously introduced into the next ion exchange resin tower 7 as the third acetic acid stream. This third acetic acid stream is enriched in acetic acid more than the second acetic acid stream continuously introduced into the distillation tower 6. That is, the acetic acid concentration of the third acetic acid stream is higher than that of the second acetic acid stream. The acetic acid concentration of the third acetic acid stream is, for example, 99.8 to 99.999 mass% as long as it is higher than that of the second acetic acid stream. In this embodiment, the extraction position of the side stream from the distillation tower 6 is higher in the height direction of the distillation tower 6 than the introduction position of the second acetic acid stream into the distillation tower 6. In another embodiment, the extraction position of the side stream from the distillation tower 6 is the same as or lower in the height direction of the distillation tower 6 than the introduction position of the second acetic acid stream into the distillation tower 6. The distillation tower 6 can be replaced by a simple still (evaporator), and if the impurities are sufficiently removed in the distillation tower 5, the distillation tower 6 can be omitted.

[0187] The ion exchange resin tower 7 is a purification unit for carrying out an adsorption removal step, which is a step for further purifying acetic acid by adsorbing and removing mainly alkyl iodides (e.g., ethyl iodide, propyl iodide, butyl iodide, hexyl iodide, decyl iodide, etc.) contained in trace amounts in the third acetic acid stream continuously introduced into the ion exchange resin tower 7.

[0188] In the ion exchange resin tower 7, an ion exchange resin having an adsorption ability for alkyl iodide is packed in the tower to form an ion exchange resin bed. Examples of such ion exchange resins include cation exchange resins in which some of the detachable protons in exchange groups such as sulfonic acid groups, carboxyl groups, and phosphonic acid groups are replaced with metals such as silver and copper. In the adsorption and removal step, for example, the third acetic acid stream (liquid) flows through the inside of the ion exchange resin tower 7 packed with such an ion exchange resin, and during the flow process, impurities such as alkyl iodide in the third acetic acid stream are adsorbed by the ion exchange resin and removed from the third acetic acid stream. In the ion exchange resin tower 7 in the adsorption and removal step, the internal temperature is, for example, 18 to 100°C, and the flow rate of the acetic acid stream [per m of resin volume] is 18 to 100°C. 3 Acetic acid processing capacity per unit (m 3 / h)] is, for example, 3 to 15 m 3 / h·m 3 (resin volume).

[0189] A fourth acetic acid stream is continuously discharged from the lower end of the ion exchange resin tower 7 to a line 47. The fourth acetic acid stream has a higher acetic acid concentration than the third acetic acid stream. That is, the fourth acetic acid stream is enriched in acetic acid than the third acetic acid stream continuously introduced into the ion exchange resin tower 7. The acetic acid concentration of the fourth acetic acid stream is, for example, 99.9 to 99.999 mass% or more, as long as it is higher than the acetic acid concentration of the third acetic acid stream. In this production method, this fourth acetic acid stream can be stored in a product tank not shown in the figure.

[0190] In this acetic acid production apparatus, a so-called product column or finishing column, which is a distillation column, may be provided as a purification unit for further purifying the fourth acetic acid stream from the ion exchange resin column 7. When such a product column is provided, the product column is, for example, a rectification column such as a plate column or a packed column. When a plate column is used as the product column, its theoretical plates are, for example, 5 to 50 plates, and the reflux ratio is, for example, 0.5 to 3000 depending on the number of theoretical plates. Inside the product column in the purification process, the column top pressure is set to, for example, -195 to 150 kPaG, and the column bottom pressure is set to, for example, -190 to 180 kPaG, higher than the column top pressure. Inside the product column, the column top temperature is set to, for example, 50 to 150 °C, which is higher than the boiling point of water at the set column top pressure and lower than the boiling point of acetic acid, and the column bottom temperature is set to, for example, 70 to 160 °C, which is higher than the boiling point of acetic acid at the set column bottom pressure. In addition, the product tower or finishing tower can be substituted with a simple still (evaporator).

[0191] When a product column is provided, all or a part of the fourth acetic acid stream (liquid) from the ion exchange resin column 7 is continuously introduced into the product column. From the top of such a product column, vapor containing trace amounts of low boiling point components (e.g., methyl iodide, water, methyl acetate, dimethyl ether, crotonaldehyde, acetaldehyde, formic acid, etc.) is continuously withdrawn as an overhead stream. This vapor is separated into a condensed portion and a gas portion in a condenser.

[0192] A portion of the condensate is continuously refluxed to the product column, and another portion of the condensate may be recycled to the reaction tank 1 or discharged to the outside of the system, or both, and the gaseous portion is supplied to a scrubber system 8. A bottom product containing a trace amount of high boiling point components is continuously withdrawn from the bottom of the product column, and this bottom product is recycled, for example, to the second acetic acid stream in line 34 before being introduced into the distillation column 6. A side stream (liquid) is continuously withdrawn as a fifth acetic acid stream from a height position between the top and bottom of the product column. The withdrawal position of the side stream from the product column is, for example, lower than the introduction position of the fourth acetic acid stream into the product column in the height direction of the product column.

[0193] The fifth acetic acid stream is enriched in acetic acid compared to the fourth acetic acid stream continuously introduced into the product column. That is, the fifth acetic acid stream has a higher acetic acid concentration than the fourth acetic acid stream. The fifth acetic acid stream has an acetic acid concentration of, for example, 99.9 to 99.999 mass% or more, as long as it is higher than the acetic acid concentration of the fourth acetic acid stream. This fifth acetic acid stream is stored, for example, in a product tank not shown. Note that the ion exchange resin tower 7 may be installed downstream of the product column instead of (or in addition to) being installed downstream of the distillation tower 6, and the acetic acid stream discharged from the product column may be treated.

[0194] Each aspect disclosed in this specification can be combined with any other feature disclosed in this specification. Each configuration in each embodiment and their combinations are merely examples, and addition, omission, substitution, and other modifications of the configuration are possible as appropriate within the scope of the present disclosure. In addition, each invention according to this disclosure is not limited by the embodiments or the following examples, but is limited only by the claims. EXAMPLES

[0195] Hereinafter, one embodiment of the present disclosure will be described in more detail based on examples.

[0196] Example 1 The separation performance of acetaldehyde and methyl iodide for each of the silica membranes shown in Table 1 was evaluated by pervaporation under heated and pressurized conditions. 2A separation membrane of 1000 g was installed. A mixture (approximately 600 g) of 79.8% by mass of acetaldehyde, 7.8% by mass of methyl iodide, and 12.4% by mass of water was introduced into the device as the feed liquid components. The mixture was heated to 40°C or 55°C (40°C for membrane types No. 1-2, and 55°C for membrane types No. 3-6) in a water bath at 75 ml / min using a liquid delivery pump, and supplied to the separation membrane while maintaining the pressure on the feed liquid side at 190 kPaG. The pressure on the permeation side was normal pressure, and the mixture was allowed to permeate for 4-5 hours, after which it was collected in a cold trap. The permeability [mol / m 2 ·s·Pa] and the transmittance ratio were calculated. The transmittance ratio was calculated as [permeability of acetaldehyde / permeability of methyl iodide]. The results are shown in Table 1. In addition, "AD" in Tables 1 to 3 indicates acetaldehyde, and "MeI" indicates methyl iodide.

[0197] The central pore diameter and water contact angle of each of the silica films shown in Table 1 were determined as follows.

[0198] (1) Central pore diameter For membrane types No. 1 and 2, the permeability was calculated by single-component gas permeation tests. 2 , CO 2 , N 2 , C.H. 4 , C.F. 4 , and S.F. 6 Gas permeability [mol / m 2 s Pa], and the dynamic molecular diameter of various single component gases is measured using H 2 :2.9Å, CO 2 :3.3Å, N 2 :3.6Å, CH 4 : 3.8 Å, CF 4 : 5.0Å, SF 6 The central pore diameters of membrane types No. 1 to No. 2 were identified to be within the ranges shown in Table 1. For membrane types No. 3 to No. 6, calculations were performed by the nanoperm porometry method.

[0199] (2) Water contact angle A contact angle meter "Drop Master 700" (manufactured by Kyowa Interface Science Co., Ltd.) was used to measure the water contact angle by dropping a drop of water onto the film surface and measuring the angle at that time.

[0200] [Table 1]

[0201] As can be seen from Table 1, when a silica membrane having a central pore size of 0.3 to 2.0 nm and a water contact angle of 0 to 70° was used as a separation membrane (membrane types No. 1 to 6), the permeability of methyl iodide was low and the permeability ratio was high. In particular, when a silica membrane having a central pore size of more than 0.5 nm and 1.0 nm or less was used (membrane types No. 3 to 5), the permeability ratio was high. This shows that a silica membrane having a central pore size of 0.3 to 2.0 nm and a water contact angle of 0 to 70° can efficiently separate acetaldehyde and methyl iodide.

[0202] Comparative Example 1 An experiment was carried out using the acetaldehyde separation and removal system shown in FIG. 5. A mixed liquid containing acetaldehyde, methyl iodide, and water was supplied to a distillation tower 91 (plate tower, actual number of plates: 80, tower top pressure: 0.18 MPaG, tower top temperature: 52°C) through a line 101, and distillation was carried out. An overhead stream (line 102) separated by distillation in the distillation tower 91 was condensed in a condenser 91a, a part of the condensate was refluxed to the top of the distillation tower 91 (line 104), and the remaining part of the condensate was supplied to an extraction tower 92 (packed tower, tower top pressure: 0.25 MPaG, tower top temperature: 40°C) (line 105). The condensate supplied to the extraction tower 92 was subjected to an extraction treatment while introducing water as an extraction solvent from a line 109, and the extract separated by the extraction treatment was extracted from a line 107, and the lower phase was extracted from a line 108. The lower phase was not recycled through a line 110. The above extract was supplied to a distillation column 93 (packed column, column top pressure: 0.17 MPaG, column top temperature: 85°C) through a line 107, where it was distilled and separated into an overhead stream (line 112) and a bottoms stream (line 113). The overhead stream through the line 112 was condensed in a condenser 93a, and a part of the condensate was refluxed to the top of the distillation column 93 (line 114), while the remaining part of the condensate was distilled through a line 115. The reflux ratio of the distillation column 91 was 182.2, and the reflux ratio of the distillation column 93 was 5.0. The flow rates and concentrations of various components at various points in the steady state during the experiment of Comparative Example 1 are shown in Table 2.

[0203] [Table 2]

[0204] Example 2 The distillate (line 115) obtained in the same manner as in Comparative Example 1 was used as the process liquid, and an experiment was performed using the membrane separation process shown in FIG. 3. The process gas was supplied to the membrane module 58 through the line 60, and membrane separation was performed under an environment of a temperature of 80° C., a pressure of 800 kPaG, and a sweep gas flow rate of 16.6 L / min. As the separation membrane, a silica membrane of the membrane type No. 6 used in Example 1 was provided on a ceramic porous substrate. By membrane separation, the permeate gas (line 84) and the concentrated liquid (line 82) were separated. The concentrated liquid was cooled by the cooler 82b, and recycled from the line 118 to the extraction tower 92 shown in FIG. 5 through the line 83. The flow rates and concentrations of various components at each point in the steady state during the experiment of Example 2 are shown in Table 3.

[0205] [Table 3]

[0206] According to Table 2, it can be seen that in Comparative Example 1, which does not have the membrane separation step, methyl iodide is discharged at a flow rate of 2.1 kg / h by distillation from line 115. On the other hand, according to Table 3, it can be seen that in Example 2, which has the membrane separation step, of the 2.1 kg / h of methyl iodide that would normally be discharged, only 0.2 kg / h is discharged from the permeation gas line 84, and a flow rate of 1.9 kg / h can be recovered. The recovery rate is 90.5%, and the concentration of methyl iodide in the discarded permeation gas (line 84) is reduced to 1.0 mass%. Therefore, according to the method for producing acetic acid of the present disclosure, the loss of methyl iodide is significantly reduced. In addition, the need to newly produce and add methyl iodide is reduced, and the work load and cost are also reduced.

[0207] In Tables 2 and 3, the concentration of each component is listed so that the total of acetaldehyde, methyl iodide, and water is 100.0 mass%. However, in an actual acetic acid production process, each process liquid in an acetaldehyde separation and removal system may only contain acetaldehyde, methyl iodide, and water as the main components, and may also contain trace components such as methanol, dimethyl ether, alkanes, and crotonaldehyde, which deteriorate the potassium permanganate test value (permanganate time), and organic iodine compounds.

[0208] Example 3 A commercially available nano-ceramic porous substrate (model number: eSep-nano-Si-1.5-2, manufactured by E-Sep Co., Ltd.) was used as a porous layer mainly composed of silica having through-holes of 1.5 to 2.0 nm. The thickness of the silica porous layer was about 500 nm, and the lower layer had a structure supported by an α-alumina porous substrate formed of macropores. The outer shape was a cylinder with a diameter of 12 mm, an inner diameter of 9 mm, and a length of 40 cm. Air with a humidity of 50% or more at 25°C was introduced to the outside of the cylindrical membrane for more than one minute. At that time, the inside of the cylindrical membrane was depressurized, and water vapor was forcibly introduced into the inside of the membrane pores. The membrane was held in a state in which water vapor remained in the pores for more than 6 hours to promote the generation of OH groups in the silica layer. The silane coupling agent 3-aminopropyltrimethoxysilane (APTMS) was diluted 100 times with water and left to stand at room temperature for more than 3 hours to allow the hydrolysis reaction to proceed sufficiently. The liquid was then soaked into a cloth, which was then used to apply the cylindrical composite membrane prepared above. The mixture was then dried at 110°C for 0.5 hours to fix the mixture and obtain a silica membrane.

[0209] Example 4 The silica membrane obtained in Example 3 was circulated with 99.5 wt % MeI (manufactured by Wako Pure Chemical Industries, Ltd.) at room temperature for 7 hours, and then left to stand for 16 hours to obtain a silica membrane.

[0210] The separation performance of acetaldehyde and methyl iodide for each of the silica membranes shown in Table 4 was evaluated by pervaporation under heated and pressurized conditions. 2 A separation membrane of 79.77% by mass was installed. Approximately 600 g of a mixture of 79.77% by mass of acetaldehyde, 7.78% by mass of methyl iodide, and 12.45% by mass of water was charged into the device as the feed liquid components. The mixture was heated to 50°C in a water bath, and while the pressure on the feed liquid side was maintained at 190 kPaG, it was fed to the separation membrane at 75 ml / min using a liquid delivery pump. The pressure on the permeation side was kept at normal pressure, and the mixture was allowed to permeate for 4 to 5 hours, after which it was captured in a cold trap and the permeated liquid was collected. The permeability of acetaldehyde [mol / m 2 The permeability ratio was calculated as [permeability of acetaldehyde / permeability of methyl iodide]. The results are shown in Table 4. In addition, "AD" in Table 4 indicates acetaldehyde. The same experiment was repeated six times for the silica membrane of Example 3 and three times for the silica membrane of Example 4, and evaluation was performed.

[0211] The water contact angles of the silica films of Examples 3 and 4 were measured by dropping one drop of water onto the film surface using a contact angle meter "Drop Master 700" (manufactured by Kyowa Interface Science Co., Ltd.). For the silica film of Example 3, the water contact angle was measured before the experiment and after six experiments, and for the silica film of Example 4, the water contact angle was measured after three experiments. The results are shown in Table 5.

[0212] [Table 4]

[0213] [Table 5]

[0214] As can be seen from Table 4, when a silica membrane having a median pore size of 0.5-5.0 nm and having an amino group was used as a separation membrane (Examples 3 and 4), the permeability of acetaldehyde was high, and the permeability ratio did not change significantly even after repeated experiments. This shows that a silica membrane having a median pore size of 0.5-5.0 nm and having an amino group can efficiently separate a mixed solution containing components with different molecular sizes and / or physical properties, i.e., acetaldehyde and methyl iodide, and its performance (separation ability) does not decrease even when used repeatedly.

[0215] Example 5 A commercially available nano-ceramic porous substrate (model number: eSep-nano-Si-1-2, manufactured by E-Sep Co., Ltd.) was used as a porous layer mainly composed of silica having through-holes of 1.0 to 2.0 nm. The thickness of the silica porous layer was about 500 nm, and the lower layer had a structure supported by an α-alumina porous substrate formed of macropores. The outer shape was a cylinder with a diameter of 12 mm, an inner diameter of 9 mm, and a length of 40 cm. Air with a humidity of 50% or more at 25°C was introduced to the outside of the cylindrical membrane for more than one minute. At that time, the inside of the cylindrical membrane was depressurized, and water vapor was forcibly introduced into the inside of the membrane pores. The membrane was held in a state in which water vapor remained in the membrane pores for more than 6 hours to promote the generation of OH groups in the silica layer. LAMBIC-771W (manufactured by Osaka Organic Chemical Industry Co., Ltd.), a hydrophilic polymer, was diluted 5 times with water, applied to the cylindrical composite membrane prepared above, dried at 80°C for 5 minutes, and washed with water to obtain a silica membrane.

[0216] Example 6 A silica membrane was obtained in the same manner as in Example 5, except that a commercially available nanoceramic porous substrate (model number: eSep-nano-Si-1.5-2.5, manufactured by E-Sep Corporation) was used as a porous layer mainly composed of silica having through pores of 1.5 to 2.5 nm.

[0217] In the same manner as in Example 4, the separation performance of the silica membranes of Examples 5 and 6 for acetaldehyde and methyl iodide was evaluated by pervaporation under conditions of heating and pressure. The results are shown in Table 6. In addition, "AD" in Table 6 indicates acetaldehyde. The same experiment was repeated four times for the silica membrane of Example 5 and three times for the silica membrane of Example 6.

[0218] The water contact angles of the silica films of Examples 5 and 6 were measured by dropping one drop of water onto the film surface using a contact angle meter "Drop Master 700" (Kyowa Interface Science Co., Ltd.). For the silica film of Example 5, the water contact angle of the silica film before the experiment and after four experiments were measured, and for the silica film of Example 6, the water contact angle of the silica film before the experiment and after three experiments were measured. The results are shown in Table 7.

[0219] [Table 6]

[0220] [Table 7]

[0221] As can be seen from Table 6, when a silica membrane having a median pore size of 0.5 to 5.0 nm and a structure derived from a hydrophilic polymer was used as a separation membrane (Examples 5 and 6), the permeability of acetaldehyde was high, and the permeability ratio did not change significantly even after multiple experiments.

[0222] Variations of the invention according to the present disclosure are described below. [Appendix 1] A process for producing acetic acid, comprising a membrane separation step of separating and obtaining an acetaldehyde-rich stream and a methyl iodide-rich stream from a process stream containing acetaldehyde and methyl iodide using a silica membrane having a median pore size of 0.3 to 2.0 nm and a surface water contact angle of 0 to 70°. [Appendix 2] The method for producing acetic acid according to Appendix 1, wherein the acetaldehyde concentration in the process stream is 0.1% by mass or more (1% by mass or more, 2% by mass or more, 5% by mass or more, 10% by mass or more, 20% by mass or more, 30% by mass or more, 40% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, 75% by mass or more, or 80% by mass or more). [Appendix 3] The method for producing acetic acid according to appendix 1 or 2, wherein the acetaldehyde concentration in the process stream is 99.9% by mass or less (99% by mass or less, 95% by mass or less, 90% by mass or less, 85% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, 10% by mass or less, or 5% by mass or less). [Appendix 4] The method for producing acetic acid according to any one of Appendices 1 to 3, wherein the concentration of methyl iodide in the process stream is 0.01% by mass or more (0.1% by mass or more, 0.5% by mass or more, 1% by mass or more, 2% by mass or more, 3% by mass or more, 5% by mass or more, 6% by mass or more, 7% by mass or more, 10% by mass or more, 12% by mass or more, 15% by mass or more, 20% by mass or more, or 30% by mass or more). [Appendix 5] The method for producing acetic acid according to any one of Appendices 1 to 4, wherein the concentration of methyl iodide in the process stream is 99% by mass or less (95% by mass or less, 90% by mass or less, 80% by mass or less, 70% by mass or less, 50% by mass or less, 20% by mass or less, 10% by mass or less, 5% by mass or less, 3% by mass or less, 1% by mass or less, or 0.1% by mass or less). [Appendix 6] The method for producing acetic acid according to any one of Appendices 1 to 5, wherein the process stream has an acetaldehyde concentration of 0.1 to 99.9% by mass and a methyl iodide concentration of 0.01 to 99% by mass. [Appendix 7] The method for producing acetic acid according to any one of Appendices 1 to 6, wherein the total concentration of acetaldehyde and methyl iodide in the process stream is 1 mass% or more (10 mass% or more, 20 mass% or more, 30 mass% or more, 40 mass% or more, 50 mass% or more, 60 mass% or more, 70 mass% or more, 80 mass% or more, or 85 mass% or more). [Appendix 8] The method for producing acetic acid according to any one of Appendices 1 to 7, wherein the total concentration of acetaldehyde and methyl iodide in the process stream is 99% by mass or less (95% by mass or less, 90% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less). [Appendix 9] The method for producing acetic acid according to any one of Appendices 1 to 8, wherein the water concentration in the process stream is 0.1% by mass or more (0.5% by mass or more, 1% by mass or more, 2% by mass or more, 5% by mass or more, 7% by mass or more, 10% by mass or more, 20% by mass or more, 30% by mass or more, 40% by mass or more, 50% by mass or more, 60% by mass or more, 70% by mass or more, 80% by mass or more, or 90% by mass or more). [Appendix 10] The method for producing acetic acid according to any one of Appendices 1 to 9, wherein the water concentration in the process stream is 95% by mass or less (90% by mass or less, 80% by mass or less, 70% by mass or less, 50% by mass or less, 20% by mass or less, 15% by mass or less, 10% by mass or less, 5% by mass or less, or 1% by mass or less).

[0223] [Appendix 11] The method for producing acetic acid according to any one of Appendices 1 to 11, wherein the membrane separation step is carried out under conditions in which the temperature of the process liquid is 10 to 180°C (e.g., 20 to 150°C, preferably 30 to 130°C, more preferably 50 to 110°C, and further preferably 70 to 90°C). [Appendix 12] The method for producing acetic acid according to any one of Appendices 1 to 11, wherein the membrane separation step is carried out under pressurized conditions (for example, an absolute pressure of 110 to 2000 kPa, preferably 200 to 1800 kPa, more preferably 300 to 1500 kPa, further preferably 400 to 1200 kPa, and particularly preferably 600 to 1000 kPa). [Appendix 13] A carbonylation reaction step of reacting methanol with carbon monoxide to produce acetic acid in the presence of a catalyst system containing a metal catalyst and methyl iodide (preferably a catalyst system containing a metal catalyst and methyl iodide, as well as acetic acid, methyl acetate, and water); A separation step of subjecting the reaction mixture obtained in the carbonylation reaction step to one or more evaporation and / or distillation processes to separate the reaction mixture into a stream containing a metal catalyst, an acetic acid stream rich in acetic acid, and a stream richer in lower boiling components than the acetic acid stream; and an acetaldehyde separation and removal system for separating acetaldehyde from at least a portion of a condensate obtained by condensing the stream rich in low boiling components by one or more distillation processes; 13. The method for producing acetic acid according to any one of claims 1 to 12, wherein an acetaldehyde-rich stream separated in the acetaldehyde separation and removal system is subjected to the membrane separation step as the process stream. [Appendix 14] The method for producing acetic acid according to Appendix 13, wherein the separation step includes an evaporation step of separating the reaction mixture obtained in the carbonylation reaction step into at least a vapor stream and a residual liquid stream by evaporation. [Appendix 15] The method for producing acetic acid according to Appendix 14, wherein the separation step comprises: (i) the separation step; and a low boiling component removal step in which the vapor stream is subjected to distillation to separate and obtain at least an overhead stream rich in low boiling components and a first acetic acid stream rich in acetic acid; or (ii) instead of the evaporation step and the low boiling component removal step, an evaporation and low boiling component removal step in which the reaction mixture obtained in the carbonylation reaction step is separated into at least a stream containing a metal catalyst, an overhead stream rich in low boiling components, and an acetic acid stream rich in acetic acid. [Appendix 16] In the low boiling point removal step, the vapor stream is subjected to distillation to separate it into an overhead stream rich in low boiling components, a side cut stream, and a bottom stream, and the side cut stream and / or the bottom stream are the first acetic acid stream. The method for producing acetic acid according to Appendix 15. [Appendix 17] The separation step includes (iii) the dehydration step and a dehydration step of distilling the first acetic acid stream to separate and obtain at least an overhead stream rich in water and a second acetic acid stream enriched in acetic acid compared to the first acetic acid stream, or (iv) a dehydration step of separating and obtaining an acetic acid stream rich in acetic acid in the dehydration step having the function of the dehydration step instead of the dehydration step and the dehydration step, or (v) a dehydration step of separating and obtaining an acetic acid stream rich in acetic acid in the dehydration step having the function of the dehydration step instead of the evaporative dehydration step and the dehydration step, according to appendix 15 or 16. [Appendix 18] A method for producing acetic acid according to any one of Appendices 13 to 17, further comprising at least one of the following steps (a) to (c): (a) A high boiling component removal step in which the acetic acid-rich acetic acid stream (e.g., the first or second acetic acid stream) obtained in the separation step is distilled to separate it into a bottom stream rich in high boiling components and a third acetic acid stream that is richer in acetic acid than the acetic acid stream before being subjected to distillation. (b) an adsorption removal step of treating the acetic acid-rich acetic acid stream (e.g., the first or second acetic acid stream) or the third acetic acid stream obtained in the separation step with an ion exchange resin to obtain a fourth acetic acid stream; (c) a product step of distilling the acetic acid-rich acetic acid stream (e.g., the first or second acetic acid stream) or the third or fourth acetic acid stream obtained in the separation step to obtain a fifth acetic acid stream that is richer in acetic acid than the acetic acid stream before being subjected to the distillation. [Appendix 19] The method for producing acetic acid according to any one of Appendices 13 to 18, wherein the acetaldehyde separation and removal system includes an extraction step of separating an aqueous phase containing acetaldehyde from at least a portion of the acetaldehyde separated by the one or more distillation treatments by one or more extraction treatments. [Appendix 20] The method for producing acetic acid according to any one of Appendices 13 to 19, wherein an acetaldehyde-rich stream separated in the distillation treatment in the acetaldehyde separation and removal system and / or an aqueous layer containing acetaldehyde separated in an extraction treatment of the acetaldehyde-rich stream are subjected to the membrane separation step as the process stream.

[0224] [Appendix 21] The method for producing acetic acid according to any one of Appendices 13 to 20, wherein a portion of the methyl iodide-rich stream separated and obtained by the membrane separation step is recycled to a process in the acetaldehyde separation and removal system (in particular, a distillation column that discharges the process stream). [Appendix 22] The method for producing acetic acid according to any one of Appendices 13 to 21, wherein the process stream comprises an overhead stream from a distillation column that performs a distillation treatment in the acetaldehyde separation and removal system. [Appendix 23] The method for producing acetic acid according to Appendices 22, wherein a portion of the methyl iodide-rich stream separated and obtained by the membrane separation step is refluxed to the distillation column. [Appendix 24] The acetaldehyde separation and removal step comprises: a distillation step (A1) of distilling at least a portion of the condensate obtained by condensing the stream rich in low boiling components to separate and obtain a first overhead stream rich in acetaldehyde and a residual stream; an extraction step (B1) for subjecting at least a portion of the condensate obtained by condensing the first overhead stream to extraction and separating it into an aqueous phase rich in acetaldehyde and an organic phase rich in methyl iodide; a distillation step (C1) for subjecting the acetaldehyde-rich aqueous phase to distillation to separate and obtain a second acetaldehyde-rich overhead stream and a water-rich bottoms stream, 24. The method for producing acetic acid according to any one of appendixes 13 to 23, wherein one or more of the first overhead stream, the condensate obtained by condensing the first overhead stream, the acetaldehyde-rich aqueous phase, the second overhead stream, and the condensate obtained by condensing the second overhead stream (preferably, the second overhead stream and / or the condensate obtained by condensing the second overhead stream) are subjected to the membrane separation step as the process stream. [Appendix 25] The method for producing acetic acid according to Appendices 24 and 25, wherein the process stream comprises a condensate obtained by condensing the second overhead stream. [Appendix 26] The method for producing acetic acid according to appendix 24 or 25, wherein the methyl iodide-rich stream obtained by separation in the membrane separation step is recycled to the distillation step (A1) and / or the extraction step (B1). [Appendix 27] The acetaldehyde separation and removal step comprises: a distillation step (A2) of distilling at least a portion of the condensate obtained by condensing the stream rich in low boiling components, and withdrawing a liquid descending from a concentration zone in the distillation column where methyl iodide and acetaldehyde are concentrated as a side stream; A separation step (B2) of separating the side stream into an aqueous phase and an organic phase; and a distillation step (C2) for subjecting the aqueous phase to distillation to separate and obtain a third overhead stream rich in acetaldehyde and a bottoms stream rich in water, The process may further include an extractive distillation step (D2) for subjecting the third overhead stream rich in acetaldehyde to extractive distillation to separate and obtain a fourth overhead stream rich in acetaldehyde and a water-rich bottoms product, 24. The method for producing acetic acid according to any one of claims 13 to 23, wherein one or more streams selected from the group consisting of the third overhead stream, a condensate obtained by condensing the third overhead stream, the fourth overhead stream, and a condensate obtained by condensing the fourth overhead stream are subjected to the membrane separation step as the process stream. [Appendix 28] The method for producing acetic acid according to appendix 27, wherein in the distillation step (A2), the extraction solvent is supplied from above (preferably near the top of the column) the supply position of the condensate. [Appendix 29] The method for producing acetic acid according to appendix 27 or 28, wherein in the distillation step (A2), the side stream is withdrawn from the distillation column at a position lower than the position at which the extraction solvent is introduced and higher than the position at which the condensate is supplied, in the height direction of the distillation column. [Appendix 30] The method for producing acetic acid according to any one of Appendices 27 to 29, wherein the process stream comprises a condensate obtained by condensing the fourth overhead stream.

[0225] [Appendix 31] The method for producing acetic acid according to any one of Appendices 27 to 30, wherein the methyl iodide-rich stream obtained by separation in the membrane separation step is recycled to one or more of the distillation step (A2), the separation step (B2), the distillation step (C2), and the extractive distillation step (D2) (preferably, the distillation step (A2)). [Appendix 32] The method for producing acetic acid according to any one of Appendices 1 to 31, wherein the process stream is a liquid. [Appendix 33] The method for producing acetic acid according to appendix 32, wherein the membrane separation step is carried out while blowing a sweep gas into the feed side. [Appendix 34] The method for producing acetic acid according to any one of Appendices 1 to 33, wherein the recovery rate of methyl iodide in the membrane separation step is 1 mass% or more (5 mass% or more, 10 mass% or more, 20 mass% or more, 30 mass% or more, 40 mass% or more, 50 mass% or more, 60 mass% or more, 70 mass% or more, 80 mass% or more, 90 mass% or more, 95 mass% or more, or 99 mass% or more). [Appendix 35] The method for producing acetic acid according to any one of Appendices 1 to 34, wherein the recovery rate of methyl iodide in the membrane separation step is 100% by mass or less (99.5% by mass or less, 95% by mass or less, 93% by mass or less, 90% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less). [Appendix 36] The method for producing acetic acid according to any one of Appendices 1 to 35, wherein the recovery rate of acetaldehyde in the membrane separation step is 1 mass% or more (5 mass% or more, 10 mass% or more, 15 mass% or more, 20 mass% or more, 30 mass% or more, 50 mass% or more, 60 mass% or more, 80 mass% or more, or 90 mass% or more). [Appendix 37] The method for producing acetic acid according to any one of Appendices 1 to 36, wherein the recovery rate of acetaldehyde in the membrane separation step is 100% by mass or less (99.5% by mass or less, 95% by mass or less, 93% by mass or less, 90% by mass or less, 80% by mass or less, 70% by mass or less, 60% by mass or less, 50% by mass or less, 40% by mass or less, 30% by mass or less, 20% by mass or less, or 10% by mass or less). [Appendix 38] The method for producing acetic acid according to any one of Appendices 1 to 37, wherein the concentration of methyl iodide in a methyl iodide-rich stream separated and obtained by the membrane separation step is higher than the concentration of methyl iodide in the process stream and is 1 mass% or more (5 mass% or more, 10 mass% or more, 15 mass% or more, 20 mass% or more, 25 mass% or more, 30 mass% or more, 40 mass% or more, or 50 mass% or more). [Appendix 39] The method for producing acetic acid according to any one of Appendices 1 to 38, wherein the concentration of methyl iodide in a methyl iodide-rich stream separated and obtained by the membrane separation step is higher than the concentration of methyl iodide in the process stream and is 100 mass% or less (90 mass% or less, 70 mass% or less, 50 mass% or less, 40 mass% or less, 30 mass% or less, 20 mass% or less, 10 mass% or less, or 5 mass% or less). [Appendix 40] The method for producing acetic acid according to any one of Appendices 1 to 39, wherein the acetaldehyde concentration in the acetaldehyde-rich stream separated and obtained by the membrane separation step is higher than the acetaldehyde concentration in the process stream and is 0.1 mass% or more (1 mass% or more, 5 mass% or more, 10 mass% or more, 20 mass% or more, 30 mass% or more, 50 mass% or more, 70 mass% or more, 80 mass% or more, or 90 mass% or more).

[0226] [Appendix 41] The method for producing acetic acid according to any one of Appendices 1 to 40, wherein the acetaldehyde concentration in the acetaldehyde-rich stream separated and obtained by the membrane separation step is higher than the acetaldehyde concentration in the process stream and is 100 mass% or less (99.5 mass% or less, 95 mass% or less, 93 mass% or less, 90 mass% or less, 80 mass% or less, 70 mass% or less, 60 mass% or less, 50 mass% or less, 40 mass% or less, 30 mass% or less, 20 mass% or less, or 10 mass% or less). [Appendix 42] The method for producing acetic acid according to any one of Appendices 1 to 41, wherein the silica membrane has an amino group and / or an ammonium group. [Appendix 43] The method for producing acetic acid according to any one of Appendices 1 to 42, wherein the silica membrane has a structure derived from a hydrophilic polymer having an amino group and / or an ammonium group or a structure derived from a silane coupling agent having an amino group and / or an ammonium group. [Appendix 44] The method for producing acetic acid according to Appendix 43, wherein the hydrophilic polymer is a polymer of a monomer mixture containing a betaine monomer and an alkoxysilyl group-containing compound, and the betaine monomer is at least one selected from the group consisting of a sulfoxybetaine monomer, a carboxybetaine monomer, and a phosphorylbetaine monomer. [Appendix 45] The method for producing acetic acid according to appendix 43 or 44, wherein the silane coupling agent comprises an alkoxy group and a monovalent organic group, each of which is bonded to a silicon atom, and at least one of the monovalent organic groups has an amino group and / or an ammonium group in the chain and / or at the end. [Appendix 46] The method for producing acetic acid according to Appendix 45, wherein in the monovalent organic group having an amino group and / or an ammonium group, the monovalent organic group is an alkyl group such as a cyclic alkyl group, a linear alkyl group, or a branched alkyl group, or a group in which two or more alkyl groups are bonded via a heteroatom such as an oxygen atom or a sulfur atom, and is preferably an alkyl group, more preferably a linear or branched alkyl group, and even more preferably a linear alkyl group. [Appendix 47] The method for producing acetic acid according to Appendices 45 and 46, wherein the monovalent organic group has 1 to 20, 1 to 10, or 1 to 7 carbon atoms. [Appendix 48] The method for producing acetic acid according to any one of Appendices 43 to 47, wherein the silane coupling agent is represented by the following formula: R x -Si-R 1 m (OR 2 ) 3-m (R 1 R represents an alkyl group having 1 to 3 carbon atoms which may have a substituent. 2 R represents an alkyl group having 1 to 3 carbon atoms which may have a substituent. Xrepresents an alkyl group having an amino group and / or the above ammonium group in the chain and / or at the end, and m represents an integer of 1 to 3. [Appendix 49] The silane coupling agent is 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, N-2-(aminoethyl)-3-aminopropyltrimethoxysilane, N-2-(aminoethyl)-3-aminopropyltriethoxysilane, trimethoxy[3-(methylamino)propyl]silane, [3-(N,N-dimethylamino)propyl]trimethoxysilane, trimethoxy[3-(phenylamino)propyl]silane, N-[2-(N-vinylbenzylamino)ethyl]-3-aminopropyltrimethoxysilane, N-[8-(trimethoxy The method for producing acetic acid according to any one of Appendices 43 to 48, wherein the aryl group is at least one selected from the group consisting of N-[3-(trimethoxysilyl)propyl]ethane-1,2-diamine, N-[3-(trimethoxysilyl)octyl]ethane-1,2-diamine, N-[3-(trimethoxysilyl)propyl]-1-butanamine, [3-(diethylamino)propyl]trimethoxysilane, 3-[(1,3-dimethylbutylidene)amino]propyltriethoxysilane, (3-mercaptopropyl)trimethoxysilane, (3-mercaptopropyl)triethoxysilane, and N,N-bis[(diphenylphosphino)methyl]-3-(triethoxysilyl)propylamine. [Appendix 50] The method for producing acetic acid according to any one of Appendices 1 to 49, wherein the silica membrane is provided on at least one surface of a porous substrate. [Appendix 51] The method for producing acetic acid according to Appendices 50, wherein the porous substrate is an alumina substrate. [Appendix 52] A method for producing an acetaldehyde-rich stream and a methyl iodide-rich stream, comprising separating and obtaining an acetaldehyde-rich stream and a methyl iodide-rich stream from a mixture containing acetaldehyde and methyl iodide using a silica membrane having a median pore size of 0.3 to 5.0 nm and a surface water contact angle of 0 to 70°. [Industrial Applicability]

[0227] According to the method for producing acetic acid disclosed herein, acetic acid can be industrially produced by a methanol carbonylation process (methanol acetic acid process). [Explanation of symbols]

[0228] 1 Reactor 2 Evaporation tank 3,5,6 Distillation tower 4 Decanter 7 Ion exchange resin tower 8. Scrubber System 9. Acetaldehyde separation and removal system 16 Reaction mixture supply line 17 Steam flow exhaust line 18,19 Residual liquid recycle line 54 Carbon monoxide-containing gas introduction line 55,56 Potassium hydroxide introduction line 57 Catalyst Circulation Pump 58 Membrane Module 59,72,73 Receiver 91 Distillation tower (First deacetaldehyde tower) 92 Extraction tower 93 Distillation tower (second deacetaldehyde tower) 94 Distillation tower (extractive distillation tower) 95 Decanter 96 Decanter 97 Distillation tower (acetaldehyde removal tower) 98 Distillation tower (extractive distillation tower) 99 Decanter 200 Chimney Tray

Claims

1. The method for producing acetic acid includes a membrane separation step of separating and obtaining an acetaldehyde-rich stream and a methyl iodide-rich stream from a process stream containing acetaldehyde and methyl iodide and having an acetaldehyde concentration of 20 mass% or more using a silica membrane having a median pore size of 0.3 to 5.0 nm and a surface water contact angle of 0 to 70°.

2. The method for producing acetic acid according to claim 1, wherein the process stream has an acetaldehyde concentration of 20 to 99.9% by mass and a methyl iodide concentration of 0.01 to 80% by mass.

3. The method for producing acetic acid according to claim 1 or 2, wherein the total concentration of acetaldehyde and methyl iodide in the process stream is 30 mass% or more.

4. a carbonylation reaction step of reacting methanol with carbon monoxide to produce acetic acid in the presence of a catalyst system including a metal catalyst and methyl iodide, as well as acetic acid, methyl acetate, and water; a separation step of subjecting the reaction mixture obtained in the carbonylation reaction step to one or more evaporation and / or distillation processes to separate the reaction mixture into a stream containing a metal catalyst, an acetic acid stream rich in acetic acid, and a stream richer in lower boiling components than the acetic acid stream; and an acetaldehyde separation and removal system for separating acetaldehyde from at least a portion of a condensate obtained by condensing the stream rich in low boiling components by one or more distillation processes; The method for producing acetic acid according to any one of claims 1 to 3, wherein an acetaldehyde-rich stream separated in the acetaldehyde separation and removal system is subjected to the membrane separation step as the process stream.

5. 5. The method for producing acetic acid according to claim 4, wherein a portion of the methyl iodide-rich stream separated and obtained by the membrane separation step is recycled to the process in the acetaldehyde separation and removal system.

6. The method for producing acetic acid according to claim 4 or 5, wherein the process stream comprises an overhead stream of a distillation column that performs a distillation treatment in the acetaldehyde separation and removal system.

7. 7. The method for producing acetic acid according to claim 6, wherein a part of the methyl iodide-rich stream separated and obtained by the membrane separation step is refluxed to the distillation column.

8. The acetaldehyde separation and removal system comprises: A distillation step (A1) of distilling at least a portion of the condensate obtained by condensing the stream rich in low boiling components to separate and obtain a first overhead stream rich in acetaldehyde and a residual stream; an extraction step (B1) for subjecting at least a portion of the condensate obtained by condensing the first overhead stream to extraction and separating it into an aqueous phase rich in acetaldehyde and an organic phase rich in methyl iodide; a distillation step (C1) for subjecting the acetaldehyde-rich aqueous phase to distillation to separate and obtain a second acetaldehyde-rich overhead stream and a water-rich bottoms stream, The method for producing acetic acid according to claim 4, wherein the second overhead stream and / or a condensate obtained by condensing the second overhead stream are subjected to the membrane separation step as the process stream.

9. 9. The method of claim 8, wherein the process stream comprises a condensate obtained by condensing the second overhead stream.

10. 10. The method for producing acetic acid according to claim 8 or 9, wherein the methyl iodide-rich stream obtained by separation in the membrane separation step is recycled to the distillation step (A1) and / or the extraction step (B1).

11. The acetaldehyde separation and removal system comprises: a distillation step (A2) of distilling at least a portion of the condensate obtained by condensing the stream rich in low boiling components, and withdrawing a liquid descending from a concentration zone in the distillation column where methyl iodide and acetaldehyde are concentrated as a side stream; A separation step (B2) of separating the side stream into an aqueous phase and an organic phase; and a distillation step (C2) for subjecting the aqueous phase to distillation to separate and obtain a third overhead stream rich in acetaldehyde and a bottoms stream rich in water, The process may further include an extractive distillation step (D2) for subjecting the third acetaldehyde-rich overhead stream to extractive distillation to separate and obtain a fourth acetaldehyde-rich overhead stream and a water-rich bottoms product, 5. The method for producing acetic acid according to claim 4, wherein one or more streams selected from the group consisting of the third overhead stream, a condensate obtained by condensing the third overhead stream, the fourth overhead stream, and a condensate obtained by condensing the fourth overhead stream are subjected to the membrane separation step as the process streams.

12. 12. The method of claim 11, wherein the process stream comprises a condensate obtained by condensing the fourth overhead stream.

13. 13. The method for producing acetic acid according to claim 11 or 12, wherein the methyl iodide-rich stream obtained by separation in the membrane separation step is recycled to the distillation step (A2).

14. The method for producing acetic acid according to any one of claims 1 to 13, wherein the process stream is a liquid.

15. The method for producing acetic acid according to any one of claims 1 to 14, wherein the recovery rate of methyl iodide in the membrane separation step is 1 mass% or more.

16. The method for producing acetic acid according to any one of claims 1 to 15, wherein the concentration of methyl iodide in a stream rich in methyl iodide separated and obtained by the membrane separation step is higher than the concentration of methyl iodide in the process stream and is 1 mass% or more.

17. The method for producing acetic acid according to any one of claims 1 to 16, wherein the acetaldehyde concentration in the acetaldehyde-rich stream separated and obtained by the membrane separation step is higher than the acetaldehyde concentration in the process stream and is 30 mass% or more.

18. The method for producing acetic acid according to any one of claims 1 to 17, wherein the silica membrane is provided on at least one surface of a porous substrate.

19. A method for producing an acetaldehyde-rich stream and a methyl iodide-rich stream, comprising the steps of: separating and obtaining an acetaldehyde-rich stream and a methyl iodide-rich stream from a mixture containing acetaldehyde and methyl iodide and having an acetaldehyde concentration of 20 mass% or more, using a silica membrane having a median pore size of 0.3 to 5.0 nm and a surface water contact angle of 0 to 70°.

20. 20. The method for producing an acetaldehyde-rich stream and a methyl iodide-rich stream as claimed in claim 19, wherein the mixture has an acetaldehyde concentration of 20 to 99.9 wt. % and a methyl iodide concentration of 0.01 to 80 wt. %.

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