JP7651274B2 - Method for producing acetic acid - Google Patents

Description

This disclosure relates to a method for producing acetic acid.

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, and 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 separation 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 produced as a by-product during the reaction, and this acetaldehyde causes a decrease in the quality of the product acetic acid.

Therefore, the condensate of the overhead stream from the low boiling 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 the 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.

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. Furthermore, in order to compensate for the loss of methyl iodide, it is necessary to produce methyl iodide as needed and add new methyl iodide to the reactor, which creates problems such as a high workload and high costs.

Methods for efficiently separating acetaldehyde and methyl iodide are known, for example, as disclosed in Patent Documents 1 to 4.

International Publication No. 2017/057142 JP 2006-94764 A Japanese Patent Application Publication No. 9-40590 Japanese Patent Application Publication No. 9-77697

However, even the methods disclosed in Patent Documents 1 to 4 do not allow complete separation of acetaldehyde and methyl iodide, and a method capable of separating them more efficiently has been desired.

Therefore, the 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.

As a result of intensive research into achieving the above-mentioned objective, the inventors of the present disclosure have discovered that by separating a mixture of acetaldehyde and methyl iodide using a specific separation membrane, acetaldehyde and methyl iodide can be separated efficiently. The present disclosure relates to a product that was completed based on these findings and through further research.

That is, the present disclosure provides a method for producing acetic acid, which includes a membrane separation step in which an acetaldehyde-rich stream and a methyl iodide-rich stream are separated from a process stream containing acetaldehyde and methyl iodide using an MFI zeolite membrane.

It is preferable that the acetaldehyde concentration in the process stream is 0.1 to 99.9 mass %, and the methyl iodide concentration is 0.01 to 99 mass %.

The combined concentration of acetaldehyde and methyl iodide in the process stream is preferably 1% by mass or more.

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 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;
and an acetaldehyde separation and removal system for separating acetaldehyde from at least a portion of a condensate obtained by condensing the low boiling component-rich stream 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.

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,
Preferably, the second overhead stream and/or a condensate obtained by condensing the second overhead stream is subjected to the membrane separation step as the process stream.

The process stream preferably includes a stream distilled from the distillation step (C1) among the condensate obtained by condensing the second overhead stream.

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).

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, a condensate obtained by condensing the third overhead stream, the fourth overhead stream, and a condensate obtained by condensing the fourth overhead stream may be subjected to the membrane separation step as the process stream.

The process stream preferably includes a stream distilled from the extractive distillation step (D2) among the condensates obtained by condensing the fourth overhead stream.

The process stream is preferably a liquid.

It is preferable to carry out the above membrane separation process under pressure conditions of 110 to 2000 kPa (absolute pressure).

It is preferable to carry out the membrane separation process under conditions where the temperature of the process stream is 10 to 180°C.

It is preferable that the recovery rate of methyl iodide by the above membrane separation process is 1% by mass or more.

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

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

The MFI zeolite membrane is preferably provided on at least one surface of the porous substrate.

The porous substrate is preferably an alumina substrate.

The present disclosure also provides a method for producing an acetaldehyde-rich stream and a methyl iodide-rich stream by separating and obtaining an acetaldehyde-rich stream and a methyl iodide-rich stream from a mixture containing acetaldehyde and methyl iodide using an MFI zeolite membrane.

It is preferable that the acetaldehyde concentration in the above mixture is 0.1 to 99.9% by mass, and the methyl iodide concentration is 0.01 to 99% by mass.

The total concentration of acetaldehyde and methyl iodide in the mixture is preferably 1% by mass or more.

According to the above-mentioned method for producing acetic acid, acetaldehyde and methyl iodide can be efficiently separated, so the loss of methyl iodide can be reduced. This also reduces the workload of adding new methyl iodide to the reaction process.

FIG. 2 is a schematic flow diagram showing an example of a membrane separation process. FIG. 1 is a production flow diagram showing one embodiment of an acetic acid production system. FIG. 1 is a schematic flow diagram showing an example of an acetaldehyde separation and removal system. FIG. 4 is a schematic flow diagram showing another example of an acetaldehyde separation and removal system. FIG. 4 is a schematic flow diagram showing yet another example of an acetaldehyde separation and removal system. FIG. 4 is a schematic flow diagram showing yet another example of an acetaldehyde separation and removal system.

The method for producing acetic acid according to one embodiment of the present disclosure includes a membrane separation step in which an acetaldehyde-rich stream and a methyl iodide-rich stream are separated from a process stream containing acetaldehyde and methyl iodide using an MFI zeolite membrane during the acetic acid production process.

The MFI zeolite membrane is a membrane formed from MFI zeolite, and mainly allows methyl iodide to pass through and concentrates acetaldehyde without allowing it to pass through. In other words, the permeate stream separated and obtained in the membrane separation process is a stream richer in methyl iodide than the process stream (feed stream), and the concentrated stream separated and obtained is a stream richer in acetaldehyde than the process stream (feed stream). This is because the MFI zeolite membrane basically allows methyl iodide to pass through due to its molecular sieve function. On the other hand, although a large amount of acetaldehyde should also pass through due to the molecular sieve function alone, it is presumed that MFI zeolite has a hydrophobic surface and therefore does not allow acetaldehyde to pass through easily. Due to the performance of the MFI zeolite membrane, acetaldehyde and methyl iodide can be separated efficiently.

The MFI zeolite membrane is preferably provided on at least one surface of a porous substrate. The porous substrate acts as a substrate for supporting the MFI zeolite membrane. The porous substrate has pores that allow methyl iodide to pass through.

The porous substrate may be any known or conventional material. Examples of materials 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. Of these, alumina is preferred. When the porous substrate is an alumina substrate, it is presumed that the alumina flows into the pores in the MFI zeolite membrane and fills a portion of the pores, making the pores even more appropriate in size, and acetaldehyde and methyl iodide can be separated more efficiently.

The process stream 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 the process streams in all the devices and piping in the acetic acid production process. The process stream may be liquid or gas.

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.

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.

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.

The process stream may contain components other than acetaldehyde and methyl iodide. The other components include components that may be generated in the acetic acid production process, 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.

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.

The methyl iodide concentration in the methyl iodide-rich stream obtained by the membrane separation step is higher than the methyl iodide concentration in the process stream, and may be, 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.

The recovery rate of methyl iodide in the membrane separation process ([methyl iodide flow rate in the permeate 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.

The acetaldehyde concentration in the acetaldehyde-rich stream obtained by the membrane separation process is higher than the acetaldehyde concentration in the process stream, and may be, 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.

The recovery rate of acetaldehyde in the membrane separation process ([acetaldehyde flow rate in the concentrated stream/acetaldehyde flow rate in the feed stream] x 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.

An embodiment of the membrane separation process will be described below. FIG. 1 is a schematic flow diagram showing an embodiment of the membrane separation process in the present disclosure in the case where the process stream to be subjected to the membrane separation process is a liquid (process liquid). According to this flow, for example, when the process liquid is subjected to the membrane separation process, the process liquid is supplied to the receiver 58 through line 62. When the liquid level controller 58a detects that the liquid level in the receiver 58 exceeds a predetermined liquid level, the valve 63b is opened, and the process liquid is supplied to the receiver 59 through lines 63 and 64 from the bottom of the receiver 58 by the pump 63a. The gas generated in the receiver 58 is connected through line 65 to line 116 or line 227, which will be described later, and disposed of.

The process liquid in the receiver 59 is supplied from the bottom of the receiver 59 by the pump 66a through lines 66 and 67 to the membrane module 60 equipped with an MFI-type zeolite membrane. At this time, the process liquid is heated by the heater 66b, the amount of steam of which is adjusted by the temperature controller 67a. At this time, in order to maintain a pressure in the receiver 59 at which the circulating liquid supplied through line 75 does not vaporize, the pressure controller 59a opens the valve 68a and supplies nitrogen to the receiver 59 from line 68. If the pressure rises more than necessary due to operational modulation, the gas in the receiver 59 is connected to line 116 or line 227, etc., described below, by opening the valve 79a, through line 79, and disposed of.

The membrane separation process is carried out in the membrane module 60. An inert gas such as nitrogen or helium may be blown into the permeation side of the membrane module 60 equipped with a zeolite membrane as a sweep gas. This increases the partial pressure difference between acetaldehyde and methyl iodide, improving the recovery rate of methyl iodide. It also prevents the permeate flow passing through the zeolite membrane from stagnating. There are no particular limitations on the temperature and pressure of the process gas fed into the membrane separation process, so long as the process flow can be maintained in a liquid state.

The membrane separation step is preferably carried out at a temperature of 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 terms. By carrying out the membrane separation step in the above temperature range and under pressure, the partial pressure difference of methyl iodide between the concentrated stream and the permeate stream can be increased to increase the permeability of methyl iodide, and acetaldehyde and methyl iodide can be separated more efficiently.

The methyl iodide-rich stream (permeate stream) that has permeated the MFI zeolite membrane is cooled by a condenser 69a through a line 69 and supplied to the receiver 61. When the liquid level controller 61a detects that the liquid level in the receiver 61 has exceeded a predetermined liquid level, a methyl iodide-rich stream (line 71) is obtained from the bottom of the receiver 61 through a line 70 by a pump 70a. 71a is a flow meter. The methyl iodide-rich stream (line 71) is directly or indirectly recycled to the reaction step and can be reused as a cocatalyst in the reaction step. The methyl iodide-rich stream (line 71) is preferably recycled to a 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 pressure inside the receiver 61 is detected by a pressure controller 72a, and when it exceeds a predetermined pressure, the gas generated in the receiver 61 is discharged by a pump 72b through lines 72 and 73 and is then incinerated.

On the other hand, when the liquid level controller 59b detects that the liquid level in the receiver 59 exceeds a predetermined level, a part of the acetaldehyde-rich stream (concentrated stream) (line 74) obtained by concentration without passing through the MFI zeolite membrane in the membrane separation process is discharged through line 77 by opening valve 76c while adjusting the flow rate with flow rate controller 76a, and is subjected to incineration treatment. When passing through line 77, the acetaldehyde-rich stream is heated by heater 76b to a temperature at which it vaporizes under the pressure after valve 76c. In order to increase the amount flowing through the membrane module 60 and increase the efficiency (permeability) of membrane separation, the flow of line 74 is returned to receiver 59 through line 75.

In this manner, the membrane separation process separates the process liquid (line 62) into an acetaldehyde-rich stream (line 77) and a methyl iodide-rich stream (line 71).

The membrane separation step has been described as one step in the acetic acid production process, but is not limited to this embodiment. It can be performed as a method for separating acetaldehyde and methyl iodide, in which an acetaldehyde-rich stream and a methyl iodide-rich stream are separated and obtained from a mixture containing acetaldehyde and methyl iodide using an MFI zeolite membrane, or as a method for producing an acetaldehyde-rich stream and a methyl iodide-rich stream. Preferred embodiments of this separation method and production method are the same as those described in the membrane separation step above.

In addition, in each embodiment, by passing a solvent such as methanol, methyl acetate, acetic acid, or water through the membrane module 60 or by immersing the membrane module 60 in such a solvent, the membrane performance can be restored even if it has deteriorated. The solvent can be passed through the membrane module 60 by external circulation using a pump or the like, or by running-through cleaning. The membrane module 60 can 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.

In the above-mentioned method for producing acetic acid, the production process of 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 processes 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 low 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, it is preferable to subject the vapor stream to distillation to separate it 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 distilled to separate and obtain at least an overhead stream rich in water and a second acetic acid stream that is more enriched in acetic acid than the first acetic acid stream.

In addition, the separation process may include, instead of the evaporation process and the dehydration process, a process (evaporation dehydration process) for separating the reaction mixture obtained in the carbonylation reaction process into a stream containing the catalyst, an overhead stream rich in low boiling components, and a first acetic acid stream rich in acetic acid. In addition, instead of the dehydration process and the dehydration process, the separation process may include a dehydration process that also has the function of the dehydration process (so-called dehydration process), that is, a process for subjecting the vapor stream to distillation to separate it 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 evaporation dehydration process may also include a process (evaporation dehydration process) that also has the function of the dehydration process. The acetic acid stream rich in acetic acid obtained from the dehydration process and the dehydration process corresponds to the second acetic acid stream.

The method for producing acetic acid may further include at least one of the following steps (a) to (c):
(a) a high boiling removal step in which the acetic acid stream (e.g., the first or second acetic acid stream) rich in acetic acid obtained in the separation step is distilled to separate the stream into a bottoms stream rich in high boiling components and a third acetic acid stream enriched in acetic acid compared to the acetic acid stream before distillation; (b) an adsorption removal step in which the acetic acid stream (e.g., the first or second acetic acid stream) or the third acetic acid stream rich in acetic acid obtained in the separation step is treated with an ion exchange resin to obtain a fourth acetic acid stream; (c) a product step in which the acetic acid stream (e.g., the first or second acetic acid stream) or the third or fourth acetic acid stream rich in acetic acid obtained in the separation step is distilled to obtain a fifth acetic acid stream enriched in acetic acid compared to the acetic acid stream before distillation.

The acetic acid production method may also have an acetaldehyde separation and removal system that separates acetaldehyde from at least a portion of the condensate obtained by condensing the low boiling point component-rich stream by one or more distillation processes. The acetaldehyde separation and removal system may also include an extraction step that separates 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.

In the above-mentioned acetic acid production method, it is preferable to subject the acetaldehyde-rich stream separated in the distillation process in the above-mentioned acetaldehyde separation and removal system, and/or the aqueous layer containing acetaldehyde separated in the extraction process from the 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 in which the above-mentioned distillation process is performed. It is preferable that at least a portion 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).

Hereinafter, one embodiment of the acetic acid production method including the membrane separation process will be described. Figure 2 is an example of a production flow diagram (methanol carbonylation process) showing one 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.

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

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 methanol and carbon monoxide as raw materials, 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)

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 vessel 1 through line 11. Carbon monoxide is continuously supplied from a carbon monoxide storage section (not shown) to the reaction vessel 1 through line 12. The carbon monoxide does not necessarily have to be pure carbon monoxide, and may contain small amounts (e.g., 5% by mass or less, preferably 1% by mass or less) of other gases such as nitrogen, hydrogen, carbon dioxide, and oxygen.

The metal catalyst in the reaction mixture is for promoting the carbonylation reaction of methanol, and for example, a rhodium catalyst or an iridium catalyst can be used. As the rhodium catalyst, for example, a rhodium complex represented by the chemical formula [Rh(CO) ₂ I ₂ ] ^- can be used. As the iridium catalyst, for example, an iridium complex represented by the chemical formula [Ir(CO) ₂ I ₂ ] ^- can be used. As the metal catalyst, a metal complex catalyst is preferred. The concentration of the catalyst in the reaction mixture (metal equivalent) 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.

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% with respect to 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 suppress side reactions. Examples of ionic iodides include alkali metal iodides such as lithium iodide, sodium iodide, and potassium iodide. The concentration of the ionic iodide in the reaction mixture is, for example, 1 to 25 mass% with respect to 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 moles (metal equivalent), preferably 0.5 to 15 moles (metal equivalent) per mole of iridium (metal equivalent).

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

The acetic acid in the reaction mixture includes acetic acid that has been charged in advance into the reaction tank 1 before the operation of the acetic acid production apparatus, and acetic acid that is generated as the 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% by mass, and preferably 60 to 80% by mass, based on the entire liquid phase of the reaction mixture.

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.

Hydrogen iodide is also a by-product contained in the reaction mixture. When the above-mentioned catalysts and co-catalysts are used, hydrogen iodide is inevitably produced 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.

Examples of by-products include hydrogen, methane, carbon dioxide, acetaldehyde, crotonaldehyde, 2-ethylcrotonaldehyde, dimethyl ether, alkanes, formic acid, propionic acid, and 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 result from corrosion of the equipment, as well as 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.".

In the reaction tank 1 in which the reaction mixture described above 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).

When the device is operating, the vapor in the gas phase in reaction vessel 1 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 reaction vessel 1 through line 13. By adjusting the amount of vapor extracted, the pressure in reaction vessel 1 can be controlled, and for example, the pressure in reaction vessel 1 can be maintained constant. The vapor extracted from reaction vessel 1 is introduced into condenser 1a.

Condenser 1a separates the vapor from reactor 1 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 condenser 1a through line 14 to reactor 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 condenser 1a through line 15 to scrubber system 8.

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

As described above, acetic acid is continuously produced in reaction tank 1 during operation of the device. Such a reaction mixture containing acetic acid is continuously withdrawn from reaction tank 1 and introduced into the next evaporation tank 2 through line 16.

Evaporation tank 2 is a unit for carrying out the evaporation process (flash process). This evaporation process is a process for partially evaporating the reaction mixture that is continuously introduced into evaporation tank 2 through line 16 (reaction mixture supply line) and separating it into a vapor stream (volatile phase) and a residual liquid stream (low volatile phase).

Evaporation may occur 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).

The ratio of the vapor stream and residual liquid stream separated in the evaporation process is, for example, 10/90 to 50/50 (vapor stream/residual liquid stream) in mass ratio. The vapor generated in this process 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 stream discharge line).

A part of the vapor flow extracted from the evaporation tank 2 is continuously introduced into the condenser 2a, and another part of the vapor flow is continuously introduced into the next distillation column 3 through the line 21. The acetic acid concentration of the vapor flow 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 flow 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).

The residual liquid stream generated in this process 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 that remain in this process without volatilization. It is continuously introduced from the evaporation tank 2 to the heat exchanger 2b through 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 line 19 to the reaction tank 1 and recycled. The lines 18 and 19 are collectively referred to as the residual liquid stream recycle line. The acetic acid concentration of the residual liquid stream is, for example, 55 to 90% by mass, preferably 60 to 85% by mass.

The condenser 2a separates the vapor flow 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 to produce 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. In other words, in this acetic acid production apparatus, the heat generated by the methanol carbonylation reaction is efficiently removed by the condenser 2a.

The distillation tower 3 is a unit for carrying out the first distillation step, and in this embodiment, it is positioned as a so-called low boiling component removal tower. The first distillation step is a step of distilling the vapor stream continuously introduced into the distillation tower 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.

The distillation tower 3 is, for example, a fractionation tower such as a plate tower or a packed tower. When a plate tower is used as the distillation tower 3, the 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 distillation tower 3, the top pressure is set to, for example, 80 to 160 kPaG, and the bottom pressure is set to, for example, 85 to 180 kPaG, higher than the top pressure. Inside the distillation tower 3, the top temperature is set to, for example, 90 to 130°C, which is lower than the boiling point of acetic acid at the set top pressure, and the bottom temperature is set to, for example, 120 to 165°C (preferably 125 to 160°C), which is higher than the boiling point of acetic acid at the set bottom pressure.

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

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

The condenser 3a separates the vapor from the distillation column 3 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, 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).

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.

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 and 51.

In the acetaldehyde separation and removal process using the acetaldehyde separation and removal system 9, acetaldehyde contained in at least a portion of the condensate (e.g., the organic phase and/or the aqueous phase) obtained by condensing the overhead stream rich in low boiling components is separated and removed by a known method, such as 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 the aqueous phase are recycled to the reaction tank 1 through lines 52 and 23 for reuse.

Figure 3 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 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 the 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 line 117. This allows the stream rich in methyl iodide to be recycled to the distillation step (A1).

The condensate (feed stream) supplied to the extraction tower 92 is extracted with water introduced from line 109 (extraction step (B1)). The extraction process 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 process is supplied to the distillation tower (second deacetaldehyde tower) 93 through line 107 and distilled to separate and obtain an acetaldehyde-rich second overhead stream (line 112) and a water-rich residual stream (line 113) (distillation step (C1)). 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). 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 by the membrane separation process. This allows the stream rich in methyl iodide to be recycled to the extraction process (B1). The stream rich in methyl iodide may also be recycled to the distillation process (C1).

The bottoms of the first deacetaldehyde column 91, i.e., the bottoms of the second deacetaldehyde column 93, i.e., the bottoms of the second deacetaldehyde column 93, i.e., the bottoms of the second deacetaldehyde column 93, i.e., the bottoms of the second deacetaldehyde column 93, i.e., the bottoms of the second deacetaldehyde column 93, i.e., the bottoms of the second deacetaldehyde column 93, i.e., the bottoms of the second deacetaldehyde column 93, i.e., the bottoms of the second deacetaldehyde column 93, i.e., the bottoms of the second deacetaldehyde column 93, i.e., the bottoms of the second deacetaldehyde column 93, i.e., the bottoms of the second deacetaldehyde column 93, i.e., the bottoms of the first ... second deacetaldehyde column 93, i.e., the bottoms of the second deacetaldehyde column 93, i.e., the bottoms of the second deacetaldehyde column 93, i

In addition, when the aqueous phase is treated in the acetaldehyde separation and removal step according to the flow of FIG. 3, for example, the aqueous phase (feed stream) is supplied to a distillation tower (first deacetaldehyde tower) 91 through 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, 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 the 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 line 117. This allows the stream rich in methyl iodide to be recycled to the distillation step (A1).

The condensate (feed stream) supplied to the extraction tower 92 is extracted with water introduced from line 109 (extraction step (B1)). The extraction process 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 process is supplied to the distillation tower (second deacetaldehyde tower) 93 through 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). The second overhead stream rich in acetaldehyde 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). 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 by the membrane separation step. This allows the stream rich in methyl iodide to be recycled to the extraction step (B1).

The water-rich residual liquid stream which is the bottoms 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 bottoms 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, 116) that are not condensed in the condensers 91a and 93a are absorbed in the scrubber system 8 or disposed of.

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, it is preferable that the process stream to be subjected to the membrane separation step is the second overhead stream and/or the condensate obtained by condensing the second overhead stream, and from the viewpoint of making it easier to apply high-pressure conditions so that more methyl iodide permeates the MFI zeolite membrane, the condensate of the second overhead stream (particularly line 115), which is a liquid, is particularly preferable. Line 115 is the distillate stream from distillation step (C1).

In addition to the above-mentioned method, the 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 can also be separated and removed by a distillation process (distillation process (A2)) using extractive distillation or azeotropic distillation, 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 extracting solvent (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.

If a relatively large amount of water is present in the distillation tower, the liquid descending from the concentration zone may be withdrawn as a side stream without introducing the extraction solvent into the distillation tower. For example, the distillation tower 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.

The extraction solvent is preferably introduced above the feed liquid, more preferably near the top of the tower. The side stream is preferably withdrawn below the extraction solvent introduction position in the height direction of the tower and above the feed liquid supply position. According to this method, acetaldehyde can be extracted at a high concentration from the concentrate of methyl iodide and acetaldehyde using the extraction solvent (usually water), and since the area between the introduction point of the extraction solvent and the side cut point is used as the extraction zone, acetaldehyde can be efficiently extracted with a small amount of extraction solvent. Therefore, for example, the number of stages of the distillation tower can be significantly reduced compared to a method in which the extract by extractive distillation is withdrawn from the bottom of the distillation tower (extractive distillation tower), and the steam load can also be reduced. In addition, by using a small amount of extraction solvent, the ratio of methyl iodide to acetaldehyde in the aqueous extract (MeI/AD ratio) can be reduced compared to the method of combining dealdehydication distillation and aqueous extraction shown in Figure 3 above, so acetaldehyde can be removed under conditions that suppress the loss of methyl iodide to the outside of the system.

The acetaldehyde concentration in the side stream is much higher than that in the feed liquid and bottoms. The ratio of acetaldehyde to methyl iodide in the side stream is also higher than that in the feed liquid and bottoms.

The organic phase (methyl iodide phase) obtained by separating the side stream may be recycled to the distillation tower. 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 tower, and preferably above the feed liquid supply position.

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.

By recycling the organic phase obtained by separating the side stream to the distillation tower, or by introducing the miscible solvent into the distillation tower, 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 suppressing the incorporation of methyl iodide into the aqueous phase.

The theoretical plates of the above distillation tower 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 to the 80 to 100 plates of conventional distillation towers and extractive distillation towers used in deacetaldehyde removal, this allows for efficient separation and removal of acetaldehyde with a smaller number of plates.

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.

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

Figure 4 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 through a line 202 to perform extractive distillation in the distillation tower 94 (extractive distillation tower) (distillation step (A2)). The feed stream (line 201) may be joined through a line 228 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 distillation step (A2).

Above the feed position of the distillation tower 94, a chimney tray 200 is provided to receive the liquid (extract) that descends from the concentration zone in the tower where methyl iodide and acetaldehyde are concentrated. In this extractive distillation, the liquid on the chimney tray 200 is preferably removed in its entirety and introduced into the decanter 95 via line 208 for liquid separation.

The aqueous phase (containing acetaldehyde) in the decanter 95 is introduced through 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 the decanter 96. Both the separation in the decanter 95 and the separation in the decanter 96 correspond to the separation step (B2). The separation step (B2) and/or the distillation step (C2) may be joined with a stream rich in methyl iodide separated by the membrane separation step.

The aqueous phase in the decanter 96 is supplied to a distillation column 97 (deacetaldehyde column) via line 216 for distillation and separation 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 line 217, and a portion 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 line 220.

Water is introduced from near the top of the distillation tower 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 condensed by leading it to the condenser 98a through line 223, and a part of the condensate (mainly methyl iodide) is refluxed to the top of the tower (line 225), and the remainder is recycled to the reaction system through line 226, but may also 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 tower 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 tower 94 through lines 213, 210, and lines 214 and 210, respectively, but may not be recycled. A portion of the aqueous phase in the decanter 95 may be used as the extraction solvent (water) in the distillation column 94. A portion of the aqueous phase in the decanter 96 may be recycled to the distillation column 94 via line 210. The methyl iodide-rich stream may also be recycled to the extractive distillation step (D2).

In some cases (e.g., when the feed stream contains methyl acetate), a miscible solvent (acetic acid, ethyl acetate, etc.) 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 distillation tower 3 can be charged into distillation tower 94 through line 215 to improve distillation efficiency. The miscible solvent is supplied to distillation tower 94 above the feed stream supply section (connection section of line 201) and below the connection section of recycle line 210. The bottoms of distillation tower 94 are recycled to the reaction system.

The vapor from the top of the distillation tower 94 is condensed in the condenser 94a via line 203, and the condensate is separated in the decanter 99. The organic phase is returned to the top of the distillation tower 94 via line 206, and the aqueous phase is sent to the decanter 95 via line 207.

The bottoms of distillation tower 97 (mainly water) and distillation tower 98 (extractive distillation tower) (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 that are not condensed in condensers 94a, 97a, and 98a (lines 211, 221, and 227) are absorbed in scrubber system 8 or disposed of.

Figure 5 is a schematic flow diagram showing another example of an acetaldehyde separation and removal system using the above-mentioned extractive distillation. 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 of the system is the same as the example in Figure 4.

Figure 6 is a schematic flow diagram showing yet another example of an acetaldehyde separation and removal system using the above-mentioned extractive distillation. In this example, the entire amount of liquid on the chimney tray 200 is extracted, and is introduced directly into a cooling cooler 95a via line 208 without passing through a decanter 95, where it is cooled and supplied to a decanter 96. The rest of the system is the same as the example in Figure 5.

One or more of the side stream 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 streams. 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 of being easy to apply high pressure conditions so that more methyl iodide permeates the MFI zeolite membrane, the condensate of the third overhead stream (particularly, line 220) and/or the condensate of the fourth overhead stream (particularly, line 226), which are liquids, are particularly preferred. Line 220 is a stream distilled from the distillation step (C2), and line 226 is a stream distilled from the extractive distillation step (D2).

In FIG. 2, 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 the absorption liquid in the scrubber system 8.

The bottoms withdrawn from the bottom of the distillation column 3 contain more components (high boiling point components) with a higher boiling point than acetic acid compared to the overhead stream and 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 portion of this bottoms is continuously introduced into the evaporation tank 2 through lines 25 and 26 and recycled, and another portion of the bottoms is continuously introduced into the reaction tank 1 through lines 25 and 23 and recycled.

The first acetic acid stream, which is continuously withdrawn 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 the acetic acid concentration 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 also 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.

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

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.

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

The distillation tower 5 is, for example, a fractionation tower such as a plate tower or a packed tower. When a plate tower is used as the distillation tower 5, the theoretical plates are, 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 tower 5 in the second distillation step, the top pressure is set to, for example, 150 to 250 kPaG, and the bottom pressure is set to, for example, 160 to 290 kPaG, higher than the top pressure. Inside the distillation tower 5 in the second distillation step, the top temperature is set to, for example, 130 to 160°C, which is higher than the boiling point of water at the set top pressure and lower than the boiling point of acetic acid, and the bottom temperature is set to, for example, 150 to 175°C, which is higher than the boiling point of acetic acid at the set bottom pressure.

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

The vapor withdrawn from the top of distillation column 5 contains more components with a boiling point lower than that of acetic acid (low boiling point components) than 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.

The condenser 5a separates the vapor from the distillation column 5 into a condensed portion and a gas portion by cooling and partially condensing it. The condensed portion includes, for example, water and acetic acid. A portion of the condensed portion is continuously refluxed from the condenser 5a to the distillation column 5 through line 35. Another portion of the condensed portion is continuously introduced from the condenser 5a through lines 35, 36, and 23 into the reaction vessel 1 and recycled. The gas portion 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 portion from the condenser 5a may be supplied to the scrubber system 8 without merging with the line 15. The hydrogen iodide in the gas that reaches the scrubber system 8 is absorbed by the absorption liquid in the scrubber system 8, and methyl iodide is produced by reaction of the hydrogen iodide in the absorption liquid with methanol or methyl acetate. The liquid containing useful components such as methyl iodide is then recycled from the scrubber system 8 to the reaction tank 1 via the recycle lines 48 and 23 and reused.

The bottoms (or side stream) drawn off from the bottom of the distillation column 5 contain, in addition to acetic acid, 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 5, such as acetic anhydride, propionic acid, acetate, 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 acetate salts include metal acetate salts such as potassium acetate formed when an alkali such as potassium hydroxide is supplied to line 27, etc. Also included are metal acetate salts formed from acetic acid and corrosion metals, etc., which are generated and released on 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 line 27, etc. 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 line 34. In addition, the bottoms (or side stream) withdrawn from the bottom of the distillation column 5 also contain the above-mentioned corroded metals, etc., as well as compounds (iodide salts) of iodine derived from corrosive iodine and the corroded metals, etc.

The second acetic acid stream is enriched in acetic acid more than 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. Furthermore, 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 withdrawn, the withdrawal 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.

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.

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

The distillation tower 6 is, for example, a fractionation tower such as a plate tower or a packed tower. When a plate tower is used as the distillation tower 6, the theoretical plates are, 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 tower 6 in the third distillation step, the top pressure is set to, for example, -100 to 150 kPaG, and the bottom pressure is set to, for example, -90 to 180 kPaG, higher than the top pressure. Inside the distillation tower 6 in the third distillation step, the top temperature is set to, for example, 50 to 150°C, which is higher than the boiling point of water at the set top pressure and lower than the boiling point of acetic acid, and the bottom temperature is set to, for example, 70 to 160°C, which is higher than the boiling point of acetic acid at the set bottom pressure.

From the top of the distillation column 6, steam 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.

The vapor withdrawn from the top of the distillation column 6 contains more components with a boiling point lower than that of acetic acid (low boiling point components) than 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, and formic acid. Such vapor is continuously introduced into the condenser 6a through line 38.

Condenser 6a separates the vapor from distillation column 6 into a condensed fraction and a gas fraction by cooling and partially condensing it. The condensed fraction 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 condensed fraction is continuously refluxed from condenser 6a through line 40 to distillation column 6. A portion of the condensed fraction (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 condensed fraction (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.

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. 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 to the reaction tank 1 through recycle lines 48 and 23 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. In addition, 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 irregularly withdrawn outside the apparatus when necessary.

When a portion of the condensed fraction (distillate fraction) is removed from the distillation processing 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.

The bottoms withdrawn from the bottom of the distillation tower 6 through line 39 contain more components (high boiling point components) with a higher boiling point than acetic acid compared to the overhead stream from the distillation tower 6, such as acetate, acetic anhydride, and propionic acid. Examples of the acetate include potassium acetate formed when an alkali such as potassium hydroxide is supplied to line 34. Examples of the acetate include metal acetate salts formed from acetic acid and corrosion metals such as metals that are 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 tower 6 through line 39 further contain the above-mentioned corrosion metals, and compounds of iodine derived from corrosive iodine and the corrosion metals. In this embodiment, such bottoms are discharged outside the acetic acid production apparatus.

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 other embodiments, 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.

The ion exchange resin tower 7 is a purification unit for carrying out the adsorption removal process. This adsorption removal process is a process 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 that is continuously introduced into the ion exchange resin tower 7.

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 a part of the eliminable protons in the exchange groups such as sulfonic acid groups, carboxyl groups, and phosphonic acid groups is replaced with a metal such as silver or copper. In the adsorption 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 removal step, the internal temperature is, for example, 18 to 100°C, and the flow rate of the acetic acid stream [acetic acid treatment amount per 1 m ³ of resin volume (m ³ /h)] is, for example, 3 to 15 ^{m 3} /h·m ³ (resin volume).

A fourth acetic acid stream is continuously discharged from the lower end of the ion exchange resin tower 7 to line 47. The acetic acid concentration of the fourth acetic acid stream is higher than that of the third acetic acid stream. In other words, the fourth acetic acid stream is enriched in acetic acid more 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.

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, for example, to -195 to 150 kPaG, and the column bottom pressure is set higher than the column top pressure, for example, to -190 to 180 kPaG. Inside the product column, the column top temperature is set, for example, to a temperature higher than the boiling point of water at the set column top pressure and lower than the boiling point of acetic acid at 50 to 150 ° C., and the column bottom temperature is set, for example, to a temperature higher than the boiling point of acetic acid at the set column bottom pressure, at 70 to 160 ° C. The product tower or finishing tower can also be replaced with a simple still (evaporator).

When a product column is provided, all or 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 is continuously extracted as an overhead stream containing trace amounts of low boiling point components (e.g., methyl iodide, water, methyl acetate, dimethyl ether, crotonaldehyde, acetaldehyde, and formic acid). This vapor is separated into a condensed portion and a gas portion in a condenser.

A portion of the condensed fraction is continuously refluxed to the product column, and another portion of the condensed fraction may be recycled to the reaction tank 1 or discharged to the outside of the system, or both, and the gas fraction is fed to the 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 the 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 in the height direction of the product column than the introduction position of the fourth acetic acid stream into the product column.

The fifth acetic acid stream is enriched in acetic acid compared to the fourth acetic acid stream continuously introduced into the product tower. 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 in the figure. Note that instead of (or in addition to) being installed downstream of the distillation tower 6, the ion exchange resin tower 7 may be installed downstream of the product tower to treat the acetic acid stream discharged from the product tower.

Each aspect disclosed in this specification may be combined with any other feature disclosed in this specification. Each configuration in each embodiment and their combinations are merely examples, and additions, omissions, substitutions, and other modifications of configurations are possible as appropriate within the scope of the spirit of this disclosure. Furthermore, each invention related to this disclosure is not limited by the embodiments or the following examples, but is limited only by the scope of the claims.

Below, one embodiment of the present disclosure is described in more detail based on an example.

Example 1
The separation performance of acetaldehyde and methyl iodide for various separation membranes described below was evaluated by pervaporation. Specifically, a mixture (1022.73 g) of 30.78% by mass of acetaldehyde, 0.99% by mass of methyl iodide, and 68.23% by mass of water was first charged into the receiver as the feed liquid components. Then, a separation membrane with a membrane area of 13.8 cm ² was immersed in the mixture, and a line was connected so that the permeation side was led to a vacuum pump. The pressure on the feed liquid side was kept at normal pressure and the temperature at 26.7 ° C., and the mixture in the receiver was stirred with a stirrer. The pressure on the permeation side was set to 10 kPaA (absolute pressure), and the mixture was permeated for about 3 hours, and the permeation liquid was collected by collecting it in a cold trap of methanol and dry ice. Then, the permeability [mol/m ² ·s·Pa] and permeability ratio (permeability of methyl iodide/permeability of acetaldehyde) of each component were calculated. As the separation membrane, the trade name "HZM-2" (Y-type zeolite membrane/alumina porous substrate), the trade name "HZM-3" (MOR-type zeolite membrane/alumina porous substrate), the trade name "HZM-4" (CHA-type zeolite membrane/alumina porous substrate), the trade name "HZM-6" (MFI-type zeolite membrane/alumina porous substrate) (all manufactured by Hitachi Zosen Corporation), and the separation membrane A (MFI-type zeolite membrane/silica porous substrate) were used. The results are shown in Table 1. In Tables 1 to 3, "AD" represents acetaldehyde, and "MeI" represents methyl iodide.

Example 2
The separation performance of the separation membrane "HZM-6" confirmed to permeate in Example 1 was evaluated by pervaporation under heated and pressurized conditions. Specifically, a separation membrane with a membrane area of 10.6 cm ² was first installed in a pervaporation device. As the feed liquid component, a mixture (698.26 g) of 71.79% by mass of acetaldehyde, 7.78% by mass of methyl iodide, 12.45% by mass of water, and 7.98% by mass of other components was charged into the device. The mixture was heated to 39.0 ° C. in a water bath at 75 ml / min with a liquid feed pump, and supplied to the separation membrane while maintaining the pressure on the feed liquid side at 170 kPaG. The pressure on the permeation side was normal pressure, and the mixture was permeated for about 4 hours, and the permeated liquid was collected by collecting it in a cold trap. Then, the permeability [mol/m ² ·s·Pa] and the permeability ratio (permeability of methyl iodide/permeability of acetaldehyde) of each component were calculated.

As can be seen from Table 1, when the MFI type zeolite membrane is used as a separation membrane, unlike other separation membranes, the permeability ratio is large, indicating that it is possible to separate these substances efficiently. On the other hand, when the Y type zeolite membrane, the MOR type zeolite membrane, and the CHA type zeolite membrane are used as separation membranes, none of acetaldehyde, methyl iodide, or water permeates, and acetaldehyde and methyl iodide cannot be separated.

Comparative Example 1
An experiment was carried out using the acetaldehyde separation and removal system shown in FIG. 3. A mixed liquid containing acetaldehyde, methyl iodide, and water was supplied to a distillation tower 91 (plate tower, actual number of stages: 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 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), and 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.

Example 3
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. 1. The process liquid was supplied to the receiver 58 through line 62. The process liquid in the receiver 58 was sequentially supplied to the receiver 59 through lines 63 and 64. The process liquid in the receiver 59 was supplied to the membrane module 60 through lines 66 and 67, and membrane separation was performed under an environment of a temperature of 90° C. and a pressure of 800 kPaG. As the separation membrane, a product name "HZM-6" (manufactured by Hitachi Zosen Corporation) was used. The permeate (line 69) and the concentrated liquid (line 74) were separated by membrane separation. The permeate was recycled to the line 105 through lines 71 and 118. 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.

From Table 2, it can be seen that in Comparative Example 1, which does not have the membrane separation process, methyl iodide is discharged at a flow rate of 2.1 kg/h by distillation from line 115. On the other hand, from Table 3, it can be seen that in Example 3, which has the membrane separation process, of the 2.1 kg/h of methyl iodide that would normally be discharged, the discharge from line 77 can be reduced to 0.3 kg/h. The recovery rate is 85.7%, and the methyl iodide concentration in the discarded concentrated liquid (line 77) is reduced to 1.1 mass%. Therefore, according to the method for producing acetic acid disclosed herein, the loss of methyl iodide is significantly reduced. In addition, the need to newly produce and add methyl iodide is reduced, and the workload and costs are also reduced.

In Tables 2 and 3, the concentrations of each component are listed so that the total of acetaldehyde, methyl iodide, and water is 100.0% by mass. However, in an actual acetic acid production process, each process liquid in the 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.

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 an MFI zeolite membrane.
[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 acetaldehyde concentration in the process stream is 0.1 to 99.9% by mass and the methyl iodide concentration is 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% 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 85% by 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).

[Appendix 11] The method for producing acetic acid according to any one of Appendices 1 to 10, wherein the membrane separation step is carried out under conditions 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).
[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 pressure (for example, an absolute pressure of 110 to 2000 kPa, preferably 200 to 1800 kPa, more preferably 300 to 1500 kPa, even more 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 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;
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 12, wherein the 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 separation step includes (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. The method for producing acetic acid according to Appendix 14.
[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 in which the first acetic acid stream is subjected to distillation to separate and obtain at least a water-rich overhead stream and a second acetic acid stream enriched in acetic acid compared to the first acetic acid stream, or (iv) a dehydration step in which an acetic acid stream rich in acetic acid is separated and obtained in the dehydration step having the function of the dehydration step instead of the dehydration step and the dehydration step, or (v) an acetic acid production method according to appendix 15 or 16, which includes an evaporative dehydration step in which an acetic acid stream rich in acetic acid is separated and obtained in the evaporative dehydration step having the function of the dehydration step instead of the evaporative dehydration step and the dehydration step.
[Appendix 18] The 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 removal step in which the acetic acid stream (e.g., the first or second acetic acid stream) rich in acetic acid 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 enriched in acetic acid compared to the acetic acid stream before distillation. (b) An adsorption removal step in which the acetic acid stream (e.g., the first or second acetic acid stream) or the third acetic acid stream obtained in the separation step is treated with an ion exchange resin to obtain a fourth acetic acid stream. (c) A product step in which the 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 is distilled to obtain a fifth acetic acid stream enriched in acetic acid compared to the acetic acid stream before distillation. [Appendix 19] The acetaldehyde separation and removal system includes an extraction step in which an aqueous phase containing acetaldehyde is separated from at least a portion of the acetaldehyde separated by the one or more distillation processes by one or more extraction processes. The method for producing acetic acid according to any one of appendices 13 to 18.
[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 process in the acetaldehyde separation and removal system, and/or an aqueous layer containing acetaldehyde separated in the extraction process from the acetaldehyde-rich stream, is subjected to the membrane separation step as the process stream.

[Appendix 21] The method for producing acetic acid according to appendix 19 or 20, wherein the process stream comprises an overhead stream from a distillation column performing the distillation treatment.
[Appendix 22] At least a part of the methyl iodide-rich stream obtained by separation in the membrane separation step is recycled to the process in the acetaldehyde separation and removal system (particularly, the distillation column that discharges the process stream). The method for producing acetic acid according to any one of Appendices 13 to 21.
[Appendix 23] 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,
23. The method for producing acetic acid according to any one of claims 13 to 22, 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 24] The method for producing acetic acid according to Appendix 23, wherein the process stream comprises a stream distilled from the distillation step (C1) among the condensate obtained by condensing the second overhead stream.
[Appendix 25] The method for producing acetic acid according to appendix 23 or 24, wherein the methyl iodide-rich stream obtained by separation in the membrane separation step is recycled to one or more steps of the distillation step (A1), the extraction step (B1), and the distillation step (C1) (preferably, the distillation step (A1) and/or the extraction step (B1)).
[Appendix 26] The acetaldehyde separation and removal step comprises:
a distillation step (A2) (preferably an extractive distillation step and/or an azeotropic distillation step) in which at least a portion of the condensate obtained by condensing the stream rich in low boiling components is distilled and a liquid descending from a concentration zone in the distillation column where methyl iodide and acetaldehyde are concentrated is withdrawn 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,
23. The method for producing acetic acid according to any one of claims 13 to 22, 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 27] The method for producing acetic acid according to appendix 26, 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 28] The method for producing acetic acid according to appendix 26 or 27, wherein in the distillation step (A2), the side stream is withdrawn from the distillation column at a position lower than the introduction position of the extraction solvent when the extraction solvent is introduced, and higher than the supply position of the condensate, in the height direction of the distillation column.
[Appendix 29] The method for producing acetic acid according to any one of Appendices 26 to 28, wherein the process stream comprises a condensate obtained by condensing the third overhead stream and/or a condensate of the fourth overhead stream.
[Appendix 30] The process stream includes a stream distilled from the extractive distillation step (D2) among the condensates obtained by condensing the fourth overhead stream. The method for producing acetic acid according to any one of Appendices 26 to 29.
[Appendix 31] The method for producing acetic acid according to any one of appendices 26 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 any one of Appendices 1 to 32, wherein the recovery rate of methyl iodide by the membrane separation step is 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).
[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 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 35] The method for producing acetic acid according to any one of appendices 1 to 34, wherein the recovery rate of acetaldehyde in the membrane separation step is 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).
[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 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 37] The method for producing acetic acid according to any one of Appendices 1 to 36, 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 38] The method for producing acetic acid according to any one of Appendices 1 to 37, wherein the methyl iodide concentration in the methyl iodide-rich stream separated and obtained by the membrane separation step is higher than the methyl iodide concentration in the process stream and is 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).
[Appendix 39] The method for producing acetic acid according to any one of appendices 1 to 38, 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).
[Appendix 40] 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% 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). The method for producing acetic acid according to any one of Appendices 1 to 39.

[Appendix 41] The method for producing acetic acid according to any one of Appendices 1 to 40, wherein the MFI zeolite membrane is provided on at least one surface of a porous substrate.
[Appendix 42] The method for producing acetic acid according to appendix 41, wherein the porous substrate is an alumina substrate.
[Appendix 43] A method for producing an acetaldehyde-rich stream and a methyl iodide-rich stream, comprising separating an acetaldehyde-rich stream and a methyl iodide-rich stream from a mixture containing acetaldehyde and methyl iodide using an MFI zeolite membrane.

According to the method for producing acetic acid disclosed herein, acetic acid can be produced industrially by a methanol carbonylation process (methanol acetic acid process).

REFERENCE SIGNS LIST 1 Reaction tank 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 Vapor flow discharge line 18, 19 Residual liquid flow recycle line 54 Carbon monoxide-containing gas introduction line 55, 56 Potassium hydroxide introduction line 57 Catalyst circulation pump 58, 59, 61 Receiver 60 Membrane module 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 column (acetaldehyde removal column)
98 Distillation tower (extractive distillation tower)
99 Decanter 200 Chimney Tray

Claims (17)

In a process for producing acetic acid, a membrane separation step is performed in which a process stream containing acetaldehyde and methyl iodide is separated into a concentrated stream rich in acetaldehyde and a permeate stream rich in methyl iodide using an MFI zeolite membrane;
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 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;
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 acetaldehyde-rich stream separated in the acetaldehyde separation and removal system is subjected to the membrane separation step as the process stream;
A method for producing acetic acid, wherein the MFI zeolite membrane is provided on at least one surface of a porous substrate. The method for producing acetic acid according to claim 1, wherein the acetaldehyde concentration in the process stream is 0.1 to 99.9 mass %, and the methyl iodide concentration is 0.01 to 99 mass %. 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 1% by mass or more. 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 method for producing acetic acid according to any one of claims 1 to 3, wherein the second overhead stream and/or a condensate obtained by condensing the second overhead stream is subjected to the membrane separation step as the process stream. The method for producing acetic acid according to claim 4, wherein the process stream includes a stream distilled from the distillation step (C1) among the condensate obtained by condensing the second overhead stream. The method for producing acetic acid according to claim 4 or 5, 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). 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,
The method for producing acetic acid according to any one of claims 1 to 3, 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. The method for producing acetic acid according to claim 7, wherein the process stream includes a stream distilled from the extractive distillation step (D2) among the condensate obtained by condensing the fourth overhead stream. The method for producing acetic acid according to any one of claims 1 to 8, wherein the process stream is a liquid. The method for producing acetic acid according to claim 9, wherein the membrane separation step is carried out under a pressure condition of 110 to 2000 kPa (absolute pressure). The method for producing acetic acid according to claim 10, wherein the membrane separation step is carried out under conditions where the temperature of the process stream is 10 to 180°C. The method for producing acetic acid according to any one of claims 1 to 11, wherein the recovery rate of methyl iodide in the membrane separation step is 1 mass% or more. The method for producing acetic acid according to any one of claims 1 to 12, wherein 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 1 mass% or more. The method for producing acetic acid according to any one of claims 1 to 13, 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. The method for producing acetic acid according to any one of claims 1 to 14, wherein the porous substrate is an alumina substrate. Separating and obtaining an acetaldehyde-rich concentrated stream and a methyl iodide-rich permeate stream from a mixture containing acetaldehyde , methyl iodide , and water using an MFI zeolite membrane;
the total concentration of acetaldehyde and methyl iodide in the mixture is 1% by mass or more;
The MFI zeolite membrane is provided on at least one surface of a porous substrate, the method for producing an acetaldehyde-rich stream and a methyl iodide-rich stream. 17. The method for producing an acetaldehyde-rich stream and a methyl iodide-rich stream as claimed in claim 16, wherein the mixture has an acetaldehyde concentration of 0.1 to 99.9 wt. % and a methyl iodide concentration of 0.01 to 99 wt. %.

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