JPH0725814A - Production of highly pure acetic acid - Google Patents

Abstract

PURPOSE:To provide a method for producing highly pure acetic acid having a sufficient quality, namely an excellent permanganate time, by a purification method comprising only a conventional purification process without requiring a purification treatment process such as an ozone treatment. CONSTITUTION:A method for producing acetic acid by continuously reacting methanol and/or a methyl acetate aqueous solution with carbon monoxide in the presence of a rhodium complex as a catalyst and methyl iodide as a cocatalyst is characterized in that acetaldehyde is separated from a process liquid circulated in a reactor to maintain the concentration of the acetaldehyde to <=1500ppm in the reaction solution.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an industrial process for producing highly pure acetic acid. In particular, it relates to a continuous production method of acetic acid having a low content of reducing impurities.

[0002]

2. Description of the Related Art Acetic acid is used in petrochemical industry, organic chemical industry,
It is one of the basic chemicals used in large quantities in the pharmaceutical and agrochemical manufacturing industry, polymer industry, etc.

Various industrial production methods of acetic acid are known. Among them, the method of producing acetic acid by continuously reacting methanol and carbon monoxide is the most industrially excellent method (special feature Kosho 47-3334). Not only is this method highly productive, but the amount of impurities by-produced is small, and acetic acid having a higher purity can be produced than other methods. However, even in this method, a small amount of impurities are by-produced, and in continuous operation over a long period of time, these impurities are mixed into acetic acid which is a product, which deteriorates the quality of the product. Therefore, a large amount of equipment and energy are used for the purification of acetic acid. Further, recently, a reaction condition and a method for improving a catalyst have been disclosed. By adding a catalyst stabilizer such as an iodide salt and reacting under a condition of lower water content than conventional conditions, the productivity of an acetic acid production catalyst is high. An industrial method for producing acetic acid has been disclosed (Japanese Patent Laid-Open No. Sho 60).
-54334, JP-A-60-239434). It is disclosed that by-products such as carbon dioxide and propionic acid are reduced by reducing the water content in the reaction solution.
However, among other minute impurities, the amount of acetic acid produced increases with the productivity of acetic acid, and when the productivity is increased by improving the catalyst or changing the reaction conditions, the quality of acetic acid is poor. It will happen. Especially, reducing substance test
In a quality test that examines the presence of extremely small reducing impurities in acetic acid, called (permanganate time),
Even with today's sophisticated instrumental analysis, it is possible to detect a minute increase in minute concentrations of impurities that are difficult to quantify, and these impurities lead to deterioration in quality. Such impurities are aldehydes, especially unsaturated aldehydes and the like.
These compounds are diverse and it is not practical to separate and remove each one. In particular, crotonaldehyde and 2-ethylcrotonaldehyde, which are dehydration condensates of acetaldehyde, have similar boiling points to acetic acid, and it is difficult to separate minute amounts of these impurities by distillation.

Therefore, in the conventional technique, crude acetic acid containing these minute reducing impurities is treated with ozone (Japanese Patent Publication No. 61-2052).
No.) or an oxidant (Japanese Patent Publication No. 56-10297). However, treatment with ozone or an oxidant has a limit on the concentration of impurities to be treated.
For example, the only compounds that can be treated with ozone are unsaturated compounds, saturated aldehydes do not decompose, and the compounds generated by the decomposition of unsaturated compounds are saturated aldehydes, and Aldecht itself is also reductive. This is because it is a compound that deteriorates the permanganate time. Therefore, after treatment with ozone, purification such as distillation to remove saturated aldehydes or treatment with activated carbon is required (JP-A 1-2115).
No. 48).

[0005]

The present inventors have identified in detail the reducing impurities that deteriorate the quality of acetic acid and studied the generation pathways of these compounds in detail. As a result, the acetaldehyde generated during the reaction is reduced by these reduction impurities. It was found to be the fundamental causative agent of trace trace impurities. In the conventional acetic acid production process, components with a boiling point lower than that of acetic acid contained in the reaction solution, that is, methyl acetate generated from methanol as a raw material, methyl iodide as a co-catalyst, water present in the reaction solution, etc. react. Almost all separated in the process of separating acetic acid from the crude liquid so as not to wastefully consume these compounds, are recycled to the reactor and reused.
That is, a small amount of acetaldehyde by-produced in the reactor circulates without being removed from the process and accumulates in the reaction solution as the reaction continues. Accumulated acetaldehyde condenses under reaction conditions to yield unsaturated aldehydes.
Further, it is reduced by hydrogen existing under the reaction conditions and further reacted to form propionic acid. Since propionic acid has a higher boiling point than acetic acid, it is extracted from the reaction system together with acetic acid in the process of separating acetic acid, and is separated from acetic acid as a high-boiling fraction in a distillation column for the purification distillation of acetic acid, and then leaves the process. Go. That is, acetaldehyde generated in the reactor is discharged from the process as propionic acid, mixed with acetic acid as a condensate and discharged out of the system, or distilled out as a low boiling point fraction and discharged out of the system.

That is, the acetaldehyde concentration of the reaction solution in the continuous reaction in the steady state is such that the amount of acetaldehyde continuously generated in the reactor is distilled off as a high boiling point component during the purification of acetic acid, and propionic acid is discharged to the outside of the system. , And a concentration that is stoichiometrically approximately equal to the total amount of low-boiling aldehydes and esters derived from acetaldehyde and mixed in acetic acid or distilled off as a low-boiling substance.

Among these, unsaturated aldehydes having a boiling point close to that of acetic acid produced by the condensation of acetaldehyde become difficult to be separated in the purification step of acetic acid and are mixed as impurities in acetic acid, resulting in deterioration of quality in a test for reducing substances of acetic acid. cause.

[0008]

[Means for Solving the Problems] From the above chemical analysis, the inventors of the present invention, by controlling the concentration of acetaldehyde in the reactor within 1500ppm, impurities derived from acetaldehyde, namely, It is possible to control the amount of impurities such as propionic acid, crotonaldehyde, and 2-ethylcrotonaldehyde that adversely affect the quality of acetic acid, and to control the amount of each compound from acetic acid. They have found that high-quality acetic acid can be produced without spending energy, and completed the present invention.

That is, the present invention uses a rhodium complex as a catalyst,
In the method for producing acetic acid by continuously reacting methanol and / or methyl acetate aqueous solution with carbon monoxide using methyl iodide as a co-catalyst, the reaction is performed by separating acetaldehyde from the process liquid circulating in the reactor. The present invention relates to a method for producing high-purity acetic acid, which comprises carrying out the reaction while keeping the acetaldehyde concentration in the liquid at 1500 ppm or less.

A method for continuously producing acetic acid using methanol and carbon monoxide as raw materials is called Monsanto method, and the process is disclosed by Monsanto Company (US Patent
4,102,922, Hydrocarbon Processing, November, 1972
Others) (Fig. 1). The raw material methanol and carbon monoxide are continuously charged in the reactor (1) and continuously reacted at a predetermined temperature and pressure. The reaction temperature is usually 150 to 250 ° C., and the reaction pressure is 15 to 40 atm. In the reaction solution, the catalyst rhodium complex was present at a concentration of 200 to 1000 ppm, the cocatalyst methyl iodide was 5 to 20 wt%, and the solvent water was 0.1 to 15 wt%. , And acetic acid as a solvent constitutes the remaining main component. In addition, since it is a continuous reaction, the methyl acetate produced when the raw material reacts with acetic acid
It exists in the range of 0.1 to 30 wt%. In addition, as a result of continuous production for a long time, which is actually performed, trace impurities accumulated in the reaction solution, that is, unsaturated aldehydes such as acetaldehyde, crotonaldehyde, and 2-ethylcrotonaldehyde, and further in the reactor. There are saturated aldehydes, saturated aliphatic carboxylic acids, such as propionic acid, their methyl esters, etc., which are products hydrogenated by the hydrogen present in. Under the conditions of high productivity disclosed in recent years (JP-A-60-54334 and JP-A-60-239434).
No.), the water concentration in the reaction solution can be reduced, and the amount of side reaction products generated can be reduced. Under this condition, the stability of the catalyst deteriorates, so 0.1 to 20 wt% of iodine ion is added as a stabilizer of the rhodium catalyst. When iodine ions are introduced, an iodide salt having an alkali metal, a quaternary ammonium ion, and a quaternary phosphonium ion as a counter cation and soluble in a reaction solution is added. Under this highly productive condition, a large amount of basic cations are present in the reaction solution as counter cations for iodine ions, which promotes the condensation of acetaldehyde and increases the amount of reducing substances generated. Be done.

In the present invention, when the reaction conditions are water of 5 to 10 wt%, the reaction solution composition contains 1 to 10 wt% of iodine ions as a catalyst stabilizing component, and the water content is 5 wt.
%, It is preferable to carry out the reaction with a reaction liquid composition containing 1 to 20 wt% of iodine ions as a catalyst stabilizing component. Alkali metal ions are preferred as counter cations for iodine ions, and lithium ions are particularly preferred.

Under such reaction conditions, the concentration of acetaldehyde in the reaction solution is 1500 ppm or less, preferably 1000 pp.
By keeping it at m or less, the amount of impurities produced is suppressed, and in the purification of crude acetic acid, high-purity acetic acid can be produced only by a basic and simple distillation operation.

The concentration of acetaldehyde in the reaction solution was 1500
Keeping below ppm can be achieved by controlling the reaction conditions or removing acetaldehyde from the process liquid circulating to the reactor. In controlling the reaction conditions, the hydrogen partial pressure is most effective. By decreasing the hydrogen partial pressure, the amount of acetaldehyde generated decreases, and as a result, the amount of propionic acid generated decreases. However, unexpectedly, a decrease in the hydrogen partial pressure causes hydrogenation of reductive unsaturated compounds that have a great adverse effect on the test for reducing substances such as crotonaldehyde and 2-ethylcrotonaldehyde, resulting in a saturated compound. It may deprive you of the opportunity to render it harmless, worsen the quality and time of permanganate, and only give you acetic acid of low purity. From this, it was found that it is preferable to remove acetaldehyde from the process liquid circulating in the reactor in order to control the acetaldehyde in the reactor to 1500 ppm or less. A method for removing acetaldehyde from the process liquid circulating in the reactor will be described below by using FIG. 1 as an example of the process.

The reaction solution is continuously withdrawn from the reactor (1), introduced into an evaporation tank (2) controlled to have a pressure lower than the reaction pressure, and flash-evaporated. The components evaporated in the evaporation tank are introduced into the distillation column (3) and distilled. In this distillation column, low boiling point components are mainly separated. Distillate at the top of the distillation column
It is introduced into (4) and separates into two phases. The upper layer consists of an aqueous phase containing acetic acid and the lower layer consists of a methyl iodide phase containing acetic acid and water. Acetaldehyde is concentrated in these two phases.
These two phases are returned to the reactor. The high boiling components of the distillation column (3) are also returned to the reactor. Distillation column containing acetic acid as a product (3)
The side cut liquid of (1) is introduced into the next distillation column (5). The crude acetic acid dehydrated in the distillation column (5) is extracted from the bottom of the column and distilled in the next distillation column (6). Acetaldehyde is also contained in the top liquid of the distillation column (5), but this liquid is also returned to the reactor. In the distillation column (6), propionic acid, which is a high-boiling product, and a small amount of high-boiling impurities are extracted from the bottom, and low-boiling impurities are extracted from the top. Product acetic acid distillation column (6)
Obtained with a side cut. Possibly distillation column (6)
Can also be separated into a high boiling column and a low boiling column to purify acetic acid.

On the other hand, although the acetaldehyde is contained in the offgas of the reactor and the offgas discharged from each distillation column, it is recovered in the absorption system (7) together with the organic components in the offgas such as methyl iodide, and is collected in the reactor. Will be returned.

As a method for separating acetaldehyde from such a process, it is preferable to separate only acetaldehyde from a process liquid containing acetaldehyde by distillation separation, extraction, a combination of extraction and distillation, extractive distillation or the like. In particular, as the process liquid that can effectively perform the separation of acetaldehyde by distillation, the upper and lower layers of the separation tank (4), the top liquid of the distillation column (5), and the liquid that has absorbed the organic matter in the offgas in the absorption system are acetaldehyde. Is preferred because of its high concentration, but among these, the upper and lower layers of the separation tank (4) are more preferred. Methyl iodide is present in the process liquid containing these acetaldehydes. Methyl iodide and acetaldehyde have the closest boiling points, and it is the most difficult to separate them. However, methyl iodide is a compound containing iodine and it is difficult to dispose of it.
It is necessary to recover and return to the reactor. As a method for separating acetaldehyde, a process liquid containing acetaldehyde can be separated and distilled in a single distillation column,
Preferably, the low boiling point component consisting of acetaldehyde and methyl iodide can be first separated from other components by distillation, and then methyl iodide and acetaldehyde can be further separated by distillation. Further, by utilizing the property that acetaldehyde is well mixed with water and methyl iodide is hardly mixed with water, water extraction may be used to separate methyl iodide and acetaldehyde, and then distillation may be performed. The amount of acetaldehyde to be separated from the process is such that the acetaldehyde concentration in the reaction solution during the steady continuous reaction can be kept at 1500 ppm or less, preferably 1000 ppm or less. In essence, the total amount of acetaldehyde generated under steady-state continuous reaction conditions and extracted from the process under steady-state continuous reaction conditions, that is, propionic acid, crotonaldehyde, 2-
The amount is approximately equal to the equivalent amount of acetaldehyde equivalent to the total amount of ethyl crotonaldehyde. In practice,
Propionic acid is the most quantitative and most of it, so it is sufficient to extract the molar amount of acetaldehyde corresponding to the molar amount of propionic acid.

That is, by extracting acetaldehyde from the process liquid, not only reducing trace impurities derived from acetaldehyde in the acetic acid product can be reduced, but also propionic acid which is a by-product of the Monsanto acetic acid can be reduced. Has the advantage that it can be easily purified.

[0018]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

Example 1 It is shown that acetaldehyde can be separated by distillation.
A model liquid of the upper layer liquid of a separating tank containing 1 wt% of acetaldehyde was prepared and distilled under the following conditions in an Oldasha distillation column having an inner diameter of 40 mm and a plate size of 30. Feed composition: Methyl iodide 7 wt% Acetic acid 44 wt% Water 48 wt% Acetaldehyde 1.0 wt% Distillation conditions: Reflux ratio 1.5 Feed amount 100 parts Extraction amount 8.5 parts from the top of the tower, 91.5 parts from the bottom of the tower 17 from the top of the feed stage Most of all acetaldehyde and methyl iodide charged in the liquid distilled from the top of the first distillation column were distilled. The composition of this liquid was as follows. Methyl iodide 82.3 wt% Water 5.9 wt% Acetaldehyde 11.8 wt% When this distillate was distilled under the following conditions in a distillation column with an inner diameter of 40 mm and 60 stages, acetaldehyde was distilled from the top of the column,
Separated from methyl iodide.

Distillation conditions: 57th from the top of the feed stage Reflux ratio 150 Feed amount 100 parts Extraction amount 11 parts from the top of the tower, 89 parts from the bottom of the tower Top liquid composition: acetaldehyde 99.99 wt% Methyl iodide 100 ppm or less.

Example 2 A model liquid of a lower layer liquid of a separating tank containing 0.4 wt% of acetaldehyde was prepared in the same manner as in Example 1, and was distilled under the following conditions in an Oldshaw distillation column having an inner diameter of 40 mm and 30 stages.

Composition of feed liquid: methyl iodide 90.2 wt% methyl acetate 5.0 wt% acetic acid 3.5 wt% water 0.9 wt% acetaldehyde 0.4 wt% Distillation conditions; reflux ratio 3.0 feed amount 100 parts withdrawal amount 94 parts from top, bottom 6 parts from the 17th stage from the top of the feed stage Most of all the acetaldehyde and methyl iodide charged were distilled in the liquid distilled from the top of the distillation column. The composition of this liquid was as follows.

Methyl iodide 93.3 wt% Methyl acetate 5.3 wt% Water 0.96 wt% Acetaldehyde 0.43 wt% When this distillate was distilled under the following conditions in a distillation column having an inner diameter of 40 mm and 60 plates, acetaldehyde was produced from the top of the column. Distilling,
Separated from methyl iodide. Distillation conditions: 57th stage from the top of the feed stage Reflux ratio 150 Feed amount 100 parts Extracted amount 0.28 parts from the top of the column (equivalent to 70 wt% of the acetaldehyde charged) Top liquid composition: Acetaldehyde 99.99 wt% Methyl iodide 100 ppm or less .

Example 3 The following experiment was conducted based on the method shown in FIG. The raw material methanol and carbon monoxide are continuously charged into the reactor (1) and continuously reacted at a temperature of 187 to 189 ° C. and a pressure of 28 kg / cm ² . The reaction solution is continuously withdrawn from the reactor, introduced into an evaporation tank (2) having a pressure lower than the reaction pressure, and flash-evaporated. The components evaporated in the evaporation tank are introduced into the distillation column (3) and distilled. The distillate at the top of the distillation column is introduced into the separation tank (4) and separated into two phases. The high boiling components of the distillation column (3) are also returned to the reactor. The side cut liquid of the distillation column (3) containing acetic acid as a product is introduced into the next distillation column (5). The crude acetic acid dehydrated in the distillation column (5) is extracted from the bottom of the column and distilled in the next distillation column (6). In the distillation column (6), propionic acid, which is a high-boiling product, and a small amount of high-boiling impurities are extracted from the bottom, and low-boiling impurities are extracted from the top. The product acetic acid is obtained by side-cutting the distillation column (6). On the other hand, although the acetaldehyde is contained in the offgas of the reactor and the offgas discharged from each distillation column, the absorption system together with the organic components in the offgas such as methyl iodide (7)
And then returned to the reactor. In this process, continuous reaction was carried out in a state where acetaldehyde was present in the reaction liquid in the reactor in an amount of 800 to 1000 ppm. The reaction conditions were a rhodium concentration of 450 ppm, methyl iodide 13 wt%, water 8 wt%, lithium iodide 4.5 wt% and methyl acetate 1.2 wt%. Under this production condition, water containing 0.5 wt% of acetaldehyde, acetic acid,
The upper layer liquid of the liquid separation tank (4) consisting of methyl iodide and methyl acetate was withdrawn at an amount of 0.25 liters per hour (1/3 of the total amount of the upper layer liquid), 1.2 kg / cm in a distillation column with an inner diameter of 60 mm and 40 stages. ^It was continuously distilled under a pressure of ² G. The preparation stage was the 20th stage from the top. The reflux ratio of the overhead liquid was 3.0. Every hour from the bottom of the tower
0.24 liters were withdrawn and returned to the reactor. Overhead liquid consisting mainly of methyl iodide and acetaldehyde is delivered from the top of the tower every hour.
Withdraw 0.015 liters and remove the liquid with an inner diameter of 50 m
Prepared at the 57th stage of a 60-stage distillation column with a pressure of 1.0 kg / cm ² G
Distilled at a reflux ratio of 40 under pressure. 0.8 g of acetaldehyde was separated from the top of the column every hour. From the bottom of the column, 0.015 liter of liquid was withdrawn every hour and returned to the reactor. The permanganate time in the acetic acid obtained by this process, that is, the acetic acid obtained by the side cut of the distillation column 6, was 240 minutes.
The concentration of acetaldehyde in the reaction solution is 800-1000.
It was kept at ppm.

Comparative Example In Example 3, the reaction was continuously performed under the same reaction conditions and operating conditions without removing acetaldehyde by distillation.
At this time, the acetaldehyde concentration in the reaction system is 1700 to 1800.
The steady state was reached at ppm and the permanganate time of the acetic acid obtained at steady state was 60 minutes.

[0026]

INDUSTRIAL APPLICABILITY According to the present invention, from a low boiling process liquid containing acetaldehyde circulating in a reactor in a continuous production process of acetic acid, acetaldehyde which is comparable to the amount generated under the reaction conditions can be separated by distillation. By removing it, the acetaldehyde concentration in the reaction solution under the reaction conditions can be increased.
By controlling the reaction within 1500 ppm, the purification process such as ozone treatment is not required, and only the purification process that is normally performed is sufficient to obtain sufficiently high quality, that is, excellent permanganate time. Acetic acid is obtained.

[Brief description of drawings]

FIG. 1 is a flow chart of a Monsanto method for continuously producing acetic acid.

[Explanation of symbols]

 1 Reaction tank 2 Evaporation tank 3 Low boiling distillation column 4 Liquid separation tank 5 Distillation tower (dehydration tower) 6 Acetic acid distillation tower 7 Absorption system

Claims (2)

[Claims] 1. A method for producing acetic acid by continuously reacting an aqueous methanol and / or methyl acetate solution with carbon monoxide using a rhodium complex as a catalyst and methyl iodide as a co-catalyst to produce acetic acid. Concentration 15
A method for producing high-purity acetic acid, which is characterized in that the reaction is carried out while keeping it at 00 ppm or less. 2. The production of high-purity acetic acid according to claim 1, wherein the reaction is carried out by separating acetaldehyde from the process liquid circulating in the reactor to keep the acetaldehyde concentration in the reaction liquid at 1500 ppm or less. Method.