JP3213392B2 - Acetic acid production method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing acetic acid from methanol and carbon monoxide.

[0002]

2. Description of the Related Art Acetic acid is one of the basic chemicals, and is an important chemical in petrochemical industry, polymer chemical industry, organic chemical industry, and pharmaceutical and agrochemical manufacturing industries. There are various methods for producing acetic acid, and among them, the method for producing acetic acid from methanol and carbon monoxide is the most industrially superior method.

In recent years, as an improved method of this method, it has been proposed to reduce the water concentration in the reaction solution (Japanese Patent Application Laid-Open Nos. 60-54334, 60-239434, 60-155147). That is, there is disclosed a technique in which the productivity of acetic acid is increased by reducing the water concentration in the reaction solution, and the amount of by-products is reduced. In these techniques, when the water concentration is 10 wt% or less, the stability of the rhodium catalyst is reduced. Therefore, it is effective to add an iodide such as an alkali metal iodide, a quaternary ammonium salt, or a quaternary phosphonium salt. It is also disclosed. Further, since the reaction rate is significantly reduced when the water concentration in the reaction solution is 5% by weight or less, a technique for increasing the reaction rate by adding 5 to 30% by weight of lithium iodide is disclosed.

[0004]

Generally, in an industrial process for producing acetic acid from methanol and carbon monoxide,
Methanol and carbon monoxide are continuously introduced into the reactor containing the reaction solution and reacted, and the reaction solution is continuously taken out of the reactor, and the components that evaporate in the evaporating tank having a lower pressure than the reactor do not evaporate. Separate into components. The components to be evaporated mainly include methyl iodide, which is one of the main catalysts, methyl acetate generated from the raw material methanol, water contained in the reaction solution, and acetic acid, which is a product and a reaction solvent. The components that do not evaporate include methyl iodide, methyl acetate, water, and acetic acid remaining after evaporation of the components contained in the evaporating components, as well as a rhodium complex as a catalyst and lithium iodide as a rhodium stabilizer. included.

[0005] This evaporation operation is most preferably performed by adiabatic evaporation from the viewpoint of use of energy. However, in the case of adiabatic evaporation, the amount of evaporation is limited.
It is preferable to reduce the amount of evaporation of components other than acetic acid.
That is, the amounts of methyl iodide, methyl acetate and water may be reduced. Of these compounds, water has the largest latent heat of vaporization, and it is most effective to reduce the amount of water. That is, it is for this reason that the productivity of acetic acid can be increased by lowering the water concentration in the reaction solution.

[0006] However, it has been found that when the water content is reduced to 5 wt% or less, in addition to the above-mentioned obstacles, there is a problem that the reaction activity gradually decreases in the case of a continuous reaction performed industrially. This is because the main catalyst, rhodium, is active.
This is caused by changing from a valent complex to an inactive trivalent complex. Under a reaction condition containing a large amount of water, a shift reaction occurs between the raw material carbon monoxide and water, and hydrogen and carbon dioxide are generated. The hydrogen generated here has the function of converting trivalent rhodium into a monovalent complex.

That is, when the amount of generated hydrogen is large, the inactive trivalent rhodium complex is rapidly converted to monovalent, so that the catalytic activity is maintained. On the other hand, under the reaction conditions with a small amount of water, the amount of generated hydrogen is small, so that the degree of conversion of the trivalent rhodium complex to monovalent rapidly decreases, and the reaction activity gradually decreases along with the catalytic activity. In response to such adverse effects caused by lowering the water content, hydrogen is charged into the reaction system, and the hydrogen partial pressure in the reaction system is maintained at a certain pressure or higher, thereby maintaining the speed of converting rhodium trivalent to monovalent. , being able to maintain the reaction activity has been disclosed (JP-62 -298549).

However, charging hydrogen into the reactor and keeping the hydrogen partial pressure high eliminates the effect of reducing by-products, which is one of the advantages obtained by reducing the water concentration in the reaction solution. That is, by reducing the water content in the reaction solution, the shift reaction between carbon monoxide and water is suppressed, and the hydrogen partial pressure is reduced, whereby propionic acid, formic acid,
The effect of reducing by-products generated by hydrogenation of acetic acid such as hydrocarbons is lost. That is, when hydrogen is added to the reactor to maintain the activity of rhodium under a reaction condition in which the water content is 5 wt% or less and the hydrogen partial pressure is maintained at a certain pressure or more, formic acid, in proportion to the hydrogen partial pressure,
By-products such as propionic acid and hydrocarbons increase. The present invention particularly, the water concentration in the reaction solution is 10% or less,
In particular, the present invention relates to an improvement in technology when the reaction rate is increased to 5 wt% or less, that is, when lithium iodide must be added to increase the reaction rate.

[0009]

The present inventors have conducted intensive studies on a method for producing acetic acid which does not have the above-mentioned disadvantages. As a result, it is not the reactor that makes rhodium inactive trivalent but monoxide. It has been found that the catalyst liquid containing rhodium or the evaporating tank in which the reaction does not occur due to the low partial pressure of carbon is returned from the evaporator to the reactor. Thus, the presence of hydrogen during the return to the reactor from the evaporation tank or the evaporation tank prevents rhodium from becoming trivalent and / or
By converting the monovalent rhodium to monovalent, the activity of the catalyst in the reactor is maintained without increasing the hydrogen partial pressure of the reactor to a certain pressure or more, and formic acid, propionic acid, hydrocarbon It has been found that the generation of hydrogenation by-products such as hydrogen can be suppressed, and the present invention has been completed.

That is, the present invention provides a method for producing acetic acid by reacting methanol and carbon monoxide in the presence of a rhodium catalyst and methyl iodide, wherein the water content of the reaction solution is 10 wt%.
A continuous reaction is performed as follows, and the reaction solution is continuously withdrawn and introduced into an evaporation step having a pressure lower than the reaction conditions to separate an evaporating component and a non-evaporating component containing rhodium. And / or evaporating under the condition that the hydrogen partial pressure is at least 0.1 atm or more, and / or (b) separating the non-evaporating component containing the separated rhodium with at least hydrogen having a hydrogen partial pressure of at least 0.1 atm and at least 0.1 atm. The present invention relates to a method for producing acetic acid, characterized in that after being treated with carbon oxide, it is returned to a reactor and recycled.

The reaction is carried out by a continuous reaction system, and methanol and carbon monoxide as raw materials are continuously charged into a reactor, respectively. The reactor may be a gas-liquid mixing tank, a stirring mixing tank having a stirrer, or a bubble column type reactor having no stirring. The reaction is carried out at a temperature of 150 to 200 ° C. and a total reaction pressure of 20 to 50 atm.

As the main catalyst, a rhodium complex and methyl iodide are used. The concentration of rhodium in the reaction solution is 200-2
Although 000 ppm is used, if the concentration of rhodium is too high, sedimentation of rhodium iodide occurs, so it is preferably used in the range of 300 to 750 ppm, depending on the composition of the reaction solution. As long as rhodium dissolves under the reaction solution composition and reaction conditions, methyl iodide may be charged into the reactor in any form, and the concentration in the reaction solution is 10 to 20 wt%. It is preferable to use in the range. The reaction is promoted when the concentration of methyl iodide is high, but it is necessary to recover methyl iodide and circulate it to the reactor.This is economically most advantageous because of the size of the circulation process and energy consumption. Methyl iodide concentrations range from 10 to 20 wt%.

The reaction solution contains 0.5 to 10% by weight of methyl acetate depending on the equilibrium between the raw material methanol and acetic acid.

In the process of the present invention, the water content in the reactor is controlled in the range of 0.1 to 10% by weight, preferably 0.1 to 5% by weight. The lower the water concentration, the less the amount of hydrogen generated by the shift reaction, and the more the amount of by-products such as formic acid, propionic acid, and hydrocarbons is reduced, which is advantageous. . For this,
For promoting the reaction and stabilizing rhodium, a quaternary ammonium iodide salt, a quaternary phosphonium iodide salt, a lithium salt such as lithium iodide or lithium acetate can be added. The added amount is 5 in the concentration in the reaction solution.
It can be used in the range of 3030 wt%. Acetic acid, which is also a product, is suitably used as a solvent.

The reaction liquid is continuously withdrawn and introduced into an evaporation tank controlled at 1 to 3 atm through a valve. Here, the evaporating component of the extracted reaction solution, that is, a part of methyl iodide as a catalyst, a part of methyl acetate derived from unreacted methanol, a part of water, and acetic acid which is both a reaction solvent and a product Is partially taken out as vapor and guided to a low-boiling-point recovery step and an acetic acid purification step. Methyl iodide, methyl acetate and water are separated in the low boiling point recovery column, and each is circulated to the reactor through a pump. A reaction solution component (a catalyst circulation solution) containing rhodium as a catalyst that has not evaporated in the evaporation tank is circulated to the reactor through a pump.

In the present invention, when hydrogen is charged into the evaporating tank, hydrogen is introduced so that the hydrogen partial pressure in the evaporating tank is maintained at least at least 0.1 atm. This hydrogen prevents rhodium from changing to trivalent, and maintains the activity in the reactor without increasing the hydrogen partial pressure in the reactor.

In the case where the catalyst circulating liquid is subjected to hydrogen treatment, trivalent rhodium is effectively converted to monovalent rhodium by performing hydrogen treatment in a treatment tank before returning to the reactor. In the hydrotreating tank, the catalyst circulating liquid is treated at a temperature of 100 to 150 ° C., a hydrogen partial pressure of at least 0.1 atm or more, and a carbon monoxide partial pressure of at least 0.1 atm or more. The hydrogen treatment may be performed continuously or batchwise, but continuous treatment is preferable because of higher productivity. Hydrogen treatment of the catalyst circulation liquid can also be performed by charging hydrogen and carbon monoxide to a pipe for returning the catalyst circulation liquid from the evaporating tank to the reactor.

[0018]

EXAMPLES Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited to these examples.

Example 1 (1) Preparation of circulating solution of catalyst 0.1 g of rhodium chloride trihydrate (RhCl ₃ .3H ₂ O) and 40 g of lithium iodide (LiI) were placed in an autoclave made of Hastelloy B-2 having a content of 500 cc. , Methyl iodide 24g, water 4
g, acetic acid 118 g, and methanol 14 g, and the air inside was replaced with carbon monoxide, and then pressurized to 40 g / cm ² G with carbon monoxide. Next, the temperature of the autoclave was raised to 100 at 185 ° C.
Allowed to react for minutes. The reaction solution was sampled at an appropriate time, and the reaction rate at the time when the water concentration in the reactor was about 4 wt% was measured by the rate of carbon monoxide absorption, that is, the rate of pressure decrease, and was 4.5 mol / hr. Met.

After the completion of the reaction, the mixture was cooled to release the remaining carbon monoxide, and then the contents of the autoclave were transferred to a flash evaporator under a nitrogen stream. From the gas chromatography analysis of the reaction solution, the amount of propionic acid produced was 30 ppm or less. (30 ppm is the quantification limit of this gas chromatography.) The reaction solution was subjected to flash distillation at a bottom temperature of 120 to 138 ° C. to reduce the liquid volume to one half. The composition of the concentrated solution is methyl iodide 0.5wt%, methyl acetate 0.2wt%, acetic acid
6.1 wt% and water 1.2 wt%.

A liquid corresponding to the catalyst circulation liquid was used in the next experiment.

(2) Hydrogen treatment The liquid equivalent to the catalyst circulating liquid obtained as described above is placed in an autoclave, the space is replaced with a mixed gas of 4 volumes of hydrogen and 1 volume of carbon monoxide, and heated to 140 ° C. Treated for 30 minutes.

(3) Reaction Next, in order to carry out a reaction with the same reaction solution composition as in the above (1), a mixed solution of 23.5 g of methyl iodide, 54 g of acetic acid, 2.5 g of water and 14 g of methanol was added to a hydrogen-treated catalyst solution. After adding
The reaction was performed under the same reaction conditions as in (1). That is, the reaction solution is sampled at an appropriate time, and
When the reaction rate at wt% was measured by the same method as in (1), it was 4.3 mol / hr. After completion of the reaction, when the propionic acid content of the reaction solution was measured by gas chromatography,
It was 30 ppm or less, which is the quantification limit of gas chromatography, and it was found that propionic acid did not increase by this reaction.

Comparative Example 1 The hydrogen treatment step of Example 1 was omitted, and methyl iodide, water, and acetic acid were added to the catalyst solution obtained in Example 1 (1) in the same manner as in Example 1 (3). Then, an appropriate amount of methanol was added, and the same reaction as in (3) of Example 1 was performed. The reaction solution was sampled at an appropriate time, and the reaction rate was determined from the gas absorption rate at a time when the water content was about 4 wt%, and it was 3.2 mol / hr. It can be seen that the reaction activity is lower than in Example 1.

Comparative Example 2 The hydrogen treatment step of Example 1 was omitted, and methyl iodide, water, acetic acid, and methanol were added in appropriate amounts to the catalyst solution obtained in Example 1, (1). The same reaction as in (3) was performed. However, instead of pressurizing to 40 g / cm ² G with carbon monoxide, pressurizing to 40 g / cm ² G with carbon monoxide containing 5 wt% of hydrogen. The hydrogen partial pressure is about 2 atmospheres. The reaction solution was sampled at an appropriate time, and the reaction rate was determined from the gas absorption rate at the time of 4 wt% of water, and it was 4.5 mol / hr. After the reaction was completed, the content of propionic acid in the reaction solution was measured by gas chromatography and found to be 185 ppm. The amount of propionic acid increased by the reaction of Comparative Example 2 was found to be about 150 ppm. Although the reaction activity was restored,
It can be seen that the amount of propionic acid generated is larger than that of the above.

Claims (4)

(57) [Claims] 1. In the presence of a rhodium catalyst and methyl iodide,
In the method of producing acetic acid by reacting methanol and carbon monoxide, a continuous reaction is carried out at a water concentration of 10 wt% or less in the reaction solution, and the reaction solution is continuously withdrawn and an evaporation step having a pressure lower than the reaction conditions. In the step of separating into components that evaporate and components that do not evaporate including rhodium,
(A) introducing hydrogen and evaporating under conditions where the hydrogen partial pressure is at least 0.1 atm or / and (b) separating the non-evaporable components containing rhodium at least with a hydrogen partial pressure of at least 0.1 atm.
A method for producing acetic acid, comprising treating with at least 1 atm of hydrogen and at least 0.1 atm of carbon monoxide, returning the reactor to a reactor, and recycling. 2. The method according to claim 1, wherein the reaction solution contains 5 to 30% by weight of an iodide salt. 3. The method according to claim 2 , wherein the iodide salt is lithium iodide. 4. The method according to claim 1, wherein the concentration of water in the reaction solution is 0.1 to 10% by weight.