JPH0640998A - Production of acetic acid - Google Patents

Abstract

PURPOSE:To obtain acetic acid while retaining catalytic activity in the reactor and also suppressing formation of hydrogenated by-products such as formic acid, propionic acid and/or hydrocarbons. CONSTITUTION:In producing acetic acid by reaction between methanol and carbon monoxide in the presence of a rhodium catalyst and methyl iodide, continuous reaction is made under such a condition as to be <=10wt.% in the water content of the reaction liquor, and the resultant reaction liquor is continuously drawn and introduced into an evaporation process lower in pressure than the reaction process, where the liquor is separated into an evaporable component and non-evaporable component containing the rhodium. In this evaporation process, (a) hydrogen gas is introduced and the evaporation is made under a partial hydrogen pressure of >=0.1atm and/or (b) the non-evaporable component is treated with hydrogen gas >=0.1atm in partial pressure and carbon monoxide >=0.1atm in partial pressure and then returned to the reactor, thus putting the component to circulatory use.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION 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 the petrochemical industry, polymer chemistry industry, organic chemistry industry, pharmaceutical and agrochemical manufacturing industry. 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 excellent method industrially.

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

[0004]

Generally, in the industrial method for producing acetic acid from methanol and carbon monoxide,
Methanol and carbon monoxide are continuously introduced into the reaction vessel containing the reaction solution to react, and the reaction solution is continuously taken out from the reaction vessel and does not evaporate with the component that evaporates in the evaporation tank whose pressure is lower than that of the reaction vessel. Separate into components. The components to be evaporated mainly include methyl iodide, which is one of the main catalysts, methyl acetate generated from methanol as a raw material, water contained in the reaction solution, and acetic acid which is a product and a reaction solvent. Components that do not evaporate include methyl iodide, methyl acetate, water, and acetic acid remaining in the components that evaporate, as well as rhodium complexes that are catalysts and lithium iodide that is a stabilizer of rhodium. included.

This evaporation operation is most preferable in terms of energy use if it is carried out by adiabatic evaporation, but in adiabatic evaporation the amount of evaporation is limited, so in order to evaporate more acetic acid,
It is preferable to reduce the evaporation amount of components other than acetic acid.
That is, the amounts of methyl iodide, methyl acetate and water should be reduced. Of these compounds, water has the highest latent heat of vaporization, and it is most effective to reduce the amount of water. That is, it is for this reason that the acetic acid productivity can be increased by decreasing the water concentration in the reaction solution.

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 an adverse effect that the reaction activity gradually decreases in the case of a continuous reaction carried out industrially. This is because the main catalyst, rhodium, is active.
This is caused by the change from a valent complex to an inactive trivalent complex. Under a reaction condition containing a large amount of water, a shift reaction between carbon monoxide as a raw material and water occurs to generate hydrogen and carbon dioxide. Hydrogen generated here has a function of converting trivalent rhodium into a monovalent complex.

That is, when a large amount of hydrogen is generated, an inactive trivalent rhodium complex is rapidly converted into a monovalent one, so that the catalytic activity is maintained. On the other hand, under a reaction condition in which the water content is low, the amount of hydrogen generated is small, so that the degree of rapid conversion of the trivalent rhodium complex into a monovalent compound becomes small, and the catalytic activity and the reaction activity gradually decrease. To prevent 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 level or higher to maintain the speed of converting trivalent rhodium to monovalent. It has been disclosed that the reaction activity can be maintained (JP-A-60-298549).

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 merits obtained by lowering the water concentration in the reaction solution. That is, by reducing the water content in the reaction solution, the shift reaction of carbon monoxide and water is suppressed and the hydrogen partial pressure is reduced, whereby propionic acid and formic acid,
The effect of reducing by-products generated by the hydrogenation reaction of acetic acid such as hydrocarbon is lost. That is, if hydrogen is added to the reactor in order to maintain the hydrogen partial pressure above a certain pressure in order to maintain the activity of rhodium under a reaction condition of water content of 5 wt% or less, formic acid is proportional to the hydrogen partial pressure.
By-products such as propionic acid and hydrocarbon increase. The present invention particularly has a water concentration of 10% or less in the reaction solution,
Particularly, it relates to the improvement of the technique in the case of 5 wt% or less, that is, when the reaction rate must be increased by adding lithium iodide.

[0009]

Means for Solving the Problems As a result of diligent studies on the method for producing acetic acid which does not have the above-mentioned drawbacks, the present inventors have found that rhodium becomes an inactive trivalent compound not in the reactor but in the monoxide. It has been found that the carbon dioxide partial pressure is low and the reaction does not occur while the catalyst liquid containing rhodium is being returned from the evaporator to the reactor. Therefore, the presence of hydrogen in the evaporation tank or during the return from the evaporation tank to the reactor prevents rhodium from becoming trivalent and / or
By converting the converted rhodium to monovalent, the catalyst activity in the reactor is maintained and the formic acid, propionic acid, and hydrocarbon are maintained without increasing the hydrogen partial pressure in the reactor above a certain pressure. The inventors have found that hydrogenated by-products such as the above can be suppressed, and have completed the present invention.

That is, the present invention is a method for producing acetic acid by reacting methanol with carbon monoxide in the presence of a rhodium catalyst and methyl iodide, wherein the water concentration in the reaction solution is 10 wt%.
In the step of performing the following continuous reaction, continuously extracting the reaction solution and introducing it into an evaporation step at a pressure lower than the reaction conditions, and separating into a component that evaporates and a component that does not evaporate containing rhodium, (a) hydrogen Is introduced and vaporization is carried out under the condition that the hydrogen partial pressure is at least 0.1 atm or more, or (b) the separated non-evaporable components including rhodium are at least hydrogen partial pressure of 0.1 atm or more and hydrogen at 0.1 atm or more. The present invention relates to a method for producing acetic acid, which comprises treating with carbon monoxide and then returning to the reactor for reuse.

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

As the main catalyst, rhodium complex and methyl iodide are used. The concentration of rhodium in the reaction solution is 200-2
000 ppm is used, but if the rhodium concentration is too high, rhodium iodide will precipitate, so it is preferably used in the range of 300 to 750 ppm, although it depends on the composition of the reaction solution. As long as rhodium is soluble in the reaction solution composition and reaction conditions, methyl iodide may be charged in the reactor in the form of any compound, and the concentration in the reaction solution may be 10 to 20 wt%. It is preferably used in the range. The reaction is promoted when the concentration of methyl iodide is high, but methyl iodide needs to be recovered and recycled to the reactor, which is economically most advantageous due to the size of the circulation process and the amount of energy used. The methyl iodide concentration is in the range of 10 to 20 wt%.

0.5 to 10 wt% of methyl acetate is present in the reaction solution due to the equilibrium between the starting materials methanol and acetic acid.

In the process of the present invention, the water content in the reactor is controlled in the range of 0.1-10 wt%, preferably 0.1-5 wt%. The lower the water concentration, the less the amount of hydrogen generated by the shift reaction, which is advantageous because the amount of by-products such as formic acid, propionic acid, and hydrocarbons decreases, but this leads to a decrease in the reaction rate and instability of rhodium. . For this,
To accelerate the reaction and stabilize rhodium, a quaternized ammonium iodide salt, a quaternized phosphonium iodide salt, a lithium salt such as lithium iodide or lithium acetate can be added. The amount added is 5 depending on the concentration in the reaction solution.
It can be used in the range of up to 30 wt%. It is suitable to use acetic acid, which is also the product, as the solvent.

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

When hydrogen is charged into the evaporation tank in the present invention, hydrogen is introduced so that the partial pressure of hydrogen in the evaporation tank is maintained at least 0.1 atm or higher. This hydrogen prevents the rhodium from changing to trivalent, and the activity in the reactor is maintained without increasing the hydrogen partial pressure in the reactor.

When the catalyst circulating liquid is subjected to hydrogen treatment, rhodium trivalent is effectively converted into rhodium monovalent by hydrogen treating in the treatment tank before returning to the reactor. In the hydrotreating tank, the catalyst circulating liquid is treated in the presence of a temperature of 100 to 150 ° C., a hydrogen partial pressure of at least 0.1 atm or higher, and a carbon monoxide partial pressure of at least 0.1 atm or higher. The hydrogen treatment may be carried out continuously or batchwise, but the continuous treatment is preferred because of higher productivity. The hydrogen treatment of the catalyst circulating liquid can also be performed by charging hydrogen and carbon monoxide into a pipe for returning the catalyst circulating liquid from the evaporation tank to the reactor.

[0018]

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

Example 1 (1) Preparation of circulating fluid of catalyst In an autoclave made of Hastelloy B-2 having an internal capacity of 500 cc, 0.1 g of rhodium chloride trihydrate (RhCl ₃ .3H ₂ O) and 40 g of lithium iodide (LiI) were added. , Methyl iodide 24g, water 4
g, acetic acid (118 g) and methanol (14 g) were added, and the air inside was replaced with carbon monoxide, and then the pressure was increased to 40 g / cm ² G with carbon monoxide. Then raise the temperature of the autoclave to 100 at 185 ° C.
Let 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 corresponded to about 4 wt% was measured by the carbon monoxide absorption rate, that is, the pressure decrease rate, and found to be 4.5 mol / hr. Met.

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

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

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

(3) Reaction Next, in order to carry out the 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 the catalyst solution which had been treated with hydrogen. After adding
The reaction was carried out under the same reaction conditions as in (1). That is, as in (1), the reaction solution was sampled at an appropriate time to remove water 4
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 found to be 30 ppm or less, which is the limit of quantification by gas chromatography, and it was found that propionic acid was not increased by this reaction.

Comparative Example 1 Omitting the hydrogen treatment step of Example 1, the catalyst liquid obtained in (1) of Example 1 was treated with methyl iodide, water and acetic acid in the same manner as in (3) of Example 1. 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 point, and the reaction rate was determined from the gas absorption rate at a time point when the water content was about 4 wt%. The result 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 (1) of Example 1, and 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 atm. The reaction solution was sampled at an appropriate time point, and the reaction rate was determined from the gas absorption rate at a water content of 4 wt% to be 4.5 mol / hr. After the reaction was completed, the propionic acid content of the reaction solution was measured by gas chromatography and found to be 185 ppm, and the amount of propionic acid increased by the reaction of Comparative Example 2 was found to be about 150 ppm. Although the reaction activity has been restored, Example 1
It can be seen that the generation amount of propionic acid is increased as compared with.

Claims (4)

[Claims] 1. In the presence of a rhodium catalyst and methyl iodide,
In the method of producing acetic acid by reacting methanol with carbon monoxide, a continuous reaction with a water concentration in the reaction solution of 10 wt% or less is performed, and the reaction solution is continuously withdrawn so that the pressure is lower than the reaction conditions. In the step of separating into a component that evaporates and a component that does not evaporate containing rhodium,
(A) Hydrogen is introduced and vaporization is performed under the condition that the hydrogen partial pressure is at least 0.1 atm or higher, and / or (b) at least the hydrogen partial pressure of the non-evaporable component containing the separated rhodium is 0.
A process for producing acetic acid, which comprises treating with hydrogen at 1 atm or more and carbon monoxide at 0.1 atm or more, returning to the reactor and recycling. 2. The method according to claim 1, wherein the reaction solution contains 5 to 30 wt% of iodide salt. 3. The method according to claim 4, wherein the iodide salt is lithium iodide. 4. The method according to claim 1, wherein the water concentration in the reaction solution is 0.1 to 10 wt%.