GB2024201A - Process for producing methacrylic acid - Google Patents

Abstract

A process for producing methacrylic acid by the vapor-phase catalytic oxidation of methacrolein with molecular oxygen in the presence of steam, which comprises feeding methacrolein, molecular oxygen and steam into a reaction tower for a first-stage oxidation step to oxidize a part of the methacrolein to methacrylic acid, feeding the resulting reaction mixture into a methacrylic acid-absorbing tower for a separating step to bring it into counter- current contact with a methacrylic acid- absorbing solvent while maintaining the temperature of the top of the tower at 60 DEG C or higher and the temperature of the bottom of the tower at 65 DEG C or higher, separating the methacrylic acid- containing solution from the bottom of the tower and simultaneously withdrawing the gaseous mixture containing the unreacted methacrolein, and steam from the top of the tower, and then feeding the gaseous mixture into a reaction tower for a second-stage oxidation step to oxidize the remaining methacrolein to methacrylic acid.

Description

SPECIFICATION Process for producing methacrylic acid This invention relates to an improved process for producing methacrylic acid by the vapor-phase catalytic oxidation of methacrolein.

In recent years, active research and development works have been done on the production of methacrylic acid by the vapor-phase catalytic oxidation of methacrolein. The use of purified methacrolein as a starting material is advantageous to the reaction involved, but is economically undesirable because of the increased cost of purification. Accordingly, a method involving the direct use of crude methacrolein has attracted attention. However, since the crude methacrolein contains isobutylene or tertiary butanol used as the starting material or tarry byproducts, the yield of methacrylic acid obtained by such a method is drastically reduced (Japanese Laid-Open Patent Publication No. 111017/75).

As a result of extensive investigations in an attempt to remove these defects of the prior art, the present inventors found that (1) the decrease of the yield of methacrylic acid caused by impurities contained in methacrolein is due mainly to the decrease of the conversion of methacrolein, and the decrease of the selectivity of methacrylic acid is relatively small, and (2) when the conversion of methacrolein is increased, acetic acid, carbon monoxide, carbon dioxide, tarry substances, etc. are formed by the decomposition, etc. of methacrylic acid to reduce the selectivity of methacrylic acid, and therefore, the increased conversion does not lead to an increase in yield, and moreover, quickens the degradation of the catalyst.With this background, the present inventors previously developed a new process in which the step of oxidizing methacrolein is divided into two stages, and a step of separating methacrylic acid is placed between the two oxidation stages (German OLS 2748050.8). This process does not require steps of separating or purifying methacrolein which is difficult to handle, and gives methacrylic acid in a high conversion and a high yield from methacrolein.

As an improvement over the aforesaid process, the present invention provides a process for producing methacrylic acid by the vapor-phase catalytic oxidation of methacrolein with molecular oxygen in the presence of steam, which comprises feeding methacrolein, molecular oxygen and steam into a reaction tower for a first-stage oxidation step to oxidize a part of the methacrolein to methacrylic acid, feeding the resulting reaction mixture into a methacrylic acid-absorbing tower for a separating step to bring it into countercurrent contact with a methacrylic acid-absorbing solvent while maintaining the temperature of the top of the tower at 600C or higher and the temperature of the bottom of the tower at 65"C or higher, separating the methacrylic acid-containing solution from the bottom of the tower and simultaneously withdrawing the gaseous mixture containing the unreacted methacrolein and steam from the top of the tower, and then feeding the gaseous mixture into a reaction tower for a second-stage oxidation step to oxidize the remaining methacrolein to methacrylic acid.

In the process of this invention, the aforesaid operation at the methacrylic acid-absorbing tower results in the separation by the absorbing solvent of substantially all of the methacrylic acid present in the reaction mixture which has been withdrawn from the first-stage oxidation step, and substantially all of the unreacted methacrolein remains in the reaction mixture. Furthermore, substantially all, or a considerable amount, of the steam introduced into the first-stage oxidation step remains in the reaction mixture. These remaining substances are effectively utilized in the second-stage oxidation step. Thus, according to this invention, no step of separating and purifying the unreacted methacrolein is required, and the resulting methacrylic acid can be effectively recovered.Furthermore, the step of separating methacrylic acid is decreased in scale, and the cost can be greatly curtailed. Whilst the presence of 20 to 80 mole% of steam is generally required in the production of methacrylic acid from methacrolein,the process of this invention does not require condensation and liquefaction of steam, and therefore, the energy efficiency can be increased greatly.

The first step in the process of this invention is to feed methacrolein, molecular oxygen and steam into a reaction tower in which the first-stage vapor-phase catalytic oxidation of methacrolein is carried out in the presence of an oxidation catalyst. This reaction can be performed in a customary manner. Recommended reaction conditions are, for example, as follows: concentration of methacrolein, 0.001 to 25% by volume; the mole ratio of methacrolein to oxygen, 1:0.1-25; the concentration of steam, 20 to 80% by volume; the reaction temperature, 250 to 500"C; the space velocity (SV), 100-10,000 h71 (STP basis); the reaction pressure, atmospheric to 10 atmospheres.Since higher conversions of methacrolein increase the consecutive reaction of methacrylic acid and may tend to induce a decomposition reaction or the formation of tarry substances, it is necessary that in the first-stage oxidation step, the oxidation of methacrolein should be partial, for example to an extent such that the conversion of methacrolein is not more than 90%, preferably 40 to 85%.

Methacrolein to be fed may be of high purity, but needs not necessarily to be so. For example, the reaction mixture containing methacrolein as a main ingredient which is obtained by the vapor-phase catalytic oxidation of isobutylene or tertiary butanol can be used as it is. Oxygen itself may be used as a sourse of molecular oxygen, but from the standpoint of economy, air is preferred.

The reaction mixture containing methacrylic acid, steam, unreacted methacrolein, and oxygen, which has been withdrawn from the reaction tower for the first-stage oxidation step, is then sent to a methacrylic acid-absorbing tower for the separating step, in which the reaction mixture is brought into countercurrent contact with a methacrylic-absorbing solvent, thereby separating methacrylic acid. When the reaction mixture is at a high temperature, it is preferably cooled to about 100 to 2000C prior to introduction into the methacrylic acid-absorbing tower. This reduces thermal load at the methacrylic acid-absorbing tower and at the same time, makes it possible to remove tarry substances which are present in traces in the reaction mixture.

The methacrylic acid-absorbing tower may be of any type capable of bringing the reaction mixture in countercurrent contact with the absorbing solvent, and a packed tower, a plate tower, a spray tower, a wet wall tower, etc. are used. The packed tower is most commonly used which contains such packings as Rashchig rings, Lessing rings, Berl saddles, Intalox saddles, Tellerette packings, or Pall rings, which are made of porcelain, metal, plastics, etc.

In the methacrylic acid-absorbing tower, the reaction mixture is contacted countercurrently with the absorbing solvent fed from the top or intermediate stage of the tower while maintaining the tempera tureofthetop of the tower at 600C or higher, preferably at 65 to 150"C, the temperature of the bottom of the tower at 65"C or higher, preferably at 70 to 1800C, and the presence of the inside of the tower at atmospheric pressure to 10 atmospheres, preferably at atmospheric pressure to 4 atmospheres.As a result, substantially all ofthe methacry lic acid is separated from the bottom of the tower as a methacrylic acid-containing solution and simultaneously, substantially all of the unreacted methac rolein and substantially all, or a considerable portion, of the steam are withdrawn from the top of the tower. Tower top temperature of less than 60"C are undesirable because the amount of methacrolein dissolved in the absorbing solvent increases. Tower bottom temperatures of less than 65"C are neither desirable from the viewpoint of the energy efficiency because the volatilization of methacrolein becomes insufficient and condensation of steam occurs.

The absorbing solvent used may be any solvent which can easily absorb methacrylic acid. Examples are water, esters, ketones and organic acids. Of these, water and aqueous solution of aliphatic carboxylic acids having 2 to 10 carbon atoms are preferred. Usually, the absorbing solvent is used in an amount at least equal to the weight of the methacrylic acid, and in some cases at least two times the weight of the methacrylic acid. The temperature of the solvent fed, which varies depend ing upon the temperature, etc. of the reaction mixture, is usually about 20 to 1500C.

The methacrylic acid-containing solution with drawn from the bottom of the tower is purified in a customary manner in the step of purifying methacry lic acid. The gaseous mixture withdrawn from the top of the tower is fed into the reactor for the second-stage oxidation step, and mixed there with methacrolein, oxygen, steam, etc. which are added as desired. The mixture is again subjected to cataly tic oxidation. Usually, the conditions for the second stage oxidation step are the same as those em ployed in the first-stage oxidation step, but need not necessarily to be the same.

Usually, in the second-stage oxidation step, sub stantially all of the unreacted methacrolein is con verted to methacrylic acid. Hence, it is only sufficient to separate methacrylic acid alone in a customary manner. However, when, for some reason or other, the amount of methacrolein remaining in the reaction mixture is large, the reaction mixture may be subjected to a third-stage oxidation step via the same methacrylic acid separating step as described above.

The present invention is specifically described below with reference to the accompanying drawings showing one embodiment of the present invention.

Methacrolein, molecular oxygen and steam are introduced into reactor 2 from pipe 1, and the vapor-phase catalytic oxidation of methacrolein is carried out in reactor 2. The resulting reaction mixture containing methacrylic acid is introduced into cooler 4 through pipe 3, and cooled there to 100 to 200"C. The cooled mixture is introduced into the bottom stage of methacrylic acid-absorbing tower 6 through pipe 5. In the methacrylic acid-absorbing tower 6, the reaction mixture is contacted countercurrently with an absorbing solvent fed from pipes 7 and 8 provided at the top and intermediate stages of the tower.The solvent which has absorbed methacrylic acid is withdrawn from the bottom of the tower through pipe 9, and in the meanwhile, the gaseous mixture of methacrolein, steam, oxygen, etc. is withdrawn from the top of the tower through pipe 10, and fed into reactor 12. As required, methacrolein, oxygen, steam, etc. can be fed through pipe 11.

The gaseous mixture is catalytically oxidized in the reactor 2, and the reaction mixture is fed into a methacrylic acid separating step through pipe 13.

The temperatures of the top and bottom of the absorbing tower 6 are controlled by heater 14.

The following Example illustrates the present invention in more detail.

Example A methacrolein-containing mixture was produced by the vapor-phase catalytic oxidation of isobutylene in the presence of an Mo-Bi-Fe-Co-Ni-Be-P-K catalyst [Mo:Bi:Fe:Co:Ni:Be:P:K= 12:1:12:4:1:3:0.5:1 (atomic ratio); calcined at 600"C; catalyst particle diameter 4 - 8 mesh; see German OLS 2735414.9] using a mixture of isobutylene, air and steam (4:55:41 in mole ratio) at a space velocity of 2,000 hr1 while maintaining the temperature of a metal bath for heating the stainless steel reaction tube at 340"C.

The resulting methacrolein-containing mixture was fed into a first-stage oxidation step, and catalytically oxidized in the vapor-phase at a temperature of 335"C and a space velocity of 2,000 hr using an Mo-P-Cs-Crcatalyst[Mo:P:Cs:Cr= 1 :0.16:0.16:0.16 (atomic ratio); calcined at 450"C; catalyst particle diameter 4-8 mesh; see British Patent 1,393,915].

The gaseous reaction mixture (17.0 Nm3/hr at 410"C) containing 1.5% by volume of methacrylic acid, 1.5% by volume of methacrolein and 45.0% by volume of steam obtained in the above step was cooled to 150"C at 3 atmospheres in a heatexchanger. The cooled mixture was fed into the bottom of a packed tower containing Raschig rings made of porcelain, and contacted countercurrently with water at 700C fed from the top of the tower at a rate of 4 liters/hour and an aqueous solution contain ing 0.5% by weight of methacrolein and 2% by weight of methacrylic acid at 700C fed from the intermediate stage of the tower at a rate of 10 liters/hou r.At this time, the temperature of the bottom of the tower was 109"C, and the temperature of the top of the tower was 108"C. As a result, 14 kg of a solution containing 8% by weight of methacrylic acid was withdrawn from the bottom of the tower, and a gaseous mixture at 2.9 atmospheres containing 1.6% by volume of methacrolein, 45.0% by volume of steam, a small amount of oxygen and a large amount of an inert gas was withdrawn from the top of the tower at a rate of 16.8 Nm3/hour. The amount of methacrylic acid withdrawn from the bottom of the tower corresponded to 96% of the methacrylic acid, and the amounts of methacrolein and steam withdrawn from the top of the tower corresponded to 98% and 100% respectively based on the amounts fed.

The gaseous mixture withdrawn from the packed tower was fed directly into the second-stage oxidation step, and oxidized in the vapor phase using the same catalyst as used in the first-stage oxidation step at a space velocity of 2,000 her~1 while maintaining the temperature of a metal bath for heating the stainless steel reaction tube at 335"C. The resulting gaseous mixture was fully cooled, and the methacrylic acid was separated and recovered.

As a result, the conversion of methacrolein was 99.6 mole%, the yield of methacrylic acid based on methacrolein was 75.4 mole%, and the selectivity of methacrylic acid was 75.7%.

Claims (8)

1. A process for producing methacrylic acid by the vapor-phase catalytic oxidation of methacrolein with molecular oxygen in the presence of steam, which comprises feeding methacrolein, molecular oxygen and steam into a reaction tower for a first-stage oxidation step to oxidize a part of the methacrolein to methacrylic acid, feeding the resulting reaction mixture into a methacrylic acidabsorbing tower for a separating step to bring it into countercurrent contact with a methacrylic acidabsorbing solvent while maintaining the temperature of the top of the tower at 600C or higher and the temperature of the bottom of the tower at 65"C or higher, separating the methacrylic acid-containing solution from the bottom of the tower and simultaneously withdrawing the gaseous mixture containing the unreacted methacrolein, and steam from the top of the tower, and then feeding the gaseous mixture into a reaction tower for a second-stage oxidation step to oxidize the remaining methacrolein to methacrylic acid.

2. A process according to claim 1 wherein the oxidation of methacrolein in the first-stage oxidation step is carried out to a conversion of not more than 90%.

3. A process according to claim 2 wherein the first stage oxidation is carried out to a conversion of 40 to 85%.

4. A process according to claim 1,2 or 3 wherein the reaction mixture from the first-stage oxidation step is fed into the separating step after it has been cooled to a temperature of 100 to 200 .

5. A process according to any one of the preceding claims wherein the top of the methacrylic acid-absorbing tower is maintained at 65 to 150"C and the bottom at 70 to 1 800C.

6. A process according to any one of the preceding claims wherein the methacrylic acid-absorbing solvent is water or an aqueous solution of an aliphatic carboxylic acid containing 2 to 10 carbon atoms.

7. A process sccording to claim 2 substantially as described with reference to the accompanying drawings.

8. A process according to claim 1 substantially as described in the Example.