As is well known, there is a vapor phase oxidation method of propylene as the formation reaction of acrylic acid for producing an acrylic ester. In a method of obtaining acrylic acid by oxidizing propylene, since condition of the oxidation to acrolein and that of the subsequent oxidation to acrylic acid are different from each other, the method includes a two-stage process of conducting the oxidation in separate reactors and a process oxidation directly to acrylic acid by one-stage oxidation.
FIG. 1 shows one embodiment of a step flow of forming acrylic acid by two-stage oxidation and subsequently esterifying it to produce an acrylic ester, in which propylene, steam and air are subjected to two-stage oxidation through a first reactor and a second reactor each charged with a molybdenum based catalyst, etc. to form an acrylic acid-containing gas. The acrylic acid-containing gas is brought into contact with water in a condensing column to form an acrylic acid aqueous solution, to which is then added a suitable extraction solvent; the mixture is subjected to extraction in an extracting column; and the extraction solvent is then separated in a solvent separating column. Subsequently, acetic acid is separated in an acetic acid separating column to obtain crude acetic acid, and a by-product is separated from the crude acetic acid in a fractionating column, thereby obtaining an acrylic acid purified product. This acrylic acid (purified product) is esterified in an esterification column, passes through an extracting column and a light-fraction separating column to obtain a crude acrylic ester, and a by-product (high-boiling fraction) is separated from the crude acrylic ester in a fractionating column, thereby obtaining an acrylic ester purified product.
Incidentally, in recent years, as the recovery of acrylic acid from the acrylic acid aqueous solution, an azeotropic separation method in which distillation is conducted using water and an azeotropic solvent, and an azeotrope of water and the azeotropic solvent is distilled away from the column top of an azeotropic separating column, whereas acrylic acid is recovered from the column bottom is also employed in place of the solvent extraction method using an extraction solvent.
In the case of the synthesis of methacrylic esters, a methacrylic ester purified product is obtained through the similar oxidation process and subsequent esterification process using isobutylene or t-butyl alcohol in place of the propylene.
Incidentally, as the method of the formation of a (meth)acrylic ester (an acrylic ester or a methacrylic ester), a method in which a (meth)acrylic ester of a lower alcohol and a higher alcohol are subjected to transesterification in the presence of a catalyst such as an acid, to produce a (meth)acrylic ester of a higher alcohol is also employed. The crude (meth)acrylic ester obtained by this transesterification is converted into a purified (meth)acrylic ester through catalyst separation, concentration, fractionation, and other steps.
A fraction separated upon distillation and purification of the foregoing crude acrylic ester or crude methacrylic ester contains a useful by-product such as a Michael adduct. Accordingly, the by-product is decomposed, and (meth)acrylic acid or an ester thereof, the starting alcohol, etc. are recovered
As the decomposition method of a Michael adduct of an acrylic ester, a method of decomposition upon heating in the presence of a Lewis acid or a Lewis base is employed (see JP-A-49-55614 and JP-A-9-110791).
In the method of decomposing a Michael addition reaction product formed by a by-product in the production step of a (meth)acrylic ester using a Lewis acid or a Lewis base as a catalyst and recovering (meth)acrylic acid, (meth)acrylic ester and alcohol, in the case where the alcohol residue of the ester is branched, an olefin was formed as a by-product at the time of decomposition reaction and became an obstacle in operation. That is, since the olefin generated in this reaction generally has a low boiling point and is hardly condensed in a decomposition reactor or a condenser, there were encountered such problems that it obstructs an adequate operation of a vacuum reactor or distillation column and that it adversely affects the reaction results or separation ability.
The problems caused by the formation of an olefin as a by-product will be described below in detail with respect to the time of production of isobutyl acrylate and the time of production of 2-ethylhexyl acrylate.
In the decomposition step of a Michael adduct at the time of production of isobutyl acrylate, isobutylene derived from isobutyl alcohol is formed as a by-product as an olefin. This by-product isobutylene has an extremely low standard boiling point as 266.1 K so that it is hardly condensed in the decomposition reactor itself to be operated in the vacuum system or a condenser of a distillation column as a destination for recovery. For that reason, if the amount of isobutylene formed as a by-product increases, the control of the vacuum system is obstructed so that the decomposition reaction or distillation under an adequate pressure condition becomes difficult, and finally, the reaction or separation operation is adversely affected.
In the decomposition step of a Michael adduct at the time of production of 2-ethylhexyl acrylate, 2-ethyl-1-hexene derived from 2-ethylhexyl alcohol and an isomer thereof are formed as by-products. In the case where a fraction containing these olefins having 8 carbon atoms are recovered into the reaction system, not only these olefins cause azeotropy with water, but also they form an azeotrope together with the starting acrylic acid, resulting a serious problem of losing acrylic acid. In general, there are the case where the esterification is carried out in a reaction distillation mode in which the equilibrium of the reaction is shifted upon removal of water formed during the reaction from the system, thereby obtaining a high conversion and the case where after the reaction, the unreacted acrylic acid, water, alcohol, and acrylic ester are separated by extraction or distillation. In any of these modes, acrylic acid is dissolved in the aqueous phase of the column top fraction and lost. Even in the case where the acrylic acid, 2-ethylhexyl alcohol and 2-ethylhexyl acrylate containing olefins recovered in the decomposition step of a Michael adduct are recovered in the purification system, a loss of acrylic acid is similarly caused in the distillation system of the purification system. Assuming that these materials are recovered into the distillation system free from acrylic acid or water, they are ultimately recycled into the reaction system together with light fractions such as 2-ethylhexyl alcohol, whereby the foregoing loss of acrylic acid is caused.
An object of the invention is to dissolve the foregoing problems of the background art and to provide a method of decomposing a by-products such as a Michael addition reaction product formed in the production step of a (meth)acrylic ester using an alcohol bearing a branched chain and having 3 or more carbon atoms, thereby recovering (meth) acrylic acid, (meth)acrylic ester, alcohol and the like, in which an olefin formed as a by-product can be decomposed at a high recovery while suppressing the formation thereof as the by-product.