Process for the continuous preparation of lower acrylates

Details Process for the continuous preparation of lower acrylates Process for the continuous preparation of lower acrylates

1991-11-29

1993-02-16

Killos, Paul J.

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acid esters

560205, C07C 1748

Patent

active

051873095

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to a process for the continuous preparation of lower acrylates from an aqueous solution of acrylic acid and a lower alcohol. More particularly, the present invention relates to a process according to which the reaction of esterification of an aqueous solution of acrylic acid with methanol or ethanol is carried out in liquid phase so as to prepare continuously methyl or ethyl esters of acrylic acid.
It is generally known to prepare the lower (methyl or ethyl) acrylates by esterification of purified acrylic acid with a lower alcohol. It is therefore necessary, in a first operation, to purify the aqueous solution of acrylic acid which is available initially and which assays at at least 50% of pure acrylic acid diluted in a large quantity of water (not more than 50%) and which generally additionally contains lower proportions of impurities which are inherent in its production, including especially acetic acid and aldehydes. The presence of water in the acrylic acid is, in fact, a feature which is unfavourable for obtaining a good conversion when the acid is esterified, because it decreases the formation of the ester by shifting the reaction equilibrium towards the hydrolysis; the acrylic acid conversion would then be so low that it would require major and costly multiple recycling of the unconverted acid and alcohol. A conventional method for purifying the aqueous solution of acrylic acid consists, for example, in carrying out an extractive azeotropic distillation thereon, methyl isobutyl ketone being, for example, an azeotropic solvent which can be employed. After this aqueous solution of acrylic acid has been treated in a purification plant, a purified acrylic acid is generally obtained, assaying at at least 98% of pure acrylic acid, containing a small quantity of water and being consequently capable of being used as a starting material for the esterification reaction. In a second stage, therefore, this purified acrylic acid is introduced directly into an esterification reactor which is simultaneously fed with a lower alcohol (methanol or ethanol). The esterification reaction takes place at a temperature of approximately between 50.degree. C. and 110.degree. C. in the presence of at least one esterification catalyst and at a pressure equal to atmospheric pressure or slightly higher. The reaction products leaving the reactor, consisting of a mixture of lower acrylate, water and unreacted starting compounds, are then sent to the bottom of a distillation column; at the foot of the said column is recovered a mixture of acrylic acid, water and heavy impurities originating from the production of acrylic acid and/or from the esterification reaction, this stream then being recycled towards the esterification reactor in order to feed the said reactor with starting material for the esterification reaction, in addition to the acrylic acid stream originating from the purification device. A mixture consisting of lower acrylate, water and alcohol is recovered at the head of the said column and is then sent towards a purification device comprising, in a first stage, washing with water in order to separate the acrylate from the unreacted alcohol, followed by a distillation of the resulting aqueous phase, this being in order to recover the alcohol which will subsequently be recycled to the reaction, as well as a distillation of the resulting organic phase, this latter distillation making it possible, in a first step, to remove the light impurities and then, in a second step, to remove the residual heavy impurities, finally ending in the production of an acrylate solution assaying at at least 99% of pure acrylate.
The disadvantage of this method lies in the need to provide a purification unit comprising costly and bulky pieces of equipment.
The problem which the present invention proposes to solve consists, therefore, in developing a process which does not have the above disadvantages and which makes it possible to prepare methyl acrylate or ethyl acrylate continuously without resorting to a spec

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