Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2000-07-19
2002-10-29
Rotman, Alan L. (Department: 1625)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
Reexamination Certificate
active
06472554
ABSTRACT:
The present invention relates to a process for the continuous preparation of alkyl esters of (meth)acrylic acid by reacting (meth)acrylic acid with alkanols of 4 to 8 carbon atoms in a reaction zone, the reaction mixture containing the desired ester being removed from the reaction zone by the liquid phase method, purified beforehand and then worked up by rectification. The present invention furthermore relates to an apparatus and a use of the process.
The term (meth)acrylic is used predominantly as abbreviated notation for acrylic and/or methacrylic.
Alkyl esters of (meth)acrylic acid are generally known and are important, for example, as reactive monoethylenically unsaturated monomers for the preparation of aqueous polymer dispersions by free radical aqueous emulsion polymerization, which polymer dispersions are used, for example, as adhesives.
Usually, the preparation of the alkyl (meth)acrylate is carried out by direct, acid-catalyzed reaction (esterification) of (meth)acrylic acid with the corresponding alkanols.
One route for the industrial production of (meth)acrylic acid is the catalytic gas-phase oxidation of suitable C
3
-/C
4
-precursors (for example propylene, acrolein, isobutene or methacrolein) with molecular oxygen. However, this procedure gives not pure (meth)acrylic acid but a gas mixture, i.e. crude (meth)acrylic acid which, in addition to (meth)acrylic acid, contains, amongst other substances, acetic acid as a byproduct, the separation of which from (meth)acrylic acid, particularly by rectification, is expensive (cf. for example DE-A 198 14 449).
Processes in which this crude (meth)acrylic acid is esterified directly by reaction with alkanols and the removal of the byproduct is thus transferred from the acid level to the ester level have therefore been proposed. Such a process is described, for example, in the non-prior-published German Patent Application 198 51 983. According to this, crude (meth)acrylic acid is reacted with alkanols of 1 to 8 carbon atoms in a phase free of solvents, at elevated temperatures and in the presence of a strongly acidic esterification catalyst in a reaction zone, the water formed during a residence time in the reaction zone and being part of a mixture comprising starting alkanol is separated from the reaction mixture by rectification in a rectification unit RI attached to the reaction zone, the reaction mixture containing the desired ester is removed from the reaction zone and first prepurified, the acid esterification catalyst being separated off preferably by washing with water and this separation being completed preferably by subsequent washing with an aqueous alkali metal-hydroxide solution, and finally, in further rectification units, the desired ester is separated from the prepurified reaction mixture by rectification.
It is an object of the present invention to provide an improved process for the continuous preparation of alkyl esters of (meth)acrylic acid by reacting (meth)acrylic acid with alkanols of 4 to 8 carbon atoms, which ensures higher availability of the plant, an improvement in the yield and a reduction of the amount of residue.
We have found that this object is achieved by a process for the continuous preparation of alkyl esters of (meth)acrylic acid by reacting (meth)acrylic acid with alkanols of 4 to 8 carbon atoms in a phase free of solvents, at elevated temperatures and in the presence of a strongly acidic esterification catalyst, in which the (meth)acrylic acid, the alkanol and the esterification catalyst are fed to a reaction zone, the water formed during a residence time in the reaction zone and being part of a mixture comprising starting alkanol is separated from the reaction mixture in a first rectification unit RI attached to the reaction zone, the distillate obtained is separated into an organic phase containing a starting alkanol and into an aqueous phase containing water, the organic phase is recycled to the rectification unit RI, the reaction mixture containing the desired ester and removed from the reaction zone is fed to a prepurification,
1. in a first prepurification stage, the predominant part of the esterification catalyst being separated off by extraction by means of washing with water and
2. in a second prepurification stage, the strongly acidic components being neutralized and extracted with an aqueous alkali solution by reactive extraction, the remaining organic reaction mixture I is passed into a further separation zone comprising rectification units and the resulting alkyl ester of (meth)acrylic acid is isolated therein.
In said process, the prepurification comprises a third stage in which
3. residual salts and aqueous foreign-phase fractions are removed by extraction with water from the organic reaction mixture I remaining after the second prepurification stage, before it is passed on into the separation zone comprising further rectification units.
It was surprisingly found that, in the process according to the non-prior-published German Patent Application 198 51 983, the prepurification of the reaction mixture containing the desired ester is decisively improved by a further, third prepurification stage, by means of which the downstream separation of the desired ester from the residual reaction mixture by rectification is improved in terms of economy and ecology.
A particularly suitable starting material for the esterification is a (meth)acrylic acid which was produced by catalytic gas-phase oxidation of the C
3
-/C
4
-precursors stated at the outset with molecular oxygen. The crude (meth)acrylic acid obtained in this manner can preferably be fed directly to the esterification, but it is just as possible to use any more highly purified (meth)acrylic acid as the acid starting material. Crude (meth)acrylic acid contains as impurities in particular up to 5% by weight of acetic acid and in addition frequently up to 1% by weight of maleic acid and/or the anhydride thereof and low molecular weight aldehydes, frequently in an amount of up to 0.5% by weight, and up to 0.5% by weight of other components, for example propionic acid, diacrylic acid and polymerization inhibitors, for example phenothiazine. The (meth)acrylic acid content of the crude (meth)acrylic acid which can preferably be used is frequently at least 95, often at least 97 or at least 98 or at least 99,% by weight.
Suitable alkanol starting materials are in principle all alkanols of 4 to 8 carbon atoms, n-butanol and 2-ethylhexanol being particularly preferred.
Suitable strongly acidic esterification catalysts for the novel process are in particular strong mineral acids, preferably sulfuric acid, but also strong organic acids, in particular sulfonic acids, such as methanesulfonic acid, benzenesulfonic acid, dodecanesulfonic acid, para-toluenesulfonic acid or mixtures of some or all of the abovementioned acids.
The esterification is carried out in the manner described in DE 198 51 983, in a reaction zone which may consist of one or more reaction regions. In one embodiment of the invention, comprising a plurality of reaction regions, it is advantageous to cascade them. The liquid discharge stream of one reaction region expediently forms the feed of the downstream reaction region. This can be effected in a simple manner with the aid of an overflow or by means of pumps. Where the individual reaction regions are apparatus separated from one another, the number thereof is expediently ≧2 and ≦4, taking into account the capital costs. If more than one reaction region is set up within one and the same reactor (for example by the use of separating plates), the number of reaction regions may also be greater than 4. In the case of a plurality of reaction regions, the vapors of the individual reaction regions can be fed to a common rectification unit RI, for example a common rectification column, whose liquid discharge is expediently fed to the first reaction region. According to the invention, however, it may be expedient to attach one rectification unit RI to each of a plurality of reaction regions, if required to all, and to recyc
Deckert Petra
Herbst Holger
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Oh Taylor V.
Rotman Alan L.
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