Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Patent
1998-12-30
2000-05-16
Keys, Rosalynd
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
562598, C07C 5143
Patent
active
060639598
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for purifying crude acrylic acid by crystallization.
Acrylic acid, whether as acid or in the form of its salts or esters, is particularly important for preparing polymers for a very wide range of application areas, for example adhesives, superabsorbents, binders.
One way of obtaining acrylic acid is by catalytic gas phase oxidation of propane, propene and/or acrolein. In the catalytic gas phase oxidation process, these starting gases are diluted, generally with inert gases such as nitrogen, CO.sub.2 and/or steam, and passed together with oxygen over transition metal mixed-oxide catalysts at elevated temperatures and, if necessary, elevated pressure for oxidative conversion into a mixture of products which includes acrylic acid. By condensing the product mixture or by absorption into a suitable absorbent (e.g. water or a mixture of 70-75% by weight of diphenyl ether and 25-30% by weight of biphenyl), it is possible to achieve a basic separation of the acrylic acid from the product gas stream (cf. for example EP-A 297 445 and DE-C-21 36 396).
Removal of the absorbent (following, if necessary, prior desorption of impurities having a low absorbent solubility, by stripping, for example with air) by extractive and/or distillative methods of separation (e.g. removal of the aqueous absorbent by distillation, azeotropic distillation or extractive separation of the acid from the aqueous solution and subsequent distillative removal of the extractant) and/or following application of other separating steps frequently yields an acrylic acid which is herein referred to as a crude acrylic acid.
This crude acrylic acid is not a pure product. On the contrary, it comprises a spectrum of different impurities typical of production by oxidative gas phase catalysis. These impurities are in particular acetic acid, propionic acid, water and low molecular weight aldehydes such as acrolein, methacrolein, propionaldehyde, n-butyraldehyde, benzaldehyde, furfurals and crotonaldehyde.
Crude acrylic acid further typically comprises polymerization inhibitors. These are added in the course of the separation processes employed for producing crude acrylic acid for the purposes of suppressing any free-radical polymerization of the .alpha., .beta.-monoethylenically unsaturated acrylic acid, which is why they are also known as process stabilizers. The preeminent acrylic acid process stabilizer is 1,4-dibenzothiazine, ##STR1##
1,4-Dibenzothiazine is a colorless substance with a melting point of 180.degree. C. (at 1 bar), which is obtainable by heating diphenylamine with sulfur. 1,4-Dibenzothiazine is typically used as sole acrylic acid process stabilizer or in combination with other possible acrylic acid process stabilizers, for example hydroquinone, which is why 1,4-dibenzothiazine is a characteristic constituent of the crude acrylic acid relevant herein.
Further undesirable concomitants of condensed-phase acrylic acid are the acrylic acid oligomers formed by Michael addition of acrylic acid to itself and to the resulting acrylic acid dimer. Whereas these Michael adducts are normally hardly present in freshly produced crude acrylic acid (their weight proportion being typically <0.01% by weight), they form therein on prolonged standing. Statistically, merely the formation of diacrylic acid ##STR2## is important, whereas the formation of higher acrylic acid oligomers (trimers, tetramers, etc.) is essentially negligible.
In fact, even the formation of diacrylic acid is a slow process. A .gtoreq.99.5% by weight pure acrylic acid left to stand at 25.degree. C. and 1 bar will generate about 150 weight ppm of diacrylic acid per day, based on acrylic acid. The total amount of any other cocomponents present in the crude acrylic acid is generally not more than 10% by weight, based on the weight of the crude acrylic acid.
Crude acrylic acid for the purposes of this invention is therefore an acrylic acid comprising, if its acrylic acid oligomer.sup.1) (Michael adducts) content is ignored,
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Eck Bernd
Lehnert Klaus
Muller-Engel Klaus Joachim
Nestler Gerhard
BASF - Aktiengesellschaft
Keys Rosalynd
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