Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2001-08-30
2002-12-24
Richter, Johann (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S598000, C562S512200
Reexamination Certificate
active
06498272
ABSTRACT:
The present invention relates to a process for the preparation of acrylic acid, in which an acrylic acid-containing hot product gas mixture is produced by gas-phase partial oxidation of propane or propene and/or acrolein with molecular oxygen under heterogeneous catalysis over catalysts present in the solid state at elevated temperature, the temperature of the hot acrylic acid-containing product gas mixture is reduced by direct cooling by means of a quench liquid, the unvaporized part of the quench liquid
1
is circulated, if required via a heat exchanger providing indirect cooling, and a part of the unvaporized quench liquid
1
is removed as a purge from the circulation, and the cooled product gas mixture is then passed into a column equipped with baffles having separation activity, allowed to ascend into itself inside the column and subjected to fractional condensation, a crude acrylic acid being removed at the side take-off and bottom liquid containing acrylic acid oligomers and/or a high boiler fraction being removed via the bottom of the column and/or via a side take-off in the vicinity of the bottom and being used as quench liquid
1
.
Acrylic acid is an important intermediate which is used, for example, in the preparation of polymer dispersions.
Acrylic acid is obtainable, inter alia, by gas-phase partial oxidation of propane or propene and/or acrolein with molecular oxygen under heterogeneous catalysis over catalysts present in the solid state, at elevated temperatures. Said starting gases, as a rule diluted with inert gases, such as nitrogen, CO
2
and/or steam, are passed as a mixture with molecular oxygen, at elevated temperature and, if required, superatmospheric pressure, over transition metal mixed oxide catalysts and are converted by oxidation into an acrylic acid-containing product gas mixture.
By absorption into a suitable absorbent, for example water, or a mixture of from 70 to 75% by weight of diphenyl ether and from 25 to 30% by weight of biphenyl, a basic separation of the acrylic acid from the product gas stream can be achieved, (c.f. for example EP-A 297445 and DE-C 2136396).
Removing the absorbent by means of distillative separation methods usually gives an acrylic acid which is not yet a pure product and is therefore usually referred to as crude acrylic acid, which can be further purified in various ways.
However, the disadvantage of the abovementioned distillative separation method is that, in spite of the presence of free radical polymerization inhibitors, they give rise to the formation of such a large amount of acrylic acid which has undergone free radical polymerization in an undesired manner that the distillation apparatus has to be switched off periodically (as a rule every few weeks) to remove the polymer.
DE-A 19740253, DE-A 19740252, DE-A 19814421, 19814387 and 19814375 disclose a process, as described at the outset, for the preparation of acrylic acid, in which the basic isolation of the crude acrylic acid is carried out by fractional condensation of the product gas mixture of the gas-phase partial oxidation under heterogeneous catalysis. Such a procedure is also the subject of DE-A 19909923.
Surprisingly and incomprehensibly, it has been found that a major advantage of the abovementioned basic isolation of crude acrylic acid by fractional condensation is that, in the presence of free radical polymerization inhibitors, it is disturbed only to a substantially reduced extent by the formation of acrylic acid which has undergone free radical polymerization.
However, the disadvantage is still that acrylic acid in the condensed phase forms acrylic acid oligomers (Michael adducts) by Michael addition with itself and with the acrylic acid dimers forming (in this publication, the term acrylic acid oligomers always means the corresponding Michael adducts and not acrylic oligomers formed by free radical polymerization). The presence of water, the unavoidable by-product of gas-phase catalytic oxidative preparation of acrylic acid, at elevated temperature promotes the formation of acrylic acid oligomers.
Since acrylic acid oligomers have a higher boiling point than acrylic acid, they accumulate in the bottom liquid both during the distillative isolation of acrylic acid and in a fraction condensation of the product gas mixture of gas-phase catalytic oxidative acrylic acid production.
In principle, the formation of acrylic acid oligomers is undesirable since they reduce the yield of the desired product.
It is generally known that the formation of acrylic acid oligomers is a reversible process which can be reversed by the action of elevated temperatures (cf. for example, DE-A 4101879, DE-A 19701737, DE-A 19536191, U.S. Pat. No. 5,734,075, DE-A 2901783, U.S. Pat. Nos. 3,086,046, 2,806,878, German Patent 1618129, Ullmanns Encyclopädie der technischen Chemie, 4th Edition, Volume 7, Verlag Chemie, Weinheim, page 83, right column, and Ullmann's Encyclopedia of Industrial Chemistry, Fifth, Completely Revised Edition, Verlag Chemie, Weinheim, Vol. A
1
1, page 167, right column).
This thermal cleavage can be carried out under both acidic and basic catalysis or in the absence of catalysts (cf. for example, EP-A 717031).
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Ullmanns Encyclopaedie der Technischen Chemie, vol. 7, p. 83 and p. 93, “Acrylsaeure”.
W. Gerharz, et al., Ullmann's Encyclopedia of Industrial Chemistry, vol. A1, pp. 166-167, “Acrylic Acid and Derivatives”.
Müller-Engel Klaus Joachim
Nestler Gerhard
Schröder Jürgen
BASF - Aktiengesellschaft
Forohar Farhad
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Richter Johann
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