Process for dehydrating unsaturated aldehydes and the resulting unsaturated aldehydes

摘要

An a , b unsaturated C3 or C4 aldehyde such as acrolein or 2-methylacrolein, which during manufacture or treatment comes into contact with water, is dehydrated by mixing with a liquid hydrocarbon before, during or after the water-contacting stage, a liquid aqueous phase practically free from unsaturated aldehyde being separated in a further stage from a liquid phase containing the hydrocarbon and substantially all the unsaturated aldehyde and of lower water content than the aldehyde-water azeotrope. If the boiling points are sufficiently apart, the hydrocarbon and aldehyde may be separated by distillation. Water may be removed from the hydrocarbon solution of aldehyde by a dehydrating agent or by distilling to give a bottoms containing hydrocarbon, most of the aldehyde, and practically no water, and a distillate of aldehyde and water which may be returned to a stage prior to separation of the liquid aqueous phase. The hydrocarbon may be aromatic, paraffinic, olefinic, cycloparaffinic or cyclo-olefinic, e.g. xylene, propane or butane. The amount added is preferably 5-10 vols. per vol. of the aldehyde. Saturated aldehydes and ketones are separated in the aqueous phase. Stratification is preferably effected at 0-20 DEG C. Extraction may be effected by flowing water and hydrocarbon in counter-current to each other through a vessel and feeding crude unsaturated aldehyde, e.g. containing saturated carboxylic compounds, at an intermediate point. The water should generally be added in an amount at least twice that of the hydrocarbon. The ratio of water to hydrocarbon may be greater in the upper part of the column, e.g. above 1, and lower in the lower part. e.g. less than 1. This may be effected by taking off a mixture, for example, just above the crude aldehyde inlet, fractionating it, separating into two phases with a hydrocarbon, and returning at suitable places. In examples, xylene and a paraffinic fraction are the hydrocarbon solvents; acrolein-water azeotrope, acrolein and 2-methylacrolein mixtures with propionaldehyde, acetaldehyde, isobutyraldehyde, and acetone the mixtures treated. A preliminary separation by distillation may be used.