Process for isolating acrylonitrile by distillation

摘要

Acrylonitrile is isolated from a hot gaseous mixture containing also hydrocyanic acid, acetaldehyde, acetonitrile, propionic nitrile and higher boiling nitriles, such as may be obtained by the catalytic dehydration of lactonitrile, by washing the hot mixture with water in a washing tower 2 and passing the resultant solution into a stripping column 8 heated at its base to about 100 DEG C. and supplied at the top with water at a temperature up to 70 DEG C. Only the acetonitrile is washed out of the vapour stream ascending the column 8, the remaining vapour containing the acrylonitrile and all the impurities except acetonitrile being condensed in a condenser 11, the condensate from which separates into an upper phase 13, from which acrylontrile is obtained by distillation in known manner, and a lower, aqueous, phase, 12, which is returned to the stripping column. The column 8 may be heated directly by steam introduced at 9. The water supplied to the top of the column 8 may be obtained by branching off at 25 some of the aqueous acetonitrile washed out of the column at 14, the acetonitrile being expelled by steam from the column 15 and the water returned by line 10 from cooler 17. All the acetonitrile may be recovered in this way from line 16 if no aqueous acetonitrile is withdrawn by line 18. Part of the aqueous acetonitrile from line 14 may also be recirculated by line 27 to heat exchangers 4 and 5 and thence to washing tower 2 which it enters at 20 DEG -30 DEG C., the aqueous solution leaving tower 2 by line 7 reaching about 85 DEG C. in heat exchanger 4. If lactic nitrile is present in the gaseous mixture its loss from the system may be avoided by heating column 8 and if desired column 15, by indirect heat exchange with steam. This avoids the necessity of having to remove excess water resulting from condensation by lines 18 and 19; if, however, column 15 is heated by indirect exchange an amount of additional water equal to that carried over with the acetonitrile in line 16 is added to water entering cooler 17. Specifications 719,911 and 802,114 are referred to.