GB840831A - Purification of acrylonitrile - Google Patents

Abstract

Acrylonitrile containing dissolved acrolein and water, e.g. as obtained in catalytic vapour phase reaction between acrolein, ammonia and oxygen, is purified by a process comprising fractionally distilling the impure acrylonitrile to separate an overhead fraction containing substantially all of the acrolein and water together with a proportion of the acrylonitrile, and a bottom fraction of pure dry acrylonitrile, mixing the overhead fraction with water to form an aqueous phase and an acrylonitrile phase, and returning the acrylonitrile phase to the fractional distillation. The impure acrylonitrile may contain up to about 3% of water and between 5 and 10% of acrolein but proportions of between 1 and 2% water and 0.3 to 0.5% acrolein are preferred, the latter being obtained by first fractionally distilling a wet acrylonitrile of 5 to 10% or more acrolein content and taking off a fraction containing the acrolein as an azeotrope with water and a small proportion of acrylonitrile, while removing wet acrylonitrile of 0.3 to 0.5% acrolein content as the base product. The amount of water added to the overhead fraction is suitably between 1/2 and 4 times the weight of feed to the still and the resultant upper acrylonitrile phase is returned to the fractionating column by combination with the feed to the column or by introduction at the top of the column. The resultant lower aqueous phase may be led off as the main outlet for acrolein in the recovery process when the starting material contains a substantial amount, e.g. 5 to 10%, of acrolein, but when only a 0.3 to 0.5% proportion of acrolein is present, the aqueous phase is recycled to a previous stage in the recovery process, e.g. the stage at which the aqueous solution of acrylonitrile, obtained by scrubbing the catalytic oxidation product with water, is concentrated.