Purification of acrylonitrile by distillation

摘要

Acrylonitrile containing acetaldehyde, hydrogen cyanide and lactonitrile as impurities is refined by feeding the crude acrylonitrile to an intermediate section of a fractionating column, withdrawing lactonitrile (together with high boiling impurities) from the base of the column, withdrawing partially purified acrylonitrile as overhead vapours or as a liquid side stream from the upper part of the column, and immediately introducing this partially purified acrylonitrile (in the liquid phase) into a second fractionating column which is operated under such conditions of temperature and pressure that all the acetaldehyde and hydrogen cyanide in the partially purified acrylonitrile are stripped therefrom and expelled in the vapours leaving the second column while refined acrylonitrile is withdrawn from the base of this second column. Acrylonitrile containing acetaldehyde (0.5 per cent), HCN (0.01 per cent), lactonitrile (2.5 per cent) and high boilers (3.0 per cent), is fed through line 2 into column 1 (fourteen theoretical plates) which is heated by reboiler 15. The column is operated at a pressure of 140 mm. Hg absolute. The temperature at the base is maintained at 50 DEG C., while the overhead temperature is controlled at 30 DEG C. The overhead vapours, which contain acetaldehyde and HCN, are passed to condenser 5; and part of the condensate is returned to the column through line 6 while the remainder is withdrawn via line 7. The bottoms withdrawn through line 4 comprise lactonitrile and high boilers. Liquid acrylonitrile is withdrawn from the sixth plate from the top of column 1, and is passed through line 8 to column 9 (five theoretical plates) which is maintained at a pressure of 140 mm. Hg., and is heated by reboiler 16 to a temperature of 300-32 DEG C. The overhead vapours from column 9 contain the remainder of the acetaldehyde and HCN; these vapours are condensed in condenser 11, and the condensate is introduced through line 10 into column 1. Pure acrytonitrile is withdrawn from the base of column 9, either as liquid through line 13, or as vapour through line 14.