GB712444A - Process for the purification of aqueous solutions of acrylic acid nitrile by distillation - Google Patents

Abstract

<PICT:0712444/III/1> Acrylic acid nitrile is purified from divinyl-acetylene and other impurities by subjecting a dilute aqueous solution of the crude nitrile (i.e. a solution containing not more than 5 per cent. of nitrile) to partial evaporation, and causing water or a dilute aqueous solution of the nitrile to flow countercurrent to the vapours, the temperature of the countercurrent stream being maintained below the boiling point of the acrylic acid nitrile/water azeotrope, whereby the divinyl acetylene is removed in the vapours and purified nitrile solution is obtained as distillation-residue. Before they are contacted with the water, the vapours are preferably diluted with an inert gas such as nitrogen, or a volatile hydrocarbon such as hexane. Crude nitrile solution, preheated to 50-60 DEG C., is introduced into the apparatus through inlet B, and flows into Vigreux distillation column A, which is surrounded by a jacket heated by steam at 100 DEG C. The solution boils in column A, and inert gas is introduced through capillary L in order to prevent bumping. The rising vapours are first washed by the countercurrent stream of feed, are next diluted with inert gas introduced through inlet C, and then enter extraction column D, which is packed with Berl saddles and is surrounded by a jacket, through which water at 30 DEG -40 DEG C. is circulated. Water at 30 DEG -40 DEG C. in an amount about equal to one-tenth of the amount of nitrile introduced at B is introduced into column D through inlet M, and flows countercurrent to the vapours, carrying evaporated nitrile back to column A. Vapours containing volatile impurities leave the top of column D at 35 DEG C., and are condensed in condenser E. Nitrile solution at 85 DEG -95 DEG C. leaves the lower end of column A, through an adjustable siphon G, and is passed to column H, where the nitrile is driven out by a countercurrent of steam. The vapours are condensed, and the condensate is collected in receiver K, where the nitrile separates as an upper layer. The nitrile may now be fractionated, and obtained in a high state of purity. In a modification, hexane (instead of inert gas) is introduced dropwise through inlet C, and is evaporated immediately. In this case column D is maintained at 60 DEG C. in order to prevent condensation of hexane therein. In a further modification, the gaseous mixture issuing from the nitrile synthesis tower (wherein acetylene is catalytically reacted with hydrocyanic acid) is introduced directly into the purification apparatus. In this case, acetylene is used to prevent bumping in column A; and gas leaving the top of column D is returned to the synthesis tower. A mixture of chlorinated benzenes is added to column A in order to dissolve cyano-butadiene, and to prevent the formation of undesirable polymerization products thereof.