GB1465511A - Continuous production of phthalimide - Google Patents

Abstract

1465511 Phthalimide BASF AG 4 July 1974 [6 July 1973] 29607/74 Heading C2C A continuous process for preparing phthalimide comprises reacting phthalic anhydride with ammonia, flowing in countercurrent at elevated temperature and in a molar ratio of 0À9 : 1 to 1À1 : 1. The offgas formed in the reaction is preferably scrubbed with aqueous ammonia solution. In a preferred embodiment scrubbing is carried out continuously and countercurrent in a column. The washing solution is introduced into the top of the column, conveniently through a spray means or spray nozzle. The off-gas enters into the bottoms in the column. The washing liquid collecting in the bottoms after the scrubbing is conveniently drawn off continuously, some of it, advantageously from one third to one half of the total amount being returned to the top of the column with fresh washing solution and the remainder being supplied to an evaporator. The washing liquid is continuously concentrated in the evaporator, and the mixture of steam and ammonia formed is conveniently supplied to the ammonia scrubbing. It is advantageous to regulate the amounts of recycled washing liquid, freshly supplied aqueous ammonia and ammonia and water supplied from the evaporator so that the offgas is continuously scrubbed at a pH of from 10 to 13 in the washing solution. From 20 to 40% by weight of the whole of the water in the evaporator is advantageously withdrawn from the liquid in the evaporator. The evaporator liquid is then advantageously continuously passed into a decomposition plant and kept at a temperature of conveniently from 160‹ to 250‹ C. The vapour/gas mixture (of water, phthalimide, phthalic anhydride and ammonia) formed in the decomposition is returned to the phthalimide manufacture conveniently to the central region of the said countercurrent column. In a further preferred embodiment the concentrated evaporator liquid withdrawn continuously from the evaporator is not passed into the decomposer but direct into the countercurrent column of the phthalimide manufacture so that the decomposition is carried out in the reactor itself.