GB468218A - Improved process for the preparation of acetic anhydride - Google Patents

Abstract

<PICT:0468218/IV/1> <PICT:0468218/IV/2> Acetic anhydride is produced by heating acetic acid vapour in presence of an anhydrideforming catalyst, cooling the reaction vapours, to condense acetic acid water and acetic anhydride as described in the parent case, and then separating the acetic acid and reacting it with the ketene which remains uncondensed. The reaction vapours from an apparatus b, Fig. I, are cooled at c, the condensate passing to a fractionating column d, water containing some acetic acid being withdrawn at the top and condensed at g, while the ketene vapour passes from the cooler c to the bottom of a trickle tower e where it reacts with acetic acid from the column d forming anhydride, and the reaction heat in e vaporizes a part of the acetic acid, which passes to the column d by a pipe m and assists the fractionation in d; reduced pressure is maintained by a pump i. In a modification, Fig. II, the vapour liberated in e is cooled at l or by an external cooler, and waste gas escapes to the pump i; if for any reason there is too little acetic acid in e, more is added through m. In an example, acetic acid vapour containing 0,1 per cent of ammonia and 0,3 per cent of triethyl phosphate is passed through a chromium alloy tube at 780 DEG C., a pressure of 30--40 mm. being maintained by the pump i, using the apparatus shown in Fig. II. The vapours are cooled to 0 DEG C., and the condensate fed to the column d, 55 per cent acetic acid distils from the top of the column and a mixture of 75 parts acetic acid and 25 parts acetic anhydride flows to the tower e, which is also fed with additional acetic acid.