GB662356A - Improvements in or relating to the production of acetic anhydride - Google Patents

Abstract

<PICT:0662356/IV (b)/1> Acetic anhydride is obtained by a continuous process which comprises pyrolysing acetone to form ketene, immediately quenching the ketene, unconverted acetone and fixed gases so formed to a temperature of 200-300 DEG F. by jet mixing them with a mixture of acetic anhydride and acetic acid, whereby the ketene reacts with the acetic acid to produce acetic anhydride, adding acetic acid to the reaction products and using at least a portion of the resultant mixture of acetic acid and acetic anhydride as the quenching medium. Referring to the drawing acetone is vaporized in vaporizer 2 and then passed through the preheater 4 which is preheated to 1150 DEG F. and then to pyrolysis chamber 6 which may be a single tube or coil of copper or of an alloy such as stainless steel containing a large amount of chromium or of the alloy disclosed in Specification 655,101. The pyrolysis temperature may be from 1150 DEG to 1300 DEG F. with a contact time of 0.3 to 0.5 seconds. The effluent gases from the pyrolysis chamber are immediately quenched to a temperature not exceeding 300 DEG F. in the venturi mixer 8 with a mixture of 75-10 per cent acetic acid and 25-90 per cent acetic anhydride, this quenching mixture being preferably drawn off from tower 10 through line 12. The quenched mixture then passes by line 9 to tower 10 where it is contacted counter-currently with glacial acetic acid introduced through lines 11 and 31. The top section of the tower 10 serves as a fractionation section to separate the acetone and acetic acid, the acetone leaves in the top gases whilst a mixture of acetic acid and acetic anhydride is removed from the bottom of the tower and after passing through cooler 13 is pumped to venturi 8 where it is mixed with the effluent vapours from the chamber 6. Another portion of the bottoms from tower 10 is passed through line 16 to the mid-section of fractionator 17 where the acetic acid and acetic anhydride are separated, the former being recycled to tower 10 through line 28, condenser 29 and line 31 and the latter withdrawn through line 32. Unconverted acetone, some acetic acid and fixed gases (largely methane, carbon monoxide and ethylene) are passed overhead from tower 10 through condenser 19 to separator 20, a portion of the acetone being returned to tower 10 through line 21 as reflux and the remainder returned as feed to the vaporizer 2. Fixed gases containing a small amount of acetone are passed by line 24 to scrubber 25, where they are contacted countercurrently at low temperature with glacial acetic acid introduced through line 26. The acetic acid enriched with acetone is then returned to the middle of tower 10. Specification 627,810 is referred to.