Improvements in or relating to processes for azeotropic dehydration of aqueous acetic acid or aqueous mixtures of acetic acid and acetic anhydride

摘要

PICT:0749018/III/1 A process for dehydrating and purifying aqueous acetic acid, or a mixture of acetic acid and acetic anhydride, which contains acetaldehyde and/or similar volatile, water-soluble impurities, by azeotropic distillation using a main water-entrainer (e.g. ethyl acetate), and an auxiliary water-entrainer (e.g. cyclohexane) which forms a water-azeotrope more volatile than that formed by the main entrainer, is characterized in that reflux liquid in the upper section of the distilling column, which liquid comprises auxiliary waterentrainer, water and only a small proportion of the volatile impurities in the feed is subjected to hot decantation, either in the column itself or by withdrawing it as a side-stream to an external decanter, the aqueous decanted layer being withdrawn from the system, and the non-aqueous layer being allowed to flow back down the column. The vapours rising above the level where the reflux liquid is decanted are subjected to further rectification, either in an auxiliary column or in an extension of the main column, and the bulk of the volatile impurities are removed from the system as the overhead fraction from this further rectification zone. If an auxiliary column is used, it is advantageously operated under pressure. Substantially pure anhydrous acetic acid (together with anhydride, if present in the feed) is withdrawn from the base of the main column. A mixture obtained by the oxidation of acetaldehyde, and containing acetic anhydride (57 per cent) acetic acid (20 per cent), water (10.2 per cent), acetaldehyde (12.5 per cent) and methyl acetate (0.3 per cent) was subjected to distillation at 100 kg. per hour in column 1, which had previously been charged with ethyl acetate and cyclohexane. The column was provided with a decanting plate 2, and above this there were four distilling plates. The lower liquid layer on plate 2 contained 2 per cent of acetaldehyde; and 0.2 kg. per hour of acetaldehyde were removed through pipe 3. Cooling in condenser 5 was so controlled that 27 kg. per hour of vapours passed through pipe 4 to auxiliary column 6, these vapours containing the remaining 12.3 kg. of acetaldehyde and 0.3 kg. of methyl acetate, mixed with the ethyl acetate/cyclohexane/water azeotropic mixture. The impurities were withdrawn through pipe 7 after condensation in condenser 8. The ethyl acetate, cyclohexane and water were returned to column 1 through pipe 9. An anhydrous mixture of acetic acid and acetic anhydride was withdrawn from the base of column 1 through pipe 10. Specification 587,269 is referred to.