Process for the preparation of acetic acid and acetic anhydride

Claim 1

A process for the preparation of acetic acid and acetic anhydride by reacting methanol and methyl acetate with carbon monoxide, wherein a) methanol and methyl acetate in the molar ratio 10 : 1 to 1 : 10 are reacted under anhydrous conditions with carbon monoxide or mixtures of carbon monoxide and hydrogen in the presence of a carbonyl complex of noble metals from group VIII of the Periodic Table of the Elements, methyl iodide and, as promoter, a catalyst system containing an acetate or iodide of an alkali metal, quaternary organophosphonium or organoammonium, in a reaction zone at temperatures of from 150 to 250 ° C and pressures of from 5 to 120 bar; b) the carbonylation mixture leaving the reaction zone at a temperature of from 150 to 250 °C is decompressed to a pressure of from 1 to 3.5 bar in a vapor-liquid deposition zone, the major part of the volatile components evaporating immediately and, in order to prevent entrainment of liquid drops, being fed via a mist eliminator to a first distillation zone for removal of the low-boiling components; the major part of the still-volatile components is distilled off in a separation zone at a pressure of from 0.05 to 1 bar and a bottom temperature of from 50 to 170°C from the liquid stream produced in the vapor-liquid deposition zone and in the mist eliminator and is likewise fed to the first distillation zone, and the catalyst solution which remains as the bottom product is fed back to the reaction zone; c) the volatile carbonylation products are split in the first distillation zone by fractional distillation under atmospheric pressure into a low-boiling component predominantly comprising methyl iodide and methyl acetate, and are fed back into the reaction zone, and the bottom product obtained is a mixture of acetic acid and acetic anhydride; d) in order to remove traces of iodine-containing compounds, the mixture of acetic acid and acetic anhydride is passed over a carrier containing a silver salt or treated with peracetic acid, and is split into the pure components, acetic acid and acetic anhydride, by fractional distillation in a second distillation zone and a third distillation zone.