Process for producing acetic acid

A process for producing acetic acid comprising:

• a reaction step comprising continuously allowing methanol to react with carbon monoxide in the presence of a catalyst system comprising a metal catalyst, a halide salt, and methyl iodide in a carbonylation reactor;
• a flash evaporation step comprising continuously feeding a flasher with a reaction mixture from the reactor to separate a volatile component (2A) containing product acetic acid, methyl acetate, methyl iodide and water, and a low-volatile component (2B) containing the metal catalyst and the halide salt;
• an acetic acid collection step comprising feeding a distillation column with the volatile component (2A), and separating an overhead (3A) containing methyl iodide, acetic acid, methyl acetate, water, by-product acetaldehyde, and hydrogen iodide, and a stream (3B) containing acetic acid to collect acetic acid;
• an acetaldehyde separation step comprising feeding an acetaldehyde distillation column with at least part of the overhead (3A) and distilling a liquid object to be treated containing the overhead (3A) to separate a lower boiling point component (4A) containing acetaldehyde and a higher boiling point component (4B);
• and
• a condensation and discharge step comprising temporarily holding the overhead (3A) in a decanter while condensing, and then discharging at least a part of the overhead (3A) from the decanter to subject to the acetaldehyde separation step;
• wherein, in the acetaldehyde separation step, the liquid object contains at least one methanol source selected from the group consisting of methanol and dimethyl ether in a concentration of 2 to 50% by weight;
• wherein, in the liquid object, the proportion of methyl iodide is 1 to 95% by weight, the proportion of methyl acetate is 0.5 to 50% by weight, the proportion of acetic acid is 0.2 to 50% by weight, the proportion of water is 0.05 to 95% by weight, and the proportion of hydrogen iodide is 1 to 1000 ppm on the basis of weight;
• wherein the ratio of the methanol source (in terms of methanol) in the liquid object is not less than 80 mol relative to 1 mol of hydrogen iodide;
• and
• wherein, in the decanter, the concentration of the methanol source in the upper layer portion of the condensed overhead (3A) is 0.3 to 50% by weight, the concentration of the methanol source in the lower layer portion of the condensed overhead (3A) is 0.1 to 20% by weight, the concentration of methyl iodide in the upper layer portion is 1 to 6% by weight, the concentration of methyl iodide in the lower layer portion is 60 to 95% by weight, and the concentrations of hydrogen iodide in the upper layer portion and in the lower layer portion are 1 to 300 ppm on the basis of weight.