Method and apparatus for producing acetic acid

A process for producing acetic acid, comprising:

• carbonylating methanol with carbon monoxide in the presence of a catalyst system, acetic acid, methyl acetate, and water, wherein the catalyst system comprises a metal catalyst, an ionic metal iodide, and methyl iodide, to obtain a reaction mixture;
• flash-evaporating the reaction mixture to separate the reaction mixture into a volatile phase and a less-volatile phase;
• first-distilling the volatile phase to separate the volatile phase into a first overhead, a side-cut acetic acid stream and a bottom stream, wherein the first overhead contains at least one permanganate reducing compound (PRC) and methyl iodide, and the PRC contains at least acetaldehyde;
• and the side-cut acetic acid stream mainly contains acetic acid;
• and
• separating at least acetaldehyde from the first overhead,
• wherein the separation comprises:
• second-distilling the first overhead to separate the first overhead into a second overhead rich in acetaldehyde and methyl iodide and a bottom or lower stream, the second distillation comprising at least one distillation column, while controlling an internal pressure of the distillation column in the second distillation by the following method (1) and/or (2):
• (1) introducing at least one gas selected from the group consisting of a nitrogen gas, an off-gas, and an inert gas constantly to the feeding line communicating with the column top of the distillation column, with remaining a control valve on the feeding line open at a predetermined degree of opening,
• (2) discharging or withdrawing a non-condensable gas, and
• extracting acetaldehyde from the second overhead to form an extract rich in acetaldehyde and a raffinate rich in methyl iodide.