Removal of permanganate reducing compounds and alkyl iodides from a carbonylation process stream

A method for reduction and/or removal of permanganate reducing compounds (PRC's) and C.sub.2-12 alkyl iodide compounds formed in the carbonylation of methanol to a product of acetic acid, wherein said methanol is carbonylated in a suitable liquid phase reaction medium comprising a Group VIII metal catalyst, an organic iodide and iodide salt catalyst promoter;

• the products of said carbonylation are separated into a volatile phase comprising product, and a less volatile phase comprising Group VIII metal catalyst, acetic acid, and iodide catalyst promoter;
• said product phase distilled in a distillation tower to yield a purified product and an overhead comprising organic iodide, methyl acetate, water, acetic acid, and unreacted methanol, directing at least a portion of the overhead to an overhead receiver decanter which separates the overhead into a light phase, comprising acetic acid and water, and a heavy phase comprising methyl acetate and organic iodide;
• and recycling the heavy phase to the carbonylation reactor, the improvement which comprises
• (a) directing the light phase comprising acetic acid and water to a distiller which separates the mixture into two streams:
• residue stream (1) comprising water and acetic acid, and overhead stream (2) comprising methyl iodide, methyl acetate, methanol, C.sub.2-12 alkyl iodides, and PRC's;
• (b) cooling stream (1) of step (a) and ultimately recycling stream 1 to the reactor, and directing stream (2) of step (a) to a second distiller which serves to strip the PRC's from the mixture, forming a PRC enriched overhead stream and a residue stream comprising methyl iodide, methyl acetate, methanol, and water;
• (c) optionally, forwarding the overhead stream of step (b) to an extractor to remove any remaining small amounts of organic iodide compounds therefrom;
• and, separating out concentrated PRC's for disposal and returning the organic iodide phase of step (b) or step (c) as a stream containing a lower percentage of PRC's and C.sub.2-12 alkyl iodides to the carbonylation reactor.