A distillation column is provided with a downcomer having an enlarged upper region to increase a processing rate for the removal of permanganate reducing compounds (PRCs) formed by the carbonylation of methanol in the presence of a catalyst to produce acetic acid. The distillation column includes a vertically extending distillation housing; and a plurality of tray assemblies, at least one of which includes a horizontally extending tray panel and a downcomer having, in a downward direction, an enlarged, funnel-shaped upper region defined by a top downcomer panel oriented at an angle to the vertical, and a lower region of substantially constant cross-sectional area defined by a vertically-oriented bottom downcomer panel.