Downcomer, distillation residence time

The downcomer area required for trays not only increases with the liquid-flow-rate, but also with the difficulty in achieving separation between the vapour and the liquid phases. The volume required for the downcomer (downcomer residence time) increases at a lower surface tension and a smaller density difference between vapour and liquid. Because of the large downcomer area required to handle the high liquid flow rates typical of high-pressure distillations, a trayed column cross-sectional area may be 40% to 80% greater than the active tray area calculated from the vapour flow rates for such distillations. Thus, the downcomer area becomes a significant factor in the determination of the diameter of a tray column. 

R. L. Bell [AIChE 18, 495 (1972)] investigated residence time distributions on commercial-scale distillation trays using a fiber optic technique. A pulse of a 10-g/L solution of Rhodamine-B dye was injected into the downcomer of the top tray. The response data are summarized below. 

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