(145b) Gravity Flow and Internal Piping in Distillation Columns – the Devil Is in the Details

Internal piping and ducts are often used in inside distillation towers. Applications include routing liquid overflow from collectors, between geometry transitions (trays to packing or changes in tray passes), complex tray geometries (some types of radial flow trays and orbit trays), and mixing internal liquid flows with externally supplied liquids (feed and pumparound returns). Internal supplied liquid may enter piping in some combination of six major flow regimes. These are (a) weirlike flow, (b) full pipe flow, (c) Borda free flow with orifice control, (d) Borda full flow, (e) perfect weir flow, and (e) Borda free flow with weir control. Each of these has a different effective entrance coefficient into the piping. The entrance coefficient and flow rate determine the actual liquid head necessary inside the column. These flow regimes are examined and recommendations are made on why every effort should be made to seal piping to encourage full pipe flow.