Unit 5 - Continuous Distillation - School of Life Sciences

Unit 5: Continuous distillationserrated edges of the cap. Clearly this is a much more expensive trayto manufacture.Floating valves (B) offer another alternative with the valve movementbeing controlled by the vapour flow. If the flow drops then the valvefalls preventing liquid weepage. Valve trays fall between sieves andbubble caps in cost.However neither bubble caps or valve trays are suitable for a feedcontaining solids, because there is too much danger of blockage.Sieves can be used in these conditions and another option is the discand donut tray (C). Here the liquid is made to cascade from theoutside to the centre, and back to the outside of the column and thevapour has to force its way through the curtains of liquid.KEY POINT: The main concerns in tray design are theproblems caused by solids and the need to avoid the dumpingand entrainment of liquid.The condensation and evaporation which occurs on the tray is similarto that already described in the Coffey still section.5.4.2 Operating Lines.In order to calculate the number of trays required in a column it isnecessary to carry out mass balances at the top and bottom of thecolumn.Consider the situation shown in Figure 8, and note that in this Figurethe trays are numbered from the top to the bottom of the column.© 2001 The International Centre for Brewing and Distilling 15