Unit 5 - Continuous Distillation - School of Life Sciences

Unit 5: Continuous distillationThe analyser trays are linked by downcomer pipes, arranged so thatthere is a weir effect determining the liquid level on the tray. Thevapour pressure is such that the vapour bubbles into the liquidthrough the perforations, while preventing the liquid from fallingthrough them. When the vapour enters the liquid some of the lessvolatile components condense, thus the vapour which passesthrough becomes richer in the more volatile components.Simultaneously the latent heat of condensation causes the morevolatile components in the liquid to vaporise, thus stripping the liquidof alcohol as it descends from tray to tray.The rectifier tray has a wash coil snaking its way across and thendown the column. Again there is a rising vapour stream and adescending liquid one, created by condensation of vapour on theoutside of the wash coil and supplemented by the refluxing of someof the condensed vapour leaving the rectifier. The temperaturegradient causes the less volatile components in the vapour tocondense in the lower trays and the more volatile ones in the highertrays, which are at lower temperatures.KEY POINT: In both Coffey still columns there arecountercurrent flows. In the analyser hot wash descends andsteam rises; the result at the top of the column is a vapourenriched in alcohol. This vapour stream then rises in therectifier, progressively condensing on the outside of thedescending wash coil, such that liquids of differing compositioncollect on the rectifier trays.The beauty of the Coffey still is that the temperature gradient in thewash coil creates different liquid profiles at each tray. A typicalconcentration of compounds in a rectifier is shown in Figure 3. Theprofile is a reflection of the equilibrium temperature reached on eachtray. This is controlled by measuring the temperatures at the bendsand adjusting flow rates.© 2001 The International Centre for Brewing and Distilling 8