Unit 5 - Continuous Distillation - School of Life Sciences

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Unit 5: Continuous distillationIt can be seen that this system closely resembles the Coffey still.Although Figure 4 shows only one column, in practice this is usuallysplit into two to reduce the height. The only significant difference isthat in the Coffey Still there is a wash coil causing condensation inthe rectifier, whereas in the column all the descending liquid in therectifying section is provided by reflux.KEY POINT: The significant difference between a Coffey Stilland a column still system is that in the latter all the descendingliquid in the rectifying section is provide by reflux.Figure 5 shows the vapour/liquid equilibrium diagram for anethanol/water mixture at atmospheric pressure. Note that molepercentages are used, as in chemical engineering this is thepreferred way of studying vapour / liquid systems.Azeotrope 97.2% abvEquilibrium curveMole % ethanol in vapour(10% abv)Mole % ethanol in liquidFigure 5. Vapour / Liquid Equilibrium Diagram.© 2001 The International Centre for Brewing and Distilling 12
Unit 5: Continuous distillationThe diagonal straight line in the diagram represents points where theconcentration in the vapour equals the concentration in the liquid;conditions under which distillation could obtain no separation of thecomponents. The equilibrium curve for ethanol:water meets thediagonal at 89 mole percent of ethanol. This is called the azeotrope(or constant boiling mixture) and indicates the maximum separationwhich can be achieved by distillation.Returning to the other end of the curve, let us consider what happenswhen a mixture containing 3.3 mole % ethanol (10% abv) is boiled ina pot. A vapour will be produced with 24 mole % ethanol (53% abv).If this is condensed and reboiled in a second pot, the vapour will be55 mole % ethanol (83%abv) and if this process is continued one willmove up the equilibrium curve producing more concentrated ethanolsolutions until the azeotrope is reached.This series of steps is oversimplified because as vapour is drawnfrom the pot so the circumstances change. However if the pot had acontinuous feed of the ethanol:water mixture and the vapourgenerated in the first pot was continually condensed, and thensupplied to the second pot, then the process would closely approachwhat happens in a distillation tower, with the trays replacing the pots.Now we need to examine the tray in more detail. Figure 6 shows across section of the perforated tray.Figure 6. Sieve Tray.The operation of such a tray is a complex balance. Low vapour flowrates result in pulsating liquid flow or dumping of liquid onto the tray© 2001 The International Centre for Brewing and Distilling 13
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Unit 5 - Continuous Distillation - School of Life Sciences

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