What Drives Distillation Towers

All machines have drivers. A distillation column is also a machine, driven by a reboiler. It is the heat duty of the reboiler, supplemented by the heat content (enthalpy) of the feed, that provides the energy to make a split between light and heavy components. A useful example of the importance of the reboiler in distillation comes from the venerable use of sugarcane in my home state of Louisiana. 

If the cut cane is left in the fields for a few months, its sugar content ferments to alcohol. Squeezing the cane then produces a rather low-proof alcoholic drink. Of course, one would naturally wish to concentrate the alcohol content by distillation, in the still shown in Fig. 7.1. 

The alcohol is called the light component, because it boils at a lower temperature than water the water is called the heavy component, because it boils at a higher temperature than alcohol. Raising the top reflux rate will lower the tower-top temperature and reduce the amount of the heavier component, water, in the overhead alcohol product. But what happens to the weight of vapor flowing up through the trays Does the flow go up, go down, or remain the same  

There are two ways to answer this question. Let s first look at the reboiler. As the tower-top temperature shown in Fig. 7.1 goes down, more of the lighter, lower-boiling-point alcohol is refluxed down the tower. The tower-bottom temperature begins to drop, and the steam flow to the reboiler is automatically increased by the action of the 

Now let s look at the reflux drum. The incremental reflux flow comes from this drum. But the liquid in this drum comes from the condenser. The feed to the condenser is vapor from the top of the tower. Hence, as we increase the reflux flow, the vapor rate from the top of the tower must increase. One way of summarizing these results is to say that the reflux comes from the reboiler. 

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