TABLE OF CONTENTS An internal-combustion engine drives a car. Pumps are driven by turbines or motors. Jet planes are pushed by the thrust of an axial air compressor.
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 sugar cane 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. 6.1.
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...
