The Compleat Distiller

THE COMPLEAT DISTILLER 99
These three equations, plotted for ethanol mol fractions ranging
from 0 to 1 and temperatures from 78.5ºC to 100ºC, result in the
following constant pressure plot. The lower curve is called the
bubble line, because it shows the temperatures at which the mix
begins to boil, and the upper curve is called the dew line,
because it indicates the temperature at which vapor first
condenses. Anything below the bubble line is liquid, and
anything above the dew line is vapor. The area enclosed by the
two curves represents points where liquid and vapor are in
dynamic equilibrium.
Fig. 8-5
This plot is extremely useful Because it lets you graphically plot the progress of a distillation. If, for
example, you start with 11% ethanol in the boiler, the first vapor to come off contains 23% ethanol. If
this is condensed and re-boiled, the resulting vapor would contain 40% ethanol, and so on up to the
maximum possible concentration of around 96% ethanol. Ethanol has a maximum concentration lower
than 100% because it forms an azeotropic mixture with water. We used charts like this in Chapter 2 to
illustrate what happens with repeated distillations.
The stages in this plot are called theoretical plates. This name comes from the plates or trays inserted
in large commercial distillation towers to facilitate the evaporation and condensation process. These
plates serve the same purpose as the packing used in small-scale columns. On the diagram, a
theoretical plate is marked out by going vertically from the bubble line to the dew line, and then
horizontally back to the bubble line.
In the example starting at 11%, 7 plates are needed to reach the top concentration, and the first plate is
the boiler itself. If a packed column is 100 cm long, and the temperature levels out at the boiling point
of ethanol 60 cm up, then the Height Equivalent Theoretical Plate (HETP) for this packing would be
60/6 = 10 cm. If you are interested in calculating the number of theoretical plates needed in a column,
you should use the McCabe-Thiele equations, and we use a simplified form of these in a moment when
discussing Reflux. On the internet, Tony Ackland's website (see Appendix 8) gives an excellent
description of them. Tony is a chemical engineer in New Zealand with a great interest in the hobby of
distillation.