26 ◾  Distillation Control, Optimization, and Tuning
The feed to the distillation column in Figure 2.1 is split into
a distillate stream and a bottoms stream. The fraction of feed
going into distillate, that is, D/F, strikes a balance between distillate purity and bottoms purity. For example, with 50% lights
in the feed, a D/F ratio of 0.52 will force heavy impurities into
the distillate. Similarly, a D/F of 0.48 will force light impurities
into the bottoms. A high D/F ratio will result in all of the temperatures going up in a distillation column, but some temperature points will increase much more than others. For example,
the top and bottom temperatures may not change much when
the impurities are in the ppm range. However, the temperature
point may respond quite significantly where the vapor concentration in the column is about 50% lights and 50% heavies. If the D/F ratio is shifted up, then the vapor composition
and temperature from the tray below will be shifted up into
the tray of interest. Similarly, if D/F is shifted down, the liquid composition and temperature from the tray above will be
shifted down into the tray of interest.
4.2 Temperature Gradient per Theoretical Stage
A steady-state simulation for the distillation of 50 wt% cyclohexane in n-heptane with 50 theoretical stages at atmospheric
pressure was run to study the temperature and vapor composition gradient above and below each theoretical stage
(Figure 4.1). The feed is on Stage 23 from the bottom. The
base case was run with 0.07% C7 (heavy) impurity in the
distillate and 0.07% C6 (light) impurity in the bottoms. The
difference between the temperature on the stage below and
the stage above can be calculated for each stage and divided
by 2. The temperature goes up almost linearly with the concentration of heavy key component in the vapor phase, so
the concentration of light component was used from the
stage above minus the stage below, that is, (y n + 1 − y n − 1 )/2.
The composition gradients for the top and bottom stage were