30 ◾  Distillation Control, Optimization, and Tuning
develop across the column. The highest temperature is found
at the bottom of the column, while the lowest temperature is
found at the top of the column. The temperature profile corresponds directly to the material balance split within the tower.
There exists a point within the tower where the temperature
will change the most as the material balance and the composition profile change. The temperature point that is most
responsive to changes in material balance split tends to be
near the point where the vapor composition is about 50% light
key component and 50% heavy key component. The most
responsive temperature point in the tower:
1. Responds the most to a change in material balance split
2. Can be anywhere in the tower
3. Tends to be near the mid-temperature (50% in vapor)
4. Can be identified by computer simulations
5. Can be seen in the control room by watching which temperature point moves the most
6. Can move within a range (operating window) while both
distillate and bottoms compositions stay within spec.
7. Tends to be closer in the tower to the end with the
smaller product stream

Exercises
4.1 With a column distilling a binary mixture of cyclohexane and n-heptane near to total reflux with 10%
cyclohexane in the bottoms and 49% cyclohexane in
the distillate and 4.0 theoretical stages, which stage
would have the most responsive temperature due to
changes in D/F material balance split?
4.2 Calculate the D/F ratio if the feed concentration is 50%
cyclohexane and 50% n-heptane, the distillate contains 0.1% n-heptane, and the bottoms contains 0.1%
cyclohexane.