Distillate/Feed Material Balance Split ◾  29
that the temperature movement at a point in a distillation column is a result from the compositions above and below that
point in the column when the pressure is steady.
In addition to the two methods described for finding the
MRT (most responsive temperature) point in a distillation column, there are three additional methods that can be used without a computer simulation. First, if there are temperature points
in the column that are monitored, just look at the history record
to see which temperature point moves the most. Second, find
the temperature point in the column that is nearest to being
halfway between the top temperature and the bottom temperature. Third, look in the area of the column where the vapor
concentration would be about 50% light key component. For
example, if the feed contains 10% light key component, there is
a high probability that the MRT would be above the feed point.
Similarly, if the feed contains 80% light key, there is a high
probability that the MRT would be below the feed point.
Many of the disturbances to a distillation column come
from the feed stream changing in flow rate, temperature, or
composition. Other disturbances include changes in steam
header pressure, changes in cooling water temperature, and
changes in cooling water supply pressure. Also, changes in
ambient temperature such as day and night temperature cycles
and sudden rainstorms can cause disturbances. The MRT point
in the distillation tower generally responds first. Most of the
disturbances can be shed by process control action before
either of the end compositions go out of spec. For vacuum
towers, it is sometimes necessary to add pressure compensation to the MRT, so the control action is primarily for changes
in composition.
4.4 Summary
The multiple stages of counter-current contact with mass
transfer in a distillation column cause a temperature profile to