Optimizing Product Quality Performance ◾  65
minimum variability and good response to load changes if
needed. If a distillation column is on manual control, then one
improvement may be to install a temperature controller for the
MRT (most responsive temperature) point in the column to
automatically shed many of the disturbances that come to the
column. In some cases, a matrix of the response of each variable for the distillation column is measured empirically, and
then a dynamic matrix control is used to reduce variability.
Online analyzers can be used for direct composition control, but data from production plant distillation columns have
shown the MRT (most responsive temperature) point responding 60 to 90 min before the online analyzer responds at one
end of the column.
7.6 Optimize with Expert System Advisor
The distillate and bottoms streams from a distillation column can be sampled and analyzed once per shift, and then
an expert system can be used to recommend adjustments in
setpoints for the separation power base and the MRT using a
set of backward chaining logic rules. For the case of an upper
specification limit of 1,000 ppm heavy key impurity in the
distillate stream and a standard deviation of 100 ppm impurity,
the desired impurity concentration may be 700 ppm, that is, 3
standard deviations below the upper specification limit. The
expert decision rules can be set to make adjustments only if
the impurity concentrations are outside of the range from 0.6
to 0.8 times the upper specification limit (USL). For example,
the first priority is to check the key impurity separation power.
If the key heavy impurity concentration in the distillate is
above 0.8 times its upper specification limit and the key light
impurity in the bottoms is above 0.8 times its upper specification limit, then increase the separation power base (Rule 1
in Table 7.2). An initial guideline is to increase the separation
power base, steam/feed, or reflux/feed by 1%.