Optimizing Product Quality Performance ◾  67
specification limit of 32% heavy key impurity, and the standard
deviation of impurity concentration may be 0.2%. Then, the
trigger points for Table 7.2 can be set at, say, 31.6% and 31.2%
heavy key impurity to be 2 standard deviations and 4 standard
deviations below the upper specification limit.
In some distillation columns, the temperature movement
from concentration changes is too small to be measured. For
example, an off-spec product with 5 ppm of light key impurity
is stripped down to 10 ppb in the bottoms and concentrated
to 50 ppm in the distillate. In that case, then, the distillate flow
rate can be set to be 10% of the feed rate to get the distillate
concentration to increase by 10 from 5 to 50 ppm. The recommended action in Table 7.2 can be to increase or decrease the
distillate, or distillate/feed, by 1%.
An expert system will run a set of backward chaining rules
in the same order each time the system is run starting with
Rule No. 1. If Rule No. 1 is not true, then Rule No. 2 is run. If
Rule No. 2 is true, then Rule No. 2 is said to fire. When Rule
No. 2 fires, the expert system stops, and the action for Rule No.
2 is recommended. If none of the first six rules fire, then Rule
No. 7 fires, and the recommendation is “No Change Necessary.”
A situation can arise when operating a distillation column where Rule No. 1 in Table 7.2 can fire to recommend
an increase in steam/feed or reflux/feed, but that cannot be
accomplished because of a constraint on a manipulated variable. The most simple case occurs when a steam valve or a
reflux valve is already 100% open and cannot be increased
any further. That is a constraint. In another case, the flowmeter for the steam rate or for the reflux rate is at 100% of span
and cannot indicate a higher value for a flow control loop.
In yet another case, the vapor traffic going up the column is
creating the maximum pressure drop that can be operated
successfully through a distillation tray or through a distillation
column packing. A higher flow rate of vapor would hold liquid
up in the column, and it would be flooded. The column must
be kept below flooding. This is another constraint.