58 ◾  Distillation Control, Optimization, and Tuning
7.1 Quality Performance Measurement
One measure of the quality of distillate and bottoms from a
distillation column is the concentration of impurities present.
The rectification section of a distillation column above the
feed point is primarily for removing heavy impurities from
the distillate. So, the quality of the distillate stream can be
measured by the concentration of heavy key impurity. The
distillate quality specification can be an upper specification
limit (USL) of heavy key impurity in the stream. If there are
lighter components present, they will essentially all appear in
the distillate.
Similarly, the stripping section of the distillation column
below the feed point is primarily for removing light impurities from the bottoms stream. The quality of the bottoms
stream can be measured by the concentration of light key
impurity. The word impurity is simply used to indicate low
concentrations. The actual light key impurity component
left in the bottoms may be the product being sold and may
represent a yield loss. The bottoms quality specification can
be an upper specification limit (USL) for the light key impurity in the stream. Essentially, all heavier components will
appear in the bottoms. A failure can be indicated by the
impurity concentration exceeding the upper specification
limit.
The concentration of key impurities in samples of distillate or bottoms can be studied with conventional statistical
calculations, for example, average concentration and standard deviation. There can be common cause variability from
everyday disturbances that occur to a distillation column and
assignable cause variability from known disturbances such
as startup, loss of steam, etc. The main emphasis of statistical measurement of product quality is on common cause
variability.