70 ◾  Distillation Control, Optimization, and Tuning
7.7 Summary
This chapter described the quality performance of the distillate stream in terms of the average concentration of heavy
key impurity and the number of standard deviations from the
upper specification limit. The quality performance of the bottoms stream is described in terms of the average concentration
of light key impurity and the number of standard deviations
from the upper specification limit. The maximum economic
return from a distillation column is achieved by reducing variability, striking a balance between the distillate and bottoms
quality, and reducing energy consumption.
Exercises
7.1 If the average impurity concentration is equal to the
USL (upper specification limit), how many samples
would be out of spec every week assuming 21 samples
per week and a normal distribution of error?
7.2 If the light key impurity USL (upper specification limit)
for ethanol in water is 0.3% and the standard deviation
is 0.025%, what should the target concentration be to
achieve a frequency of failure rate of 1 sample out of
790?
7.3 For a binary distillation of benzene and toluene with a
USL of 0.2% toluene in the distillate and 1% benzene in
the bottoms, how should the MRT setpoint be changed
if the distillate contained 0.1% toluene and the bottoms
contains 1.5% benzene?
7.4 For a binary distillation of benzene and toluene with
a USL of 0.2% toluene in the distillate and 1% benzene
in the bottoms, how could the column performance be
improved if the toluene in the distillate was 0.05% and
the benzene in the bottoms was 0.1%?