68 ◾  Distillation Control, Optimization, and Tuning
Table 7.3 Distillate Rules When Steam or
Reflux is Constrained
Rule
No.
Impurity in
Distillate
Recommended Action
Temperature (D/F)
1
>0.8 Dist USL Decr .3(MRT/stage)
2
<0.6 Dist USL Incr .3(MRT/stage)
3
No change necessary
When the separation power is at a maximum constrained
value because of boilup or reflux, then Rule No. 1 in Table 7.2
is no longer valid, and a new set of rules is needed. When
either boilup or reflux is constrained, then one degree of freedom for control is lost, and only one of the product streams
can be kept within specification. A decision would need to
be made to keep the distillate within specification or bottoms
within specification, but it may not be possible to keep both
streams in spec. If the distillate stream is to be kept in spec,
then the three rules in Table 7.3 can be used. If the bottoms
stream is to be kept in spec, then the three rules in Table 7.4
can be used.
When the operating conditions were optimized for a
number of distillation columns, there was an average reduction in energy consumption of 18% valued in the range of
$30,000 to $50,000/year, which was also accompanied by
Table 7.4 Bottoms Rules When Steam or
Reflux is Constrained
Rule
No.
Impurity in
Bottoms
Recommended Action
Temperature (D/F)
1
>0.8 Bott USL Incr .3(MRT/stage)
2
<0.6 Bott USL Decr .3(MRT/stage)
3
No change necessary