Std Dev’s = 100, USL’s = 1000, Bott DNS = 4σ, Dist DNS = 4σ
Fraction of Samples
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
–1000
–800
–600
–400
–200
0
200
400
600
800
1000
–ppm Light Key in Bottoms (Tray Temp) ppm Heavy Key in Distillate
Figure 7.4 High steam consumption in distillation column.
Optimizing Product Quality Performance ◾  63
7.5 Reduce Variability
One of the best ways of improving product quality at both
ends of the distillation column is to reduce variability, as
shown in Figure 7.5; that will reduce the failure rate. Robinson 2
described the benefits from improved dryer control, such as
reduced variability in product moisture content and reduced
energy consumption because the average concentration of
moisture in the product could be run closer to the upper
specification limit.
The maximum economic return from a distillation column
can be achieved by minimizing variability and optimizing
the quality performance between the distillate and bottoms
streams (Figure 7.6). The optimum DNS for a product being
sold is usually in the range of 3.0 to 3.5σ. The optimum DNS
for a stream that is being recycled back in the process is usually in the range of 1.5 to 2.0σ.
Reduction of variability can be accomplished by the selection of the best control strategy for the distillation column to
shed disturbances and by tuning process control loops for