10 ◾  Distillation Control, Optimization, and Tuning
cause a surge in vaporization. An increase in pressure can
cause a surge in condensation.
The separation power base in the classic McCabe–Thiele 1
graphical model of a binary distillation column is established
by the reflux ratio, R/D, which is the ratio of the reflux flow
rate divided by the distillate flow rate. For example, with a
distillation column that is fed 1,000 kg/h of feed that produces
85 kg/h of distillate with 425 kg/h of reflux, the reflux ratio
is 425/85 = 5. A minimum reflux ratio is required to achieve
the desired separation with an infinite number of theoretical
stages. The maximum reflux ratio, called total reflux, with zero
distillate flow rate can be used in design calculations to determine the minimum number of theoretical stages required to
achieve a desired separation.
The desired material balance split, D/F, that is, the ratio
of the distillate flow rate divided by the feed rate, is generally determined by the weight fraction of light components in
the feed stream. For example, a distillation column for separating ethanol from water that produces 85 kg/h of distillate
from 1,000 kg/h of feed would be running with a D/F ratio of
85/1000 = 0.085.
2.4 Summary
This chapter presented a concept drawing of a distillation
process that included flow rate sensors, liquid level sensors,
temperature sensors, and process control valves. The process
streams were identified and labeled. The standard definition
of reflux ratio was presented, and the concepts of separation
power and material balance split were introduced.