Page 130 - Separation process engineering

reboiler and condenser pressures are not independent but are related by the pressure drop in the
column (~0.1 psi per stage for column with p > 1 bar).

Heuristics are rules of thumb that often disagree with each other. If they disagree, the most important
heuristic should be followed. For distillation the most important pressure heuristic is often avoiding the
use of refrigeration in the condenser because refrigeration is expensive.
In design problems, the desired separation is set, and a column is designed that will achieve this
separation. For binary distillation we would usually specify the mole fraction of the more volatile

component in the distillate and bottoms products. In addition, the external reflux ratio, L /D in Figure 4-6,
0
is usually specified. Finally, we usually specify that the optimum feed location be used; that is, the feed
location that will result in the fewest total number of stages. The designer’s job is to calculate distillate
and bottoms flow rates, the heating and cooling requirements in the reboiler and condenser, the number of
stages required and the optimum feed stage location, and finally the required column diameter. Alternative
specifications such as the splits (fraction of a component recovered in the distillate or bottoms) or
distillate or bottoms flow rates are common. Four sets of possibilities are summarized in Table 3-2.

Table 3-2. Specifications and calculated variables for binary distillation for design problems

In simulation problems, the column has already been built and we wish to predict how much separation