Page 132 - Separation process engineering

mass and energy balances around the entire column. For binary design problems, these balances can
usually be solved without doing stage-by-stage calculations. Figure 3-8 shows the schematic of a
distillation column. The specified variables for a typical design problem are circled. We will assume that
the column is well insulated and can be considered adiabatic. All of the heat transfer takes place in the
condenser and reboiler. Column pressure is assumed to be constant.

Figure 3-8. Binary distillation column: Circled variables are typically specified in design problems

From the balances around the entire column we wish to calculate distillate and bottoms flow rates, D and
B, and the heat loads in the condenser and reboiler, Q and Q . We can start with mass balances around
R
c
the entire column using the balance envelope shown by the dashed outline in the figure. The overall mass
balance is

(3-1)

and the more volatile component mass balance is

(3-2)

For the design problem shown in Figure 3-8, Eqs. (3-1) and (3-2) can be solved immediately, since the
only unknowns are B and D. Solving Eq. (3-1) for B, substituting this into Eq. (3-2), and solving for D,
we obtain

(3-3)

and