Page 145 - Separation process engineering

Figure 4-2. Stripping section balance envelopes; (A) below feed stage (stage f + 1), (B) stage k, (C)
partial reboiler

The equilibrium relationships are Eqs. (4-4) written for stage f + 1.

(4-4, stage f + 1)

These six equations have six unknowns: L , , x , y , H , and h . x is specified in the problem
f
B
f+1
f
f
f+1
statement; B and Q were calculated from the column balances; and y (required for the last equation) was
R
f
obtained from the solution of Eqs. (4-1, stage j) to (4-4c, stage j) with j = f − 1. At the feed stage we
change from one set of balance envelopes to another.
Note that the same equations will be obtained if we write the balances above stage f + 1 and around the
top of the distillation column (use a different balance envelope). This is easily illustrated with the overall
mass balance, which is now

Rearranging, we have

However, since the external column mass balance says F − D = B, the last equation becomes

which is Eq. (4-5, stage f + 1). Similar results are obtained for the other balance equations. Thus the
balance envelope we use is arbitrary.

Once the six Eqs. (4-4a) to (4-7) for stage f + 1 have been solved, we can proceed down the column to
the next stage, f + 2. For a balance envelope around general stage k as shown in Figure 2-2B, the
equations are

(4-5, stage k)