Page 171 - Separation process engineering

The open steam example illustrated one specific case. It is useful to generalize this analysis procedure. A
section of the column is the segment of stages between two input or exit streams. Thus in Figure 4-15
there are two sections: top and bottom. Figure 4-17 illustrates a column with four sections. Each section’s
operating equation can be derived independently. Thus, the secret (if that’s what it is) is to treat each
section as an independent subproblem connected to the other subproblems by the feed lines (which are
also independent).

Figure 4-17. Distillation column with four sections

An algorithm for any problem is the following:

1. Draw a figure of the column and label all known variables (e.g., as in Figure 4-15). Check to see if
CMO is valid.
2. For each section:

a. Draw a mass balance envelope. We desire this envelope to cut the unknown liquid and vapor
streams in the section and known streams (feeds, specified products or specified side-streams). The
fewer streams involved, the simpler the mass balances will be. This step is important, since it
controls how easy the following steps will be.
b. Write the overall and most volatile component mass balances.

c. Derive the operating equation.
d. Simplify.

e. Calculate all known slopes, intercepts, and intersections.
3. Develop feed line equations. Calculate q values, slopes, and y = x intersections.

4. For operating and feed lines:
a. Plot as many of the operating lines and feed lines as you can.

b. If all operating lines cannot be plotted, step off stages if the stage location of any feed or side stream
is specified.

c. If needed, do external mass and energy balances (see Example 4-5). Use the values of D and B in
step 2.