Page 131 - Separation process engineering

can be achieved for a given feed. Since the column has already been built, the number of stages and the
feed stage location are already specified. In addition, the column diameter and the reboiler size, which
usually control a maximum vapor flow rate, are set. There are a variety of ways to specify the remainder
of the problem (see Table 3-3). The desired composition of more volatile component in the distillate and
bottoms could be specified, and the engineer would then have to determine the external reflux ratio, L /D,
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that will produce this separation and check that the maximum vapor flow rate will not be exceeded. An
alternative is to specify L /D and either distillate or bottoms composition, in which case the engineer
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determines the unknown composition and checks the vapor flow rate. Another alternative is to specify the
heat load in the reboiler and the distillate or bottoms composition. The engineer would then determine the
reflux ratio and unknown product composition and check the vapor flow rate. The thread that runs through
all these alternatives is that since the column has been built, some method of specifying the separation

must be used.
Table 3-3. Specifications and calculated variables for binary distillation for simulation problems

The engineer always specifies variables that can be controlled. Several sets of possible specifications

and calculated variables are outlined in Tables 3-1 to 3-3. Study these tables to determine the difference
between design-type and simulation-type problems. Note that other combinations of specifications are
possible.
In Table 3-1 we find five specified variables common to both types of problems. For design problems
(Table 3-2), four additional variables must be set. Note that whereas column diameter is a specified
variable in simulation problem C, it serves as a constraint in simulation problems A, B, and D (Table 3-
3). Column diameter will allow us to calculate V max and then we can check that V < V max . However, we

have not specified a variable for simulation. In problem C, where we specify V = fraction × V max , the
column diameter serves as a variable for simulation.

Chapter 4 starts with the simple design problem and progresses to simulation and other more complicated
problems.

3.4 External Column Balances

Once the problem has been specified, the engineer must calculate the unknown variables. Often it is not
necessary to solve the entire problem, since only limited answers are required. The first step is to do