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3. Perform a material balance for the connected columns to estimate the approximate
flows and compositions of the intercolumn streams.
4. Design each column rigorously with loose specifications.
5. Tighten the specifications gradually.
6. Connect the columns, loosening the specifications.
7. If convergence is difficult, provide starting estimates for the intermediate streams.
8. If necessary, change the default convergence method.
9. When convergence is achieved, tighten the specifications gradually.
Examples of some of the steps above are given at Examples/etohtolueneazeos, to
calculate azeotrope composition as a function of pressure, ExampleTwoMB the material
balance - simple blocks calculation, and ExampleTwo the rigorous solution.
14.3 EXTRACTIVE DISTILLATION
Benzene -cyclohexane is an example of a close-boiling system. Kojima et al.'s (1968)
vapor - liquid equilibrium data for this system are shown in Figure 14.4. Note that the
vapor - liquid equilibrium curve is almost the same as the
y
=
x
line, which makes it
virtually impossible to separate using standard distillation methods. Systems such as
these can be separated by introducing a third component, the solvent, with a lower
volatility than either component, into a distillation column. The solvent must have the
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
x - mole fraction benzene
Figure 14.4
Vapor - liquid equilibrium benzene - cyclohexane at 760 mmHg.
