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342 Applied Process Design for Chemical and Petrochemical Plants

Figure 11-36. Condenser duty for single-stage propylene refrigeration system. (Excerpted by special permission: Mehra, Y. R. Chemical Engi-
neering, Jan. 15, 1979. ©McGraw-Hill, Inc., New York. All rights reserved.)

Temperature, Propane, Butane,
(Text continued from page 337) °F Mol Frac. Mol Frac.
Top 104.7 0.938 0.062
Again using trial and error, find the correct
Bottom 98.2 0.977 0.023
temperature for the bottom of the condenser, knowing
Average 101.5 0.957 0.043
that Y c2 1.000.
Trial 1 Trial 2 The average temperature being 101.5°F (close enough)
Constant (T 100°F) (T 98°F) means that the assumed pressure of 180 psia holds.
Gas X c2 K Y c2 K Y c2 Step 4. To check the original assumed equilibrium com-
position in the evaporator, the total amount of propane
Propane 0.938 1.06 0.995 1.04 0.975
and butane in the system must be determined. This
Butane 0.062 0.38 0.0235 0.37 .023
total must be equal to the original charge. Therefore,
1.0185 0.998
the next step will be to calculate the volume of the total
The composition of the vapor at the bottom of the con- system. These calculations must necessarily be some-
denser is, by interpolation, 97.7% propane, 2.3% what approximate because the exact installation condi-
butane, and the temperature 98.2°F. tions are not known.