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Unformatted text preview: Perry's Handbook (p. 3-203, 6th edition), at P = 415 psia (28.6 bar), the saturation
temperature of methane is 176 K or -143oF. To condense at just below this temperature, could
use a partial condenser with refrigerant R-14 (carbon tetrafluoride), which from Perry's
Handbook (p. 12-26, 6th edition) has a saturation pressure of 70 psia at -150oF. Exercise 7.4
Subject: Operating pressure for distillation of an ethylene-ethane mixture Find: Suitable operating pressure.
Analysis: Assume that the distillate is nearly pure ethylene, the most volatile of the two
components. To use cooling water, the distillate temperature would be approximately 120oF.
But the critical temperature of ethylene is 9.7oC or 49.4oF . Therefore could not condense it. The
critical pressure of ethylene is 50.5 atm or 742 psia. Using Fig. 7.16, could consider operation at
415 psia because that is safely below the critical pressure (P/Pc = 0.56 and could operate as high
as a P/Pc = 0.8). From Perry's Handbook (p. 3-203, 6th edition), at P = 415 psia (28.6 bar), the
saturation temperature of ethylene is 258 K or 4.7oF. To condense at just below this temperature
could use a partial condenser with refrigerant R-1270 (propylene), which from Perry's Handbook
(p. 12-26, 6th edition) has a saturation pressure of 35 psia at 0oF. Exercise 7.5
Subject: Need for testing or piloting a distillation separation. Find: Circumstances that require laboratory or pilot-plant testing of a proposed distillation.
Analysis: Laboratory and/or pilot-plant testing is recommended for:
A new mixture not previously separated by distillation.
A sharp and critical separation.
A mixture with uncertain vapor-liquid equilibria data.
A lack of experience with tray efficiency for the mixture. Exercise 7.6
Subject: Economic tradeoff in distillation. Find: Reasons for tradeoff between trays and reflux.
Analysis: It is well known that for a given separation, as the number of trays is increased, the
reflux ratio can be decreased. Thus, as the tower height is increased, the vapor and liquid traffic
up and down the column can be decreased. Therefore, the column diameter can be decreased.
Also the condenser and reboiler duties and sizes, and the utility requirements can be decreased.
Therefore, there is a tradeoff. Exercise 7.7
Subject: McCabe-Thiele method for binary distillation. Find: Reasons for the survival of the McCabe-Thiele graphical method.
Analysis: For a binary mixture, the McCabe-Thiele method shows clearly the ease or difficulty
of the separation. Pinched regions are readily seen. The effect of feed location is readily seen.
The method is reasonably accurate. Azeotropes are readily accommodated. Exercise 7.8
Subject: Separation of ethyl alcohol and water at 1 atm. with two countercurrent cascades. Given: One cascade (a) with given liquid feed to top stage and given vapor feed to bottom
stage. Another cascade (b) with total condenser and reflux, and given vapor feed to bottom
stage. Vapor-liquid equilibrium data for 1 atm.
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- Spring '11
- The Land