Capitulo 7 Seader

Exercise 7.1
Subject: Comparison of absorption, distillation, and stripping

Find: Differences between absorption and distillation, and stripping and distillation Analysis: Absorption Feed is gas Second phase (absorbent) is added Operation can be adiabatic Single-section cascade Can be almost isothermal Can not separate a closeboiling mixture Tray efficiency can be low Stripping Feed is liquidDistillation Feed is liquid, vapor or a mixture of the two

Second phase (stripping agent) Second phase is created by is added heat transfer Operation can be adiabatic Single-section cascade Can be almost isothermal Can not separate a closeboiling mixture Tray efficiency can be moderate Must have heat transfer at the top and bottom stages Two-section cascade Can have a large temperature rangeCan separate a close-boiling mixture Tray efficiency can be high

Exercise 7.2
Subject: Emergence of packing to replace trays.

Find: Reasons why some existing trayed towers are being retofitted with packing, and some large-diameter columns are being designed for packing. Analysis: New random packings and structured packings have been introduced with higher capacity, lower pressure drop, andhigher efficiency than trays. Also for packed columns, liquid distributors have been greatly improved. Channeling of liquid in packed columns is now much less of a problem.

Exercise 7.3
Subject: Condenser coolant for distillation of a methane-ethane mixture.

Find: Appropriate coolant. Analysis: Assume that the distillate is nearly pure methane, the most volatile of the two components. Touse cooling water, the distillate temperature would be approximately 120oF. But the critical temperature of methane is -115oF. Therefore could not condense it. The critical pressure of methane is 673 psia. Using Fig. 7.16, could consider operation at 415 psia because that is safely below the critical pressure (P/Pc = 0.62 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 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 anethylene-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 fromPerry'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: 1. A new mixture not previously separated by distillation. Asharp and critical separation. 2. 3. A mixture with uncertain vapor-liquid equilibria data. 4. 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....