ch07 - Exercise 7.1 Subject Comparison of absorption...

Exercise 7.1
Subject: Comparison of absorption, distillation, and stripping
Find: Differences between absorption and distillation, and stripping and distillation
Analysis:
Absorption Stripping Distillation
Feed is gas Feed is liquid Feed is liquid, vapor or a
mixture of the two
Second phase (absorbent) is
added
Second phase (stripping agent)
is added
Second phase is created by
heat transfer
Operation can be adiabatic Operation can be adiabatic Must have heat transfer at the
top and bottom stages
Single-section cascade Single-section cascade Two-section cascade
Can be almost isothermal Can be almost isothermal Can have a large temperature
range
Can not separate a close-
boiling mixture
Can not separate a close-
boiling mixture
Can separate a close-boiling
mixture
Tray efficiency can be low Tray efficiency can be
moderate
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, and higher 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. To use cooling water, the distillate temperature would be approximately 120
o
F.
But the critical temperature of methane is -115
o
F. 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/P
c
= 0.62 and could operate as high as a P/P
c
= 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 -143
o
F. 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 -150
o
F.
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 120
o
F.
But the critical temperature of ethylene is 9.7
o
C or 49.4
o
F . 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/P
c
= 0.56 and could operate as high
as a P/P
c
= 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 4.7
o
F. 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 0
o
F.

Exercise 7.5