Unformatted text preview: S=KV/L. The entering gas rate, V, is 946 lbmol/h. Column operating conditions are about 94
psia and a maximum temperature of 126oF.
Pertinent properties and factors for the three potential absorbents are:
Absorbent
C5 s
light oil
medium oil gpm
115
36
215 ρ, lb/gal
5.24
6.0
6.2 lb/h
36,200
13,000
80,000 MW
72
130
180 lbmol/h
500
100
440 Kvalue
0.9
0.005
0.0005 S=KV/L
2.3
0.013
0.0013 The light oil can not be used because its flow rate of 100 lbmol/h is much lower than the
necessary 368 lbmol/h. The C5s can not be used because their stripping factor is very high. The
only possible alternative is the medium oil. Exercise 6.5
Subject: Stripping of VOCs from water effluents by air and water Given: Packed tower for stripping
Find: Advantages and disadvantages of air over steam Analysis: Advantages: Air is available anywhere.
Air is inexpensive.
Disadvantages: Air can form a flammable or explosive mixture with the VOC.
With steam, the exit gas can be condensed and the VOC
recovered as a liquid. Exercise 6.6
Subject: Preferred operating conditions for absorbers and strippers. Find: Best conditions of temperature and pressure, and the tradeoff between number of stages
and flow rate of separating agent, using equations.
Analysis: Absorbers:
For high performance, want a large absorption factor, A = L/KV
Therefore, want a small Kvalue.
Assume that:
γ iL Pi s
Ki =
P
s
Pi = vapor pressure, which increases with increasing temperature
Therefore, operate at a high pressure and a low temperature.
Strippers:
For high performance, want a large stripping factor, S = KV/L
Therefore, want a large Kvalue.
Therefore, operate at low pressure and a high temperature.
For the tradeoff between number of equilibrium stages, N, and flow rate of mass separating
agent, L or V. Consider absorption. The fraction of a component absorbed is given from a
modification of Eq. (548),
A −1
A N +1 − A
1 − φ A = 1 − N +1
= N +1
A −1 A −1
Thus, a large fraction absorbed can be achieved with either N or A = L/KV. The tradeoff is most
clearly shown in Fig. 5.9. For low fractions absorbed (i.e. high φA), N has little effect and (1 φA) is approximately equal to A. But for high fractions absorbed, the larger the value of N, the
smaller the required value of A, and, thus, the smaller the required value of the flow rate of the
liquid separating agent, L.
The tradeoff for stripping is similar. The fraction of a component stripped is given by a
modification of Eq. (551),
S −1
S N +1 − S
1 − φ S = 1 − N +1
= N +1
S −1 S
−1
Thus, a large fraction stripped can be achieved with either N or S = KV/L. The tradeoff is most
clearly shown in Fig. 5.9. For low fractions stripped (i.e. high φS), N has little effect and (1  φS)
is approximately equal to S. But for high fractions absorbed, the larger the value of N, the
smaller the required value of S, and, thus, the smaller the required value of the flow rate of the
vapor separating agent, V. xercise 6.7
Subject: Absorption of CO2 f...
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 Spring '11
 Levicky
 The Land

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