Capitulo 6

Exercise 6.1
Subject:
Given:

Simultaneous absorption and stripping.
Fig. 6.1 in which simultaneous absorption and stripping occur.

Find: Whether absorption and stripping is more important.
Analysis:

Stripping of water = 22 kmol/h
Absorption of acetone = 10.35 kmol/h
Therefore, more stripping than absorption occurs.
However, the operation is primarily absorption because a verylarge percentage, 99.5%, of the
acetone is absorbed. Only a very small fraction of the water is stripped.

Exercise 6.2
Subject:
Given:

Column packings since 1950.
History of the development of column packings.

Find: Advantages of new packings. Advances in packing design and fabrication. Need for
structured packings.
Analysis: The newer packings provide more surface area for masstransfer, a higher flow
capacity, and a lower pressure drop. They provide more effective through flow. Many are
plastics made from molds or thin metal strips that can be inexpensively fabricated into intricate
shapes. Structured packings largely eliminate the problems of channeling and at the same time
give improvement in efficiency, capacity, and pressure drop.

Exercise 6.3
Subject:Advantages of bubble-cap trays

Given: Characteristics and performance of bubble-cap trays
Find: Characteristics that give bubble-cap trays a very high turndown ratio.
Analysis: Unlike sieve and valve trays, bubble-cap trays do not allow liquid to weep.
Therefore, bubble-cap trays can be operated at very low liquid flow rates. Also, bubble caps
force the vapor to flow out sideways, rather thanvertically up, thus allowing a relatively high
vapor rate.

Exercise 6.4
Subject:
Given:

Selection of alternative absorbent for Example 6.3
Flow rate, density, and MW of three potential absorbents listed below.

Find: Select the best absorbent with reasons why. Are any of the absorbents unacceptable?
Analysis: In Example 6.3, the rich gas contains C1 to nC6 hydrocarbons, with mostly C3.Object
of absorber is to absorb most of the nC4. The absorbent is an oil of 250 MW and 21oAPI, at a
flow rate, L, of 368 lbmol/h. An alternative absorbent must also be of higher molecular weight
than nC4 and must have a flow rate of at least 368 lbmol/h. If its molecular weight is too low, a
significant amount of it will be stripped. This can be judged by its K-value and stripping factor,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

K-value
0.9
0.005
0.0005S=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 oftemperature and pressure, and the trade-off 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 K-value.
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...