Chapter6
Absorption and Stripping of
Dilute Mixtures
Purpose and Requirements:


Know Equipment of Absorption and Stripping
Learn to Design a trayed Tower and a packed Tower
Key and Difficult Points:
Key Points
 Equipment of Absorption and Stripping
 Graphical Equilibrium-Stage Method for Trayed Towers
 Algebraic Method for Trayed Towers
 Rate-Based Method for Packed Towers
Difficult Points
 Algebraic Method for Trayed Towers
 Rate-Based Method for Packed Towers
Outline









6.1 EQUIPMENT
6.2 GENERAL DESIGN CONSIDERATIONS
6.3 GRAPHICAL EQUILIBRIUM-STAGE METHOD FOR TRAYED
TOWERS
6.4 ALGEBRAIC METHOD FOR DETERMINING THE NUMBER OF
EQUILIBRIUM STAGES
6.5 STAGE EFFICIENCY
6.6 TRAY CAPACITY, PRESSURE DROP, AND MASS TRANSFER
6.7 RATE-BASED METHOD FOR PACKED COLUMNS
6.8 PACKED COLUMN EFFICIENCY, CAPACITY, AND PRESSURE
DROP
6.9 CONCENTRATED SOLUTIONS IN PACKED COLUMNS
Absorption
(Gas Absorption/Gas Scrubbing/Gas Washing吸收)


Gas Mixture (Solutes or Absorbate)
Liquid (Solvent or Absorbent)

Separate Gas Mixtures
Remove Impurities, Contaminants, Pollutants, or
Catalyst Poisons from a Gas(H2S/Natural Gas)

Recover Valuable Chemicals


Physical Absorption

Chemical Absorption
(Reactive Absorption)
Figure 6.1 Typical Absorption Process
Absorption Factor
(A吸收因子)

A = L/KV
Component
Water
Acetone
Oxygen
Nitrogen
Argon
A = L/KV
1.7
1.38
0.00006
0.00003
0.00008
K-value
0.031
2.0
45,000
90,000
35,000
•Larger the value of A，Fewer the number of stages required
•1.25 to 2.0 ，1.4 being a frequently recommended value
Stripping
(Desorption解吸)

Stripping
Distillation

Stripping Factor

(S解吸因子)

S = 1/ A= KV/L
High temperature
Low pressure is desirable
Optimum stripping factor ：1.4.
6.1 EQUIPMENT
trayed tower
packed column
bubble column
spray tower
centrifugal contactor
Figure 6.2 Industrial Equipment for Absorption and Stripping
Trayed Tower
(Plate Clolumns板式塔)
Figure 6.3 Details of a contacting tray in a trayed tower
(a) perforation
(b) valve cap (c) bubble cap
(d) Tray with valve caps
Figure 6.4 Three types of tray openings for
passage of vapor up into liquid
Froth
Liquid carries no vapor bubbles
to the tray below
Vapor carries no liquid droplets
to the tray above
No weeping of liquid through the
openings of the tray
(a) Spray(b) Froth(c) Emulsion(d) Bubble(e)Cellular Foam
Equilibrium between the exiting
vapor and liquid phases
is approached on each tray.
Figure 6.5 Possible vapor-liquid flow regimes for a contacting tray
Packed Columns
Figure 6.6 Details of internals
used in a packed column
Packing Materails
•More surface area for mass transfer
•Higher flow capacity
•Lower pressure drop
(a)
(b) Random
Structured
Packing
Packing
Materials
Materials
•Expensive
•Far less pressure drop
•Higher efficiency and capacity
Figure 6.7 Typical materials used in a packed column
6.2 ABSORBER/STRIPPER DESIGN








6.2.1 General Design Considerations
6.2.2 Trayed Towers
6.2.2.1 Graphical Equilibrium-Stage
6.2.2.2 Algebraic Method for Determining
the Number of Equilibrium
6.2.2.3 Stage Efficiency
6.2.3 Packed Columns
6.2.3.1 Rate-based Method
6.2.3.2 Packed Column Efficiency, Capacity,
and Pressure Drop
6.2.1 General Design Considerations
Design or analysis of an absorber (or stripper) requires
consideration of a number of factors, including:
1. Entering gas (liquid) flow rate, composition,
6. Number
of equilibrium
stages
temperature,
and pressure
7. Heat effects and need for cooling (heating)
2. Desired degree of recovery of one or more solutes
8. Type of absorber (stripper) equipment
3.
of absorber
absorbent
(stripping agent)
9. Choice
Height of
(stripper)
4.
and(stripper)
temperature, and allowable
10.Operating
Diameter pressure
of absorber
gas pressure drop
5. Minimum absorbent (stripping agent) flow rate and
actual absorbent (stripping agent) flow rate as a
multiple of the minimum rate needed to make the
separation
SUMMARY





1. A liquid can be used to selectively absorb one or more components from a
gas mixture. A gas can be used to selectively desorb or strip one or more
components from a liquid mixture.
2. The fraction of a component that can be absorbed or stripped in a
countercurrent cascade depends on the number of equilibrium stages and the
absorption facto: A = L/KV, or the stripping factor, S = KV/L, respectively.
3. Absorption and stripping are most commonly conducted in trayed towers
equipped with sieve or valve trays, or in towers packed with random or
structured packings.
4. Absorbers are most effectively operated at high pressure and low
temperature. The reverse is true for stripping. However, high costs of gas
compression, refrigeration and vacuum often preclude operation at the most
thermodynamically favorable conditions.
5. For a given gas flow rate and composition, a desired degree of absorption of
one or more components, a choice of absorbent, and an operating temperature
and pressure, there is a minimum absorbent flow rate, given by (6-9) to (6-11),
that corresponds to the use of an infinite number of equilibrium stages. For the
use of a finite and reasonable number of stages, an absorbent rate of 1.5 times
the minimum ' is typical. A similar criterion, (6-12), holds for a stripper.



6. The number of equilibrium stages required for a selected absorbent or
stripping agent flow rate for the absorption or stripping of a dilute solution can
be determined from the equilibrium line, (6-1), and an operating line, (6-3) or (65), using graphical algebraic, or numerical methods. Graphical methods, such
as Figure 6.11, offer considerable visual insight into stage-by-stage changes in
compositions of the gas and liquid streams.
7. Rough estimates of overall stage efficiency, defined by (6-21), can be made
with the correlations of Drickamer and Bradford, (6-22), O'Connell, (6-23), and
Figure 6.15 More accurate and reliable procedures involve the use of a small
Oldershaw column , or semitheoretical equations, e.g., of Chan and Fair, based
on mass transfer considerations, to determine a Murphree vapor-point
efficiency, (6-30), from which a Murphrtf vapor tray efficiency can be estimated
from (6-31) to (6-34), which can then be related to the overall efficiency using
(6-37).
8. Tray diameter can be determined from (6-44) based on entrainment flooding
considerations using Figure 6.24. Tray vapor pressure drop, the weeping
constraint, entrainment, and downcomer backup can be estimated from (6-45),
(6-64), (6-65), (6-66), respectively.


9. Packed column height can be estimated using the HETP, (669), or HTU/NTU,(6-85), concepts, with the latter having a more
fundamental theoretical basis in the two-film theory of mass
transfer. For straight equilibrium and operating lines, HETP is
related to the HTU by (6-90), and the number of equilibrium
stages is related to the NTU by (6-91).
10. Below a so-called loading point, in a preloading region, the
liquid holdup in a packed column is independent of the vapor
velocity. The loading point is typically about 70% of the flooding
point and most packed columns are designed to operate in the
preloading region at from 50% to 70% of flooding. From the
GPDC chart of Figure 6.36, the flooding point can be estimated,
from which the column diameter can be determined with (6-98



11. One significant advantage of a packed column is its
relatively low pressure drop per unit of packed height, as
compared to a trayed tower. Packed column pressure drop can
be roughly estimated from Figure 6.36 or more accurately from
(6-100).
12. Numerous rules of thumb are available for estimating the
HETP of packed columns. However, the preferred approach is
to estimate hog from separate semitheoretical mass transfer
correlations for the liquid and gas phases, such as those of (6123) and (6-124) based on the extensive experimental work of
Billet and Schultes.
13. Determination of theoretical stages for concentrated
solutions involves numerical integration because of curved
equilibrium and/or operating lines.
REFERENCES










1. Washburn, E.W., Ed.-in-Chief, International Critical Tables, McGrawHill, New York, Vol. Ill, p. 255 (1928).
2. Lockett, M., Distillation Tray Fundamentals, Cambridge University
Press, Cambridge, UK, p. 13 (1986).
3. Okoniewski, B.A., Chem. Eng. Prog., 88 (2), 89-93 (1992).
4 Sax, N.I., Dangerous Properties of Industrial Materials, 4th ed., Van
Nostrand Reinhold, New York, pp. 440-441 (1975).
5. Lewis, W.K., Ind. Eng. Chem., 14, 492-497 (1922).
6. Drickamer, H.G., and J.R. Bradford, Trans. AICHE, 39, 319-360
(1943).
7. Jackson, R.M., and T.K. Sherwood, Trans. AIChE, 37, 959(1941).
8. S- O'Connell, H.E., Trans. AIChE, 42, 741-755 (1946).
9.Walter, J.F., and T.K. Sherwood, Ind. Eng. Chem., 33, 493-XH (1941).
10. Edmister, W.C., The Petroleum Engineer, C45-C54 (Jan. 1949).










11. Lockhart, F.J., and C.W. Leggett, in K.A. Kobe and J.J. McKetta,
Jr., Ed., Advances in Petroleum Chemistry and Refining,vol. 1,
Interscience, New York, Vol. 1, pp. 323-326 (1958).
12. Holland, C.D., Multicomponent Distillation, Prentice-Hall,
Englewood Cliffs. NJ, 1963.
13. Murphree, E.V., Ind. Eng. Chem., 17, 747 (1925).
14. Hausen, H., Chem. Ing. Tech., 25, 595 (1953).
15．Standart, G., Chem Eng. ScL, 20, 611 (1965).
16.Lewis, W.K., Ind. Eng. Chem., 28, 399 (1936).
17. Gerster, J.A., A.B. Hill, N.H. Hochgraf, and D.G. Robinson, Tray
Efficiencies in Distillation Columns," Final Report from the University of
Delaware, American Institute of Chemical Engineers, V* York (1958).
18. Bubble-Tray Design Manual, AIChE, New York (1958).
19.Gilbert. T.J., Chem. Eng. ScL, 10, 243 (1959).
20.Barker, P.E., and M.F. Self, Chem. Eng. ScL, 17, 541 (1962).










21. Bennett, D.L., and H.J. Grimm, AIChE J., 37, 589 (1991).
22. O'dershaw, C.F., Ind. Eng. Chem. Anal. Ed., 13, 265 (1941).
23.Fair. J.R., H.R. Null, and W.L. Holies, Ind. Eng. Chem. Process Des.Dev., 22,
53-58 (1983).
24.Souders, M., and G.G. Brown, Ind. Eng. Chem., 26, 98-103 (1934).
25.Fair,J.R., Peiro/Chem. Eng., 33, 211-218 (Sept. 1961).
26. Sherwood, T.K., G.H. Shipley, and F.A.L. Holloway, Ind. Eng. Chem., 30,
765-769 (1938).
27. Glitsch Ballast Tray, Bulletin No. 159, Fritz W. Glitsch and Sons, Dallas, TX
(from FRI report of Sept. 3, 1958).
28. Glitsch V-l Ballast Tray, Bulletin No. 160, Fritz W. Glitsch and Sons, Dallas,
TX (from FRI report of Sept. 25, 1959).
29. Oliver, E.D., Diffusional Separation Processes. Theory, Design, and
Evaluation, John Wiley and Sons, New York, pp. 320-321 (1966).
30. Bennett, D.L., R. Agrawal, and P.J. Cook, AIChE J., 29, 434-442 (1983).










31. Smith, B.D., Design of Equilibrium Stage Processes, McGraw-Hill, New
York (1963).
32. Klein, G.F., Chem. Eng., 89 (9), 81-85 (1982).
33. Kister, H.Z., Distillation Design, McGraw-Hill, New York (1992).
34. Lockett, M.J., Distillation Tray Fundamentals, Cambridge University Press,
Cambridge, UK, p. 146 (1986).
35. American Institute of Chemical Engineers (AIChE), Bubble-Tray Design
Manual, AIChE, New York, (1958).
36. Chan, H., and J.R. Fair, Ind. Eng. Chem. Process Des. Dev., 23,814-819
(1984).
37. Chan, H., and J.R. Fair, Ind. Eng. Chem. Process Des. Dev., 23,820-827
(1984).
38. Scheffe, R.D., and R.H. Weiland, Ind. Eng. Chem. Res., 26,228-236 (1987).
39. Foss, A.S., and J.A. Gerster, Chem. Eng. Prog., 52, 28-J to34-J (Jan. 1956).
40. Gerster, J.A., A.B. Hill, N.N. Hochgraf, and D.G. Robinson, "Tray
Efficiencies in Distillation Columns," Final Report from University of Delaware,
American Institute of Chemical Engineers (AIChE), New York (1958).










41. Fair, J.R., Petro./Chem. Eng., 33 (10), 45 (1961).
42. Colburn, A.P., Ind. Eng. Chem., 28, 526 (1936).
43. Chilton, T.H., and A.P. Colburn, Ind. Eng. Chem., 27, 255-260, 904 (1935).
44. Colburn, A.P., Trans. AIChE, 35, 211-236, 587-591 (1939).
45. Billet, R., Packed Column Analysis and Design, Ruhr-University Bochum
(1989).
46. Stichlmair, J., J.L. Bravo, and J.R. Fair, Gas Separation and Purification, 3,
19-28 (1989).
47. Billet, R., and M. Schultes, Packed Towers in Processing and
Environmental Technology, translated by J. W. Fullarton, VCH Publishers, New
York (1995).
48. Leva, M., Chem. Eng. Prog. Symp. Ser., 50 (10). 51 (1954).
49. Leva, M., Chem. Eng. Prog., 88, (1) 65-72 (1992).
50. Kister, H.Z., and D.R. Gill, Chem. Eng. Prog., 87 (2), 32-42(1991).










51. Billet, R., and M. Schultes, Chem. Eng. Technoi, 14, 89-95(1991).
52. Ergun, S., Chem. Eng. Prog., 48 (2), 89-94 (1952).
53. Kunesh, J.G., Can. J. Chem. Eng., 65, 907-913 (1987).
54. Whitman, W.G., Chem. and Met. Eng., 29, 146-148 (1923).
55. Sherwood, T.K., and F.A.L. Holloway, Tram. AIChE., 36,39-70
(1940).
56. Cornell, D., W.G. Knapp, and J.R. Fair, Chem. Eng. Prog., 56(7)
68-74 (1960).
57. Cornell, D., W.G. Knapp, and J.R. Fair, Chem. Eng., Prog., 56(8),
48-53 (1960).
58. Bolles, W.L., and J.R. Fair, Inst. Chem. Eng. Symp. $er 3/35
(1979).
59. Bolles, W.L., and J.R. Fair, Chem. £>ig.,89(14), 109-116 (1982)
60. Bravo, J.L., and J.R. Fair. Ind. Eng. Chem. Process
Des.Devel.21,162-170(1982).








61. Bravo, J.L., J.A. Rocha, and J.R. Fair, Hydrocarbon Processing 64
(1), 56-60 (1985).
62. Fair, J.R., and J.L. Bravo, Chem. E. Symp. Ser., 104, A183-A201
(1987).
63. Fair, J.R., and J.L. Bravo, Chem. Eng. Prog., 86 (1). 19-29 (1990).
64. Shulman, H.L., C.F. UHrich, A.Z. Proulx, and J.O. Zimmerm, AIChE
J., 1, 253-258 (1955).
65. Onda, K., H. Takeuchi, and Y.J. Okumoto, J. Chem. Eng .jpn., 1,
56-62 (1968).
66. Billet, R., Chem. Eng. Prog., 63 (9), 53-65 (1967).
67. Billet, R., and M. Schultes, Beitrage zur Verfahrens-Und
Umweltechnik, Ruhr-Universitat Bochum. pp. 88-106 (1991).
68. Higbie, R., Trans. AIChE, 31, 365-389 (1935).