absorption

1. Absorption andAbsorption and
Stripping ofStripping of
Dilute MixturesDilute Mixtures
Chapter6Chapter6

2. 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
Purpose and Requirements:
 Know Equipment of Absorption and Stripping
 Learn to Design a trayed Tower and a packed Tower

3. 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

4. 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

5. Figure 6.1 Typical Absorption Process
 Chemical Absorption
(Reactive Absorption)
 Physical Absorption

6. Absorption Factor
(A 吸收因子 )
 A = L/KV
Component A = L/KV K-value
Water 1.7 0.031
Acetone 1.38 2.0
Oxygen 0.00006 45,000
Nitrogen 0.00003 90,000
Argon 0.00008 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

7. 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.

8. 6.1 EQUIPMENT
Figure 6.2 Industrial Equipment for Absorption and Stripping
trayed tower packed column
spray towerbubble column
centrifugal contactor

9. Figure 6.3 Details of a contacting tray in a trayed tower
Trayed Tower
(Plate Clolumns 板式塔 )

10. Figure 6.4 Three types of tray openings for
passage of vapor up into liquid
(d) Tray with valve caps
(b) valve cap (c) bubble cap(a) perforation

11. Figure 6.5 Possible vapor-liquid flow regimes for a contacting tray
(a) Spray(b) Froth(c) Emulsion(d) Bubble(e)Cellular Foam
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
Equilibrium between the exiting
vapor and liquid phases
is approached on each tray.

12. Packed Columns
Figure 6.6 Details of internals
used in a packed column

13. Figure 6.7 Typical materials used in a packed column
Packing Materails
(a) Random Packing
Materials
(b) Structured Packing
Materials
•More surface area for mass transfer
•Higher flow capacity
•Lower pressure drop
•Expensive
•Far less pressure drop
•Higher efficiency and capacity

14. 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

15. 6.2.1 General Design Considerations
1. Entering gas (liquid) flow rate, composition,
temperature, and pressure
2. Desired degree of recovery of one or more solutes
3. Choice of absorbent (stripping agent)
4. Operating pressure and temperature, and allowable
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
Design or analysis of an absorber (or stripper) requires
consideration of a number of factors, including:
6. Number of equilibrium stages
7. Heat effects and need for cooling (heating)
8. Type of absorber (stripper) equipment
9. Height of absorber (stripper)
10. Diameter of absorber (stripper)

16. 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 (6­9) to (6­11),
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, (6­12), holds for a stripper.

17.  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, (6­1), and an operating line, (6­3) or (6­
5), using graphical algebraic, or numerical methods. Graphical methods, such
as Figure 6.11, offer considerable visual insight into stage­by­stage changes in
compositions of the gas and liquid streams.
 7. Rough estimates of overall stage efficiency, defined by (6­21), can be made
with the correlations of Drickamer and Bradford, (6­22), O'Connell, (6­23), 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 vapor­point
efficiency, (6­30), from which a Murphrtf vapor tray efficiency can be estimated
from (6­31) to (6­34), which can then be related to the overall efficiency using
(6­37).
 8. Tray diameter can be determined from (6­44) based on entrainment flooding
consid­erations using Figure 6.24. Tray vapor pressure drop, the weeping
constraint, entrainment, and downcomer backup can be estimated from (6­45),
(6­64), (6­65), (6­66), respectively.

18.  9. Packed column height can be estimated using the HETP, (6­
69), or HTU/NTU,(6­85), concepts, with the latter having a more
fundamental theoretical basis in the two­film theory of mass
transfer. For straight equilibrium and operating lines, HETP is
related to the HTU by (6­90), and the number of equilibrium
stages is related to the NTU by (6­91).
 10. Below a so­called 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 (6­98

19.  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
(6­100).
 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 (6­
123) and (6­124) 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.

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