Transcript 幻灯片 1

Chapter7
Distillation of
Binary Mixtures
Purpose and Requirements:
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Know the importance and mechanism of separation
Learn to select feasible separation process for an industry
process
Key and Difficult Points:
Key Points
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Mechanism of Separation
Component Recoveries and Product Purities
Separation Power
Selection of Feasible Separation Processes
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Difficult Points
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Mechanism of Separation
Selection of Feasible Separation Processes
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Outline
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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)
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Gas Mixture (Solutes or Absorbate)
Liquid (Solvent or Absorbent)
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Separate Gas Mixtures
Remove Impurities, Contaminants, Pollutants, or
Catalyst Poisons from a Gas(H2S/Natural Gas)
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Recover Valuable Chemicals
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Physical Absorption
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Chemical Absorption
(Reactive Absorption)
Figure 6.1 Typical Absorption Process
Absorption Factor
(A吸收因子)
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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)
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Stripping
Distillation
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Stripping Factor
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(S解吸因子)
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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
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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
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of absorber
absorbent
(stripping agent)
9. Choice
Height of
(stripper)
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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
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1. A binary liquid and/or vapor binary mixture can be separated into two nearly
pure products (distillate and bottoms) by distillation, provided that the value of
the relative volatility of the two components is high enough, usually greater than
1.05.
2. Distillation is the most mature and widely used separation operation, with
design procedures and operation practices well established.
3. The purities of the products from distillation depend on the number of
equilibrium stages in the rectifying section above the feed entry and in the
stripping section below the feed entry, and on the reflux ratio. Both the number
of stages and the reflux ratio must be greater than the minimum values
corresponding to total reflux and infinite stages, respectively. The optimal
reflux-to-minimum-reflux ratio is usually in the range of 1.10 to 1.50.
4. Distillation is most commonly conducted in trayed towers equipped with sieve
or valve trays, or in columns packed with random or structured packings. Many
older towers are equipped with bubble-cap trays.
5. Most distillation towers are equipped with a condenser, cooled with cooling
water to provide reflux, and a reboiler, heated with steam, to provide boilup.
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6. When the assumption of constant molar overflow is valid in each of the two
sections of the distillation tower, the McCabe-Thiele graphical method is
convenient for determining stage and reflux requirements. This method
facilitates the visualization of many aspects of distillation and provides a
procedure for locating the optimal feed-stage location.
7. Miscellaneous considerations involved in the design of a distillation tower
include selection of operating pressure, type of condenser, degree of reflux
subcooling, type of reboiler, and extent of feed preheat.
8. The McCabe-Thiele method can be extended to handle Murphree stage
efficiency, multiple feeds, side streams, open steam, and use of interreboilers
and intercon-densers.
9. Rough estimates of overall stage efficiency, defined by (6-21), can be made
with the Drickamer and Bradford, (7-42), or O'Connell, (7-43), correlations.
More accurate and reliable procedures use data from a small Oldershaw
column or the same semitheoretical equations for mass transfer in Chapter 6
that are used for absorption and stripping.
10. Tray diameter, pressure drop, weeping, entrainment, and downcomer
backup can all be estimated by the procedures in Chapter 6.
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11. Reflux and flash drums are sized by a procedure based on
avoidance of entrainment and provision for adequate liquid residence
time.
12. Packed column diameter and pressure drop are determined by the
same procedures presented in Chapter 6 for absorption and
stripping. , . •
13. The height of a packed column may be determined by the HETP
method, or preferably from the HTU method. Application of the latter
method is similar to that of Chapter 6 for absorbers and strippers, but
differs in the manner in which the curved equilibrium curve must be
handled, as given by (7-47).
14. The Ponchon-Savarit graphical method removes the assumption of
constant molar overflow in the McCabe-Thiele method by employing
energy balances with an enthalpy-concentration diagram. However,
the Ponchon-Savarit method has largely been supplanted by rigorous
computer-aided methods.
REFERENCES
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Forbes. R.J., Short History of the Art of Distillation, E.J. Brill,
McKetta, Jr., Eds., Advances in Petroleum Chemistry and
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V;x. T W.. J.S. Dweck, M. Weinberg, and R.C. Armstrong,
Chem.
Zuiderweg, F.J., H. Verburg, and F.A.H. Gilissen, Proc.
Interna- Pro%. 74 (4), 49-55 (1978). tional Symposium on
Distillation, Institution of Chem. Eng.. London,
ksjct. H.Z., Distillation Design, McGraw-Hill, New York (1992).
202-207 (1960).
Uter. H.Z., Distillation Operation, McGraw-Hill, New York 15.
Gautreaux, M.F., and H.E. O'Connell. Chem. Eng. Prog., 51(5)
232-237 (1955).
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Chan tf and j R Fair /nd £ng Chem Pfocess Des Dcuhe. W.L.,
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f;and K'D-Ti/nmerhaus' Plant Desi*n an<! e™™™Fair, J.R.,
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TJST' J A • A'B- Hil!- N-H- Hochgrof, and D.B. Robinson,
21. Ponchon, M., Tech. Moderne, 13, 20, 55 (1921).
T" m Distillation Columns, Final Report from the
Savarit, R.,Arts et Metiers, pp. 65. 142, 178. 241. 266, 307
(1922).
fttlaware, AIChE, New York (1958).
Henley, E.J., and J.D. Seader, Equilibrium-Stage Separation
OpR- Bradford, Trans. AIChE, 39,319- erations in Chemical
Engineering, John Wiley and Sons. NewYork (1981).TransAIChE, 42, 741-755 (1946).
Glitsch Ballast Tray, Bulletin 159, Fritz W. Glitsch and Sons,J .
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report of September 3, 1958).