Chapter9

Approximate Methods for Multicomponent, Multistage Separations

 

Purpose and Requirements:

Learn Fenske-Underwood-Giliiland Method Learn Kremser Group Method

Key and Difficult Points:

Key Points

 

Fenske-Underwood-Giliiland Method for Multicomponent, Multistage Separations Kremser Group Method for Multicomponent, Multistage Separations

 

Difficult Points

Kremser Group Method for Multicomponent, Multistage Separations

Outline

9.1 FENSKE-UNDERWOOD-GILLILAND METHOD 9.2 KREMSER GROUP METHOD

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(H 2 S/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 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

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 bubble column packed 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

•

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.

6. Entering gas (liquid) flow rate, composition, 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. Choice of absorbent (stripping agent) 10. Diameter of absorber (stripper) 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

. The Fenske-Underwood-Gilliland (FUG) method for simple distillation of ideal and nearly ideal multicomponent mixtures is useful for making preliminary estimates, of stage and reflux requirements.

2

. Based on a specified split of two key components in the feed, mixture, the theoretical1, Fenske equation is used to determine the minimum number of equilibrium stages at total reflux. The theoretical Underwood equations are used to determine the minimum reflux ratio for an infinite number of stages. The empirical Gilliland correlation relates the minimum stages and minimum reflux ratio to the actual reflux ratio and the actual number of equilibrium stages.

  

3.

Estimates of the distribution of nonkey components and the feed-stage location can be made with the Fenske and Kirkbride equations, respectively.

4.

The Underwood equations are more restrictive than the Fenske equation and must be used with care and caution!

5

The Kremser group method can be applied to simple strippers and liquid-liquid, extractors to make approximate estimates of component recoveries for specified values of entering flow rates and number of equilibrium stages.

REFERENCES

       1. Kremser. A.. Nail. Petroleum News, 22 (21), 43-49 (1930).

2

.

Edmister. W.C.. AIChE J., 3 , 165-171 (1957). 3. Kobe, K.A.. and J.J. McKetta. Jr., Eds,

Advances in Petroleum Chemistry and Refining,

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Ind. Eng.

Chem.,42 ,871-876(1950).

7.

Underwood, A.J.V.. Trans. Inst. Chem. Eng., 10 ,112-158 (1932).

        . Gilliland. E.R., Ind. Eng. Chem., 32 , 1101-1106 (1940).

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Ind. Eng.

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Elements of Fractional Distillation,

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       Van Winkle, M., and W.G. Todd. Chem. Eng., 78 (21). 136 148(1971).

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Diffusional Separation Processes: Theory, Design, and Evaluation,

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Ind. Eng. Chem., Process

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     . Stupin, W.J., and FJ. Lockhart, "The Distribution of Non-Key Components in Multicomponent Distillation," presented at the 61st Annual Meeting of the AIChE, Los Angeles, CA, December 1-5, 1968.

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