Equilibrium-Based Methods for Equilibrium-Based Methods for Multicomponent Absorption, StriMulticomponent Absorption, Stripping, Distillation, and Extractiopping, Distillation, and Extractio

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Chapter10Chapter10

Key and Difficult Points:Key Points Theoretical Model for an Equilibrium Stage General Strategy of Mathematical SolutionDifficult Points Equation-Tearing Procedures Simultaneous Correction Procedures Inside-Out Method

Purpose and Requirements: Know Equilibrium-Based Methods for Multicomponent Learn to use ASPEN PLUS, ChemCAD, HYSIM, PRO/II

Outline

10.1 THEORETICAL MODEL FOR AN EQUILIBRIUM STAGE10.2 GENERAL STRATEGY OF MATHEMATICAL SOLUTION10.3 EQUATION-TEARING PROCEDURES10.4 SIMULTANEOUS CORRECTION PROCEDURES10.5 INSIDE-OUT 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(H2S/Natural Gas) Recover Valuable Chemicals

Figure 6.1 Typical Absorption Process

Chemical Absorption (Reactive Absorption)

Physical Absorption

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

Figure 6.2 Industrial Equipment for Absorption and Stripping

trayed tower packed column

spray towerbubble column

centrifugal contactor

Figure 6.3 Details of a contacting tray in a trayed tower

Trayed Tower(Plate Clolumns板式塔 )

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

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 belowVapor carries no liquid droplets to the tray aboveNo weeping of liquid through the openings of the tray

Equilibrium between the exiting vapor and liquid phases is approached on each tray.

Packed Columns

Figure 6.6 Details of internals used in a packed column

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

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

1. Entering gas (liquid) flow rate, composition, temperature, and pressure

2. Desired degree of recovery of one or more solutes3. Choice of absorbent (stripping agent)4. Operating pressure and temperature, and allowable

gas pressure drop5. 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 stages7. Heat effects and need for cooling (heating)8. Type of absorber (stripper) equipment9. Height of absorber (stripper)10. Diameter of absorber (stripper)

SUMMARY

1. Rigorous methods are readily available for computer-solution of equilibrium-based models for multicomponent, multistage absorption, stripping, distillation, and liquid-liquid extraction.

2. The equilibrium-based model for a countercurrent-flow cascade provides for multiple feeds, vapor side streams, liquid side streams, and intermediate heat exchangers. Thus, the model can handle almost any type of column configuration.

3. The model equations include component material balances, total material balances, phase equilibria relations, and energy balances.

4. Some or all of the model equations can usually he grouped so as to obtain tridiagonal matrix equations, for which an efficient solution algorithm is available.

5. Widely used methods for iteratively solving all of the model equations are the bubble-point (BP) method, the sum-rales (SR) method, the simultaneous correction (SO method, and the inside-out method.

6. The BP method is generally restricted to distillation problems involving narrow-boiling feed mixtures.

7. The SR method is generally restricted to absorption and stripping problems involving wide-boiling feed mixtures or in the ISR form to extraction problems.

8. The SC and inside-out methods are designed to solve any type of column configuration for any type of feed mixture. Because of its computational efficiency, the inside-oi method is often the method of choice; however, it may fail to converge when highly! nonideal liquid mixtures are involved, in which case the slower SC method should j be tried. Both methods permit considerable flexibility in specifications.

9. When both the SC and inside-out methods fail, resort can be made to the much slower relaxation and continuation methods.

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