Presentation on theme: "Equilibrium-Based Methods for Multicomponent Absorption, Stripping, Distillation, and Extraction Chapter10."— Presentation transcript:
Equilibrium-Based Methods for Multicomponent Absorption, Stripping, Distillation, and Extraction Chapter10
Key and Difficult Points: Key Points Theoretical Model for an Equilibrium Stage General Strategy of Mathematical Solution Difficult 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 STAGE 10.2 GENERAL STRATEGY OF MATHEMATICAL SOLUTION 10.3 EQUATION-TEARING PROCEDURES 10.4 SIMULTANEOUS CORRECTION PROCEDURES 10.5 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(H 2 S/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 Acetone Oxygen ,000 Nitrogen ,000 Argon ,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.
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 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.
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 General Design Considerations Trayed Towers Graphical Equilibrium-Stage Algebraic Method for Determining the Number of Equilibrium Stage Efficiency Packed Columns Rate-based Method 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 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)
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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