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Overview of Mass Exchange Operations

This lecture provides an overview of mass exchange operations, including the concept of a mass exchanger, equilibrium relationships, interphase mass transfer, modeling techniques, and various types of mass exchangers.

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Overview of Mass Exchange Operations

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  1. Mass Integration CHEN 4470 – Process Design Practice Dr. Mario Richard EdenDepartment of Chemical EngineeringAuburn University Lecture No. 7 – Overview of Mass Exchange Operations January 31, 2013

  2. What is a Mass Exchanger? • Mass Exchanger • A mass exchanger is any direct-contact mass-transfer unit which employs a Mass Separating Agent (or a lean phase) to selectively remove certain components (e.g. pollutants) from a rich phase (e.g. a waste stream). • Absorption, Adsorption, Extraction, Ion Exchange, ….

  3. Equilibrium 1:4 • Generalized Description • The composition of the rich stream (yi) is a function of the composition of the lean phase (xj) • Dilute Systems • For some applications the equilibrium functions may be linearized over the operating range

  4. Mole fraction of solute in gas • Vapor pressure of solute at T • Mole fraction of solute in liquid • Total pressure of gas • Mole fraction of solute in gas • Mole fraction of solute in liquid • Henry’s coefficient • Liquid-phase solubility of the pollutant at temperature T Equilibrium 2:4 • Special Cases • Raoult’s law for absorption • Henry’s law for stripping

  5. Solute composition in liquid • Solute composition in solvent • Distribution coefficient Equilibrium 3:4 • Special Cases • Distribution function used in solvent extraction • Interphase Mass Transfer • For linear equilibrium the pollutant composition in the lean phase in equilibrium with yi can be calculated as:

  6. Overall mass transfer coefficient for rich phase • Overall mass transfer coefficient for lean phase Equilibrium 4:4 • Interphase Mass Transfer (Continued) • For linear equilibrium the pollutant composition in the rich phase in equilibrium with xj can be calculated as: • Rate of Mass Transfer Correlations for estimating overall mass transfer coefficients can be found in McCabe et al. (1993), Perry and Green (1984), King (1980) and Treybal (1980).

  7. Mass Exchangers – I 1:2 • Multistage Contactors • Multistage countercurrent tray column

  8. Mass Exchangers – I 2:2 • Multistage Contactors (Continued) • Multistage Mixer-Settler System

  9. Modeling – I 1:5 • Stagewise Columns • A generic mass exchanger • Schematic of a multistage mass exchanger

  10. Modeling – I 2:5 • Stagewise Columns (Continued) • Operating line (material balance) • The McCabe-Thiele diagram

  11. Modeling – I 3:5 • Stagewise Columns (Continued) • The Kremser equation • Isothermal • Dilute • Linear equilibrium

  12. Modeling – I 4:5 • Stagewise Columns (Continued) • Other forms of the Kremser equation

  13. Modeling – I 5:5 • Stagewise Columns (Continued) • Number of actual plates • Stage efficiency can be based on either the rich or the lean phase. If based on the rich phase, the Kremser equation can be rewritten as:

  14. Mass Exchangers – II 1:3 • Differential (Continuous) Contactors • Countercurrent packed column

  15. Mass Exchangers – II 2:3 • Differential (Continuous) Contactors (Continued) • Spray column

  16. Mass Exchangers – II 3:3 • Differential (Continuous) Contactors (Continued) • Mechanically agitated mass exchanger

  17. Modeling – II • Continuous Mass Exchangers • Height of a differential contactor

  18. Crash Course in Economics 1:5 • Which Car is Cheaper? • Fixed cost: The car itself, i.e. body, engine, tires, etc. $500 $21,000

  19. Crash Course in Economics 2:5 • Which Car is Cheaper? (Continued) • Annual Operating Cost (AOC): How much to run and maintain the car. $ vs. $/year ??? We need to annualize the fixed cost of the car $4,000/year $700/year

  20. Crash Course in Economics 3:5 • Which Car is Cheaper? (Continued) • Annualized Fixed Cost (AFC) • Total Annualized Cost (TAC)

  21. Crash Course in Economics 4:5 • Which Car is Cheaper? (Continued) Useful Life: 2 Years Salvage Value: $200 AFC = ($500-$200)/2 yr = $150/yr Useful Life: 20 Years Salvage Value: $1000 AFC = ($21,000-$1,000)/20 yr = $1000/yr

  22.  Crash Course in Economics 5:5 • Which Car is Cheaper? (Continued) TAC = $4,000 + $250 = $4,250/yr TAC = $1,000 +$700 = $1,700/yr

  23. Minimizing Cost of MENs 1:3 • Total Annualized Cost of Mass Exchange System • Fixed cost: Trays, shell, packing, etc. • Operating cost: solvent makeup, pumping, heating/cooling, etc. • Driving Force • Minimum allowable composition • difference • Must stay to the left of • equilibrium line

  24. Minimizing Cost of MENs 2:3 • Driving Force (Continued) • Minimum allowable composition difference at rich end of mass exchanger When the minimum allowable composition difference εj increases, then the ratio of L/G increases. AOC increases, due to higher MSA flow AFC decreases, due to smaller equipment, e.g. fewer stages

  25. Minimizing Cost of MENs 3:3 • Driving Force (Continued) Trade-off between reducing fixed cost and increasing operating cost Composition driving force, becomes a optimization variable OPTIMUM

  26. Other Business • Next Lecture – February 5 • Synthesis of mass exchange networks part I • SSLW pp. 297-308

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