1. CHEN 4470 – Process Design Practice Dr. Mario Richard Eden Department of Chemical Engineering Auburn University Lecture No. 3 – Overview of Mass Exchange Operations January 23, 2007 Mass Integration

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. Generalized Description –The composition of the rich stream (y i ) is a function of the composition of the lean phase (x j ) Dilute Systems –For some applications the equilibrium functions may be linearized over the operating range Equilibrium 1:4

4. Special Cases –Raoult’s law for absorption –Henry’s law for stripping Equilibrium 2:4 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

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

6. Interphase Mass Transfer (Continued) –For linear equilibrium the pollutant composition in the rich phase in equilibrium with x j can be calculated as: Rate of Mass Transfer Equilibrium 4:4 Overall mass transfer coefficient for rich phase Overall mass transfer coefficient for lean phase 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. Multistage Contactors –Multistage countercurrent tray column Mass Exchangers – I 1:2

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

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

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

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

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

13. 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: Modeling – I 5:5

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

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

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

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

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

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

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

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

23. 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 Minimizing Cost of MENs 1:3

24. Driving Force (Continued) –Minimum allowable composition difference at rich end of mass exchanger Minimizing Cost of MENs 2:3 When the minimum allowable composition difference ε j 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. Driving Force (Continued) Minimizing Cost of MENs 3:3 OPTIMUM Trade-off between reducing fixed cost and increasing operating cost Composition driving force, becomes a optimization variable

26. Next Lecture – January 25 –Synthesis of mass exchange networks part I –SSL pp. 367-383 Other Business