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Lecture 19.pptx

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Lecture 19.pptx

  1. 1. Separation Processes-I (ChE-206) Lecture No. 19 Ion Exchange
  2. 2. Ion Exchange • In an ion-exchange process, ions of positive charge (cations) or negative charge (anions) in a liquid solution, usually aqueous, replace dissimilar and displaceable ions, called counterions, of the same charge contained in a solid ion exchanger, which also contains immobile, insoluble, and permanently bound co-ions of the opposite charge. • Thus, ion exchange can be cation or anion exchange. • Example: Water softening by ion exchange involves a cation exchanger, in which a reaction replaces calcium ions with sodium ions:
  3. 3. • The exchange of ions is reversible and does not cause any permanent change to the solid ion-exchanger structure. • Thus, it can be used and reused unless fouled by organic compounds in the liquid feed that attach to exchange sites on and within the ion exchange resin. • In ion-exchange, the solid separating agent becomes saturated or nearly saturated with the molecules, atoms, or ions transferred from the fluid phase. Which means the sorbent needs to be recovered.
  4. 4. Industrial Applications
  5. 5. Ion-Exchangers • The first ion exchangers were naturally occurring inorganic aluminosilicates (zeolites). • Naturally occurring-porous sands. • Ion-exchange resins are generally solid gels in spherical or granular form, which consist of: • (1) a three-dimensional polymeric network • (2) ionic functional groups attached to the network • (3) counterions • (4) a solvent
  6. 6. Properties of Ion Exchange Resins Exchange Capacity: “quantity of counter-ions that can be exchanged onto the resin”. • Total capacity is dependent on the quantity of functional groups on a resin. • Important in selecting an ion exchange resin. • Reported as milliequivalents per gram of dry resin. • For typical strong acid cation exchange resin; exchange capacity falls in the range of 3.6 to 5.5 meq/g.
  7. 7. Selectivity: “Preference or affinity of the resin for the ions in solution is called selectivity”. • For a binary exchange, selectivity may be expressed as a selectivity coefficient ( Ki j). • The greater the selectivity coefficient ( K ), the greater is the preference for the ion by the exchange resin.
  8. 8. Process operation (Co-current) Service Backwash Regeneration Slow Rinse Fast Rinse
  9. 9. Process operation Service: The raw water is passed downward through the column until the hardness exiting the column exceeds the design limits. The column is taken out of service and another column is brought on line. Backwash: A flow of water is introduced through the under-drain. It flows up through the bed sufficient to expand the bed by 50 percent. The purpose is to relieve hydraulic compaction and to move the finer resin material and fragments to the top of the column and remove any suspended solids that have accumulated during the service cycle. Regeneration: The regenerating chemical, for example, sodium chloride, flows downward through the bed at a slow rate to allow the reactions to proceed toward complete regeneration. Slow rinse: Rinse water is passed through the column at the same flow rate as the regenerating flow rate to push the regenerating chemical through the bed. Fast rinse: This is a final rinse step. The fast rinse flows at the same flow rate as the service flow rate to remove any remaining regenerating solution. Return to service: The column is put back in use.
  10. 10. Counter-current • Regenerate is passed though the resin in the opposite direction to that of the water being treated. • Characteristics of countercurrent operation: • Lower leakage. • higher chemical efficiency • More expensive design • More complicated to operate
  11. 11. Ion-Exchange Equilibria • Ion exchange differs from adsorption in that one sorbate (a counterion) is exchanged for a solute ion, the process being governed by a reversible, stoichiometric, chemical-reaction equation. • Thus, selectivity of the ion exchanger for one counterion over another may be just as important as the ion-exchanger capacity. • Accordingly, the law of mass action is used to obtain an equilibrium ratio rather than to fit data to a sorption isotherm such as the Langmuir or Freundlich equation.
  12. 12. Case 1 • As discussed by Anderson, two cases are important. • In the first, the counterion initially in the ion exchanger is exchanged with a counterion from an acid or base solution, e.g., • Note that hydrogen ions leaving the exchanger immediately react with hydroxyl ions to form water, leaving no counterion on the right-hand side of the reaction. • Accordingly, ion exchange continues until the aqueous solution is depleted of sodium ions or the exchanger is depleted of hydrogen ions.
  13. 13. Case 2 • In the second, more-common, case, the counterion being transferred from exchanger to fluid remains as an ion. For example, exchange of counterions A and B is expressed by: • where A and B must be either cations (positive charge) or anions (negative charge).
  14. 14. Equilibrium Constant • For this case, at equilibrium, a chemical-equilibrium constant based on the law of mass action can be defined: • where molar concentrations ci and qi refer to the liquid and ion-exchanger phases, respectively. • The constant, KA,B is not a rigorous equilibrium constant because (15-38) is in terms of concentrations instead of activities. • Although it could be corrected by including activity coefficients, it is used in the form shown, with KA,B referred to as a molar selectivity coefficient for A displacing B. • For the resin phase, concentrations are in equivalents per unit mass or unit bed volume of ion exchanger. For the liquid, concentrations are in equivalents per unit volume of solution. For dilute solutions, KA,B is constant for a given pair of counterions and a given resin.
  15. 15. • When exchange is between two counterions of equal charge, the equation reduces to a simple equation in terms of equilibrium concentrations of A in the liquid solution and in the ion-exchange resin. The total concentrations, C and Q, in equivalents of counterions in the solution and the resin, remain constant during the exchange. • For unequal counterion charges, KA,B depends on the ratio C/Q and on the ratio of charges, n.
  16. 16. Equipment for Ion-Exchange • Contacting modes for ion exchange are same as that of adsorption. • (a) Stirred-tank, slurry operation • (b) Cyclic fixed-bed, batch operation • (c) Continuous countercurrent operation • Although use of fixed beds in a cyclic operation is most common, stirred tanks are used for batch contacting, with an attached strainer or filter to separate resin beads from the solution after equilibrium is approached. • Agitation is mild to avoid resin attrition, but sufficient to achieve suspension of resin particles.
  17. 17. Continuous, countercurrent contactors • To increase resin utilization and achieve high efficiency, efforts have been made to develop continuous, countercurrent contactors: • The Higgins contactor (moving, packed bed) • Himsley fluidized-bed process
  18. 18. The Higgins contactor (moving, packed bed) • This operates as a moving, packed bed by using intermittent hydraulic pulses to move incremental portions of the bed from the ion-exchange section up, around, and down to the backwash region, down to the regenerating section, and back up through the rinse section to the ion-exchange section to repeat the cycle. • Liquid and resin move counter currently.
  19. 19. Himsley fluidized-bed process • This has a series of trays on which the resin beads are fluidized by upward flow of liquid. • Periodically the flow is reversed to move incremental amounts of resin from one stage to the stage below. • The batch of resin at the bottom is lifted to the wash column, then to the regeneration column, and then back to the top of the ion-exchange column for reuse.
  20. 20. Book • Seader, J. D.; Henley, E. J.; Roper, D. K., Separation Process Principles: Chemical and Biochemical Operations. 3rd Ed.; John Wiley & Sons, Inc.: 2011. • Chapter 15: Adsorption, Ion Exchange, Chromatography, • and Electrophoresis

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