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. • 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.
6. 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
7. 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.
8. 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.
10. 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.
11. 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
12. 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.
13. 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.
14. 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).
15. 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.
16. • 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.
17. 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.
18. 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
19. 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.
20. 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.
21. 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
