8/13/2019 lecture water quality control and design: ion exchange
1/35
Department of Chemical &
Environmental Engineering
ENVE 120, Fall 2013
Ion ExchangeChapter 5
Mark MatsumotoChemical & Environmental Engineering A213 Bourns Hall and 454 Chung Hall, x2-3197
8/13/2019 lecture water quality control and design: ion exchange
2/35
Department of Chemical &
Environmental Engineering
Ion Exchange
Purpose – Removal of _______________ from Solution
Water Treatment
__________
Demineralization (deionization)
Wastewater Treatment
________________________
Demineralization
Metal removal and recovery (industrial wastewater)
Remediation
___________________________________________________
Cations not mobile in soils
8/13/2019 lecture water quality control and design: ion exchange
3/35
Department of Chemical &
Environmental Engineering
Ion Exchange
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Ca2+
Ca2+
Ca2+
Na+
Ca2+
Ca2+
Ca2+
Mg2+
Mg2+
Mg2+
Mg2+
Mg2+
Ca2+
Ca2+
Ca2+
Ca2+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+Na+
Na+
Na+
Na+
Na+
Na+ Na+
Na+ Na+
Na+
Na+
Na+
Na+Na+
Na+
Na+
Na+
Ca2+
Ca2+
Ca2+
Na+
Ca2+
Mg2+
Mg2+
Mg2+
Mg2+
Mg2+
Ca2+
Ca2+
Ca2+
Na+
Na+
Na+
Na+
8/13/2019 lecture water quality control and design: ion exchange
4/35
Department of Chemical &Environmental Engineering
Ion Exchange Material (Resins)
Ion exchange resins are __________ created by cross-linking hydrocarbon
chains. These resins are _________, inert and relatively rigid. Ionic functionalgroups are attached to this framework.
StyreneDivinyl-benzene
Polystyrene chain
Divinylbenzenecrosslink
8/13/2019 lecture water quality control and design: ion exchange
5/35
Department of Chemical &Environmental Engineering
Ion Exchange Material (Resins)
8/13/2019 lecture water quality control and design: ion exchange
6/35
Department of Chemical &Environmental Engineering
Resin Types
Resins are classified based on the type of ________________ they contain and
their % of ________________. Cationic exchangers
Strongly acidic – functional groups derived from ______ acids e.g., R-SO3H(sulfonic) – work at a wide range of pH values.
Weakly acidic – functional groups derived from _____ acids, e.g., R-COOH(carboxylic) – work at a narrow range of pH values.
Anionic exchangers Strongly basic – functional groups derived from ___________________________,
R-N-OH – work at a wide range of pH values.
Weakly basic – functional groups derived from ___________________________,R-NH3OH or R-R’-NH2OH – work at a narrow range of pH values.
8/13/2019 lecture water quality control and design: ion exchange
7/35
Department of Chemical &Environmental Engineering
Ion Exchange Reactions
__________________ must be maintained in the bulk fluid
Two monovalent ions exchanged for one divalent ion
Generalized reactions:
⇌
⇌
(cationic)
(anionic)
where R = ionic group attached to a solid exchange resin A = soluble functional group that can be exchanged with similar ion in the bulk water
8/13/2019 lecture water quality control and design: ion exchange
8/35
Department of Chemical &Environmental Engineering
Equilibrium Expression (Cationic)
In ion exchange, the equilibrium expression is termed the _________________
K , which varies with T , pH , ionic strength.
If the power term is ignored, then the equilibrium expression is termed the
________________ or selectivity quotient
When 1 B will be preferentially adsorbed over A . The higher is, thegreater the affinity or preference.
8/13/2019 lecture water quality control and design: ion exchange
9/35
Department of Chemical &Environmental Engineering
8/13/2019 lecture water quality control and design: ion exchange
10/35
Department of Chemical &Environmental Engineering
Selectivity or Preference of Exchange
The selectivity or ______________________ varies with resin. However, theselectivity preference depends on primarily two factors.
Valence or charge
Ions with _______ valence are preferred. Typical preferences:
Exceptions due occur:
Hydration radius
If ions have the same valence, ions with _________ hydration radii are preferred. Thepercentage of cross-linking is interrelated since it affects pore size. Typical preferences based
on hydration radii:
Caution: Selectivity coefficientsand preference series only apply
for low ionic strength waters,typical of drinking water.
8/13/2019 lecture water quality control and design: ion exchange
11/35
Department of Chemical &Environmental Engineering
Selectivity Coefficients
Selectivity coefficients for ______________________ can be easily determined.
Example: Home water softener systems often use ion exchange resins that areinitially charged with Na + by using high concentrations of NaCl (brine).However, as noted on the prior table, the selectivity coefficients are based onhaving hydrogen ion as the initial ion on the resin. Using the values given in thetable, the selectivity coefficient between Na+ and Ca2+ can be determined for
the 4% cross-linked resin:
3.14
1.20 2.62
8/13/2019 lecture water quality control and design: ion exchange
12/35
Department of Chemical &Environmental Engineering
Exchange Capacity Concepts
As noted before, a ______________ must be maintained. Thus, it is useful to
track ion concentrations in terms of ___________ rather than moles. This leadsto two definitions:
This latter term is also known as the ___________________.
During the exchange process, we want to know how much of the target ion forremoval is in the liquid and solid phases. Let be the fraction of A + insolution compared to the total amount of ions in solution and let be thefraction of A + on the resin compared to the total amount of exchangeable ions
on the resin:
Total concentration of exchangeable ions in the liquid phase, eq/L
Total concentration of exchangeable sites on the resin, eq/L of bulk volume
8/13/2019 lecture water quality control and design: ion exchange
13/35
Department of Chemical &Environmental Engineering
Equilibrium Expression (Cationic)
If the total exchange capacity of the ion exchange resin is defined as:
Then, the _____________ of counterion i at any time is:
8/13/2019 lecture water quality control and design: ion exchange
14/35
Department of Chemical &Environmental Engineering
Exchange Capacity Concepts
For divalent ions:
For now consider monovalent exchange between A and B. Recall:
Assuming that A and B are the only cations:
yields
yields
1 → 1
⇌
8/13/2019 lecture water quality control and design: ion exchange
15/35
Department of Chemical &Environmental Engineering
Exchange Capacity Concepts
Using similar logic and analysis on the resin:
Substituting into the selectivity coefficient:
Rearranging:
1
1
1
· 1
· 1
1
1
8/13/2019 lecture water quality control and design: ion exchange
16/35
Department of Chemical &Environmental Engineering
Exchange Capacity Concepts
For a divalent exchange it can be shown that:
These ion exchange equations (monovalent, divalent) form the relationship forthe amount of target ion exchanged as a function of the amount in solution.
2 ⇌ 2
2 ⇌ 2
1
1
8/13/2019 lecture water quality control and design: ion exchange
17/35
Department of Chemical &Environmental Engineering
Example
Nitrate NO3- is to be removed by ion exchange on a strong base anionic resin
initially charged with chloride Cl-. The resin has the following characteristics:
The influent has the following characteristics:
Determine how much water can be treated by an ion exchanger with 1 ft3 of thisresin assuming rapid kinetics and equilibrium between the water and resin.
4 1.3
3
1.5
8/13/2019 lecture water quality control and design: ion exchange
18/35
Department of Chemical &Environmental Engineering
Example
First, determine at equilibrium with the influent relative to the influent
water composition.
Solving for leads to 0.67. This value is the maximum fraction ofresin sites that can be occupied by NO3
- for this water. When this fraction hasbeen reached the resin is exhausted .
The total amount of NO3
- that this resin can exchange is:
1
1 4
0.33
10.33 2
1.5
1.5 3.0 0.33
1.3 0.67 0.87
8/13/2019 lecture water quality control and design: ion exchange
19/35
Department of Chemical &Environmental Engineering
Example
The volume of water that can be treated assuming a fully Cl- charged resin is:
Assuming rapid kinetics (narrow exchange zone), the effluent (exiting thecolumn) nitrate concentration will be near zero
0.87
1.5
,
580
580
580
7.48
4,300
8/13/2019 lecture water quality control and design: ion exchange
20/35
Department of Chemical &Environmental Engineering
Example 2
Calcium Ca2+ is to be removed by ion exchange on a strong acid cationic resin
initially charged with chloride Na+. The resin has the following characteristics:
The influent has the following characteristics:
Determine how much water can be treated per L of ion exchange resinassuming rapid kinetics and equilibrium between the water and resin.
1.9 2.0
2.6
1.4
8/13/2019 lecture water quality control and design: ion exchange
21/35
Department of Chemical &Environmental Engineering
Example 2
An alternative method to the previous method is to use the separation factor
and the relationship.
Recall that i represents the ion in solution that is to be removed and j is the ion
on the resin that will be exchanged. Thus,
Leading to:
1
1
1.9 0.526
1.4 2.0 1,0001.4 2.6
0.526
1,010
8/13/2019 lecture water quality control and design: ion exchange
22/35
Department of Chemical &Environmental Engineering
Example 2
The volume of water that can be treated assuming a fully Na+ charged resin is:
Assuming rapid kinetics (narrow exchange zone), the effluent (exiting thecolumn) nitrate concentration will be near zero
1,010 1.4
720
8/13/2019 lecture water quality control and design: ion exchange
23/35
Department of Chemical &Environmental Engineering
Continuous-Flow Operation
____________ dominated process Transport of ions from bulk solution
to the ________ or film layerDiffusion of ions through the filmlayer
Diffusion of ions into the ______where the exchange sites are
Exchange of ions via reversiblereaction
Diffusion of exchanged ions outwardthrough the ______
Diffusion of exchanged ions through
the film layer toward the film layerTransport of ions from the film layerinto the bulk solutionBulk solution
Film layer
8/13/2019 lecture water quality control and design: ion exchange
24/35
Department of Chemical &Environmental Engineering
Continuous-Flow Operation
Concentration
L e n g t h
a l o n
g
r e a c t o r
8/13/2019 lecture water quality control and design: ion exchange
25/35
Department of Chemical &Environmental Engineering
Continuous-Flow Operation
Concentration
L e n g t h
a l o n
g
r e a c t o r
8/13/2019 lecture water quality control and design: ion exchange
26/35
Department of Chemical &Environmental Engineering
Continuous-Flow Operation
Concentration
L e n g t h
a l o n
g
r e a c t o r
8/13/2019 lecture water quality control and design: ion exchange
27/35
8/13/2019 lecture water quality control and design: ion exchange
28/35
Department of Chemical &Environmental Engineering
Service Cycle
Service : Normal operation
Backwash : Upflow flow to expandbed by 50% to _________ andremove any trapped particles
Regeneration : Regenerant solution(concentrate) passed slowly through
the bed to ____________________(follows principles of equilibrium)
Slow rinse : Clean water slowlypassed through bed to remove
________________
Fast rinse : Final rinse of the resin
Return to service
8/13/2019 lecture water quality control and design: ion exchange
29/35
Department of Chemical &Environmental Engineering
Regeneration
After all of the available exchange sites are utilized, the spent or _________
column must be regenerated. As its name implies ion exchange is a reversible process that can beaccomplished by exposing the resin to an appropriate regeneration solution or ______________.
For home water softening system, resins are initially charged with Na+. After a
period of time, most of the exchange sites are filled with ____________ ionsand the softening process ceases.
To restore the softening capacity of the resins, the system must be regeneratedby soaking the resin in a solution containing a high concentration of ______.Because of the high concentration of Na+ in solution, Na+ attachment to theexchange sites are favored over the Ca2+ and Mg2+ ions. The Ca2+ and Mg2+
ions on the resin are released into the brine (salt) solution and Na+ ions areexchanged back onto the resin.
After regeneration, the brine solution is purged from the resin and the ionexchange system is put back into service.
8/13/2019 lecture water quality control and design: ion exchange
30/35
Department of Chemical &Environmental Engineering
Example
A spent water softening column that primarily removes Ca2+ is to beregenerated in a batch mode back to the Na+ form. A strong NaCl brine iscontacted with the exhausted resin to replace Ca2+ with Na+. The compositionof the brine (regenerant) after equilibration with the exhausted resin is:
For this resin, the exchange capacity and selectivity coefficient are:
Note: Most of the Ca2+
is from the spent column. Na+
in the fresh brine wasslightly higher than 2 eq/L.
Determine the effectiveness of the regeneration. In other words, what is after the regeneration is complete?
2 46 /
0.2 4 /
2
4
8/13/2019 lecture water quality control and design: ion exchange
31/35
8/13/2019 lecture water quality control and design: ion exchange
32/35
Department of Chemical &Environmental Engineering
Example
Only 76.5% regeneration can be accomplished with this regenerant becauseeven small amounts of Ca can have a significant effect because of the highselectivity coefficient in favor of Ca. To get higher regeneration the NaClconcentration must be higher or the total volume of regenerant must be greaterto dilute the Ca that comes off the exhausted column. Both options cost moneyand there needs to be a tradeoff evaluated between higher column utilizationand more costly regeneration.
8/13/2019 lecture water quality control and design: ion exchange
33/35
Department of Chemical &Environmental Engineering
Design Considerations
Selection of resin: ________________ (meq/mL) – less than total
capacity (see previous example) _________________
Service flow rate (SFR) = Q/V resin Typical 200 to 1,000 m3 /m3-d
Typical 8 to 40 BV/hr
Empty-bed contact time (EBCT) = V resin /Q Typical 1.5 to 7.5 min
Surface loading rate (SLR) = Q/A s Typical 175 to 880 m/d (depending on headloss)
Limit headloss to
8/13/2019 lecture water quality control and design: ion exchange
34/35
Department of Chemical &Environmental Engineering
Water Softening: Typical Design Criteria
___________________: 400-800 m3 /d · m2 of bed cross-sectional area
Backwash rate: Want __________ expansion of the resin bed. Rate isdependent on density of the resin and temperature of the backwashwater.
Regeneration: For strong acid and strong base resins: 2 to 10%
solutions, weak acid and base resins: 1 to 5% solutions.
8/13/2019 lecture water quality control and design: ion exchange
35/35
Department of Chemical &Environmental Engineering
Water Softening: Typical Design Criteria
Regeneration:
Minimum contact time of ________
Flow rate of 60 -120 m3 /d · m2 of cross sectional area
Quantity of resin depends on manufacturer specifications
Rinsing to remove excess regenerant:
___________ the bed volume (BV) of resin
Bed depth: Minimum of 0.9 mFreeboard: Length of 50 to 75% of the bed depth