lecture water quality control and design: ion exchange

8/13/2019 lecture water quality control and design: ion exchange

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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

[email protected]


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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


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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+


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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


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Ion Exchange Material (Resins)


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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.


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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


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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.


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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.


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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


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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


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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:


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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

⇌


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Using similar logic and analysis on the resin:

Substituting into the selectivity coefficient:

Rearranging:

1

1

1

· 1

· 1

1

1


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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


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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


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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


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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


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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


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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


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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


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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


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Concentration

L e n g t h

a l o n

g

r e a c t o r


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Concentration

L e n g t h

a l o n

g

r e a c t o r


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Concentration

L e n g t h

a l o n

g

r e a c t o r


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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


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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.


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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


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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.


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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


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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.


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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

lecture water quality control and design: ion exchange

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