Ion Exchange Operations

Matt Roth

Content

• Performance Drivers – Equilibrium

• Selectivity

• Equilibrium leakage

– Kinetics

• Performance Factors – Operating capacity

• Cycle end point

– Reaction zone

– Flow distribution

– Regeneration

• Plant Operation – Feed water

– Demineralization service cycle: WAC, SAC, WBA, SBA

– Throughput; water quality

– Regeneration: backwash, chemical injection, displacement, fast rinse

Equilibrium and Kinetics

-Controlling principles and performance

Performance drivers content

• Equilibrium

– Selectivity

– Equilibrium leakage

• Kinetics

Equilibrium as a Driving Force

5

A

Potential Difference

B

Equilibrium

Equilibrium Equation

R-X + O R-O + X

K

K O

X =

[RO]

[RX]

[X]

[O]

K is “Selectivity Coefficient” O

X

Describes the relative affinity (equilibrium) of an IER for two different ions

Selectivity

• Relative preference for one ion over a different ion

– Typically cations vs. H+ and anions vs. OH-

• Divalent ions more selectively held than monovalent

• Highest selectivity ion loads at the top of the bed

• Least selective ion leaks first and indicates exhaustion

• Sodium typically leaks first for cations

• Silica typically leaks first for anions

7

Selectivity

8

In general (dilute solutions)

Trivalent+++ > Divalent++ > Monovalent+ ions

Sulfonic (SAC) resins

Ba > Pb > Sr > Ca > Ni > Cu > Mg

Ag >> Cs > K > NH4 > Na > H > Li

Quaternary Ammonium (SBA)

SO4 > CrO4 > NO3 > CH3COO > I > Br > Cl > F > OH

MARATHON C

MARATHON A

Relative selectivity of common ions

Cation Exchanger Anion Exchanger

Inlet

Outlet

Inlet

Outlet

Ca2+

Mg2+

Na+

H+

SO42-

CO32-

Cl-, HSiO3-

OH-

Example of equilibrium leakage

10

R-H + Na+

K

K Na

H @

[R Na]

[RH]

[H+]

[Na+]

R-Na + H+

1.7 =

Na Leakage = 0.92 ppm Na+

From dissociation [H+] = 10-2

Solve for [R Na]

[RH]

0.7% Sites Na+ Form

pH out of Cation unit = 2

Observation: Cond = 10 µS/cm

Cond, µS/cm Na+ leakage % R-Na

10 920 ppb 0.7%

5 460 ppb 0.34%

2 185 ppb 0.14%

1 92 ppb 0.07%

0.5 44 ppb 0.03%

0.1 8 ppb 0.006%

Equilibrium calculations -Predict leakage

Ion Exchange Kinetics

• Rate at which ion moves to the resin surface and is removed

• In ion exchange film diffusion is the rate limiting step

• Surface fouling slows down removal of ions.

Single Bead Kinetic Model

Stagnant Film

Solid Bead

C*

C L

Bulk Liquid Flow

C = Concentration of Ions in Bulk Liquid C* = Concentration of Ions at Bead Surface L = Film Thickness

In Ion Exchange processes, overall

exchange kinetics is controlled by film

diffusion

R

What is controlling performance?

• Equilibrium Controlled

– Baseline Leakage determined by residual impurities at the outlet of the

column. (what is left on the resin after regeneration)

– Regeneration efficiency-controlled by equlibrium

• Kinetic Controlled

– Mass transfer zone width

– Time to breakthrough and shape of the curve

– SO4 leakage at ppb levels – anion surface fouling

Ion exchange resin breakthrough curve

Time

Baseline Leakage of O

Zero

Influent [O]

Concentration of O

IX Vessel Performance Factors

Performance factors content

• Operating capacity

– Cycle end point

• Reaction zone

• Flow distribution

• Regeneration

Column operation

Fluid to be treated

(Influent)

Treated fluid

(Effluent)

Resin bed

1 Bed volume

(BV)

Total capacity

2.1 eq/l resin

Operating capacity

Exhausted resin

Regenerated resin 0 100

Exhaustion, %

Bed

Depth Reaction zone

Cycle end point

Exhausted resin

Leakage

0 100

Exhaustion, %

Operating

capacity

1.3 eq/l resin

Regenerated resin

Reaction zone

Flow distribution in service vessels is

critical

21

Flow Distribution

• Effect in Service

– Widen mass transfer zone

– Reduces throughput

– Could increase leakage

• Effect in Regeneration

– Reduces throughput

– Increase leakage

– Increases rinse time

– Chemical hideout

Operating capacity- After 1st Cycle

Exhausted resin

Operating capacity

60% of total

Reaction

zone

Begin End

Not all total capacity

available

Co-flow regeneration

Regenerant

Eluate (spent regenerant) Leakage

Liquid to

be treated

Eluate (spent

regenerant)

Counter-flow regeneration

Regenerant

Liquid to

be treated

Clean

polishing

zone

Plant operation

Feed Water -TDS • Total Dissolved Solids (TDS)

• Amount of saltsin the water: NaCl, Na2SO4 CaCl2, etc.

Feed Water Analysis, ppm as Ion Cations Anions

Ca 150 SO4 280

Mg 75 NO3 70

Na 250 Cl 300

K 20 HCO3 545

Fe 0.5 SiO2 35

Total

Cations 496

Total

Anions 1230

Total Dissolved

Solids 1726

TDS is the load on the ion exchange Resin

Na

Ca + Mg HCO3

Cl + SO4

SiO2

Cations Anions

Feed Water

-Other contaminants

• Organics measured as TOC

– Oil/hydrocarbons

– Surfactants

– Natural Organics

• Oxidants

– Chlorine

– Chloramines

• Particulate matter

– TSS – Dirt, Fe, flocculent, etc.

– Turbidity

Not part of the load, but have an impact on performance

Demineralisation

• Trying to remove all ionic contaminates

• Cation resin removes cations from the water

• Anion resin removes anions from the water

• You have created new water molecules

Na

Na

ClCl

Cl

Na

H

H

H OH OH

OH

(charges not shown)

Service cycle

• What happens in each bed?

30

Four Bed System

WAC SAC WBA SBA

WAC SAC WBA SBA

DI – Weak Acid Cation

• Removes hardness associated with alkalinity

• Very efficient regeneration

– 1.05 to 1.15 eq of acid per eq of operating capacity

• Use Rule of thumb

– Total hardness >50% of total cations, Alkalinity/hardness ratio ~1

2R-H + Ca2+(HCO3)2 R2-Ca2+ + 2CO2 + 2H2O

WAC

DI – Strong Acid Cation

• Most common is 8% DVB

– Best balance of total capacity and regeneration efficiency

• Uniform particle size (UPS) resins needed for reverse flow systems;

also preferred for Co-flow systems

– Need less fines

– Improved rinse with UPS

• Higher DVB resins used when oxidant concentration is high > 0.2

ppm

SAC

DI – Weak Base Anion

• Removes only strong acids

• Very efficient regeneration

– 1.20 to 1.30 eq of NaOH per equivalent of operating capacity

• Use Rule of thumb

– Total FMA >50% of total anions

– High organics protect SBA

R-N: + H+Cl- R-N: HCl

CO2

WBA

DI – Strong Base Anion

• Resin types

• Type 1 vs. Type 2

• Acrylic vs. Styrenic

• Gel vs. Macroporous

• UPS vs. Gaussian

• High solids vs. Low solids

• Selection criteria

• Temperature

• Organic loading

• Chemical efficiency

• Silica leakage

• Silica load

CO2

SBA

• Many choices

Throughput/Run Time

• Feed water can vary greatly

• Throughput/run time varies with amount of contaminants in

water

– Higher load, lower throughput

– Lower load, higher throughput

• Flow distribution problems

• Resin degradation

• Excess loading of other contaminants

– Fe

– NOM

Water Quality

• Measured by instrumentation and testing

– Instruments need routine calibration

– Testing very important to ensure water quality measured by

instruments

• Boiler reliability and Process reliability dependent on

effluent water quality

Regeneration Steps

• Backwash

• Chemical Injection

– Acid and Caustic

• Chemical Displacement

– Also called slow rinse

• Fast Rinse

Backwash

• Necessary on every regeneration for Co-flow Cation

and Anion Systems

– Must remove particulate matter and Fe from cation

– Must remove anion fines from anion

– Counter-flow systems only backwashed when necessary

• Poor backwash can create pressure drop and flow

distribution problems

– Lower throughput/shorter runs

– Lower water quality

Backwash

• Extreme care must be taken to prevent loss of resin;

ESPECIALLY anion resin

• Expand bed to 1 to 1.5 feet below the outlet

– Ideally 80% expansion, minimum 60%

• Backwash time starts when bed is fully expanded

• Minimum of 20 minutes

• Water temperature affects expansion!

Backwash

Chemical Injection

-Critical parameters

• Quantity of Acid or Caustic

• Concentration of Acid and Caustic

• Temperature of Caustic

• Flow Rate of Acid and Caustic

• Contact Time

Chemical Injection -Quantity of Acid or Caustic

• Must use correct amount of chemical

• Need mass action to drive equilibrium

• Direct effect on throughput/run time

• Direct effect on leakage in co-flow vessels

Chemical Injection -Concentration of Acid and Caustic

• As with quantity of chemical, concentration is critical

for mass action

• Weak Acid Cation < 0.7% H2SO4

– Critical to prevent CaSO4 precipitation

• All other resins typical is 4-8%

– Below 4% effectiveness decreases

– Above 8% high potential for resin breakage

Chemical Injection

-Resin breakage

• Resin shrink and swell as they change forms

– Osmotic stress on the resin is very strong

– Shrink/swell too fast causes beads to break

• WBA are particularly susceptible

– Regenerate very easily, shrink very fast

– Macroporous to increase strength, but still not enough

– Least physically stable resin in the system

– Backwash critically important for good WBA performance

Chemical Injection

-Temperature of caustic

• For more efficient SiO2 removal and prevention of SiO2

precipitation

• Typical temperature 120°F for co-flow systems

• Hot water alone can remove SiO2 from the anion

Chemical Injection -Flow rate

• For cation, flow rate critical to keep

precipitation from occurring inside bed.

• For all other resins, typical flow rate

0.25 – 1 gpm/ft3 (2-8 BV/hr)

– Too low flow rate results in poor

distribution of chemical throughout the

bed.

– Too fast flow rate results in inefficient

regeneration and waste of chemical

Chemical Injection -Contact time

• Given quantity, concentration, and flow rate, contact time

is set

• Needs to be >20 minutes

• Balanced to maximize effectiveness, minimize waste,

and minimize precipitation.

Chemical Displacement

• Continuation of chemical injection

• Must be done at same flow rate as chemical injection

• Completes regeneration

• Rinses out bulk of chemical

• If fast rinse is started too quickly, fast rinse time may

be extended due to mixing in vessel.

Fast Rinse

• Typically performed at service flow rate

• Rinse to final conductivity and SiO2 requirements

• Can be extended for many reasons

– Resin degradation

– NOM loading on anion resin

– Fe loading on cation resin

– Poor flow distribution

Thank You!

For more information please visit our web site or

contact your local Dow representative.

http://www.dowwaterandprocess.com/