ION EXCHANGE

WATER TREATMENT PROCESSESProcessDriving force

Ion Exchange (IX)ChemicalElectrodialysis (ED)ElectricalReverse Osmosis (RO)PressureDistillation (MSF)Thermal

WATER TREATMENT PROCESSES

Process

Feed TDS

Product TDS

mg/I max.

mg/I

IX

1500

ION EXCHANGEThe feedwater to an ion exchange system must meet the following specificationsSuspended solidsNilTurbidity

ION EXCHANGE PROCESSES

Process

Function

Softening

Substitutes hardness with sodium

Dealkalization

Removes hardness

Reduces alkalinity

Demineralization

Reduces TDS

SOFTENINGService stage is represented by the following reactions Ca(HCO3)2(l) + 2 NaR(s) CaR2(s) + 2NaHCO3(l)MgCI2(l) + 2 NaR(s) MgR2(s) + 2NaCl (l)Hard Regenerated Exhausted SoftWater ResinResin Water

SOFTENINGThe regeneration stage is represented by the following reactions CaR2(s) + 2 NaC1(l) 2NaR(s) + CaCl2(l) MgR2(s) + 2 NaCl (l) 2NaR(s) + MgCl2(l) Exhausted Regenerant Regenerated Waste resin (Common Salt) resin effluent

DEALKALIZATIONThe dealkalization processes are

Split stream dealkalization

Dealkalization with a weak acid cation (WAC) resin.

Combination of WAC and softener

SPLIT STREAM DEALKALIZATION

SPLIT STREAM DEALKALIZATIONReactions during servicea) SoftenerCa(HCO3)2(l) + 2 NaR(s) CaR2(s) + 2NaHCO3(l)MgCI2(l) + 2 NaR(s) MgR2(s) + 2NaCl (l)Hard Regenerated Exhausted Softwater resinresin water

SPLIT STREAM DEALKALIZATIONReaction during serviceb) HI UnitCa(HCO3)2(l) + 2HR(s) CaR2(s) + 2 H2CO3(l)Temporary Regenerated Exhausted Carbonichardness resin resin acidMgCl2(l) + 2HR(s) MgR2(s) + 2 HCl (l)Permanent Regenerated Exhausted Freehardness resin resin mineral acid Na2SO4(l) + 2HR(s) 2NaR(s) + H2SO4(l)Sodium Regenerated Exhausted Freesalt resin resin mineral acid

SPLIT STREAM DEALKALIZATIONReaction during servicec) BlendNaHCO3(l) + HCl (l)NaCl (l) + H2CO3(l)2NaHCO3(l) + H2SO4(l)Na2SO4(l) + 2H2CO3(l)Alkalinity FreeNeutral Carbonic mineral acidsaltacid

Carbonic acid is unstable and dissociates to carbon dioxide.H2CO3(l) H2O(l) + CO2 (g)The carbon dioxide is stripped in an atmospheric degasser.

SPLIT STREAM DEALKALIZATIONa) SoftenerCaR2(s) + 2NaCl (l) 2NaR(s) + CaCl2(l)MgR2(s) + 2NaCl (l) 2NaR(s) + MgCl2(l) Exhausted Sodium Regenerated Wasteresin chloride resin effluent

b) HI UnitCaR2(s) + 2HCl (l) 2HR(s) + CaCl2(l)MgR2(s) + 2HCl (l) 2HR(s) + MgCl2(l)NaR(s) + HCl (l) HR(s) + NaCl (l)Exhausted Hydrochloric Regenerated Wasteresinacid resin effluentReactions during regeneration

DEALKALIZATION WITH A WEAK ACID CATION (WAC) RESINa) Service

Ca(HCO3)2(l) + 2HR(s)CaR2(s) + 2H2CO3(l)Mg(HCO3)2(1) + 2HR(s)MgR2(s) + 2H2CO3(I)NaHCO3(1) + HR(s)NaR(s) + H2CO3(I)Alkaline Regenerated Exhausted Carboniccation resinresin acidReactions

...DEALKALIZATION WITH A WAC RESINb) Regeneration

CaR2(s) + 2HCl (l)2HR (s) + CaCl2(l)MgR2(s) + 2HCl (l)2HR (s) + MgCl2(l)NaR (s) + HCl (l)HR (s) + NaCl (l)Exhausted HydrochloricRegenerated Wasteresin acidresin effluent

Reactions

COMBINATION OF WAC AND SOFTENERThe WAC resin reacts with alkaline salts only and not with neutral salts in water.

The combination of WAC and Softener is used therefore when permanent hardness is present in water.

The WAC efficiently removes the temporary hardness and the downstream Softener exchanges the permanent hardness.

DEMINERALIZATIONBut there are several extensions and combinations of these basic processes which progressively improve the quality of water and the efficiency of the processes.There are two basic processes Two bed Mixed bed (MB)

...DEMINERALIZATIONThe reactions during service of the strong acid cation (SAC) exchanger are as follows Ca(HCO3)2(l) + 2HR(s) CaR2(s) + 2 H2CO3(l)Temporary Regenerated Exhausted Carbonichardness resin resin acidMgCl2(l) + 2HR(s) MgR2(s) + 2 HCl (l)Permanent Regenerated Exhausted Freehardness resin resin mineral acid Na2SO4(l) + 2HR(s) 2NaR(s) + H2SO4(l)Sodium Regenerated Exhausted Freesalt resin resin mineral acidCarbonic acid is unstable and dissociates to carbon dioxide.H2CO3(l) H2O(l) + CO2 (g)

...DEMINERALIZATIONThe reactions during service of the strong base anion (SBA) exchanger are as followsH2SO4(l) + 2ROH(s) R2SO4(s) + 2H2O(l)HCl(l) + ROH (s) RCl(S) + H2O(l)Free Regenerated Exhausted Demineralisedmineral acid resin resin waterCO2(l) + ROH(s) RHCO3(s)Carbon Regenerated Exhausteddioxide resin resinSiO2(l) + ROH(s) RHSiO3(s)Silica Regenerated Exhausted resin resinNaCl(l) + ROH(s) RCl(s) + NaOHCation Regenerated Exhausted P-Alkalinityslip resin resin

...DEMINERALIZATIONCaR2(s) + 2HCl (l) 2HR(s) + CaCl2(l)MgR2(s) + 2HCl (l) 2HR(s) + MgCl2(l)NaR(s) + HCl (l) HR(s) + NaCl (l)Exhausted Hydrochloric Regenerated Wasteresinacid resin effluentThe reaction during regeneration of the strong acid cation (SAC) exchanger are as follows

...DEMINERALIZATIONR2SO4(s)+2NaOH(l)2ROH(s) +Na2SO4(l)

RCl(s)+NaOH(l)ROH(s) +NaCl(l)

RHCO3(s)+NaOH(l)ROH(s) +NaHCO3(l)

RHSiO3(s)+NaOH(l)ROH(s) +NaHSiO3(l)

ExhaustedSodiumRegenerated Wasteresinhydroxideresineffluent

The reactions during regeneration of the strong base anion (SBA) exchanger are as follows

...DEMINERALIZATIONAnother two bed demineralization process utilises a weak base anion (WBA) exchanger after the strong acid cation (SAC) exchanger when silica removal is not required.

The reactions of the WBA resin during service are HCl(l)+R .(s)R HCl (s)H2SO4(l)+R .(s)R H2SO4 (s)Free mineralRegeneratedExhaustedacidresinresin

...DEMINERALIZATIONThe reactions of the WBA resin during regeneration are

R HCl (s) + NaOH(l) R .(s) + NaCl(l) + H2O(l)

R H2SO4(s) + 2NaOH(l) R .(s) + Na2SO4(l) + 2H2O(l)

Exhausted Sodium Regenerated Wasteresin hydroxide resin effluent

...DEMINERALIZATIONA partial list of demineralization treatment schemes.SAC- SBASAC- DG - SBASAC- DG - SBA - MBSAC- DG - SBA - CATPOLSAC- DG - WBA - SBA - MBWAC - SAC - DG - WBA - SBA - MBLBC - DG - LBA - MBDG - Degasser LBC - Layered Bed Cation LBA - Layered Bed Anion CATPOL - Cation Polisher

REGENERATION TECHNIQUESIndividual units - softener, cation, anion - are regenerated by the coflow or counterflow method

Pair of units - WAC - SAC, WBA - SBA - are regenerated in thoroughfare mode with the regenerant flowing from the secondary unit to the primary unit.

Mixed beds are regenerated sequentially or simultaneously.

COFLOW/COUNTERFLOW REGENERATIONThe direction of flow of the regenerant can be the same as that of the service flow or opposite to it.

When the regenerant flow is in the same direction as the service flow the regeneration process is called cocurrent or coflow regeneration.

When the regenerant flow is in the direction opposite to the service flow, the regeneration process is called countercurrent or counterflow regeneration.

FLOW DIAGRAM OF COFLOW UNITSERVICECOFLOW UNIT

INTERNAL ARRANGEMENT OF COFLOW UNIT

COFLOW REGENERATIONCoflow regeneration comprises the following basic steps

Backwash Regenerant injection Rinse

The rinse step may include a slow rinse followed by the final rinse

COUNTERFLOW REGENERATIONCounterflow regeneration provides superior quality of treated water as compared to coflow regeneration.

Consequently counterflow regeneration results in a lower consumption of chemicals as compared to coflow regeneration.

Counterflow regeneration needs some special requirements.

COUNTERFLOW REGENERATIONThe key requirements for a successful counterflow regeneration are

Use of treated water (from the unit) for dilution and injection of regenerant and in the subsequent rinse stage.

Maintenance of a static bed during the regeneration process.

Avoidance of backwashing of the resin bed at every regeneration.

Provision of an additional collector for spent regenerant and maintenance of an inert/ dead/ sacrificial resin layer above this collector.

FLOW DIAGRAM OF COUNTERFLOW UNITSERVICECOUNTERFLOW UNIT

INTERNAL ARRANGEMENT OF COUNTERFLOW UNIT

COUNTERFLOW REGENERATIONCounterflow regeneration comprises the following basic steps Regenerant injection Regenerant rinse Final rinse

Prior to regenerant injection a flushing of the spent acid collector is done to facilitate smooth regeneration.

Counterflow units are backwashed once in seven regenerations. After the backwash the regeneration proceeds as above with twice the quantity of regenerant (normally used) injected into the unit.

REGENERANT INJECTION CONCENTRATIONSThe table below lists the regenerants and the injection concentrations into various units.Regenerant Unit Concentration (% w/v max.)NaClSoftener15HClWAC/SAC/MB5H2SO4WAC0.8SAC/MB1.5 - 5NaOHWBA/SBA/MB5WBA-SBA2 - 4

THOROUGHFARE REGENERATIONIn thoroughfare regeneration, the primary invariably is a coflow unit while the secondary may be a coflow or counterflow unit.

In WAC - SAC system, the WAC is regenerated coflow and the SAC is regenerated counterflow always.

In WBA - SBA pair of units, the WBA is regenerated coflow while the SBA may be either coflow or counterflow regenerated dependent on the silica residual desired.

MIXED BED REGENERATIONMixed Bed regeneration comprises the following basic steps.

BackwashRegenerant injectionRegenerant rinseAir mixFinal rinse

When the regenerants are injected and rinsed in sequence, the regeneration is termed sequential.When the regenerants are injected and rinsed simultaneously, the regeneration is termed simultaneous.

The simultaneous regeneration results in a saving of time and waste water besides valving.

FLOW DIAGRAM OF MB SEQUENTIALSERVICEMIXED BED - SEQUENTIAL

FLOW DIAGRAM OF MB SIMULTANEOUSSERVICEMIXED BED - SIMULTANEOUS

INTERNAL ARRANGEMENT OF MIXED BED UNIT