Ion Exchange (IEX) Process - دستگاه تصفیه آب · Ion Exchange (IEX) Process Mohammad...

Ion Exchange (IEX) Process

Mohammad Mahdi BazriChemical and Biological Engineering Department

University of British ColumbiaVancouver, BC, CANADA

December 2011-Tehran-Iran

Water analysis

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Introduction3

Initially developed around 5o’s A powerful technology to soften and demineralise water Industrial and domestic applications Industrial and drinking water treatment applications Chemistry and chromatography

Applications Water Treatment Food Industry

• Sugar industry• Dairy products

Chemical Industry• Metal recovery• Selective removal of various elements

Pharmaceutical Industry• Extract and purification of drug

Other Applications• Mining • Enzyme immobilization• Analytical chromatography

Fundamentals5

Exchange of the ions on the resin structure with the ones present in the solution

Schematic cation and anion exchange resin beads

Fundamentals6

Exchange of the ions on the resin structure with the ones present in the solution

Schematic cation and anion exchange resin beads

Resin Structure7

Polystyrene

Polyacrylic Weakly anionic

Strongly anionic

DVB

Resin Types8

Remove hardness/alkalinity and other cations:

• Weakly Acidic Cation Exchange Resins (-COOH)• Strongly Acidic Cation Exchange Resins (-HSO3

- )

Remove anions (NO3, SO4, HCO3) and NOM:

• Weakly Basic Anion Exchange Resins (-N(CH3)2)• Strongly Basic Anion Exchange resins (-N(CH3)3 OH)

o Type I (trimethyl amine)o Type II (dimethyl ethanol amine, lower basicity, better

regenerability)

Terminologies: capacity

Exchange capacity (eq/L)

The number of ion exchange sites

Operating Capacity (useful capacity)

The number of ion exchange sites where exchange has really taken place during the loading run.

Operating capacity < Total capacity

The typical operating capacity of a weak base anion exchange resin is 70 to 90 % of the total capacity.

For weak acid cation resin, operating capacity depends on several parameters, so there is no such simple estimate.

The typical operating capacity of SAC and SBA resins is about 40 to 60 % of their total capacity.

Operating capacity depends on:

Concentration and type of ions to be adsorbed

Service flow rate

Temperature

Type, concentration and quantity of regenerant

Type of regeneration process (co-flow, reverse flow)

Bed depth (reverse flow regeneration only)

Particle size of the ion exchange resins

Performance data and computer programs

Terminologies: exchange zoneThe area between fully regenerated and fully exhausted resin is called exchange zone or reaction zone.

Ion leakage endpoint of run

Weak acid and weak base resins are sensitive to flow rate, higher flow rate longer reaction zones.

SAC and SBA resins are less sensitive to flow rate.

Shorter reaction zone, higher utilization of total capacity , higher achievable operating capacity.

Fine resins have generally higher kinetics: shorter path for the ions to travel inside the resin beads

Water SofteningRemove hardness mainly Ca+2 and Mg+2

Strongly acidic cation exchange resin in Na+ form

Applications• Domestic and industrial water boilers• Laundries, Dish washer• Soft drink plants

Resins used• Amberlite IR120 Na; SR1L Na , AmberjetTM 1000 Na

Treated water quality• Residual hardness < 0.02 meq/L (1 mg/L as CaCO3)

with reverse flow regeneration Regeneration: brine (NaCl as a 10 % solution)

Water Softening

SAC (Na+)

Water Softening

The water salinity is unchanged, only the hardness is replaced by sodium.

Sodium salts have much higher solubility, so they don't form scale or deposits.

A small residual hardness is still there, its value depends on regeneration conditions.

Water De-alkalizationRemove bicarbonate and temporary associated hardnessWeakly acidic cation exchange resin in H+ form

Applications• Beverages, Soft drink plants, municipal water

Resins used• Amberlite IRC86 (industrial), Amberlite PWC13

(municipal), ImacTM HP333 and HP335 (household filter cartridges)

Treated water quality• Endpoint at 10 % of the raw water alkalinity• Contains CO2 which requires degasifier

Regeneration: Acid (preferably HCl at 5 % concentration)

Degasification2 R-H + Ca (HCO3)2 R2-Ca+2 H+ + 2 HCO3

-

H+ + HCO3CO2 + H2O (CO2 solubility @ 25 : 1.5 g/L)

• Reduce the ionic load thus regenerant• CO2 residual 10 mg/L

Degasifier

Degasifier• Atmospheric degasser (bicarbonate + CO2 < 0.6

meq/L)

• Forced draft degasser

• Thermal degasser, (O2-CO2)

• Vacuum degasser, (1-5 kPa, O2-CO2)

• Membrane degasser

Small size, for RO permeates with low pH and high

free CO2, small demineralization systems

Water DemineralizationRemove all the ions from water CIX (H+) - AIX (OH-)Treated water contains only traces of sodium and silica

Applications• Beverages, soft drink plants, municipal water

Resins used• Amberlite IRC86 (WAC), Amberlite IR120 or Amberjet

1000 or 1200 (SBA), Amberlite IRA96 or IRA67 (WBA), Amberlite IRA402 or Amberjet 4200 or 4600 (SBA)

Treated water quality (lower than RO or Distillation)• Conductivity: 0.2 to 1 µS/cm • Residual silica 5 to 50 µg/L

Regeneration: strong acid and caustic soda

Operational aspect Column Operation

• Co-flow regeneration• Counter flow regeneration

Batch (well-mixed)• MIEX, other IEX resins

Fluidized or Suspended • MIEX or any other IEX resin

Column Operation

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Column operationFreeboard columns

• Air or water hold down vessels (requires inert resin)• Stratified (weak/strong mix) vessels• Split flow regeneration unit (avoid disturb, hard to adjust)

Packed bed columns• Very little extra space, Smaller and cheaper• Usually higher depth• Floating bed with almost not fluidization • WAC/SAC and WBA/SBA are separated via a plate with

nozzlesMix beds

• Hard to regenerate • Not very efficient due to shallow depth

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Practical limitations

Affinity difference

Effective for low concentrations (brackish or sea water )

Only ionized targets can be eliminated

Regeneration (cost, environmental burden)

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Practical limitations

Affinity difference

Effective for low concentrations (brackish or sea water )

Only ionized targets can be eliminated

Regeneration (cost, environmental burden)

Regenerant

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• NaCl or KCl (10% ) for SAC and SBA• HCl for nitrate removal• Demineralization (SAC)

o HCl (5%), efficient no precipitationo H2SO4 (0.7-6%), cheaper and easier to store , less

efficient, potential for precipitationo HNO3, exothermic reaction, dangerous, not

recommended

• De-alkalization WAC , HCl, H2SO4 (0.7%)• SBA , NaOH (4%)• WBA, NaOH, NH3, bicarbonate

Regeneration

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Co-flow regenerationReverse exchange (displacement of low affinity with high affinity ions)Requires large excess of solution to fully regenerateLeakage in the next run

Counter-flow regenerationHigher reg. Efficiency, lower reg. quantityLower elution leakageLower resin inventoryImproved water qualityHard to keep the resin bed consolidatedResin mixingPurity of the regenerant

Regeneration steps

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• Backwash the bed to remove the suspended solids (co-flow)• Regenerant injection at low flow-rate (20-40 minutes)• Rinsing • Washing with service water

Regeneration ratioAmberjet 1200 regenerated with 55 g HCl per litreoperating capacity : 1.20 eq/L 55 g/L HCl = 55/36.5 = 1.507 eq/L Regenerant ratio = 1.507/1.20 = 1.26 = 126 %

Regeneration ratio

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• WAC resins: A safe number is 105 to 110 %.

• WBA resins: 115 to 140 %, due to presence of some strongly basic functional groups.

• SAC resins: Regenerated with sulphuric acid, larger excess (at least 40%) than those regenerated with HCl.

• SBA resins: Depends on the type of SBA resin (styrenictype I, type II or acrylic resins). Not economical to regenerate the resin totally.

• SAC and SBA resins: Larger excess than their weak counterparts.

Thoroughfare regeneration

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Throughput32

Volume produced until the cartridge has to be replaced

• Salinity of the feed water • Volume of resin in the cartridge (throughput is

approximately proportional to resin volume) • Type of resin used • Quality and efficiency of the off-site regeneration process • Endpoint (conductivity at which the unit is considered

exhausted)

With good resins and good regeneration, the throughput can be approximately calculated as:

Throughput [L] = 500 × (Resin volume [L]) / (Salinity [meq/L])

Kinetics33

Kinetics34

In normal practice, solution is withdrawn at the outlet of the column so the equilibrium is permanently shifted to the right.

R-A+ + B+ R-B+ + A+

Na+-form SAC resins initially remove divalent cations fromwater, but not other monovalent cations (selectivity)

2 R-Na + Ca+2 (HCO3–)2 R2-Ca + 2 Na+ HCO3

–

Selectivity35

WAC (H-form) resin removes only hardness, and only when alkalinity is present.

For softening WAC resin must first be converted to the Na-form with an alkali.

OH– form resins are not used to remove bicarbonate or carbonate from neutral water when it contains hardness (precipitation risk)

Selectivity36

OH-form resin reacts with very weak acids, such as silica (SiO2) or boric acid (H3BO3).

Special SBA resins with different functional groups are used with an increased nitrate, sulphate, perchlorateselectivity.

R-OH + H+HSiO3– R-HSiO3 + H+OH–

Selectivity37

Selectivity38

Selectivity39

Selectivity40

Selectivity41

Selectivity42

Selectivity43

Nitrate removal

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RSBA-Cl + NO3- RSBA-NO3 + Cl-

SAB (Cl-)

Amberlite PWA5 –PWA15Purolite A520E

SIR-100-HP

Methemoglobinemia, also known as “blue baby syndrome

Nitrate removal

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Anion resins are less dense and require a backwash flow rate about 1/3 that of softening resin.

salt concentrations are typically applied at 4 percent to 8 percent.

Regenerant levels must be high enough in nitrate removal to ensure that nitrate leakages are kept to acceptably low levels.

Natural Organic Matter

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Removal Mechanisms by IEX resins :• Adsorption (blocking exchange sites)• Size Exclusion (blocking exchange sites)• Ion exchange (favourable)

Easier regeneration (lower conc., lower flow rate/contact time)

Hydrophobic Hydrophilic

Neutral

Very Complex Mixture

IEX for NOM removal47

• A relatively recent concept

• Polystyrene resins are more hydrophobic /selective thanPolyacrylic resins.

• Polystyrene resins have shown stronger affinity witharomatic moieties.

• Competition kinetics exist between NOM and inorganicIons

SO4 > NO3 > NOM > Cl > HCO3 > OH > F

Important characteristics of resin48

• Functional group (strong or weak basic)

• Backbone structure (acrylic or styrene)

• Resin water content

• Physical form (gel or macroporous)

• Bead size

• Treatment process

IEX for NOM removal49

• Efficient reactor design

• Enforce IEX as the dominant removal mechanism• Shorter contact time

• Combined different IEX resins

• Enhance the NOM removal• Combination of weakly-strongly basic IEX resin• Combination of Polystyrene-Polyacrylic IEX resins

• Microbial growth on IEX resins

• Disinfection• set-up configuration