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Table 1 Major water treatment processes

Process Resins used Significant property Application areas

Softening SAC Hardness selectivity Domestic, industrial processes,

food processing, etc.

Partial softening

Dealkalization

WAC

WAC

Temporary hardness

removal and

alkalinity removal

Potable water, beverage industry,

industrial processes, laundry, glass

washing

Demineralization WAC, SAC Removal of cations Industrial water processes

WBA, SBA Removal of anions Steam generation, food processing

industry, process water for pharma-

ceutical use, etc.

Nitrate removal SBA Nitrate selectivity Potable water, food and beverage

processing

Metals removal WAC chelate resins Selectivity for heavy metals Wastewater treatment

Sorption Macronets WBA, SBA Selectivity for organics Organic scavenging

Key: WAC, weak acid cation resin; WBA, weak base anion resin; SAC, strong acid cation resin; SBA, strong base anion resin; chelate,

chelating ion exchange resin; macronet, special resin with adsorption properties.

WATER TREATMENT

Overview: Ion Exchange

J. Irving, Purolite International Limited, Pontyclun,Mid Glamorgan, Wales, UK

Copyright^ 2000 Academic Press

Introduction

Ion exchange resins are used for many water treat-ment applications. Of these applications, in termsof the volume of resins used, water softening anddemineralization of water are the most signiRcant.

Water softening has been practiced commercially fora century or more, making use of a wide range of natural and synthetic products. As the variety of usesfor puriRed water has increased, so has the need tosoften and demineralize water. Demineralization hasonly been practiced since the discovery of syntheticanion exchange resins in the 1920s. Their useful-ness increased greatly with the invention of stronglybasic anion exchange resins, which can remove weak-ly acidic compounds such as silica and carbon diox-ide, as well as mineral acids. This process of ionexchange can be used as a simple method to producewater of very high purity. In general, as industrialand domestic requirements have grown, speciRca-

tions for water quality have become progressivelymore stringent, and regulations to enforce these have

become more strict. Hence the choice of resin typesfor a particular application becomes increasinglycomplex.

Applications of Ion Exchange

in Water Treatment

A wide variety of new water treatment applicationsemploy ion exchange resins in limited volume andthere is limited use in niche areas for many specialresin types. However, reverse osmosis (RO) is

increasingly being used instead of ion exchangewhere treated water quality requirements are notparticularly high. The use of RO followed by anion exchange polishing process is often used forthe production of high purity water, for examplein the manufacture of silicon chips for the computerindustry. Table 1 lists the major water treatment pro-cesses in which ion exchange resins are used.

Principles of Ion Exchange Applicable

to Water Softening

The properties and theoretical principles of ion ex-change resins are fully covered elsewhere. This article

III / WATER TREATMENT / Overview: Ion Exchange 4469


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discusses only those principles that directly relateto operating performance of the ion exchange resinsthat are currently used in water treatment.

Ion Exchange Equilibria

Water softening is a very ef Rcient process. Water

containing hardness ions (calcium and magnesium) ispassed through a cation exchange resin in the sodiumform. In dilute solution, the hardness ions are selec-tively held:

KCaNa"([CaR]/ [Na2R])([Na

2S]/ [CaS])CS/ CR [1]

where K is a simpliRed selectivity coef Rcient describ-ing the equilibrium. [Ca] is the calcium concentra-tion, [Na] is the sodium concentration, and C is theoverall ionic concentration. Subscripts R and S rep-resent resin and solution phase, respectively. This

equation takes no account of activity coef Rcients, butnevertheless can be used, certainly for comparativepurposes, and usually gives fairly accurate predic-tions. Clearly the larger the value of KCaNa, the greaterthe fraction of calcium residing in the resin phase.Tables of selectivity coef Rcients have been compiledfor a wide variety of cations and anions.

Selectivity for Hardness in Softening and Resin

Regeneration

The more dilute the ionic concentration, the higherthe calcium (and magnesium) fraction in the resin

phase, and the less calcium is needed in the solutionphase to satisfy the equation. However, when consid-ering regeneration, the ionic concentration in solutionis much higher, so as CS/ CR tends to a value greaterthan 1, so less calcium is needed in the resin phase tosatisfy the equation. The poorer selectivity of theresin sites for calcium present in high ionic concentra-tions ensures that the regeneration stage is veryef Rcient. The principle of this equation appliesto all comparisons between monovalent ions such assodium and divalent ions such as calcium. Since mag-nesium, the other main contributor to total hardness,is also divalent, the principles described apply equallyto magnesium. In order that the regeneration processis reasonably ef Rcient, there must be an excess of sodium ions.

Reverse Osmosis

One of the advantages of RO is that no regenerantchemicals are required. The disadvantages of RO isthat capital costs are higher and pumping costscan also be high. In addition, the volume of the rejectwaste can be large, even though the ionic concentra-

tion within the reject is quite low. There is also a risk

of membrane fouling. Generally when the total dis-solved solids (TDS) in the treated water are high, ROis preferred. However, as both processes are constant-ly changing in ef Rciency, the commercial breakevencost point is constantly changing.

Water Quality and Regeneration Ef\ciency

Clearly the use of smaller quantities of regenerantwould make the ion exchange softening process moreef Rcient and competitive. It has been demonstratedthat the ion exchange process must be carried out ina column if the ef Rciency is to be optimized. Forexample, regenerating calcium from a resin with so-dium chloride solution simply by adding the regener-ant to the resin while stirring in a beaker is veryinef Rcient. It suffers from the disadvantage that allthe calcium displaced from the ion exchange resin hasthe opportunity to re-enter the resin and re-occupy

the ion exchange sites. On the other hand, in a col-umn operation, the displaced ion is carried away, andcannot return to the same beads at the regenerationentry point. As more fresh regenerant is added thissame principle applies to the exchange process furtherdown the column. It has been shown that, as theregenerant contact time in the column was increasedto 24 h or more, the ef Rciency of regeneration de-creased to the point where some 25}30% fewer siteswere regenerated with the same quantity of regener-ant. The Rnal lower value tended asymptotically tothat obtained under batch equilibrium conditions.

Counter]ow and Co-]ow Regeneration

A second advantage of column operation is that thosebeads situated at the point of entry of the regenerantwill come in contact with a vast excess of pure regen-erant. This ensures that almost all of the hardnessions loaded in the previous cycle are carried awayfrom that part of the resin bed. In a stirred batchsystem, the ratio of regenerant ions to hardness ionswould be approximately equal in every bead, depend-ing on the quantity and concentration of regenerant

used. It follows that since the water being treated is incounterSow to that of the regeneration, this allowsthe treated water to pass the most highly regeneratedand rinsed resin at the point of exit, thus ensuringnear zero leakage of hardness. Of course, the resinbed must not be disturbed for this advantage to apply.In co-Sow regeneration, any regenerant at the treatedwater outlet has previously been in contact with theions to be removed, hence this part of the column isleast ef Rciently regenerated. In the following exhaus-tion cycle the sodium in the water at the outlet can‘back-exchange’ for the hardness residual at the col-

umn outlet. This results in a signiRcant hardness leak-

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age. To optimize the operating ef Rciency, the use of regenerant chemical should be cut to a minimum.This has the added advantage that less excess regener-ant will pollute the environment. From the abovearguments, it is easy to see why counterSow regenera-tion is more ef Rcient.

Diffusion in the Regeneration Stage

Returning to eqn [1], it can be shown that reductionof the regenerant quantity to a minimum can createsigniRcant disadvantages in the softening process.When the regenerant enters the column, it will inevi-tably suffer some dilution with the water it displaces.As has already been explained, the lower the ionicconcentration in solution, the higher the selectivityfor the divalent hardness ions, so any dilution willreduce the ef Rciency of the regeneration process.However, this dilution at the start of the regeneration

process is not a signiR

cant disadvantage, because theregenerant is in contact with resin heavily loaded withhardness ions and thus some of these ions are easy toremove. However, as the regeneration proceeds, theconcentration of regenerant increases and the beadsare increasingly invaded by sodium chloride. This istermed ‘Donnan invasion’. In fact, the Rrst moleculeof sodium chloride is not effective in achieving anyregeneration of divalent hardness because only one of the two hardness bonds is released. The positivecharge of the single displaced calcium ion is satisRedwith the negatively charged chloride ion.

R}Ca2#}R#NaCl"R}Na##R}Ca2#}Cl

[2]

where R is the matrix and functional group of thestrong acid cation (SAC) resin.

A second molecule of sodium chloride is clearlyneeded to free the hardness (calcium) ion and soallow it to start the diffusion process towards theoutside of the beads.

R}Ca2#}Cl#Na#Cl"RNa##Ca2#Cl2

[3]

The general direction of the regenerant is towards thecentre of the beads as the diffusion of the regenerantproceeds from the solution surrounding the beads. Anyhardness ion has to move against the regenerant cur-rent to leave a particular bead. Once the regeneranthas completed its diffusion path to the centre of eachbead, then the outward diffusion of the regeneratedhardness can presumably proceed more rapidly.

As the regeneration draws to a close, the regenerantis followed by the displacement rinse. Dilution of the

regenerant by the displacement water will cause selec-

tivity reversal, thus any hardness ion still situatedinside the beads, and travelling towards the outside, willpromptly displace two regenerated sodium sites. If theregenerant quantity is cut to a minimum, then therelease of the calcium will be slower (see eqn [2]) andthe proportion of diluted regenerant to concentratedregenerant will be greater. It follows that the shorter

difusion path offers a more effective regeneration result-ing from improved column ef Rciency.

The use of beads of a narrow size range and core shellbeads with a limited diffusion path offers a superiorperformance; this has already been seen.

Resin Kinetics

Beads of narrow size range can also present a largersurface area of exchange to the water being treated,and therefore the kinetics of exchange may be im-proved. Larger beads with a shell core formation also

provide ease of access to each individual ion exchangesite. These features can be important where high Sowrates are used. Other signiRcant factors such as resinbed depth, operating temperature, ef Rcient distribu-tion and collection of the water being treated are alsoimportant.

Principles of Ion Exchange Applied

to Demineralization

The principles applicable to the softening process alsoapply to demineralization. It is useful to discuss some

of these in more detail.

Resin Selectivity

Demineralization requires an ion exchange process of at least two stages. In the Rrst essential stage, thecations in the water to be treated are replaced withhydrogen by passing the water through strong acidcation (SAC) resin in the hydrogen form. When theresin becomes exhausted to the extent that the wateris not treated to the required quality, it must beregenerated with acid.

The Rrst stage of the demineralization process maybe improved to combine the use of both a SAC resinand a weak acid cation (WAC) resin. The latter ispositioned upstream. This is one of the many ways inwhich the ef Rciency of this regeneration process canbe varied. Returning to the properties of SAC resins,the process is unlike softening in that there are noadvantages from changes in ionic concentration whenregenerating sodium. The selectivity for sodium, ascompared with hydrogen, may be simpliRed by thefollowing equation.

KNa

H"([NaR]/ [HR])([HS]/ [NaS]) [4]



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where H is the hydrogen ion; all other symbols as foreqn [1].

It has been reported that KNaH varies according tothe cross-linking of the SAC resin.

Counter]ow Regeneration

The advantages to be obtained in water quality fromoperation in the counterSow mode are, perhaps, evenmore important than for softening. The differencebetween the selectivity of sodium and hydrogen isvery small (in fact the coef Rcient lies between 1 and2). Hence any residual sodium located near the outletof the bed is easily displaced. Low sodium leakage isclearly an essential parameter where high water pu-rity is required. It follows that either the whole resinbed must be highly regenerated, or the resin bed hasto be operated in the counterSow mode. This ensuresthat the treated water at the bed outlet is in contact

with highly regenerated resin containing only minutetraces of sodium.

Mixed Bed

The use of a mixture of SAC resins and strong baseanion (SBA) resins, regenerated separately beforemixing, has generally been considered the best way toachieve treated water of the highest purity. The pas-sage of water alternately via cation and anion resinsaffords the more or less continuous neutralization of acids and bases produced by contact with the pre-vious bead of opposite charge. The disadvantage is

that the component resins have to be separated beforeregeneration. Incomplete separation will cause theoffending beads to be regenerated with the wrongregenerant, resulting in a signiRcant deterioration intreated water quality. Techniques of rinse recycle andef Rcient counterSow regeneration are now producingwater qualities close to that of mixed bed polishing.

Effects of Sodium Leakage

One further problem occurs if the sodium leakage ishigh. The treated water from the cation resin outlet

(decationized water) passes through the anion resin,regenerated to the hydroxide form. In general, this isa very ef Rcient reaction, because the process is one of neutralization of acids, so the exchange reaction isessentially non-reversible, and the only by-product of the neutralization reaction is water. However, anysodium leakage from the cation resin must be accom-panied by an anion to preserve electroneutrality. Atthe start of the cycle, when the anion resin is freshlyregenerated, the anion that accompanies sodium willusually be hydroxide. This is not particularly desir-able, but is easily removed by the following mixed

bed resins. However, as the resin bed exhausts, the

least selective anion, in exhaustion, will accompanythe sodium. This anion is usually silica. Silica cancause deposits in superheaters, boilers, turbines andcondensers, so accelerating corrosion. Thus it is clearthat sodium leakage carries a two-fold danger.

Anion Leakage

The very high selectivity of the anions of mineralacids for ion exchange resins usually prevents highmineral anion leakage, even though regeneration toremove all of these is rarely complete. As with soften-ing, the ef Rciency of regeneration is crucial if goodcapacity and low leakage are required. The use of narrow size range resins can shorten the diffusionpath in the regeneration process.

Regeneration Ef\ciency

The regeneration ef Rciency of cation resins has al-ready been discussed. In the demineralization process,the regeneration ef Rciency of anion resins is of evenmore importance.

During the actual water treatment process, theremoval of anions is essentially an acid}base neu-tralization. Hence it is driven rapidly more or lessto completion. However, the regeneration stageinvolves the exchange of the loaded ions for thehydroxide ion. Hence this stage is a true ion ex-change process where the ions being removed arein competition with the ion being Rxed on theresin. It follows that the extent of regeneration isthe limiting step, dictating the operating capacityof the resin. In fact, when operating SBA resins atrecommended Sow rates the operation capacity isnormally only 5}10% below the available regen-erated capacity. In other words, the chromato-graphic proRle is extremely sharp.

Type I SBA resins do not regenerate easily. In factthe selectivity coef Rcient KClOH is approximately15}20 for gel Type I SBA resins, and even higherfor equivalent macroporous types.

Type II and acrylic SBA resins are more easilyregenerated, but have the disadvantage that theyremove silica less ef Rciently (especially the Type II);both types have poor thermal stability.

Recently strong base resin types have been com-pared in their operating performance and relatedproperties. A Type III resin has been developed thatis comparable with Type II or acrylic resins in itsease of regeneration, while having thermal stabilityand silica removal similar to that of a Type I resin.

Weak base anion (WBA) resins regenerate quite eas-ily, and can be used in conjunction with SBA resins to

improve overall regeneration ef Rciency. However,



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Figure 1 Regeneration efficiency of chloride form SBA Type I

clear gel anion at 65 g L1 NaOH.

they do not remove silica or carbon dioxide, sothe proportion of WBA to SBA must be controlledto balance the needs of the process. Also the freshregenerant must be used to regenerate the resins inthe order SBAPWBA, otherwise the advantage islost.

The mechanisms for regenerability of anion resinshave been discussed in the literature. BrieSy, the highselectivity for chloride, sulfate, and nitrate arises fromthe fact that these ions are less hydrated than is thehydroxide ion. Hence the hydroxide ion prefers toremain in the aqueous phase. As Figure 1 shows, thelower the moisture retention of the resin, the moredif Rcult is the regeneration process.

These mechanisms also offer good explanationsfor the variation in thermal stability between resin

types. It has been shown that nucleophilic attack onthe nitrogen of the active group by the hydroxide ionis responsible for the thermal degradation. It followsthat the more hydrated the hydroxide ion, the lowerthe electron charge density and the lower the rate of degradation. This is conRrmed by experimental data.This subject is further complicated by the differencesin selectivity of chloride and sulfate ion in the regen-eration process.

Principles of Dealkalization

The removal of alkalinity is a less common processthan softening or demineralization, hence it will onlybe dealt with brieSy. It has long been recognized thathardness associated with bicarbonates (termed tem-porary hardness) is more of a problem than thatassociated with mineral acids (permanent hardness).When water is heated the bicarbonate decomposes tocarbon dioxide and insoluble calcium carbonate. Thisforms the scale that is found in the equipment used toheat or transport water. The removal of scale may beachieved by using a weak acid cation resin regen-

erated to the hydrogen form.

The reaction:

2RCOOH#CaCO38Ca(RCOO)2#CO2#H2O

[5]

proceeds quite easily, while the reaction

RCOOH#CaCl2Ca(RCOO)2#2HCl [6]

does not, because the ionized acid produced inhibitsfurther reaction.

The WAC resin is easily regenerated witha stoichiometric amount of acid. This makes the pro-cess chemically ef Rcient but it is kinetically slow. Italso has the advantage that the total dissolved solidsare reduced by the removal of the hardness. Thevolatile carbon dioxide can be removed by deaer-ation. The main disadvantage is that acid is neededfor regeneration.

An alternative process uses a SBA resin in thechloride form. Here the bicarbonate is exchanged forchloride directly. Thus the offending temporary hard-ness is exchanged for permanent hardness. There isno reduction in total dissolved solids and the operat-ing capacity is lower, but the advantage is that theSBA resin may be regenerated with common salt.

Principles of Nitrate Removal

The World Health Organization (WHO) limit forpotable water is 50 ppm of nitrate. Many water sour-

ces have higher levels, partly because of the use ni-trate-based fertilizers to boost crop yields. Althoughthis practice has been curtailed in recent years, theproblem will remain for many years to come. Ionexchange resins, regenerated with sodium chloride,selectively remove nitrate in preference to bicarbon-ate, but not in preference to sulfate. This means thatthe sulfate in the water is also needlessly removed.The exchange of both nitrate and sulfate can result inthe chloride level increasing above WHO limits.Where sulfate levels in the water are signiRcant, theuse of a specially developed nitrate-selective (oversulfate) resin can give increased capacity and betteroverall water quality.

Physical Stability of Ion Exchange

Resins

Ion exchange resins have to be physically strong tolast for the expected life span of 4}6 years, dependingupon the resin type, temperature of operation andregeneration, ability to resist irreversible fouling fromtrace contaminants in the water to be treated, and

adherence to the recommended operating conditions.



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Table 2 Typical water analysis information

Influent water specification

Origin Mains water

Pretreatments None

Temperature 83COrganic matter 20.000 mg L1 KMnO4

Cation Concentration

(meq L 1 )

Anion Concentration

(meq L 1 )

Other Concentration

(meq L 1 )

Ca 0.900 HCO3 0.580 CO2 0.010

Mg 0.140 CO3 0.000 SiO2 0.060

Na 0.300 Cl 0.450

K SO4 0.310

Fe NO3 0.000

TC 1.340 TA 1.340

TC, total cation concentration; TA, total anion concentration. Note that TC should equal TA. In general if there is a difference the

analysis is probably incorrect. If the pH is not neutral, the hydrogen and hydroxide ions should be included in the balance.

This means that they have to be mechanically strongand also able to withstand rapid changes in swell-ing/ shrinking arising from changes in ionic form andchanges in ionic concentration. It is most importantthat the ion exchange plant is designed to accommod-ate the anticipated changes in bed depth, and resinsshould be free to adjust to the volume changes occur-

ring naturally through the process. Tests have beendeveloped to evaluate osmotic shock resistance; theseare mentioned in other articles.

The elimination of the larger beads in the resin bedwill reduce the chances of breakdown resulting fromosmotic shock, because the build-up of differentialstress with changes in ionic concentration is less insmaller beads. Up to now partially activated beads,which have the advantages of a smaller diffusion pathdescribed above, have not been physically stable be-cause of the stresses developing between the swellingand shrinking of the activated part of the resin, com-pared with the properties of the inert core. There areindications that this dif Rculty is now being overcome.

Plant Design

Water analyses vary considerably, depending on thesource of the water supply and the geographical area.Thus it has been impossible over the years to standar-dize on one type of plant or on which are the mostsuitable resins. Table 2 gives on example of the wateranalysis information needed to design a treatmentplant. Changes in total concentration of dissolvedsalts, the proportion of sodium and silica (as alreadydiscussed), the temperature of the water, the ratio of alkalinity to total anions, and the proportion of sul-fates, chlorides and nitrates, all have an inSuence onthe operating capacity and treated water quality.

The presence of iron can severely affect the perfor-mance of softeners. Iron can slowly accumulate andcause irreversible fouling. It also acts as a catalyst,promoting oxidation of resins that causes an increasein moisture retention and swelling of the resin.This in turn can cause stress within the resin bed,not only on the resin itself, but also on the internal

collectors and distributors. The performance of de-mineralizers can also be affected, especially if onlysulfuric acid is available for resin regeneration andresin cleaning.

Natural organic matter can vary both in quantityand quality from area to area, hence the choice of resins and preferred cycle times, the quantity of so-dium hydroxide used for regeneration and its temper-ature as well as cleaning regimes, can vary fromregion to region.

Basic Principles

Developments in ion exchange resins since the 1940shave been accompanied by developments in engineer-ing equipment and processes, from the invention of styrene-divinyl benzene resins to advances in tech-niques for TOC removal. The latter is now of utmostimportance for the production of ultrapure water.

The basic principles of sizing an ion exchangevessel are quite simple. Let us suppose that it isnecessary to treat F m3 of water per hour, and it isrequired to remove C eq m3. Then the load per houris FC mol, and FCh is the total load presented for anexhaustion cycle of h h. It is therefore possible tocalculate the volume of resin needed, provided theoperating capacity (O eq m3) for the particular resinis known for the speciRed operating conditions. If wehave V m3 of resin, then for the resin plant to treat the



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Table 3 Design requirements

Operating conditions

Flow rate per line 2.1 m3 h1

Running time 9.8 h

Net run 21 m3

Treated water quality

Achieved Specified End point

Conductivity

(S cm1)

0.70 1.00 2.00

Silica leakage (ppb) 16 20 500

Sodium leakage

(ppm)

0.038

Residual CO2 after

SAC filter

0.59 meq L1

Process options

Ion exchange

process

Demineralization

Plant layout SACPSBA

No. of lines (as required)

Resins chosen SAC SBA

water ef Rciently, the following equation must besatisRed.

FCh"OV

where h is the time in hours between successive regen-erations.

Many factors can affect the operating capacity in

a particular situation. This must always be lower thanthe total volume capacity of the resin. The mostimportant of these factors are the regeneration leveland the particular analysis of the water being treated.In certain cases other factors such asSow rate or cycletime, operating and regenerant temperature, treatedwater quality and cycle end point, and resin beddepth may also need to be taken into account. Fur-thermore, the plant itself will need to treat some extrawater in order to operate successfully. This extra loadneeds to be allowed for in the design. The moreconcentrated the feed water the more extra load isplaced on the resin beds. Indeed, the water may con-tain such a high level of dissolved salts that treatmentmay not be economic. In such cases RO often pro-vides a satisfactory alternative, or as a pretreatment.When all these factors are carefully considered, theoptimization of the plant becomes quite complicated,especially when taking into account the various pro-cess options and the possible choices of resin typesand their various combinations. Such an exercise re-quires a vast experience of water treatment. To helpdesign engineers and water treatment plant operatorsto make suitable design plans, computer programs

are now available. In certain cases they are suitableboth for new plant and to revamp or modify existingplant. They may also be used to check the currentperformance of a particular ion exchange line in op-eration, and to evaluate possible changes in operatingparameters. These programs or computer printoutscan be obtained from various resin manufacturers

and from original engineering manufacturers to helpexperts in these calculations. They can also serve asuseful training for those engineers learning theirtrade, and for water treatment chemists who need tounderstand the operations of a particular plant inwhich they have an interest.

Table 3 gives an example of data provided bya plant design computer program on the design re-quirements for a speciRed Sow rate and treated waterquality. Table 4 gives an example of the engineeringdata provided by a computer program. Its use cansave many hours in calculation time and allow explo-ration of the many options available before makinga Rnal choice.

Choice of Resins

The optimization of any given process requires con-siderable skill on the part of the design engineer. It isimportant to understand the strengths and weak-nesses of each resin type so that the correct type andparticle size are chosen. The design program is ex-tremely useful to balance and match the conditions of regeneration to produce the correct quality and

quantity of treated water. The combined experienceof the customer and the engineer, together with theexpertise and support of resin specialists, is generallyregarded as the best approach to determining theoptimized conditions of operation and choice of thecorrect equipment.

Resin Life

Provided that the design has been optimized, in allbut the most dif Rcult cases the resin life should be inthe range of 4}6 years, depending on the type of resin.

In fact SAC resins have been known to last very muchlonger, provided they are used at the optimum tem-perature and are kept free of contaminants and poten-tial oxidizing agents. There must also be the provisonthat regenerate quantities, concentrations and Sowrates are designed to avoid stress on the resins. Inmany cases regular checks on the state of resins can bebeneRcial. This will prevent build-up of chemical con-taminants and highlight any maloperation before realdamage is done.

See Colour Plates 125, 126.

See also: I/Ion Exchange.



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Table 4 Calculation of full plant design details for an ion exchange plant

Filter

SAC SBA

Ion exchange load

Gross run (m3) 21 21

Ionic load (eq) 28 30

Resin data

Resin type SAC SBA

Resin grade } }

Theoretical capacity (eq L1 R) 1.15 0.62

Operational capacity (eq L1 R) 0.48 0.56

Resin volume (L) 58 53

Flow rate (BV h1) 36.9 40.2

Organic load (g L1 KMnO4) 7.880

Regeneration data

Regeneration mode CTF : FB CTF : FB

Regenerant HCl NaOH

Concentration % 5.0 4.0% of Theory 345 180Level (g L1 R) 61.0 40.0

Total (kg 100%) 4 2Excess (eq) 69 24

Temperature (3C) 25

Dilution water (m3) 0.1 0.0

Slow rinse (m3) 0.1 0.2

Fast rinse (m3) Recycling Recycling

Plant size data

Bed depth (changing from supplied form as shown)

Supplied form (mm) 586 538

Exhausted form (mm) 562

Regenerated form (mm) 609 634

Vessel diameter (mm) 365 365

Cross-section (m2) 0.10 0.10

Cylindrical height (mm) Per design Per design

Hydraulic data

Linear velocity (m h1) 21.2 21.2

Pressure drop (kPa) 19.0 14.8

Design factor 0.42 0.90

(Limitations caused by flow rate)

Note: BV, bed volume; R, resin.

Further Reading

Abrams MI and Benezra L (1967) Encyclopedia of PolymerScience and Technology, pp. 692}742. Chichester: JohnWiley & Sons.

Chu B, Whitney DC and Diamond RM (1962) Journal of Inorganic Nuclear Chemistry 24: 1405}1415.

Dale J and Irving J (1992) Comparison of strong base resintypes. In: Slater MJ (ed.) Ion Exchange Advances,pp. 33}40. London: Elsevier Applied Science.

Diamond RM and Whitney DC (1966) Resin selectivity in

dilute to concentrated aqueous solutions. In: Marinsky

JA (ed.) Ion Exchange, vol. 1, pp. 277}349. New York:Marcel Dekker.

Dorfner K (1991) Introduction to ion exchange and ionexchangers. In: Dorfner K (ed.) Ion Exchangers. Berlinand New York: Walter de Gruyter.

Harland CE (1994) Some engineering notes. In:Ion Exchange: Theory and Practice, 2nd edn,pp. 261}276. Cambridge: Royal Society of Chem-istry.

Helfferich F (1962) Ion exchange equilibria. In: IonExchange, pp. 151}248. New York: McGraw-

Hill.



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Newell PA, Wrigley SP, Sehn P and Whipple SS (1996)An economic comparison of reverse osmosis and ionexchange in Europe. In: Greig JA (ed.) Ion ExchangeDevelopment and Applications, pp. 58}66. Cambridge:Royal Society of Chemistry.

Nolan J and Irving J (1984) The effect on the capacity of strong base anion exchange resins of the ratio of chlorideto sulphate in the feed water. In: Naden D and StreatM (eds) Ion Exchange Technology, pp. 160}168. Chi-chester: Ellis Horwood.

Anion Exchangers: Ion Exchange

W. H. Ho ll, Karlsruhe Nuclear Research Center,

Karlsruhe, Germany

Copyright^ 2000 Academic Press

Introduction

Anion exchange resins consist of a polymeric matrixto which different functional groups are attached.Most weakly basic anion exchangers contain tertiaryamino groups; in a few cases primary and secondarygroups are also encountered. In many cases weaklybasic anion exchangers are not monofunctional butpossess a variety of amino groups. Strongly basicresins contain quaternary ammonium groups. Stan-dard commercially available exchangers con-tain either }N#(CH3)3 groups (type 1 resins) or}N#(CH3)2C2H4OH groups (type 2 resins). Bothweakly and strongly basic exchange resins are avail-able in gel-type or macroporous modiRcations.

Properties and Relds of application mainly dependon the dissociation properties of the functionalgroups in which dissociation plays the most impor-tant role. By means of the mass action law, dissoci-ation constants of the protonated amino andammonium groups can be estimated. The respectivenumerical values are in the range of pKa'13 forstrongly basic resins and 5}8 for weakly basic resins.Therefore, strongly basic resins are protonated overthe entire pH range but weakly basic exchangers areprotonated at pH values below 5}8, depending on thetype. As a consequence, strongly basic resins willexchange anions in both acid and alkaline solutions.

In addition, these exchangers can adsorb weak acidsand even ionize very weakly dissociated acids. Weak-ly basic resins, however, can operate only in acidicmedia and are unable to convert neutral salts to therespective hydroxides (e.g. NaCl to NaOH). Further-more, they cannot normally adsorb weak acids.

The uptake of anions by resins is subject to speciRcinteractions between counterions and co-ions and thedistribution of exchangeable ions depends on theproperties of both the exchanger and the ions. Conse-quently, a favoured sorption of certain types of anions occurs. The sequence of af Rnities is given

either qualitatively by the selectivity series or quantit-

atively by, for example, separation factors. For weak-ly basic anion exchangers the sequence of most com-mon anions in fresh water is:

OHSO24 'NO3'Cl

Due to the dissociation properties of the functionalgroups, hydroxyl ions are strongly preferred. This isimportant for the conversion of these resins to the

hydroxyl (or free base) form in the regeneration step,in which only slightly more than the stoichiometricamount of OH} bearing solutions is required.

For strongly basic anion exchangers the selectivityseries is:

SO24 'NO3'Cl'HCO3'OH

For these resins hydroxyl ions are the least prefer-red among the standard anions. Conversion of theresins to the hydroxyl form therefore requires com-paratively large excess amounts of sodium hydroxide.

For elimination of nitrate anions from drinkingwater the preferred sorption of sulfate ions causesconsiderable disadvantages. Extensive research dur-ing the 1980s has shown that this drawback can beovercome by introducing functional groups which aremore hydrophobic and bulkier. By these means, theability to adsorb sulfate ions is considerably de-creased. The so-called nitrate-selective resins whichare now commercially available contain triethyl in-stead of trimethyl groups and, therefore, exhibit a re-versed preference for nitrate and sulfate ions.

The rate of exchange depends mainly on the inter-

nal interdiffusion of exchanging ions. On the com-pletely ionized strong base resins this diffusion israther quick and the overall rate of exchange mainlydepends on the particle size distribution. With weaklybasic exchangers, however, the poor dissociation andfurther speciRc interactions between functional sitesand diffusing ions considerably slow the rate of ex-change. For these resins, both the uptake of acids andconversion to the free base form by means of sodiumhydroxide strongly depend on the concentration of the liquid phases.

Strongly basic anion exchangers in the hydroxyl

form are subject to considerable degradation of

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