Ion Exchange Basics

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ION EXCHANGE BASICS

Introduction

Ion exchange is a powerful chemical technology, little known to thegeneral public. This simple page attempts to show what ion exchange is to those who are not

chemical experts.

When my friends ask me what is my professional activity, I tell them "ion exchange". Mostof them have no clue. In Western Europe, the majority of my friends have one or two ion

exchange devices in their household. So, I return a question: "Why do you think you put salt

in your dishwasher?" Very few know, and if you are in this case, you will discover why

below.

Water

Water looks simple: it is made of water molecules (formula H2O). You know

however that this apparent simplicity is more complex in reality: otherwise, bottled waterproducers would not make such a fuss about its mineralisation.

All natural waters contain some foreign substances, usually in small amounts. The water in

the river, in a well or from your tap at home is not just H2O, it contains a little of:

Solid, insoluble substances, such as sand or vegetal debris. You can in principle filterthese solid substances out.

Soluble substances, that you most often cannot see and that cannot be filtered out.These substances can be inorganic or organic, they can be ionised(electricallycharged) or not ionised.

The soluble, non-ionisedsubstances are present in the water in form of moleculesof various

sizes and formulas, for instance:

Carbon dioxide is a small molecule with a simple formula: CO2. Sugar is a larger molecule with a complicated formula abbreviated as C12H22O11.

Want to see the3 D formula?Sugars are not removed by ion exchange, though.

You may want to remove these foreign substances from the water. You can remove the

ionisedsubstances by ion exchange.

Ions

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The soluble, ionisedsubstances are present in water as ions, which are electrically charged

atoms or molecules. The positively charged ions are called cations, and the negatively

charged ions are called anions. Because water is globally neutral electrically (otherwise you

would get an electric shock when you put your hand in water) the number of positive charges

is the same as the number of negative charges.

Ions can have one charge or more, the most usual range being 1 to 3. Ions can be made of one

atom only (monoatomiic ions) , or several atoms linked permanently together, like molecules

(polyatomic ions).

Examples:

A monovalent monoatomic cation: the sodium ion Na+ A divalent monoatomic cation: the calcium ion Ca++ A monovalent polyatomic cation: the ammonium ion NH4+ A monovalent monoatomic anion: the chloride ion Cl A monovalent polyatomic anion: the nitrate ion NO3 A divalent polyatomic anion: the carbonate ion CO3= Another divalent polyatomic anion: the chromate ion (metallic complex) CrO4= The trivalent monoatomic aluminium cation Al+++exists only in very acidic solution,

not in normal water.

Similarly, there are no monoatomic di or trivalent anions in normal waterIons are able to move around in water, they are not fixed, and they are not attached to ions of

the opposite charge. Only the sum of the chargesis the same for all cations and all anions.

See figure 1 for a schematic representation of ions in water.

Figure 1: Ions in water are not attached to each other. The sum of charges is constant.

Saltsare crystallised substances containing a fixed proportion of cations and anions. For

instance, table salt has exactly the same number of sodium cations (Na +) and chloride anions

(Cl). Its formula is given as NaCl. When you dissolve a salt into water, its cations and anions

aredissociated

, and free to wander as seen on figure 1.

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The dissolved ions are surrounded by water. They are said to be hydrated. They are loosely

connected to water molecules, cations attracted by the O atom, anions by the H atoms of the

water molecule, as shown in figure 2.

I onic compound dissolved in water

Figure 2: Hydrated ions in water e.g. Na+and Cl(table salt NaCl)

Magnesium sulphate is a salt with exactly the same number of magnesium cations (with

double charge: Mg++) and sulphate anions (also with double charge, SO4=) so that the formula

is MgSO4.

Calcium chloride is made of calcium ions (with 2 charges, Ca++) and chloride ions (with 1

charge only, Cl). You need 2 chloride anions to balance each calcium cation. Therefore the

formula of calcium chloride is CaCl2.

Similarly, in sodium carbonate you have sodium cations Na+and carbonate anions CO3=, so

that you need 2 sodium ions for each carbonate ion, and the formula is Na2CO3.

When you boil and evaporate water for a long time, you are left with a dry residual which is

made of salts and possibly other residues, such as silica and organic compounds. Only in sea

water do you have a sizeable quantity of dry residual, 35 to 40 g dry residual for one litre of

sea water. In river or tap water, the dry residual is usually very low, ranging from 50 to 500

mg/L. The dry residual is also called Total Dissolved Solidsand abbreviated as TDS.

You may want to remove these foreign substances from the water. You can remove the

ionised substances by ion exchange. See details of thewater analysisandunits ofconcentrationused in ion exchange.

Ion Exchange

Impurities in water

Water, as we have seen, contains small amounts of foreign substances. In many cases, these

substances cause no problem. Drinking water containing some salinity is much better for

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health than ultra-pure water. For specific applications, however, these foreign substances are

regarded as impuritiesand must be removed from water.

Insoluble substances (sand etc.) can be removed by filtration. There are many different sorts

of filtration technologies, down to ultrafiltration that can remove sub-micron particles. For

soluble substances other techniques must be used.

Soluble ionised substancescan be removed by ion exchange.

Ion exchange resins

These are very small plastic beads, with a diameter of about 0.6 mm. These beads are porous

and contain invisible water inside the beads, measured as humidity or moisture content.

The structure of the resin is a polymer (like all plastics) on which a fixed ionhas been

permanently attached. This ion cannot be removed or displaced; it is part of the structure. To

preserve the electrical neutrality of the resin, each fixed ion must be neutralised with a

counterion. This counterion is mobile and can get into and out of the resin bead. Figure 3shows a schematic cation exchange resin bead. The dark lines represent the polymeric

skeleton of the resin bead: it is porous and contains water. The fixed ions of this cation

exchange resin are sulphonates (SO3) that are attached to the skeleton. In this picture, the

mobile ions are sodium (Na+) cations. Cation exchange resins such as Amberjet 1200 are

often delivered in the sodium form.

Figure 3: Schematic cation and anion resin beads

The anion resin bead has a very similar skeleton. The functional groups are here quaternary

ammonium cations shown in the picture as N+R3; a more accurate formula would be CH2-N+-

(CH3)3. Each ion going into the bead has to be replaced by an ion getting out of the bead,

again to preserve electrical neutrality. This is what is calledion exchange. Only ions of the

same electric sign are exchanged. You cannot make a resin that can exchange cations as well

as anions, because the fixed cations inside the resin beads would neutralise the fixed anions

and no exchange with the outside world would be possible. Therefore you need separate

cation exchange resins and anion exchange resins.

Details aboutresin structureare given in a separate page.

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Water softening

Among the substances dissolved in water, hardnessis very commonly found. Hardness is apopular word to represent principally calcium and magnesium dissolved in the water; these

ions can precipitate under certain conditions and form the scale that you may have seen in

your boiling pan, and that can obstruct pipes and damage water boilers. The softening of

water is the exchange of the hardness cations (Ca++and Mg++) for another cation that cannot

form scale because it is much more soluble: the sodium ion Na+.

To soften water, you take a cation exchange resin in which the mobile ion inside the beads is

sodium (Na+) and you pass the hard water through a column filled with the sodium form

resin. The hardness ions Ca++and Mg++move into the resin beads and each of these divalent

cations is replaced by two sodium ions getting out of the resin. The exchange reaction can be

written as:

2 RNa + Ca++ R2Ca + 2 Na+

Figure 4 illustrates the reaction: the resin beads are initially loaded with sodium (Na +) ions.

As shown schematically, each calcium or magnesium ion entering the resin bead is

compensated by two sodium ions leaving it. Anions from the water cannot enter the resin

bead because they would be repelled by the fixed sulphonate (SO3) anions inside the beads.

Figure 4: Softening (sodium exchange) in a single resin bead

This cation exchange can only take place efficiently because the cation exchange resin has a

higher affinityfor the hardness ions than for sodium. In plain English, the resin prefers

calcium and magnesium over sodium. The result of the softening process is not a net removal

of the hardness ions from water, it is the replacement of the hardness ions by sodium ions.

The salinity of the water has not changed, only the constituents of the salinity are different at

the end of the softening process.

Obviously, this exchange is not unlimited: when the resin has removed so much hardness

from the feed water that no room is left on the resin for removing more, the exhaustion run

has to be stopped. At this stage, the resin will be replaced by a fresh resin, or regenerated.

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Demineralisation

If you replace all cations dissolved in water by H+ions and all anions by OHions, these willrecombine and form new molecules of water. To do this, you need a cation exchange resin in

the H form and an anion exchange resin in the OH form. All cations and anions will be

exchanged, and in this case the net result is a complete disappearance of the ionic

contaminants. The cation exchange reactions will be:

2 RH + Ca++ R2Ca + 2 H+

RH + Na+ RNa + H+

In these equations, R represents the cation exchange resin. This is shown on figure 5. The

resin is initially in the hydrogen (H+) form. In this picture the anions in water are not shown,

but the sulphonic functional groups SO3are. You can see that one Ca++ion getting in causes

two H+ions to leave the resin, whilst one Na+cation is exchanged for one H+ion.

Figure 5: Decationisation (all cations replaced by H+)

Similarly, an anion exchange resin initially in the OHform can remove all anions. The anion

exchange reactions will be:

ROH + Cl RCl + OH

2 ROH + SO4= R2SO4+ 2 OH

where R represents the anion exchange resin. All anions are replaced by hydroxide (OH)

ions. There is no picture for this anion exchange, as it is very similar to the cation exchange

picture in figure 5 above.

At the end of the exchange process, the resin beads have loaded all cations and anions from

the water and released H+and OHions. The resin beads are nearly exhausted (fig. 6).

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These H+and OHions will immediately combine and form water:

H++ OH HOH H2O

The ionic contaminants are now sitting on the two resins (Na and Ca on the cation resin, Cl

and SO4on the anion resin) and the water has been completely demineralised. Its salinity isreduced to almost nothing, a few ions that have escaped from the resin columns, and that

are called ion leakage.

Figure 6: Resin beads are exhausted.

H+and OHions have been released into the water

Water demineralisation can thus be summarised in a small single picture:

Figure 7: Demineralisation summary!

Regeneration

When the resins are exhausted, you can bring them back to the fresh state and start over

again. Regeneration of ion exchange resins is a reversal of the exchange reactions shown

above.

Regeneration of a water softener

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The softening resin is regenerated with sodium (Na+) ions supplied by a salt (common salt:

NaCl) solution. The regeneration reaction is:

R2Ca + 2 NaCl 2 RNa + CaCl2

Regeneration can only be performed when the concentration of the

regenerant is high, typically 1000 times higher than the concentration in normal water. For

instance, salt is used as a brine with 10 % (about 100 g/L) concentration.

At this stage, you will have understood why you put salt in your dishwasher: the salt is

diluted with water and regenerates the invisible softening cartridge usually located at the

bottom of the machine, out of sight.

Regeneration of a demineraliser

In the case of demineralisation, strong acids such as hydrochloric acid (HCl) or sulphuric acid

(H2SO4) are fully dissociated and can supply H+ions to replace the cations that have been

exchanged and are sitting in the cation exchange resin beads at the end of the exhaustion run:

RNa + HCl RH + NaCl

Similarly, strong alkalis, of which in practice only caustic soda (NaOH) is used, can supply

OHions to replace the anions sitting on the anion exchange resins beads at the end of the

run:

RCl + NaOH ROH + NaCl

As can be seen from the regeneration reactions, the regeneration step produces saline waste.

This is the principal disadvantage of ion exchange.

See a page with co-flow and reverse flowregeneration methods.

How resins look like

Click on the pictures

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A sample of AmberliteTM

FPC23 AmberjetTMmixed bed resins AmberjetTM4400

There is a full page withmany other resin picturesunder the microscope.

Column operation

In the laboratory as

well as in industrial plants, ion exchange resins are used in columns. The water or solution to

be treated flows through the resin. On the picture at the right, you see the fresh resin, then

you see how the resin gets progressively loaded with the ions from the feed solution. Ions

from the resinnot shown on the pictureare released into the treated solution. At the

end some of the ions from the feed escape into the pure solution, and operation is stopped.

The next pictures show a typical laboratory column, a simple industrial column and aphotograph of an existing Amberpack plant.

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Laboratory setup Industrial column AmberpackTMcolumn

The jug showed at thetopof this page contains a small filter filled with activated carbon and

ion exchange resin. The quantity of resin is around 150 ml. For comparison, a large industrial

ion exchange column can contain 20'000 L of resin, sometimes more.

Ion exchange capacity

Total capacityThe number of "active groups", or "functional groups" in an ion exchange resin is its total

capacity. As there are billions of individual active groups in a single bead of resin, the

capacity is usually expressed in equivalents per litreof resin. One equivalent is 6.021023

active groups. You don't have to remember this very large number called Avogadro number.

A typical strong acid cation exchange resin has a total capacity of 1.8 to 2.2 eq/L

A typical weak acid cation exchange resin has a total capacity of 3.7 to 4.5 eq/L

A typical weak or strong base anion exchange resin has a total capacity of 1.1 to 1.4 eq/L

Operating capacityIn the "column operation" picture above, the resin is 100% regenerated at the beginning of

the run, and not completely exhausted at the end of the run. The definition of operating

capacity is: the difference of regenerated sites between the beginning and the end of the ion

exchange run. It is also measured in equivalents per litre.

In operation, the operating capacity of the resin amounts to about half the total capacity. The

actual range is 40 to 70 % of the total capacity depending on the operating conditions. See

other detailsin a specific page.

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It is the numberof ions and their charge (one, two, or three charges per ion), not their

mass or weight, that is important for ion exchange. Therefore allfeed water analysesmust

have the mass of ions converted to equivalents.

Why the resin quantity is expressed in volume, not weightWhen an ion exchange resin filter is designed and built, it is the volume of filtering media

that is important to determine the column size, not its mass. Ion exchange resins have

different density values (see resinproperties), so the resins are sold by volume, in litres or

cubic metres, or in cubic feet in the USA. Many of the resin properties are also related to the

resin volume.

Treated water quality

In a typical demineralisation system regenerated in reverse flow (seeregeneration methods),

the treated water quality, expressed in water conductivity, is below 1 S/cm. Considering that

feed water from rivers and deep wells has a conductivity of 100 to more than 1000 S/cm,

the efficiency of ion exchange ranges from 99 to more than 99.9 %. Other processes, such as

reverse osmosis, are far from this high salt rejection number.

Limits of ion exchange

For ion exchange to be efficient there must be a difference in affinitybetween the ion in the

resin and the ion or ions you want to remove from solution. The resin must have a higher

affinity for the ion in solution compared to the ion in the resin.

The ion exchange technology is a perfect tool to remove or exchange contaminants present in

low concentrations. In such a case the running time until the resin column is exhausted can

be very long, ranging from a few hours to several months. When however the concentration

of contaminants is high, say several grams per litre of water, the ion exchange cycles become

exceedingly short and the quantity of regenerants increases to uneconomical levels. In the

case of brackish water (underground water with high salinity as often found in arid countries)or sea water, ion exchange is not suitable and other technologies must be used, such as

reverse osmosis or distillation.

Also, any contaminant that is not ionised cannot be removed by ion exchange. Other

technologies are available for this purpose, using activated carbon, polymeric adsorbents,

molecular sieves and other media.

Selective ion exchange

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See a separate page with details of the aboveapplications.

Conclusion

Ion exchange is a very powerful technology to removeimpurities from water and other solutions. Many industries depend on ion exchange for the

production of extremely pure water. Examples are:

Nuclear and thermal power stations Semiconductor, computer chips and display panel production Selective removal of toxic contaminants from drinking water

There are also many applications in areas other than water treatment, as mentioned above.

Go to thesite mapfor several detailed pages on applications, processes, resin properties and

more.

Amberjet, Amberlite, and Amberpack are original trademarks of Rohm and Haas, a

subsidiary of the Dow Chemical Company.

Ion exchange capacity

Introduction

Ion exchange is a cyclic process: ions are loaded to resins, the resins get progressively

exhausted, and when there is no place to load more ions, the loading phase is interrupted, and

the resins must be regenerated. Ion exchange capacity indicates the quantity of ions loaded to

the resin.

Definitions

Total capacity

The total capacity of a resin sample is the number of ion exchange sites. See detailsat the end

of the page. Typical capacity values are givenbelow.

Operating capacity

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Also called useful capacity, it is the number of ion exchange sites where exchange has really

taken place during the loading run. It is also the number of resin chargesnot the number of

ions because some ions have more than one chargepicked up by the resin in one cycle.

The ion exchange capacity is expressed as eq/L(equivalents per litre of resin).

The unit of mole should be avoidedaltogether in ion exchange, as it does not take valenceinto account and brings only confusion. For reference: 1 eq = 1 mole / valence.

The operating capacity is always smaller than the total capacity. We will see why.

Zone of exchange

Ideal case

Start of the run Middle of the run End of the run

In an ideal case, we would start with a fully regenerated resin. During the exhaustion run, the

exchange front would be absolutely flat, meaning that each infinitesimal resin layer would be

instantaneously converted from regenerated to exhausted, capturing the incoming ions with

an infinite speed of exchange. This flat front would move down the column as more ions are

removed from water. At some point, the flat front would reach the bottom of the column, and

the resin would then be totally exhausted. In such a case, the operating capacity would be

equal to the total capacity of the resin. This case does not exist in practice, as the exchangefront is not flat and the resin is not always fully regenerated at the beginning of the run.

In the real world, there are two cases:

Case 1: the resin is totally regenerated at the beginning of the run (WAC & WBA)

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Start of the run Middle of the run End of the run

At the start of the run, the resin is totally regenerated. In the course of the loading run, the top

layers of the bed get progressively exhausted. However, the exchange reaction is not

infinitely fast, as the ions must find their way to available sites inside each resin bead.

Therefore, some ions find their way to lower levels of the bed before the layers above are

totally exhausted. The area between fully regenerated and fully exhausted resin is called

exchange zoneor reaction zone, as shown in the pictures.

At some stage, the ions begin to "leak" iinto the treated water. The loading run is stopped at

the time the concentration of this ion leakagereaches a preset value. This is called the

endpointof the run. At this stage, the ion exchange resin is not fully exhausted, so the

operating capacity is smaller than the total capacity.

The operating capacity is, as defined above, the difference between the exhausted resin at the

start and at the end of the run. The behaviour shown here is typical of weakly acidicand

weakly basicion exchange resins, that can be fully regenerated with a minimum amount of

regenerant, close to the stoichiometric value. A stoichimetric regenerant quantity is the

quantity of chemical equivalents exactly equal to the ionic load during the exchange cycle. In

practice, weak resins are regenerated with a small excess over the stoichiometric quantity.

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, sothere is no such simple estimate. However, WAC resins having a high total capacity and

being regenerated almost without an excess (see regenerant ratio), their use is very helpful for

waters containing a high concentration of alkalinity and hardness.

Case 2: the resin is partially exhausted at the beginning of the run (SAC & SBA)

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Start of the run Middle of the run End of the run

This second case is typical of strongly acidicand strongly basicresins, which are more

difficult to regenerate, so that an excess of regenerant chemical must be passed through the

resin bed. Even so, it is not economical to regenerate the resin totally, which would mean a

very high regenerant dosage, so in practice the resin bed is only partially regenerated. The

pictures here indicate a resin bed with downflow loading and upflow regeneration. See the

page aboutregeneration details.

Typically the operating capacity of SAC and SBA resins is 40 to 60 % of their total capacity.

Case 2b: co-flow regenerated resins

Start of the run Middle of the run End of the run

The feed water during the exhaustion run and the regeneration solution are both flowing from

top to bottom. As a result, the top layers of the resin bed are well regenerated at the start of

the run, but the bottom of the bed is not. During the exhaustion run, a fraction of the ions

from the feed not removed during regeneration leak into the treated water, as shown in theregeneration page.

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For example, if the resin is regenerated with acid, some of the H+ions released by the

removal of Na+ions from the feed wander down the column and displace a few of the Na+

ions left at the bottom after the previous regeneration. The sodium leakage is thus much

higher than with reverse flow regeneration.

Ion exchange kinetics

Weak acid and weak base resins are sensitive to flow rate. When the flow rate increases, the

reaction zone becomes longer.

Low flow rate:

the reaction zone is short

High flow rate:

the reaction zone is long

When the reaction zone is short, the achievable operating capacity is higher, because a higher

fraction of the total capacity can be used before the leakage reaches its endpoint.

SAC and SBA resins are less sensitive to flow rate.

Fine resins have generally higher kinetics; this is especially true for WAC and WBA resins.

The reason is a shorter path for the ions to travel inside the resin beads.

Parameters affecting operating capacity

The operating capacity depends on a number of process variables including:

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

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The ion exchange resin manufacturers publish performance data allowing users to calculate

the operating capacity of their resins under the most common conditions of use. Several of

them also have computer programs to make basic designs for ion exchange systems. I have

developed such a program for Rohm and Haas under the name of IXCalc.

Measurement of the total capacity

The total capacity of a resin sample is measured by titration and expressed in eq/L. The

procedure involves a volume measurement and must be carried out under strict conditions. As

the volume changes according to the ionic form of the resinsome ions have a higher mass

and their volume is different from othersthe ionic form of measurement must always be

reported.

The total capacity must also be reported as dry weight capacityafter drying of the resin

sample. The dry weight capacity measures the number of active groups per kg of dry resin,

i.e. without the moisture content. It is expressed in eq/kg. Mention of the ionic form is critical

here as well, as different ions have different masses.

Dry weight capacity is important for two different purposes:

1. For new resins, it gives information about the efficiency of the activation process: forinstance, if every aromatic ring has been sulphonated in a strongly acidic resin, the

theoretical maximum total dry weight capacity is about 5.5 eq/kg in H+form.

2. For used resins, it gives information about a possible fouling: a fouled resin samplecontains foreign matter, which increases the dry weight, and as a consequence the dryweight capacity (number of active groups per kg of dry matter) decreases, even if no

functional group has been lost.

Operating capacity in practice

We have seen that the operating capacity of an ion exchange resin is a fraction of the total

capacity. It is also expressed in eq/L(equivalents per litre of resin) and indicates the quantity

of ions (more precisely the number of charges) that can be exchanged during a cycle.

The following table shows typicaltotal and operating capacity values for the common resins

(all values in eq/L, most common value in brackets):

Resin type* Total capacity Operating capacity

WAC 3.7 to 4.5 [4.2] 1.0 to 3.5

SAC 1.7 to 2.2 [2.0] 0.6 to 1.7

WBA 1.1 to 1.7 [1.3] 0.8 to 1.3

SBA 0.9 to 1.4 [1.2] 0.4 to 0.9

* Seeabbreviationsandresin types

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Let's consider for example a strongly acidic cation exchange resin (SAC) used for softening,

and let's assume the water to be treated has a hardness concentration of 4.4 meq/L and the

resin, under specific operating conditions, has an operating capacity of 1.1 eq/L, which is

1100 meq/L.

Each litre of ion exchange resin will thus be able to treat 1100 / 4.4 = 250 litres of the hardwater before having to be regenerated. In ion exchange jargon, this means that the throughput

is 250 bed volumes. If the water hardness is higher, the throughput will be less, and vice-

versa. See alsoconcentration and capacity units.

Regeneration methodsfor ion exchange units

Introduction

Most ion exchange resins are used in columns. Ion exchange operation is basically

discontinuous: a loading phase, called service run, is followed by regeneration of the

exhausted resins. There are two main methods for the regeneration process:

Co-flow regeneration, where the fluids are flowing from the top to the bottom of thecolumn both during the service run as well as during regeneration.

Reverse flow regeneration, where the fluids are flowing alternatively upwards anddownwards during service and regeneration.

We will also give information about regenerant quantities (regeneration ratio),thoroughfare

regeneration, and regeneranttypes and concentrations.

See also the page aboutcapacity.

Co-flow regeneration (CFR)

This regeneration technique has been used used at the beginning of ion exchange: the solution

to treat flows from the top to the bottom of the column, and the regenerant uses the same

path.

The problem is that strongly acidic and strongly basic resins are not completely converted to

the H or OH form at the end of the regeneration, because this would require too large an

excess of chemical regenerant. As a result, the bottom layers of the resin bed are more

contaminated than the top layers at the end of regeneration, so that when the next loading run

begins the leakageis high due to the displacement of the contaminating ions by the H+(or

OH

) ions produced in the exchange.

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The dark zone in the picture above represents the

proportionof exhausted resin, the yellow zone the proportion of regenerated resin. The

small picture on the right explains what I mean: for instance, at level A, the resin is 50%

exhausted and 50% regenerated. Above the exchange zone, the resin is fully exhausted, and

below it is fullly regenerated.

With co-flow regeneration, the only way to reduce thispermanent leakageis to increase the

quantity of regenerant so as to leave less contaminating ions at the outlet of the column.

Reverse flow regeneration (RFR)

This is also called "counterflow regeneraton". In the past, it was called counter-current

regeneration, but the term is not strictly correct as the resin bed does not move. With reverse

flow regeneration the regenerant is injected in the opposite direction of the service flow.

There are two sub-cases:

1. Upflow loading and downflow regeneration, as in the floating bed and AmberpackTMprocesses.

2. Downflow loading and upflow regeneration, as in the UFDTMand UpcoreTMprocesses.In this case, the regenerant doesn't have to push the contaminating ions through the whole

resin bed. The layers which are less exhausted will be regenerated first and will be the

cleanest when the next loading run (exhaustion) starts.

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At the end of regeneration, the exit layer of the column regenerated in CFR has the highest

concentration of impurities, whereas in RFR the exit layer contains the most highly

regenerated resin. This is why in CFR the contaminants at the bottom find their way into the

treated water, more at the beginning than in the middle of the run, due to a "self regeneration"

effect, whereas in RFR any displaced contaminant from the inlet layer gets immediately

removed from a layer underneath.

The graph shows the typical leakage profile during the loading phase (e.g. conductivity in

S/cm but it can be any other leakage depending on the process). The ionic leakage obtained

with reverse flow regeneration is usually so low, that it does not depend on the amount of

regenerant used. With co-flow, low leakage values are obtained only with high regenerantdosage.

No backwash with RFR

The whole effect of reverse flow regeneration relies on undisturbed resin layers. The resin

with the highest degree of regeneration should always be at the column outlet. Therefore, the

resin bed should notbe backwashed before regeneration, and should not be allowed to

fluidise at any time. So either the columns are completely filled with resin (packed beds) orthe bed is held down during regeneration. See the "column design"page for the concepts of

holddown and packed beds.

Regeneration steps

The general regeneration procedure for ion echange vessels is as follows:

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1. Backwash resin bed (co-flow regeneration only) to remove suspended solids anddecompact the bed.

2. Inject regenerant diluted in appropriate water quality. The injection is at a low flowrate, so that the contact time is 20 to 40 minutes.

3. Displace the regenerant with dilution water at the same flow rate.4. Rinse the bed at service flow rate with feed water until the desired treated waterquality is obtained.

The above is valid for most ion exchange columns, e.g. softening, nitrate removal, de-

alkalisation. For demineralisation, the cation column is regenerated first with acid, then the

anion column with caustic soda; alternatively, both are regenerated at the same time.

The regeneration of a mixed bedunit is more complicated. The steps are:

1. Backwash resin bed to separate the cation from the anion resin.2. Let the resins settle.3. Optionally: drain the water down to the resin bed surface.4. Inject caustic soda diluted in demineralised water.5. Displace the caustic with dilution water.6. Inject acid diluted in demineralised water.7. Displace the acid with dilution water.8. Drain the water down to the resin bed surface.9. Mix the resins with clean compressed air or nitrogen.10.Refill the unit slowly with water.11.Do the final rinse with feed water at service flow rate until the desired treated water

quality is obtained.

Note: Cation and anion resin can be regenerated simultaneously to save time. Otherwise,

always start with the anion resin.

Regeneration ratio

Definition:

Introduction

The regeneration ratioor regenerant ratiois calculated as the total amount ofregenerant (in equivalents) divided by the total ionic load (also in equivalents) during

one cycle.

It is is also equal to the number of eq/L regenerant per eq/L of resin operatingcapacity.

A (theoretical) regenerant ratio of 1.00 (i.e. 100 %) would correspond to thestoichiometric quantity.

All resins need a certain excess of regenerant above the stoichiometric quantity.

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Example

Amberjet 1200 regenerated with 55 g HCl per litre operating 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 %

Excess

The difference between ionic load and regenerant quantity is called excess regenerant.

Excess [in eq]= regenerant [eq] - ionic load [eq]

Excess [in %] = 100 x (1 - regenerant ratio)

Minimum values

WAC resins require just above the stoichiometric quantity. A safe number is 105 to110 %.

WBA resins require 115 to 140 %, because most of them they have some stronglybasic functional groups.

When regenerated with ammonia or sodium carbonate, WBA resins require aregenerant ratio of 150 to 200 %. These regenerants can be used for WBA only, not

for SBA resins.

SAC and SBA resins require a larger excess than their weak counterparts. Co-flow regenerated SAC and SBA resins require more than those regenerated in

reverse flow.

SAC resins regenerated in reverse flow with hydrochloric acid need an absoluteminimum of 110 % regeneration, but a safer value is 120 %. If the water contains high

hardness or low alkalinity, the minimum value must be increased.

SAC resins regenerated with sulphuric acid require a larger excess than thoseregenerated with HCl. At least 40 % more.

For SBA resins, there is no easy way to estimate a minimum, as it depends on the typeof SBA resin (styrenic type 1 vs type 2 or acrylic resins).

Important note:when calculating the regenerant ratio for SBA resins, one must take2 equivalents of NaOH for each equivalent of CO2or SiO2.

WAC/SAC couples can be regenerated with a global ratioof about 105 %. WBA/SBA couples can be regenerated with a global ratioof 110 to 120 %. More is

required if the silica level is high in the feed water.

The regenerant ratio for silicashould be at least 800 %. This should be calculatedseparately as the quantity of NaOH (in eq) divided by the load of silica (in eq) during

one cycle. One equivalent of silica is taken as 60 g as SiO2.

Thoroughfare regeneration

When a weak and a strong resin are used in series, the following two rules must apply:

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1. The feed water must pass first through the weak, then only through the strong resin.2. The regenerant must pass first through the strong, then through the weak resin.

Separate columns in service Separate columns in regeneration

Why is it so?

1. The weak resin has a high capacity and good regeneration efficiency, but does notremove all ions. Therefore it must be placed first, and the strong resin will be used toremove whatever the weak resin has not removed, albeit with a lower efficiency.

2. The strong resin requires a high excess of regenerant. The weak resin requires almostno excess. Therefore the regenerant passes through the strong resin first, and the weak

resin is regenerated with the excess regenerant coming out of the strong resin.

The above pictures are for old-fashioned, separate columns with co-flow regeneration. Below

the same for an Amberpack double compartment column.

Amberpack in service Amberpack in regeneration

All the above applies equally to a couple of weak acid and strong acid cation exchange resins.

Regenerant types and concentrations

Types of regenerant

Sodium chloride(NaCl) is normally used to regenerate SAC resins in thesofteningprocess, and SBA resins used fornitrate removal.

For softening, potassium chloride(KCl) can also be used when the presence ofsodium in the treated solution is undesirable.

In some hot condensate softening processes, ammonium chloride(NH4Cl) can beused.

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For nitrate removal, the SBA resin can be regenerated with other compoundsproviding chloride ions, such as hydrochloric acid(HCl).

Fordecationisationthe first step of ademineralisationprocessSAC resins mustbe regenerated with a strong acid. The most common acids are hydrochloric and

sulphuric acids.

o Hydrochloric acid(HCl) is very efficient and does not cause precipitations inthe resin bed.

o Sulphuric acid(H2SO4) is sometimes cheaper and easier to store and tohandle in general, but less efficient than hydrochloric acid: the operating

capacity of the SAC resin is lower. Additionally, its concentration must be

carefully adjusted to prevent calcium sulphate precipitation (see below). Once

a CaSO4precipitate is formed, it is very difficult to remove from the resin bed.

o Nitric acid(HNO3) can also be used in principle, but is not recommendedasit can cause exothermic reactions; explosions have been observed in some

cases, so that the use of nitric acid is considered dangerous.

Fordealkalisation,the WAC resin is best regenerated with hydrochloric acid(HCl).When using sulphuric acid, the concentration must be kept under 0.8 % to avoidcalcium sulphate precipitation. Other, weaker acids can also regenerate WAC resins,

such as acetic acid(CH3COOH) or citric acid, a molecule containing threeCOOH

groups: (CH2COOH-C(OH)COOH-CH2COOH = C6H8O7). Have a look at the3-

dimensional formula.

SBA resins are always regenerated with caustic soda(NaOH) in thedemineralisationprocess. Caustic potash(potassium hydroxide KOH) is in principle also applicable,

but usually more expensive.

WBA resins are usually also regenerated with caustic soda, but other regenerantsweaker alkaliscan also be used, such as:

o Ammonia(NH3)o Sodium carbonate(soda ash, Na2CO3)

Concentrations

The most usual concentrations are:

NaCl(softening and nitrate removal): 10 % HCl(decationisation, de-alkalisation and demineralisation): 5 % NaOH(demineralisation): 4 % H2SO4: for SAC resins, the acid concentration must be carefully selected between 0.7

and 6 % as a function of the proportion of calcium in the feed water (which is thesame in the SAC resin). For WAC resins, the concentration is usually 0.7 %. Too high

a concentration may cause calcium sulphate precipitation.

For SAC resins, stepwiseconcentrations are often used: after a first step at a low

concentration, a second step is carried out at a higher concentration once a great part

of the calcium on the resin has been eluted. In rare cases, three steps are used. The

steps at higher concentrations reduce the quantity of dilution water and increase the

sulphuric acid efficiency.

There are cases where different concentrations (often lower, rarely higher) must be selected.

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Neutralisation of the regenerants

See a separate page on the way toneutraliseregenerants and increase ion exchange capacity.