86577388 what are zeolites

8/14/2019 86577388 What Are Zeolites

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What are Zeolites ?Zeolites are microporous crystalline solids with well-defined structureGenerally they contain silicon, aluminium and oxygen in their framew

and cations, water and/or other molecules wthin their pores. Many occur naturally as minerals, and aextensively mined in many parts of the world. Others are synthetic, and are made commercially forspecific uses, or produced y research scientists trying to understand more a out their chemistry.

!ecause of their uni"ue porous properties, #eolites are used in a variety of applications with a glo almarket of several milliion tonnes per annum. $n the western world, ma%or uses are in petrochemicacracking, ion-exchange &water softening and purification', and in the separation and removal of gasand solvents. Other applications are in agriculture, animal hus andry and construction. (hey are ofte

also referred to as molecular sieves .

Framework Structure

) defining feature of #eolites is that theirframeworks are made up of *-connected netwof atoms. One way of thinking a out this is interms of tetrahedra, with a silicon atom in themiddle and oxygen atoms at the corners. (hesetetrahedra can then link together y their corn&see illustration' to from a rich variety of eastructures. (he framework structure may conta

linked cages, cavities or channels, which are oright si#e to allow small molecules to enter - ithe limiting pore si#es are roughly etween +

in diameter.

$n all, over + different framework structures are now known. $n addition to having silicon oraluminium as the tetrahedral atom, other compositions have also een synthesised, including thegrowing category of microporous aluminophosphates, known as ) 0Os.

Catalysis

Zeolites have the a ility to act as catalysts for chemical reactions which take place within the internacavities. )n important class of reactions is that catalysed y hydrogen-exchanged #eolites, whoseframework- ound protons give rise to very high acidity. (his is exploited in many organic reactions,including crude oil cracking, isomerisation and fuel synthesis. Zeolites can also serve as oxidation oreduction catalysts, often after metals have een introduced into the framework. 1xamples are the usof titanium Z2M-3 in the production of caprolactam, and copper #eolites in 4Ox decomposition.

5nderpinning all these types of reaction is the uni"ue microporous nature of #eolites, where the shapand si#e of a particular pore system exerts a steric influence on the reaction, controlling the access oreactants and products. (hus #eolites are often said to act as shape-selective catalysts . $ncreasi


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attention has focused on fine-tuning the properties of #eolite catalysts in order to carry out veryspecific syntheses of high-value chemicals e.g. pharmaceuticals and cosmetics.

Adsorption and Separation

(he shape-selective properties of #eolites are also the asis for their use molecular adsorption. (he a ility preferentially to adsor certain molecuwhile excluding others, has opened up a wide range of molecular sievinapplications. 2ometimes it is simply a matter of the si#e and shape of pocontrolling access into the #eolite. $n other cases different types of moleenter the #eolite, ut some diffuse through the channels more "uickly,leaving others stuck ehind, as in the purification of para -xylene ysilicalite.

6ation-containing #eolites are extensively usedas desiccants due to their high affinity for water,

and also find application in gas separation, where molecules aredifferentiated on the asis of their electrostatic interactions with the metalions. 6onversely, hydropho ic silica #eolites preferentially a sor organicsolvents.Zeolites can thus separate molecules ased on differences of si#e,shape and polarity.

Ion Exchange

(he loosely- ound nature of extra-framework metal ions &such as in #eolite4a), right' means that they are often readily exchanged for other types ofmetal when in a"ueous solution. (his is exploited in a ma%or way in watersoftening, where alkali metals such as sodium or potassium prefer toexchange out of the #eolite, eing replaced y the 7hard7 calcium and magnesium ions from the waMany commercial washing powders thus contain su stantial amounts of #eolite. 6ommercial wastewater containing heavy metals, and nuclear effluents containing radioactive isotopes can also ecleaned up using such #eolites.

Zeolites and the Environment

Zeolites contri ute to a cleaner, safer environment in a great numof ways. $n fact nearly every application of #eolites has een driveenvironmental concerns, or plays a significant role in reducing toxwaste and energy consumption.

$n powder detergents, #eolites replaced harmful phosphate uildernow anned in many parts of the world ecause of water pollutionrisks. 6atalysts, y definition, make a chemical process more effici

thus saving energy and indirectly reducing pollution. Moreover, processes can e carried out in fewesteps, miminising unecessary waste and y-products. )s solid acids, #eolites reduce the need forcorrosive li"uid acids, and as redox catalysts and sor ents, they can remove atmospheric pollutants,such as engine exahust gases and o#one-depleting 686s. Zeolites can also e used to separateharmful organics from water, and in removing heavy metal ions, including those produced y nuclea

(he shape of para -xylene meansthat it can diffuse freely in the

channels of silicalite

2odium Zeolite ), used as softener in detergent po


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fission, from water.

Zeolites in the !

Zeolite science and technology has traditionally een very strong in the 59. 0artly this has een due the scientific legacy of the late 0rofessor :ichard !arrer, the 7father of #eolite science7 who, during acareer of over 3 years in various !ritish universities, laid the foundations for the study of #eolites andiscovered many of their important properties. (he prominence of the 59 has also een associatedwith the strength of its chemical industry, particularly in areas where #eolites have applications, suchas petrochemicals, detergents, fine chemical synthesis and nuclear processing. Many !ritishcompaniescontinue to have ma%or :;< pro%ects in these areas.

R.G. Bell, May 2

Zeolites In recent years, zeolites have been mentioned in the conservation literature, butinformation on what they are and how they work has been scarce. (A recent

paper on passive conservation techniques was given by Yvonne hashoua ofthe !ritish "useum at the resins conference in Aberdeen, eptember #$%#&,#'' . he compared the performance of gaseous )*l absorbers and found thatthe best absorber was zeolite.+

The PQ Corporation's Zeolites and Catalysts Division has given permission to

reprint parts of their brochure, "Zeolites and the Environment The !ear ###,"$hich should fill part of that need% &or more information, call the company at()##*+ -(.)). or fa/ them at 0* + (.0 #%

What is Zeolite?

Composition

The term 1eolite $as originally coined in the )th century by a 2$edishmineralogist named Cronstedt $ho observed, upon rapidly heating a natural1eolite, that the stones began to dance about as the $ater evaporated% 3sing the4ree5 $ords $hich mean "stone that boils," he called this material 1eolite% 6commonly used description of a 1eolite is a crystalline aluminosilicate $ith acage structure% Technically, $e spea5 of a 1eolite as a crystalline hydratedaluminosilicate $hose frame$or5 structure encloses cavities 7or pores8occupied by cations and $ater molecules, both of $hich have considerablefreedom of movement, permitting ion e/change and reversible dehydration%


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This definition places it in the class of materials 5no$n as "molecular sieves%"9The pores in dehydrated 1eolite are : ;ngstroms in si1e, $hile those of atypical silica gel average about 0# ;, and activated carbon averages #0 ;%a(6 >a ? @ 6l? @ 2i? @ -%0 A?

Natural and Synthetic Formation

Zeolites form in nature as a result of the chemical reaction bet$een volcanicglass and saline $ater% Temperatures favoring the natural reaction range from

.BC to 00BC, and the pA is typically bet$een + and #% >ature re uires 0# to0#,### years to complete the reaction%

>aturally occurring 1eolites are rarely phase(pure and are contaminated tovarying degrees by other minerals 9e%g% &e, 2? - (, uart1, other 1eolites, andamorphous glass


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The most fundamental consideration regarding the adsorption of chemicalspecies by 1eolites is molecular sieving% 2pecies $ith a 5inetic diameter $hichma5es them too large to pass through a 1eolite pore are effectively "sieved%"This "sieve" effect can be utili1ed to produce sharp separations of molecules bysi1e and shape%

The particular affinity a species has for an internal 1eolite cavity depends onelectronic considerations% The strong electrostatic field $ithin a 1eolite cavityresults in very strong interaction $ith polar molecules such as $ater% >on(polarmolecules are also strongly adsorbed due to the polari1ing po$er of theseelectric fields% Thus, e/cellent separations can be achieved by 1eolites even$hen no steric hindrance occurs%

6dsorption based on molecular sieving, electrostatic fields, and polari1abilityare al$ays reversible in theory and usually reversible in practice% This allo$sthe 1eolite to be reused many times, cycling bet$een adsorption anddesorption% This accounts for the considerable economic value of 1eolite inadsorptive applications%

Ion Exchan e

Fecause cations are free to migrate in and out of 1eolite structures, 1eolites areoften used to e/change their cations for those of surrounding fluids% The preference of a given 1eolite among available cations can be due to ion sievingor due to a competition bet$een the 1eolite phase and a ueous phase for thecations that are present%

%%% 2odium 1eolite 6 is among the $orld's most efficient removers of $aterhardness ions% This is its principal function as a detergent builder%

Catalysis

Zeolites ma5e e/tremely active catalysts%%%% 2teric phenomena are veryimportant in 1eolite catalysis, and a ne$ term, "shape selective catalysis," $ascoined to describe these effects% E/tremely selective reactions can be made tooccur over 1eolites 9$hen certain products, reactants or transition states are5ept from forming $ithin the pores because of si1e or shape


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&ar and a$ay, the largest outlet for 1eolite volume is the global laundrydetergent mar5et% Fy the end of ++ 9it $as e/pected that the $orld $ould< beconsuming detergent 1eolite 71eolite 68 at the rate of %-- million anhydrousmetric tons per year%

3nli5e phosphates, 1eolite 6 cannot contribute to the eutrophication of la5es,streams or bays%%%%

?ther 6pplications Gefrigeration

The heat of $ater adsorption for 1eolites is high% They also possess highadsorption capacity, undergo reversible adsorption*desorption, and arestructurally stable% These properties enable 1eolite to be used in solar(po$eredrefrigerators and to store energy during off(pea5 periods and release it during pea5 periods% Zeolites can also be used in refrigeration and air cooling systemsto reduce $ater in the air to very lo$ concentrations, allo$ing very effectiveevaporative cooling to occur%

Natural Zeolites

Consumption

Zeolites are found in abundance throughout the $orldH% Ao$ever, the value ofthe synthetic 1eolites sold is far higherH% =any of the uses for natural 1eolitesare environmentally related%

En#ironmentally !ri#en $ar%et Applications

&adioacti#e Waste 'reatment

>atural 1eolites are being used to treat lo$ and intermediate a ueous $aste%Current users are Fritish >uclear &uels in 4reat Fritain, Iest Jalley >uclearand Date Gidge >ational Kaboratory% >atural 1eolite has been used in theclean(up at Three =ile sland and Chernobyl%

$unicipal Waste Water 'reatment

Certain natural 1eolites have a high affinity for ammonium ions and are beingused in a tertiary $ater treatment system at Truc5ee, California% =unicipaleffluent is treated by passing it through columns pac5ed $ith a natural 1eoliteclinoptilolite to reduce the ammonium ion concentration to less than ppm%


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Pet (itter and )dor Control

>atural 1eolites are uni uely effective in adsorbing ammonia and also adsorbhydrogen sulfide% These properties ma5e natural 1eolites ideal for use in petlitter to prevent emanation of irritating odors% &or similar reasons, natural1eolites can be used for effective control of irritating gases in horse stalls, barns, 5ennels, etc%

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All natural waters contain, in various concentrations, dissolved salts which dissociate in water to formcharged ions. Positively charged ions are called cations; negatively charged ions are called anions. Ionicimpurities can seriously affect the reliability and operating efficiency of a boiler or process system.Overheating caused by the buildup of scale or deposits formed by these impurities can lead to catastrophictube failures, costly production losses, and unscheduled downtime. Hardness ions, such as calcium andmagnesium, must be removed from the water supply before it can be used as boiler feedwater. For highpressure boiler feedwater systems and many process systems, nearly complete removal of all ions,including carbon dio!ide and silica, is re"uired. Ion e!change systems are used for efficient removal ofdissolved ions from water.

Ion e!changers e!change one ion for another, hold it temporarily, and then release it to a regenerantsolution. In an ion e!change system, undesirable ions in the water supply are replaced with more acceptableions. For e!ample, in a sodium #eolite softener, scale forming calcium and magnesium ions are replacedwith sodium ions.

HISTORY

In $%&', (ans, a (erman chemist, used synthetic aluminosilicate materials )nown as #eolites in the first ione!change water softeners. Although aluminosilicate materials are rarely used today, the term *#eolitesoftener* is commonly used to describe any cation e!change process.

+he synthetic #eolite e!change material was soon replaced by a naturally occurring material called(reensand. (reensand had a lower e!change capacity than the synthetic material, but its greater physicalstability made it more suitable for industrial applications. apacity is defined as the amount of e!changeableions a unit "uantity of resin will remove from a solution. It is usually e!pressed in )ilograins per cubic foot ascalcium carbonate.

Figure - $. icroscopic view of cellular resin beads /0& '& mesh1 of a sulfonated styrene divinylben#enestrong acid cation e!hcanger. / ourtesy of 2ohm and Haas ompany.1

+he development of a sulfonated coal cation e!change medium, referred to as carbonaceous #eolite,e!tended the application of ion e!change to hydrogen cycle operation, allowing for the reduction of al)alinityas well as hardness. 3oon, an anion e!change resin /a condensation product of polyamines andformaldehyde1 was developed. +he new anion resin was used with the hydrogen cycle cation resin in an


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attempt to deminerali#e /remove all dissolved salts from1 water. However, early anion e!changers wereunstable and could not remove such wea)ly ioni#ed acids as silicic and carbonic acid.

In the middle $%4&5s, ion e!change resins were developed based on the copolymeri#ation of styrene crosslin)ed with divinylben#ene. +hese resins were very stable and had much greater e!change capacities thantheir predecessors. +he polystyrene divinylben#ene based anion e!chan ger could remove all anions,including silicic and carbonic acids. +his innovation made the complete deminerali#ation of water possible.

Polystyrene divinylben#ene resins are still used in the ma6ority of ion e!change applications. Although thebasic resin components are the same, the resins have been modified in many ways to meet there"uirements of specific applications and provide a longer resin life. One of the most significant changes hasbeen the development of the macroreticular, or macroporous, resin structure.

3tandard gelular resins, such as those shown in Figure - $, have a permeable membrane structure. +hisstructure meets the chemical and physical re"uirements of most applications. However, in some applicationsthe physical strength and chemical resistance re"uired of the resin structure is beyond the capabilities of thetypical gel structure. acroreticular resins feature discrete pores within a highly cross lin)ed polystyrenedivinylben#ene matri!. +hese resins possess a higher physical strength than gels, as well as a greater

resistance to thermal degradation and o!idi#ing agents. acroreticular anion resins /Figure - 01 are alsomore resistant to organic fouling due to their more porous structure . In addition to polystyrenedivinylben#ene resins /Figure - 71 , there are newer resins with an acrylic structure, which increases theirresistance to organic fouling.

In addition to a plastic matri!, ion e!change resin contains ioni#able functional groups. +hese functionalgroups consist of both positively charged cation elements and negatively charged anion elements. However,only one of the ionic species is mobile. +he other ionic group is attached to the bead structure. Figure - 4 isa schematic illustration of a strong acid cation e!change resin bead , which has ionic sites consisting ofimmobile anionic /3O 7 81 radicals and mobile sodium cations /9a : 1. Ion e!change occurs when raw waterions diffuse into the bead structure and e!change for the mobile portion of the functional group. Ionsdisplaced from the bead diffuse bac) into the water solution.

CLASSIFICATIONS OF ION EXCHANGE RESINS

Ioni#able groups attached to the resin bead determine the functional capability of the resin. Industrial watertreatment resins are classified into four basic categories

3trong Acid ation /3A 1


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+he e!change reaction is reversible.


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strong al)ali, such as caustic soda, to return the resin to the hydro!ide form.


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stagnant vessel of water, some hardness will diffuse into the bul) water. +herefore, at the initiation of flow,the water effluent from a #eolite softener can contain hardness even if it has been regenerated recently.

After a few minutes of flow, the hardness is rinsed from the softener, and the treated water is soft.

+he duration of a service cycle depends on the rate of softener flow, the hardness level in the water, and theamount of salt used for regeneration. +able - $ shows the effect of regenerant level on the softeningcapacity of a gelular strong cation resin. 9ote that the capacity of the resin increases as the regenerantdosage increases, but the increase is not proportional. +he regeneration is less efficient at the higherregenerant levels. +herefore, softener operating costs increase as the regenerant level increases. As shownby the data in +able - $, a $'&> increase in regenerant salt provides only a @ > increase in operatingcapacity.

+able - $. Bffect of regenerant salt level on strong acid cation resin softening capacity.

Table 8-1. Bffect of regenerant salt level on strong acid cation resin softening capacity .

S"l #l$%f&' C"p"ci ( #!r%f &'

@ $-,&&&- 0&,&&&

$& 04,&&&$' 7&,&&&

E)*ip+en

+he e"uipment used for sodium #eolite softening consists of a softener e!change vessel, control valves andpiping, and a system for brining, or regenerating, the resin. Csually, the softener tan) is a vertical steelpressure vessel with dished heads as shown in Figure - @ . a6or features of the softening vessel include aninlet distribution system, free board space, a regenerant distribution system, ion e!change resin, and aresin retaining underdrain collection system.

+he inlet distribution system is usually located at the top of the tan). +he inlet system provides evendistribution of influent water. +his prevents the water from hollowing out flow channels in the resin bed,which would reduce system capacity and effluent "uality. +he inlet system also acts as a collector forbac)wash water.

+he inlet distributor consists of a central headerDhub with distributing lateralsDradials or simple baffle plates,which direct the flow of water evenly over the resin bed. If water is not prevented from flowing directly ontothe bed or tan) walls, channeling will result.

+he volume between the inlet distributor and the top of the resin bed is called the free board space. +hefree board allows for the e!pansion of the resin during the bac)wash portion of the regeneration without lossof resin. It should be a minimum of '&> of the resin volume /-&> preferred1.

+he regenerant distributor is usually a header lateral system that evenly distributes the regenerant brineduring regeneration. +he location of the distributor, @ in. above the top of the resin bed, prevents the dilutionof regenerant by water in the free board space. It also reduces water and time re"uirements fordisplacement and fast rinse. +he regenerant distributor should be secured to the tan) structure to preventbrea)age and subse"uent channeling of the regenerant.


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re"uired depends on the water flow, total hardness, and time desired between regeneration cycles. Aminimum bed depth of 04 in. is recommended for all systems.

+he underdrain system, located at the bottom of the vessel, retains ion e!change resin in the tan), evenlycollects the service flow, and evenly distributes the bac)wash flow. Cneven collection of water in service oruneven distribution of the bac)wash water can result in channeling, resin fouling, or resin loss.

Although several underdrain designs are used, there are two primary typesEsubfill and resin retaining. Asubfill system consists of multiple layers of support media /such as graded gravel or anthracite1 whichsupport the resin, and a collection system incorporating drilled pipes or subfill strainers. As long as thesupport layers remain intact, the resin will remain in place. If the supporting media becomes disturbed,usually due to improper bac)wash, the resin can move through the disrupted layers and e!it the vessel. Aresin retaining collector, such as a screened lateral or profile wire strainer, is more e!pensive than a subfillsystem but protects against resin loss.

+he main valve and piping system directs the flow of water and regenerant to the proper locations. +he valvesystem consists of a valve nest or a single multiport valve. A valve nest includes si! main valves serviceinlet and outlet, bac)wash inlet and outlet, regenerant inlet, and regenerantDrinse drain. +he valves may be

operated manually, or automatically controlled by air, electrical impulse, or water pressure. In some systems,a single multiport valve is used in place of the valve nest. As the valve rotates through a series of fi!edpositions, ports in the valve direct flow in the same manner as a valve nest. ultiport valves can eliminateoperational errors caused by opening of the incorrect valve but must be properly maintained to avoid lea)sthrough the port seals.

+he brining system consists of salt dissolvingDbrine measuring e"uipment, and dilution control e"uipment toprovide the desired regenerant strength. +he dissolvingDmeasuring e"uipment is designed to provide thecorrect amount of concentrated brine /appro!imately 0@> 9a l1 for each regeneration, without allowing anyundissolved salt into the resin. ost systems use a float operated valve to control the fill and draw down ofthe supply tan), thereby controlling the amount of salt used in the regeneration. Csually, the concentratedbrine is removed from the tan) by means of an eductor system, which also dilutes the brine to the optimumregenerant strength /- $&> 9a l1. +he brine can also be pumped from the concentrated salt tan) and mi!edwith dilution water to provide the desired regenerant strength.

Sof ener Oper" ion

A sodium #eolite softener operates through two basic cycles the service cycle, which produces soft waterfor use, and the regeneration cycle, which restores resin capacity at e!haustion.

In the service cycle, water enters the softener through the inlet distribution system and flows through thebed. +he hardness ions diffuse into the resin and e!change with sodium ions, which return to the bul) water.3oft water is collected in the underdrain system and discharged. 3ervice water flow to the softener should beas constant as possible to prevent sudden surges and fre"uent on off operation.

?ue to resin re"uirements and vessel designs, the softening operation is most efficient when a service flowrate between @ and $0 gpm per s"uare foot of resin surface area is maintained. ost e"uipment is designedto operate in this range, but some special designs utili#e a deep resin bed to permit operation at $' 0&gpmDft . ontinuous operation above the manufacturer5s suggested limits can lead to bed compaction,channeling, premature hardness brea)through, and hardness lea)age. Operating well below themanufacturer5s recommended flow rates can also negatively affect softener performance. At low flow rates,the water is not sufficiently distributed, and the optimum resin water contact cannot ta)e place.


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*plug* of displacement water which gradually moves the brine completely through the bed.

Fast 2inse. After completion of the displacement rinse, water is introduced through the inlet distributor at ahigh flow rate. +his rinse water removes the remaining brine as well as any residual hardness from the resinbed. +he fast rinse flow rate is normally between $.' and 0 gpm per s"uare foot of resin. 3ometimes it isdeter mined by the service rate for the softener.

Initially, the rinse effluent contains large amounts of hardness and sodium chloride. Csually, hardness isrinsed from the softener before e!cess sodium chloride. In many operations, the softener can be returned toservice as soon as the hardness reaches a predetermined level, but some uses re"uire rinsing until theeffluent chlorides or conductivity are near influent levels. An effective fast rinse is important to ensure higheffluent "uality during the service run. If the softener has been in standby following a regeneration, a secondfast rinse, )nown as a service rinse, can be used to remove any hardness that has entered the water duringstandby.

HOT ZEOLITE SOFTENING

eolite softeners can be used to remove residual hardness in the effluent from a hot process lime or lime

soda softener. +he hot process effluent flows through filters and then through a bed of strong acid cationresin in the sodium form /Figure - 1 . +he e"uipment and operation of a hot #eolite softener is identical tothat of an ambient temperature softener, e!cept that the valves, piping, controllers, and instrumentation mustbe suitable for the high temperature /00& 0'& F1. 3tandard strong cation resin can be used at temperaturesof up to 0 & F, but for a longer service life a premium gel or macroreticular resin is recommended.


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only #eolite softeners have e!perienced problems with silica and al)alinity levels in their boilers.

=ecause the resin is such an efficient filter, sodium #eolite softeners do not function efficiently on turbidwaters. ontinued operation with an influent turbidity in e!cess of $.& J+C causes bed fouling, short serviceruns, and poor effluent "uality. ost city and well waters are suitable, but many surface supplies must beclarified and filtered before use.

+he resin can be fouled by heavy metal contaminants, such as iron and aluminum, which are not removedduring the course of a normal regeneration. If e!cess iron or manganese is present in the water supply, theresin must be cleaned periodically. efficient, the F A from the e!changer would be e"ual to the theoretical mineral acidity/+ A1 of the water. +he F A is usually slightly lower than the + A because a small amount of sodium lea)s

through the cation e!changer. +he amount of sodium lea)age depends on the regenerant level, the flowrate, and the proportion of sodium to the other cations in the raw water. In general, sodium lea)ageincreases as the ratio of sodium to total cations increases.

As a cation e!change unit nears e!haustion, F A in the effluent drops sharply, indicating that the e!changershould be removed from service. At this time the resin should be regenerated with an acid solution, whichreturns the e!change sites to the hydrogen form. 3ulfuric acid is normally used due to its affordable cost andits availability. However, improper use of sulfuric acid can cause irreversible fouling of the resin with calcium


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sulfate.

+o prevent this occurrence, the sulfuric acid is usually applied at a high flow rate /$ gpm per s"uare foot ofresin1 and an initial concentration of 0> or less. +he acid concentration is gradually increased to @ -> tocomplete regeneration.

3ome installations use hydrochloric acid for regeneration. +his necessitates the use of special materials ofconstruction in the regenerant system. As with a sodium #eolite unit, an e!cess of regenerant /sulfuric orhydrochloric acid1 is re"uired up to three times the theoretical dose.

+o complete the deminerali#ation process, water from the cation unit is passed through a strong base anione!change resin in the hydro!ide form. +he resin e!changes hydrogen ions for both highly ioni#ed mineralions and the more wea)ly ioni#ed carbonic and silicic acids, as shown below

+he above reactions indicate that deminerali#ation completely removes the cations and anions from thewater. In reality, because ion e!change reactions are e"uilibrium reactions, some lea)age occurs. ostlea)age from cation units is sodium. +his sodium lea)age is converted to sodium hydro!ide in the anionunits. +here fore, the effluent pH of a two bed cation anion deminerali#er system is slightly al)aline. +hecaustic produced in the anions causes a small amount of silica lea)age. +he e!tent of lea)age from theanions depends on the chemistry of the water being processed and the regenerant dosage being used.

?eminerali#ation using strong anion resins removes silica as well as other dissolved solids. Bffluent silicaand conductivity are important parameters to monitor during a deminerali#er service run. =oth silica andconductivity are low at the end of the fast rinse, as shown in Figure - % .


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E)*ip+en "n, Oper" ion

+he e"uipment used for cation anion deminerali#ation is similar to that used in #eolite softening. +he primarydifference is that the vessels, valves, and piping must be made of /or lined with1 corrosion resistantmaterials. 2ubber and polyvinyl chloride /PK 1 are commonly used for ion e!change vessel linings. +hecontrols and regenerant systems for deminerali#ers are more comple!, to allow for such enhancements asstepwise acid and warm caustic regenerations.

?eminerali#ers are similar in operation to #eolite softeners. +he service flow rate guidelines for ademinerali#er range from @ to $& gpm per s"uare foot of resin. Flow rates of over $& gpm per s"uare foot ofresin cause increased sodium and silica lea)age with certain waters. Anion resin is much lighter than cationresin. +herefore, the bac)wash flow rates for anion e!change resins are much lower than those for cationresins, and anion resin e!pansion is affected by the temperature of the water more than cation resine!pansion. +he water used for each step of anion resin regeneration should be free from hardness, toprevent precipitation of hardness salts in the al)aline anion resin bed.

ontinuous conductivity instruments and silica analy#ers are commonly used to monitor anion effluent water"uality and detect the need for regeneration. In some instances, conductivity probes are placed in the resin

bed above the underdrain collectors to detect resin e!haustion before silica brea)through into the treatedwater occurs.

A,-"n "!es "n, Li+i " ions

?eminerali#ers can produce high purity water for nearly every use. ?eminerali#ed water is widely used forhigh pressure boiler feedwater and for many process waters. +he "uality of water produced is comparable todistilled water, usually at a fraction of the cost. ?eminerali#ers come in a wide variety of si#es. 3ystemsrange from laboratory columns that produce only a few gallons per hour to systems that produce thousandsof gallons per minute.

Gi)e other ion e!change systems, deminerali#ers re"uire filtered water in order to function efficiently. 2esinfoulants and degrading agents, such as iron and chlorine, should be avoided or removed prior todeminerali#ation. Anion resins are very susceptible to fouling and attac) from the organic materials presentin many surface water supplies. 3ome forms of silica, )nown as colloidal, or non reactive, are not removedby a deminerali#er. Hot, al)aline boiler water dissolves the colloidal material, forming simple silicates that aresimilar to those that enter the boiler in a soluble form. As such, they can form deposits on tube surfaces andvolatili#e into the steam.

DEAL/ALIZATION

Often, boiler or process operating conditions re"uire the removal of hardness and the reduction of al)alinitybut not the removal of the other solids. eolite softening does not reduce al)alinity, and deminerali#ation istoo costly. For these situations, a deal)ali#ation process is used. 3odium #eoliteDhydrogen #eolite /splitstream1 deal)ali#ation, chloride anion deal)ali#ation, and wea) acid cation deal)ali#ation are the mostfre"uently used processes.

So,i*+ Zeoli e%H(,ro!en Zeoli e #Spli S re"+' De"l0"li." ion

In a split stream deal)ali#er, a portion of the raw water flows through a sodium #eolite softener. +heremainder flows through a hydrogen form strong acid cation unit /hydrogen #eolite1. +he effluent from thesodium #eolite is combined with the hydrogen #eolite effluent. +he effluent from the hydrogen #eolite unitcontains carbonic acid, produced from the raw water al)alinity, and free mineral acids.


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bicarbonate al)alinity in the sodium #eolite effluent to carbonic acid as shown below

arbonic acid is unstable in water. It forms carbon dio!ide gas and water. +he blended effluents are sent toa decarbonator or degasser, where the carbon dio!ide is stripped from the water by a countercurrent streamof air. Figure - $& shows a typical split stream deal)ali#ation system .

+he desired level of blended water al)alinity can be maintained through control of the percentage of sodium#eolite and hydrogen #eolite water in the mi!ture. A higher percentage of sodium #eolite water results inhigher al)alinity, and an increased percentage of hydrogen #eolite water reduces al)alinity.

In addition to reducing al)alinity, a split stream deal)ali#er reduces the total dissolved solids of the water.+his is important in high al)alinity waters, because the conductivity of these waters affects the process andcan limit boiler cycles of concentration.

So,i*+ Zeoli e%C1lori,e Anion De"l0"li." ion

3trong base anion resin in the chloride form can be used to reduce the al)alinity of a water. but does not reduce total solids.


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in ppm as a O 71. In waters that are higher in al)alinity than hardness, the al)alinity is not removed to itslowest level. In waters containing more hardness than al)alinity, some hardness remains after treatment.Csually, these waters must be polished by a sodium #eolite softener to remove hardness. ?uring the initialportion of a wea) acid cation service run /the first 4& @&>1 some cations associated with mineral anionse!change, producing small amounts of mineral acids in the effluent. As the service cycle progresses,

al)alinity appears in the effluent. of the influent al)alinity, theunit is removed from service and regenerated with a &.'> sulfuric acid solution. +he concentration ofregenerant acid should be )ept below &.' &. >, to prevent calcium sulfate precipitation in the resin.


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by counterflow cation e!changers and mi!ed bed e!changers.

Co*n erflo5 C" ion E6c1"n!ers

In a conventional deminerali#er system, regenerant flow is in the same direction as the service flow, downthrough the resin bed. +his scheme is )nown as co current operation and is the basis for most ion e!change

system designs. ?uring the regeneration of a co current unit, the contaminants are displaced through theresin bed during the regeneration. At the end of the regeneration, some ions, predominately sodium ions,remain in the bottom of the resin bed. =ecause the upper portion of the bed has been e!posed to freshregenerant, it is highly regenerated. As the water flows through the resin during service, cations aree!changed in the upper portion of the bed first, and then move down through the resin as the bed becomese!hausted. 3odium ions that remained in the bed during regeneration diffuse into the decationi#ed waterbefore it leaves the vessel. +his sodium lea)age enters the anion unit where anion e!change producescaustic, raising the pH and conductivity of the deminerali#ed water.

In a counterflow regenerated cation e!changer, the regenerant flows in the opposite direction of the serviceflow. For e!ample, if the service flow is downward through the bed, the regenerant acid flow is up throughthe bed. As a result, the most highly regenerated resin is located where the service water leaves the vessel.

+he highly regenerated resin removes the low level of contaminants that have escaped removal in the top ofthe bed. +his results in higher water purity than co current designs can produce. +o ma!imi#e contactbetween the acid and resin and to )eep the most highly regenerated resin from mi!ing with the rest of thebed, the resin bed must stay compressed during the regenerant introduction. +his compression is usuallyachieved in one of two ways

a bloc)ing flow of water or air is used the acid flow is split, and acid is introduced at both the top and the bottom of the resin bed /Figure

- $$1

Mi6e, 4e, E6c1"n!ers

A mi!ed bed e!changer has both cation and anion resin mi!ed together in a single vessel. As water flowsthrough the resin bed, the ion e!change process is repeated many times, *polishing* the water to a very highpurity. ?uring regeneration, the resin is separated into distinct cation and anion fractions as shown in Figure- $0. +he resin is separated by bac)washing, with the lighter anion resin settling on top of the cation resin.2egenerant acid is introduced through the bottom distributor, and caustic is introduced through distributorsabove the resin bed. +he regenerant streams meet at the boundary between the cation and anion resin anddischarge through a collector located at the resin interface. Following regenerant introduction anddisplacement rinse, air and water are used to mi! the resins. +hen the resins are rinsed, and the unit isready for service.

ounterflow and mi!ed bed systems produce a purer water than conventional cation anion deminerali#ers,

but re"uire more sophisticated e"uipment and have a higher initial cost. +he more complicated regenerationse"uences re"uire closer operator attention than standard systems. +his is especially true for a mi!ed bedunit.

OTHER DEMINERALIZATION PROCESSES

+he standard cation anion process has been modified in many systems to reduce the use of costlyregenerants and the production of waste. odifications include the use of decarbonators and degassers,wea) acid and wea) base resins, strong base anion caustic waste /to regenerate wea) base anion


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e!changers1, and reclamation of a portion of spent caustic for subse"uent regeneration cycles. 3everaldifferent approaches to deminerali#ation using these processes are shown in Figure - $7 .

Dec"r$on" ors "n, De!"ssers

?ecarbonators and degassers are economically beneficial to many deminerali#ation systems, because they

reduce the amount of caustic re"uired for regeneration.


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into the steam. ondensate polishers filter out the particulates and remove soluble contaminants by ione!change.

ost paper mill condensate polishers operate at temperatures approaching 0&& F, precluding the use ofanion resin. ation resin, which is stable up to temperatures of over 0 & F, is used for deep bed condensatepolishing in these applications. +he resin is regenerated with sodium chloride brine, as in a #eolite softener.In situations where sodium lea)age from the polisher adversely affects the boiler water internal chemicalprogram or steam attemperating water purity, the resin can be regenerated with an ioni#ed amine solution toprevent these problems.

+he service flow rate for a deep bed polisher /0& '& gpm per s"uare foot of resin surface area1 is very highcompared to that of a conventional softener. High flow rates are permissible because the level of solubleions in the condensate can be usually very low. Particulate iron and copper are removed by filtration, whiledissolved contaminants are reduced by e!change for the sodium or amine in the resin.

+he deep bed cation resin condensate polisher is regenerated with $' lb of sodium chloride per cubic foot ofresin, in a manner similar to that used for conventional sodium #eolite regeneration. A solubili#ing orreducing agent is often used to assist in the removal of iron. 3ometimes, a supplemental bac)wash header

is located 6ust below the surface of the resin bed. +his subsurface distributor, used prior to bac)washing,introduces water to brea) up the crust that forms on the resin surface between regenerations.

An important consideration is the selection of a resin for condensate polishing. =ecause high pressure dropsare generated by the high service flow rates and particulate loadings, and because many systems operate athigh temperatures, considerable stress is imposed on the structure of the resin. A premium grade gelular ormacroreticular resin should be used in deep bed condensate polishing applications.

In systems re"uiring total dissolved solids and particulate removal, a mi!ed bed condensate polisher may beused. +he temperature of the condensate should be below $4& F, which is the ma!imum continuousoperating temperature for the anion resin. Additionally, the flow through the unit is generally reduced toappro!imately 0& gpmDft .

Ion e!change resins are also used as part of a precoat filtration system, as shown in Figure - $4, forpolishing condensate . +he resin is crushed and mi!ed into a slurry, which is used to coat individual septumsin a filter vessel. +he powdered resin is a very fine filtering medium that traps particulate matter and removessome soluble contaminants by ion e!change.


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compositions vary widely over time. A $&> increase in the hardness of the water to a sodium #eolitesoftener causes a $&> decrease in the service run length. An increase in the ratio of sodium to total cationscauses increased sodium lea)age from a deminerali#er system. 2egular chemical analysis of the influentwater to ion e!changers should be performed to reveal such variations.

Other causes of ion e!change operational problems include

Improper regenerations, caused by incorrect regenerant flows, times, or concentrations.anufacturer5s recommendations should be followed when regenerating ion e!change resins.

hanneling, resulting from either high or low flow rates, increased suspended solids loading or poorbac)washing. +his causes premature e!haustion even when much of the bed is in a regeneratedstate.

2esin fouling or degradation, caused by poor "uality regenerant. Failure to remove silica from the resin, which can result from low regenerant caustic temperature.

+his can lead to increased silica lea)age and short service runs. B!cess contaminants in the resin, due to previous operation past e!haustion loads. =ecause the

resin becomes loaded with more contaminants than a normal regeneration is designed to remove, a

double regeneration is re"uired following an e!tended service run.

Mec1"nic"l Pro$le+s

+ypical mechanical problems associated with ion e!change systems include

Gea)ing valves, which cause poor "uality effluent and prolonged rinses. =ro)en or clogged distributor, which leads to channeling. 2esin loss, due to e!cessive bac)washing or failure in the underdrain screening or support media. ation resin in the anion unit, causing e!tended rinse times and sodium lea)age into the

deminerali#ed water. Instrumentation problems, such as faulty totali#ers or conductivity meters, which may indicate a

problem when none e!ists, or may introduce poor "uality water to service. Instrumentation in thedeminerali#er area should be chec)ed regularly.

RESIN FOULING AND DEGRADATION

2esin can become fouled with contaminants that hinder the e!change process. Figure - $ shows a resinfouled with iron. +he resin can also be attac)ed by chemicals that cause irreversible destruction. 3omematerials, such as natural organics /Figure - $-1, foul resins at first and then degrade the resin as timepasses . +his is the most common cause of fouling and degradation in ion e!change systems, and isdiscussed under *Organic Fouling,* later in this chapter.

C"*ses of Resin Fo*lin!

Iron "n, M"n!"nese . Iron may e!ist in water as a ferrous or ferric inorganic salt or as a se"uesteredorganic comple!. Ferrous iron e!changes in resin, but ferric iron is insoluble and does not. Ferric iron coatscation resin, preventing e!change. An acid or a strong reducing agent must be used to remove this iron.Organically bound iron passes through a cation unit and fouls the anion resin. It must be removed along withthe organic material. anganese, present in some well waters, fouls a resin in the same manner as iron.

Al*+in*+ . Aluminum is usually present as aluminum hydro!ide, resulting from alum or sodium aluminateuse in clarification or precipitation softening. Aluminum floc, if carried through filters, coats the resin in a


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sodium #eolite softener. It is removed by cleaning with either acid or caustic. Csually, aluminum is not afoulant in a deminerali#er system, because it is removed from the resin during a normal regeneration.

H"r,ness Precipi " es . Hardness precipitates carry through a filter from a precipitation softener or formafter filtration by post precipitation. +hese precipitates foul resins used for sodium #eolite softening. +hey areremoved with acid.

3ulfate Precipitation. alcium sulfate precipitation can occur in a strong acid cation unit operated in thehydrogen cycle. At the end of a service cycle, the top of the resin bed is rich in calcium. If sulfuric acid isused as the regenerant, and it is introduced at too high a concentration or too low a flow rate, precipitation ofcalcium sulfate occurs, fouling the resin. After calcium sulfate has formed, it is very difficult to redissolve;therefore, resin fouled by calcium sulfate is usually discarded. ild cases of calcium sulfate fouling may bereversed with a prolonged soa) in hydrochloric acid.

=arium sulfate is even less soluble than calcium sulfate. If a water source contains measurable amounts ofbarium, hydrochloric acid regeneration should be considered.

Oil Fo*lin! . Oil coats resin, bloc)ing the passage of ions to and from e!change sites. A surfactant can be

used to remove oil. are must be e!ercised to select a surfactant that does not foul resin. Oil fouled anionresins should be cleaned with nonionic surfactants only.

icrobiological Fouling. icrobiological fouling can occur in resin beds, especially beds that are allowed tosit without service flow. icrobiological fouling can lead to severe plugging of the resin bed, and evenmechanical damage due to an e!cessive pressure drop across the fouled resin. If microbiological fouling instandby units is a problem, a constant flow of recirculating water should be used to minimi#e the problem.3evere conditions may re"uire the application of suitable sterili#ation agents and surfactants.

Silic" Fo*lin! . 3ilica fouling can occur in strong base anion resins if the regenerant temperature is too low,or in wea) base resins if the effluent caustic from the 3=A unit used to regenerate the wea) base unitcontains too much silica. At low pH levels, polymeri#ation of the silica can occur in a wea) base resin. It canalso be a problem in an e!hausted strong base anion resin. 3ilica fouling is removed by a prolonged soa) inwarm /$0& F1 caustic soda.

C"*ses of Irre-ersi$le Resin De!r"," ion

O6i," ion . O!idi#ing agents, such as chlorine, degrade both cation and anion resins. O!idants attac) thedivinylben#ene cross lin)s in a cation resin, reducing the overall strength of the resin bead. As the attac)continues, the cation resin begins to lose its spherical shape and rigidity, causing it to compact duringservice. +his compaction increases the pressure drop across the resin bed and leads to channeling, whichreduces the effective capacity of the unit.

In the case of raw water chlorine, the anion resin is not directly affected, because the chlorine is consumedby the cation resin. However, downstream strong base anion resins are fouled by certain degradation

products from o!idi#ed cation resin.If chlorine is present in raw water, it should be removed prior to ion e!change with activated carbon filtrationor sodium sulfite. Appro!imately $.- ppm of sodium sulfite is re"uired to consume $ ppm of chlorine.

O!ygen saturated water, such as that found following forced draft decarbonation, accelerates the destructionof strong base e!change sites that occurs naturally over time. It also accelerates degradation due to organicfouling.

T1er+"l De!r"," ion . +hermal degradation occurs if the anion resin becomes overheated during the


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service or regeneration cycle. +his is especially true for acrylic resins, which have temperature limitations aslow as $&& F, and +ype II strong base anion resins, which have a temperature limit of $&' F when in thehydro!ide form.

Or!"nic Fo*lin!

Organic fouling is the most common and e!pensive form of resin fouling and degradation. Csually, only lowlevels of organic materials are found in well waters. However, surface waters can contain hundreds of partsper million of natural and man made organic matter. 9atural organics are derived from decaying vegetation.+hey are aromatic and acidic in nature, and can comple! heavy metals, such as iron. +hese contaminantsinclude tannins, tannic acid, humic acid, and fulvic acid.

Initially, organics bloc) the strong base sites on a resin. +his bloc)age causes long final rinses and reducessalt splitting capacity. As the foulant continues to remain on the resin, it begins to degrade the strong basesites, reducing the salt splitting capacity of the resin. +he functionality of the site changes from strong baseto wea) base, and finally to a nonactive site. +hus, a resin in the early stages of degradation e!hibits hightotal capacity, but reduced salt splitting capacity. At this stage, cleaning of the resin can still return some, butnot all, of the lost operating capacity. A loss in salt splitting capacity reduces the ability of the resin to remove

silica and carbonic acid.

Organic fouling of anion resin is evidenced by the color of the effluent from the anion unit dur ingregeneration, which ranges from tea colored to dar) brown. ?uring operation, the treated water has higherconductivity and a lower pH.

Pre-en ion . +he following methods are used, either alone or in combination, to reduce organic fouling

Prechlorination and clarification.


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organic material but does not improve unit performance. +he condition of the resin should be closelymonitored to identify the optimum schedule for cleaning.

RESIN TESTING AND ANALYSIS

+o trac) the condition of ion e!change resin and determine the best time for cleaning it, the resin should be

periodically sampled and analy#ed for physical stability, foulant levels, and the ability to perform the re"uiredion e!change.

3amples should be representative of the entire resin bed. +herefore, samples should be collected atdifferent levels within the bed, or a grain thief or hollow pipe should be used to obtain a *core* sample.?uring sampling, the inlet and regenerant distributor should be e!amined, and the condition of the top of theresin bed should be noted. B!cessive hills or valleys in the resin bed are an indication of flow distributionproblems.

+he resin sample should be e!amined microscopically for signs of fouling and crac)ed or bro)en beads .Itshould also be tested for physical properties, such as density and moisture content /Figure - $%1 . +he levelof organic and inorganic foulants in the resin should be determined and compared to )nown standards and

the previous condition of the resin. Finally, the salt splitting and total capacity should be measured on anionresin samples to evaluate the rate of degradation or organic fouling.

86577388 what are zeolites

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