Chlor alkali

Modern Industrial production of chlor alkali

1. Chlor-alkali

3. Chloralkaliprocess
Achlor-alkaliplantproducestwoprincipleproducts;chlorine(Cl
2
)andsodiumhydroxide,whichiscommonlyknownascausticsoda
(NaOH).Thetermchlor-alkaliisderivedfromthetwoproductsproduced;chlorfromchlorineandalkalifromsodiumhydroxide(sodium
hydroxideispartofagroupofchemicalscalledalkalis).Thechlorineandsodiumhydroxideareproducedfromtheelectrolysisofan
aqueoussolutionofsodiumchloridebrine(saltwatersolution).Asanalternative,apotassiumchloridebrinecanbeusedtoproduce
potassiumhydroxide,whichiscommonlyknownascausticpotash(KOH).Hydrogengasisproducedasabyproductofthechloralkali
process.
Atypicalchlor-alkaliplantconsistsofsixmainprocessareas:
-brinepreparation
-electrolysis
-chlorinehandling
-caustichandling
-hydrogenhandling
-hypochlorite
Inadditiontothemainprocess,therearefourotherplantsupportareas:
-rawmaterialstorageandhandling
-productstorageandhandling
-effluentprocessing
-utilities
Inthebrinepreparationsectionoftheplant,anaqueoussolutionofsodiumchloride(NaCl)ispreparedandpurified.Thepurifiedbrineis
thenfedtotheelectrolyticarea,where,inthemembraneelectrolyzer,chlorine,sodiumhydroxide,andhydrogenareproduced.Handling
andprocessingofthesechemicalsdependsontheenduseoftheproducts.Typically,thechlorinegasiscooled,dried,compressed,
liquifiedandtransferredtostorage.Sodiumhydroxideisoftenonlycooledbeforebeingtransferredtostorage,althoughitcanalsobe
concentratedtohigherstrengthsorconvertedtoasolidform.Thebyproducthydrogengasiseitherventedtotheatmosphereoriscooled
andutilizedintheproductionofhydrochloricacid,usedasachemicalfeedstocktootherprocesses(suchastheproductionofhydrogen
peroxide),bottledandsold,orusedasfuelinboilersforsteamgeneration.Allchlorinecontainingventsandreliefsintheplantunder
continuousoremergencydutiesareprocessedinahypochloritesystem.Thehypochloritesystemuseseitherasodiumhydroxideor
calciumhydroxidesolutiontoabsorbandreactthechlorinetoformeithersodiumorcalciumhypochloritesolutions.Ifahypochlorite
solutionisnotrequired,itcanbere-processedintheplant.

4. Raw Material (Rock salt) storage

5. BrineTreatment
Themainchemicalfeedtoachloralkaliplantissalt,whichmustbedissolvedinwatertoformabrinesolution.However,toobtainlongterm
performancefromtheelectrolyzersitisnecessarytoproduceabrinewithverylowimpuritylevels.Calcium,magnesiumandsulphatearethe
majorimpuritieswhichoccurnaturallyintherawsaltandmustbecontrolledduringplantoperation.Inaddition,sodiumchlorateisproduced
toasmallextentduringelectrolysisandmustalsobecontrolled.
Thebrinepreparationsectionofthechloralkaliplantconsistsoffiveprocessfunctions.Theseare:
-saltdissolving,
-primarybrinetreatment,
-secondarybrinetreatment,
-deionizedbrinestorage,and
-weakbrinedechlorination.
PrimaryTreatment
Inthefirststepofbrinepreparation,rawsalt(sodiumchloride)isdissolvedinamixtureofwarmwaterandrecycledweakdechlorinated
brine(fromtheelectrolyticarea)toproduceasaturatedbrinesolution.Thesaturatedbrinesolutionisthenpumpedtotheprimarybrine
treatmentsystemwheremostofthecalciumandmagnesium(dissolvedimpurities)areremovedbychemicalprecipitation.Sodaash(sodium
carbonate)andsodiumhydroxideareaddedtothebrineintheprimarytreatmentsectionandreactwiththecalciumandmagnesiumtoformcarbonate)andsodiumhydroxideareaddedtothebrineintheprimarytreatmentsectionandreactwiththecalciumandmagnesiumtoform
insolublesolidprecipitates.Demineralizedwaterisalsoaddedtoadjustthesaltcontentofthebrinetoaspecificconcentration.Depending
uponthelevelsofimpuritiespresentintherawsalt,thebrineisthenclarifiedandfiltered.Ifrelativelyhighqualitysaltisused,thennormally
onlyfiltrationofthebrineisrequiredtoremovethesolidsformedintheprimarytreatmentsystem.
IonExchange
Afterfiltration,thebrineisfurtherprocessedinthesecondarybrinetreatmentsystemwhichremovesessentiallyalltheremainingcalcium
andmagnesiumimpuritiesbyionexchangebeforebeingtransferredtothepurifiedbrinestoragetank.Thepurifiedbrineisthenheatedand
pumpedtotheelectrolyzerswherepartofthesodiumchlorideandwaterareconsumedtoproducechlorineandsodiumhydroxide.After
passingthroughtheelectrolyzer,theweakordepletedbrinesolutioncontainsdissolvedchlorinegas.Beforethisweakbrinecanbere-usedin
saltdissolving,thedissolvedchlorineisremovedintwosteps.Inthefirststep,hydrochloricacidisaddedtothebrinetoreducethepHwhich
forcessomeofthechlorinegasoutofsolution.Theremainingdissolvedchlorineisfirstremovedbyairstrippingthebrinesolution,andthen
anyresidualdissolvedchlorineischemicallyremovedbytheadditionofsodiumhydroxideandsodiumsulphiteinthebrinedechlorination
step.
Theconcentrationofsodiumsulphate,whichenterswiththerawsalt,andsodiumchlorate,whichisproducedinsmallquantitiesinthe
electrolyticsystem,mustbecontrolledtomaintainefficientoperationoftheelectrolyzers.Typicallyasmallpurgeofbrineisusedtocontrol
theleveloftheseimpurities.Chemicalprecipitationofthebrineimpuritiescanalsobeusedtocontrolthelevelofimpurities.Theweakbrine
isthenrecycledbacktothesaltdissolvingsystem.

6. Brine Purification Section
Caustic Concentration and Solidification
Plant

7. ElectrolyzerOptions
Chlorineandsodiumhydroxide,aswellasthebyproducthydrogen,areproducedintheelectrolyzersbytheelectrolysisofanaqueoussolution
ofsodiumchloride.Electrolyzerstypicallyconsistofanalternatingseriesofanodeandcathodeplateswithaselectivesemi-permeable
membranebetweeneachanodeandcathode.Directcurrent(DCelectricity)deliveredtotheelectrolyzerflowsfromtheanode(positive
electrode)throughthebrineintheanodecompartment,throughthemembrane,tothesodiumhydroxideinthecathodecompartment,andinto
thecathode(negativeelectrode).Thepurifiedbrineflowingthroughtheanodecompartmentoftheelectrolyzerisoftencalledtheanolyte,
becauseitistheelectricallyconductivesolutionpassingovertheanode.Similarly,thesodiumhydroxideflowingthroughthecathode
compartmentoftheelectrolyzerisoftencalledthecatholyte,becauseitistheelectricallyconductivesolutionpassingoverthecathode.
Chemicalreactionsandexchangeofmaterialstakingplaceinamembraneelectrolyzerduringelectrolysis.Purifiedbrineisfedtotheanode
compartmentwherechlorinegasisevolvedanddepletedorweakbrineisdischarged.Cooled,recycledsodiumhydroxideisfedtothecathode
compartmentwherehydrogengasisevolvedandstrengthenedsodiumhydroxideisdischarged.Thesemi-permeablemembranebetweenthe
anodeandcathodeselectivelypermitssodiumions(Na
+
)andwatermoleculestopassthroughthemembranebutpreventsthediffusionof
chlorideions(Cl
-
)andhydroxylions(OH
-
)throughthemembrane.Asaresult,thecurrentefficiencyforsodiumhydroxidegenerationishigh,
andthesaltcontentofthesodiumhydroxidesolutionisverylow.Thechemicalreactionsoccurringareasfollows:
2Na
+
+ 2Cl
-
→ 2Na
+
+ Cl
2
+ 2e
-
(anode)
sodium chloride goes to sodium ions and chlorine
2H
+
+ 2OH
-
+ 2e
-
→ H
2
+ 2OH
-
(cathode)
water goes to hydrogen and hydroxyl ions
2NaCl + 2H
2
O → Cl
2
+ 2NaOH + H
2
(net reaction) sodium chloride and water goes to chlorine, sodium hydroxide and hydrogen
Thesodiumionsthatpassthroughthemembranecombinewithhydroxylionsproducedatthecathodetoformsodiumhydroxide.
Thecatholytesystemconsistsofatank,pump,andheatexchangerinwhichsodiumhydroxideisrecirculatedthroughtheelectrolyzersandbled
offasproduct.Fromtheelectrolyzers,hotstrengthenedsodiumhydroxidedrainsintothecatholytetank.Acontrolledflowofsodiumhydroxide
ispumpedtothesodiumhydroxideproducthandlingsystemwhiletherestisrecycledtotheelectrolyzers.Therecycledstreamiscooledand
dilutedbytheadditionofdemineralizedwaterandrecirculatedtotheelectrolyzers.
Asdiscussedinthebrinepreparationsection,weakbrine(anolyte)containingdissolvedchlorinedrainsfromtheelectrolyzerstotheweakbrine
tank.HydrochloricacidisaddedtothebrinetolowerthepHandreleasemostofthedissolvedchlorine.Thebrineisthenpumpedtothebrine
dechlorinationsystemforfurtherchlorineremoval.

8. Membrane cell

10. Evaporationandsaltseparation
Thedecompositionefficiencyofthecellsbeingintherangeofonly50%,abouthalfoftheNaClchargedisintheweak
causticandisrecoveredbyreasonofitslowsolubilityincausticsodasolutionsafterconcentration.Hencetheweak,
about10or12%NaOHsolutionisevaporatedtoabout50%NaOHinadoubleortriple-effectevaporatorwithsalt
separatorsandthenpassesthroughasettlerandwashingfilter.Thesaltsorecoveredisagainmadeintochargingbrine.
Intheevaporators,nickeltubesaregenerallyusedtolessenironcontamination.Sincethecausticsodaliquorfromthe
evaporatorcontains50%NaOH,itwilldissolveonlyabout1%NaClandothersodiumsaltsaftercooling.Thisliquor
maybesold,aftersettling,asliquidcausticsoda.
Finalevaporation
Thecooledandsettled50%causticisconcentratedinasingleeffectfinalevaporatorto70to75%NaOH.Thisvery
strongcausticshouldbehandledinsteam-jackettedpipestopreventsolidification.Itisruntothefinishingpots.Another
methodofdehydrating50%causticutilizestheprecipitationofNaOHmonohydrate.Thismonohydratecontainsless
waterthanintheoriginalsolution.Precipitationisaccomplishedbytheadditionofammoniatothe50%solution.Ifthe
50%causticistreatedwithanhydrousammoniainacounter-currentmanner,free-flowinganhydrouscrystalsseparate
fromtheresultingaqueousammonia.Theprocesshastobecarriedoutinpressurevessels.fromtheresultingaqueousammonia.Theprocesshastobecarriedoutinpressurevessels.
Finishingcausticinpots
50%causticwasonetimefinishedincastirondirect-firedpots.Theheatefficiencyissolowthatthatnowithandles
only70to75%NaOH.Thefinaltemperatureis500to600
0
Candboilsoffallbutabout1%orlessofwater.Thesepots
arenowbeingreplacedbyDowthermheatedevaporatorsforcausticevaporationabove50%.Thehotanhydrouscaustic
istreatedwithsulfurtoprecipitateironandsettled.Themoltencausticispumpedoutintothinsteeldrumsorintothe
flakingmachine.
Specialpurificationofcaustic
50%causticcontainsimpuritieslikecolloidaliron,NaClandNaClO
3
.Theironisoftenremovedbytreatingthecaustic
with1%byweightof300-meshcalciumcarbonateandfilteringtheresultingmixture.Thechlorideandchloratemaybe
removedbyallowingthe50%caustictodropdownthroughacolumnof50%aqueousammoniasolution.Thistreatment
producescausticalmostasfreeofchloridesandchloratesasthatmadebythemercuryprocess.Thesepurification
methodsgivehigh–gradecausticwithlessthan1%impurities(anhydrousbasis)whilststandardgradecontains2½to3%
impurities(anhydrousbasis).

11. CausticHandling
Thesodiumhydroxideorcausticsodaproducedintheelectrolyzersishotandataconcentrationofapproximately32%.
Aswithchlorinehandling,theprocessequipmentrequiredforthecausticsodahandlingsystemwilldependontheenduse
ofthecaustic.Typically,thepure(orproduct)causticsodaiscooledandthentransferredtostoragetanksforholdingprior
todeliverytocustomers.Thecausticsodacanalsobeconcentrated,usuallyto50%inacausticevaporator.Steamisused
toboilthecausticandremovewaterfromthesolutionundervacuumconditions.Ifdesired,thecausticsodacanalsobe
furtherconcentratedtosolidflakeform.
ChlorineHandling
Thechlorinegasproducedintheelectrolyzersishotandsaturatedwithwatervapour.Theprocessequipmentrequiredfor
chlorinehandlingwilldependupontheenduseofthechlorinegas.Thechlorinehandlingsectionofatypicalchloralkali
plantconsistsofthefourprocessfunctions.Theseare:
-cooling
-drying
-compression-compression
-liquefaction
Ifthechlorineistobeusedinproducinghydrochloricacid(HCl),thechlorineusuallyonlyneedstobecooledand
compressedpriortouseinaHClsynthesisunit.
Afterthehotsaturatedchlorinegasfromtheelectrolyzersiscollectedabovetheelectrolyzersinapipeheaderitiscooled
andfilteredtoremovethebulkofthemoistureinthegasandanysaltwhichmayhavebeencarriedoverinthechlorine
gas.Condensedwaterfromthecoolerisdrainedtotheweakbrinesystem.Theremainingmoistureisessentiallyremoved
inthedryingsystem.Thechlorinedryingsystemusuallyconsistsoftwochlorinedryingtowersthatcirculateconcentrated
sulphuricacidwhichabsorbsmoisturefromthechlorinegas.Strongsulphuricacidiscontinuallyaddedtothedrying
systemwhileweaksulphuricacidisbledoffandremovedfromtheplant.Thedrychlorinegasisthencompressedbya
sulphuricacidringcompressor.Aftercompression,thechlorinegasiscooledandliquifiedandthentheliquidchlorineis
transferredtostoragetanks.

12. Chlorine Storage Tanks & Freon
compressors
Liquid Caustic Storage Tanks

13. Themostcommonchloralkaliprocessinvolvestheelectrolysisofaqueoussodiumchloride(abrine)inamembranecell.
Saturatedbrineispassedintothefirstchamberofthecellwherethechlorideionsareoxidisedattheanodetochlorine:
2Cl
–
→ Cl
2
+ 2e
–
Atthecathodehydrogeninthewaterisreducedtohydrogengas,releasinghydroxideionsintothesolution:
2H
2
O + 2e
–
→ H
2
+ 2OH
–
Thenon-permeableionexchangemembraneatthecenterofthecellallowsthesodiumions(Na
+
)topasstothesecond
chamberwheretheyreactwiththehydroxideionstoproducecausticsoda(NaOH).Theoverallreactionforthe
electrolysisofbrineisthus:
2NaCl + 2H
2
O → Cl
2
+ H
2
+ 2NaOH
Amembranecellisusedtopreventthereactionbetweenthechlorineandhydroxideions .Ifthisreactionweretooccur
thechlorinewouldbedisproportionatedtoformchlorideandhypochloriteions:thechlorinewouldbedisproportionatedtoformchlorideandhypochloriteions:
Cl
2
+ 2OH
–
→ Cl
–
+ ClO
–
+ H
2
O
Athighertemperatures,333K,chloratecanbeformed:
3Cl
2
+ 6OH
–
→ 5Cl
–
+ ClO
3
–
+ H
2
O
Becauseofthecorrosivenatureofthechlorineproduced,theanodehastobemadefromannon-reactivemetalsuchas
titanium,whereasthecathodecanbemadefromsteel .
Inthemembranecell,theanodeandcathodeareseparatedbyanion-permeablemembrane.Saturatedbrineisfedto
thecompartmentwiththeanode(theanolyte).Directcurrent(D.C.)ispassedthroughthecellandtheNaClsplitsinto
itsconstituentcomponents.ThemembranepassesNa+ionstothecathodecompartment(catholyte),whereitforms
sodiumhydroxideinsolution.Thechlorideionsareoxidisedtochlorinegasattheanode,whichiscollected,purifiedand
stored.HydrogengasandHydroxideionsareformedatthecathode.

14. Membrane Electrolyzers

15. Diaphragmcell
Inthediaphragmcellprocess,therearetwocompartmentsseparatedbyapermeablediaphragm,oftenmadeofasbestos
fibers.Brineisintroducedintotheanodecompartmentandflowsintothecathodecompartment.Similarlytothe
MembraneCell,chlorideionsareoxidizedattheanodetoproducechlorine,andatthecathode,waterissplitintocaustic
sodaandhydrogen.Thediaphragmpreventsthereactionofthecausticsodawiththechlorine.Adilutedcausticbrine
leavesthecell.Thecausticsodamustusuallybeconcentratedto50%andthesaltremoved.Thisisdoneusingan
evaporativeprocesswithaboutthreetonnesofsteampertonneofcausticsoda.Thesaltseparatedfromthecausticbrine
canbeusedtosaturatedilutedbrine.Thechlorinecontainsoxygenandmustoftenbepurifiedbyliquefactionand
evaporation.

16. Mercurycell
Inthemercury-cellprocess,alsoknownastheCastner-Kellnerprocess,asaturatedbrinesolutionfloatsontopofthe
cathodewhichisathinlayerofmercury.Chlorineisproducedattheanode,andsodiumisproducedatthecathode
whereitformsasodium-mercuryamalgamwiththemercury.Theamalgamiscontinuouslydrawnoutofthecelland
reactedwithwaterwhichdecomposestheamalgamintosodiumhydroxideandmercury.Themercuryisrecycledinto
theelectrolyticcell.Mercurycellsarebeingphasedoutduetoconcernsaboutmercurypoisoningfrommercurycell
pollution.
Metallic Sodium at the mercury cathode side is kept in continuous flow. Depleted brine saturated with Cl
2
overflows
from the outlet end of the cell and flows by gravity to the depleted brine receiver. The cell is operated under a slight
negative pressure in order to withdraw Cl
2
gas to the processing system by a chlorine blower.
Metallic Na amalgamates with the flowing Hg cathode and leaves the cell through a weir seal entering into the outlet
end box. From the end box the amalgam flows by gravity to the decomposer.end box. From the end box the amalgam flows by gravity to the decomposer.
Cell Reaction:
Anodic : Cl
-
------------->½ Cl
2
+ e
Cathodic: Na
+
+x Hg + e-------------->NaHgx
Mercury –Sodium amalgam enters the decomposer. Decomposer is a short circuited primary cell in which the sodium
amalgam is the soluble anodeand the graphite is the insoluble cathode in an electrolyte of sodium hydroxide.
Decomposer Reaction:
2Na(Hg)x-------------------->2Na
+
+ x Hg + 2e
2H
2
O + 2e------------------->2OH
-
+ H
2
2Na(Hg)x + 2H
2
O------------>2NaOH + H
2
+ xHg

17. Mercury cells
Schematic of a mercury cell

18. Chlorinecoolinganddrying
Chlorinegasexitingthecelllinemustbecooledanddriedsincetheexitgascanbeover80°C(176°F)andcontains
moisturethatallowschlorinegastobecorrosivetoironpiping.Coolingthegasallowsforalargeamountofmoisture
fromthebrinetocondenseoutofthegasstream.Thisreducesboththecoolingrequirementsandfeedflowofsulfuric
acidrequiredinthedryingtowers.Coolingalsoimprovestheefficiencyofboththecompressionandtheliquefaction
stagethatfollows.Chlorineexitingisideallybetween18°C(64°F)and25°C(77°F).Aftercoolingthegasstream
passesthroughaseriesoftowerswithcounterflowingsulfuricacid.Thesulfuricacidisfedintothefinaltowerat98%
andthefirsttowertypicallyhasastrengthbetween66%and76%dependingonmaterialsofconstruction.Thesetowers
progressivelyremoveanyremainingmoisturefromthechlorinegas.Afterexitingthedryingtowersthechlorineis
filteredtoremoveanyremainingsulfuricacid.
Compressionandliquefaction
Severalmethodsofcompressionmaybeused:liquidring,reciprocating,orcentrifugal.Thechlorinegasiscompressed
atthisstageandmaybefurthercooledbyinter-andafter-coolers.Aftercompressionitflowstotheliquefiers,whereit
iscooledenoughtoliquefy.Noncondensablegasesandremainingchlorinegasareventedoffaspartofthepressureiscooledenoughtoliquefy.Noncondensablegasesandremainingchlorinegasareventedoffaspartofthepressure
controloftheliquefactionsystems.Thesegasesareroutedtoagasscrubber,producingsodiumhypochlorite,orusedin
theproductionofhydrochloricacid(bycombustionwithhydrogen)orethylenedichloride(byreactionwithethylene).
Storageandloading
Liquidchlorineistypicallygravity-fedtostoragetanks.Itcanbeloadedintorailorroadtankersviapumpsorpadded
withcompresseddrygas.
Hydrogenhandling
Hydrogenproducedmaybeventedunprocesseddirectlytotheatmosphereorcooled,compressedanddriedforusein
otherprocessesonsiteorsoldtoacustomerviapipeline,cylindersortrucks.Somepossibleusesincludethe
manufactureofhydrochloricacidorhydrogenperoxide,aswellasdesulfurizationofpetroleumoils,oruseasafuelin
boilersorfuelcells.

19. Chlorineprocessing,storageandhandling
Generally,beforethechlorinecanbeused,itgoesthroughaseriesofprocessesforcooling,cleaning,drying,compression
andliquefaction.Thechlorineprocessusuallytakeshot,wetcellgasandconvertsittoacold,drygas.Chlorinegas
exitingthecelllinemustbecooledanddriedsincetheexitgascanbeover80°C(176°F)andsaturatedwithwaterthat
allowschlorinegastobecorrosivetoironpiping.
Cooling
Intheprimarycoolingprocess,thetotalvolumeofgastobehandledisreducedandalargeamountofmoistureis
condensed.Coolingisaccomplishedinoneorseveralstageswithwater,brineorotherfluids.Careistakentoavoid
excessivecoolingbecause,ataround10ºC,chlorinecancombinewithwatertoformasolidmaterialknownaschlorine
hydrate(Cl
2
·nH
2
O;n=7–8).Maintainingtemperaturesabove15°Cpreventsblockagesintheprocessequipment
Twomethodsaremostfrequentlyusedtocoolchlorinegas.
Onemethodisindirectcoolingthroughatitaniumsurface(usuallyinasingle-passverticalshell-and-tubeheat
exchanger).Theresultantcondensateiseitherfedbackintothebrinesystemofthemercuryormembranecelltechniqueexchanger).Theresultantcondensateiseitherfedbackintothebrinesystemofthemercuryormembranecelltechnique
orisdechlorinatedbyevaporationinthecaseofthediaphragmcelltechnique.Thismethodcauseslesschlorinetobe
condensedorabsorbedandgenerateslesschlorine-saturatedwaterfordisposal.Indirectcoolingcanbecarriedoutin
once-through,open-recirculating,orclosed-loopsystems.
Anothermethodisdirectcoolingwithwater(orbrineorotherfluids).Thechlorinegasiscooledbypassingitdirectlyinto
thebottomofatower.Waterissprayedfromthetopandflowscountercurrentlytothechlorine.Thecoolingwateris
generallyfreeoftracesofammoniumsalts,toavoidtheformationofnitrogentrichloride.Thismethodhastheadvantage
ofbettermasstransfercharacteristicsandhigherthermalefficiencies.Directcoolingisusuallycarriedoutinclosed-loop
systems.
Cleaningofwetchlorine
Followingprimarycooling,waterdropletsandimpuritiessuchasbrinemistareremovedmechanicallybyusingspecial
filterswithglasswoolfillingsorporousquartzgranules,orbymeansofanelectrostaticprecipitator.Chlorineisthen
passedtothedryingtowers.

20. Drying
Chlorinefromthecoolingsystemismoreorlesssaturatedwithwatervapour.Thewatercontentistypically1–3vol-%.
ThismustbereducedinordertoavoiddownstreamcorrosionandtominimisetheformationofhydratesThedryingof
chlorineiscarriedoutalmostexclusivelywithconcentratedsulphuricacid(96–98wt-%)incountercurrentcontacttowers
intwotosixstages,whichreducethemoisturecontenttolessthan20mg/m
3
.Theremainingmoisturecontentdependson
thetemperatureandconcentrationofthesulphuricacidinthelastdryingstage.Forlowtemperatureliquefaction,alower
moisturecontentisrequired,whichcanbeachievedbyaddingmoreequilibriumstagestothedryingtowersorbyusing
molecularsievestolevelsof3–9mg/m
3
.Thenumberofstagesisusuallyincreasedtolowerthefinalstrengthofthespent
sulphuricacid.Forexample,threestagesareneededtoreachaspentacidconcentrationof50–65wt-%whilesixstages
areneededforafinalconcentrationof30–40wt-%.Thecolumnscontainplasticpackingresistanttochlorineand
sulphuricacidtoimprovefluidsdistribution,increaseefficiencyandlowerpressuredrops,andthusreduceenergy
consumption.Theheatliberatedduringdilutionofthecirculatingacidisremovedbytitaniumheatexchangers,andthe
spentacidisdechlorinatedchemicallyorbystripping.Theconcentrationofthespentaciddependsonthenumberof
dryingstagesandthefurtherpotentialuseormethodofdisposal.Insomecases,theacidisreconcentratedto96wt-%by
heatingitundervacuumandthenitissubsequentlyrecirculated.Sometimestheacidissoldorusedforotherpurposes.
Rarely,itbecomeswaste.Rarely,itbecomeswaste.
Cleaningofdrychlorine
Whenleavingthetopofthedryingtower,drychlorinepassesthroughhighefficiencydemistersorapackedbedto
preventtheentrainmentofsulphuricaciddroplets.
Compression
Afterdryingandcleaning,chlorinegasmaybecompressedbyavarietyofcompressors,dependingonthethroughputand
thedesiredpressure:
Rotarycompressors,suchas:
sulphuricacidliquidringcompressorsforthroughputsof150t/dpercompressorandforpressuresof4baror,intwo-
stagecompressors,12bar;
screwcompressorsforlowthroughputsandforpressuresofupto16bar.
Reciprocatingcompressors,suchas:
dryringcompressorsforthroughputsof200t/dpercompressorandforpressuresofupto16bar.
Centrifugalcompressors,suchas:
turbocompressorsinmono-ormulti-stageoperationforthroughputsofupto~1800t/dperunitandforpressuresof
upto16bar;

21. Becauseofheatbuild-upfromcompression,multi-stageunitswithcoolersbetweenstagesareusuallynecessary.
Compressorsealsaregenerallyfittedwithapressurisedpurgetoinhibittheleakageofchlorinetotheatmosphere.Dry
chlorineathightemperaturescanreactspontaneouslyanduncontrollablywithiron.Chlorinetemperaturesaretherefore
usuallykeptbelow120°C.
Liquefaction
Liquefactioncanbeaccomplishedatdifferentpressureandtemperaturelevels:atambienttemperatureandhighpressure
(forexample18ºCand7–12bar),atlowtemperatureandlowpressure(forexample-35ºCand1bar)oranyother
intermediatecombinationoftemperatureandpressure.
Importantfactorsforselectingappropriateliquefactionconditionsincludethecompositionofthechlorinegas,thedesired
purityoftheliquidchlorineandthedesiredyield.Increasingtheliquefactionpressureincreasestheenergyconsumption
ofcompression,althoughthenecessaryenergyforcoolingdecreases,resultinginanoverallreductioninenergy
consumption.
Theliquefactionyieldistypicallylimitedto90–95%inasingle-stageinstallation,ashydrogenisconcentratedinthe
residualgasanditsconcentrationneedstobekeptbelowthelowerexplosivelimit.residualgasanditsconcentrationneedstobekeptbelowthelowerexplosivelimit.
Thechoiceoftherefrigerantinacertainstageoftheliquefactiondependsonthepressureofthechlorine.Whenthe
pressureissufficientlyhigh,watercanbeusedasanindirectrefrigerant.Whenthepressureisrelativelylow,other
refrigerantssuchasammonia(indirectcooling)orliquidchlorine(directcooling)areused.
Theresidualchlorineinthetailgascanbeusedtoproducehypochlorite,iron(III)chlorideorhydrochloricacid.The
residualchlorinewhichcannotbevaporisedisthenledtothechlorineabsorptionunit.Insomecases,itisrecoveredbyan
absorption-desorptionprocesswithcarbontetrachloride.Thelatterhasthedisadvantageofusingatoxicsubstancewitha
highozonedepletionandglobalwarmingpotential.
Handlingandstorage
Liquefiedchlorineisstoredatambientorlowtemperature.Thepressurecorrespondstothevapourpressureofthe
liquefiedchlorineatthetemperatureinthestoragetank.Pressurestorageatambienttemperatures(~7barat20°C)has
advantagesofsimplicityofoperation,easeofvisualexternalinspections,aswellaslowerenergyandinvestmentcosts.
Low-pressurestorageoperatingaroundtheboilingpointofliquidchlorine(-34°C)requiresmorecomplexinfrastructure,
particularsafetymeasuresandhigherenergycosts.

23. Sulphate
Removal
Demin. Water
Chlorine
Packing, Filling
Vaporization
Hypo
Product
Chlorine
Storage
Brine
Saturation
Primary
Treatment
Secondary
Treatment
Electrolysis
Salt
Hydrogen
Handling
Caustic
Storage
Chlorine
Compressio
n
Chlorin
e
Drying
HCl
Storage
Chlorine
Liquefaction
Caustic
Product
HCl
Product
Hypo
Production
Chlorine
Product
Sodium Sulphite
Hypo
Storage
HCl
Production
Caustic
Concentratio
n
Sulphuric Acid
Carbon Dioxide
To Hypo
HC
l
Sulphuric
Acid
NaOH
NaO
Chlor-Alkali
Brine
Dechlorinatio
n
DC
Rectificatio
n
AC Power Supply
Demineralized Water
Storage
Hypo
Destructio
n
Chlorate
Destruction
n
HCl
NaO
H

24. Schematic of a diaphragm cell

25. Schematic of a membrane cell

26. Bleaching Powder
Calcium oxy-chloride is the chemical name of bleaching powder.
Preparation of Bleaching Powder
Passing chlorine gas over dry slaked lime (Ca(OH)
2
), gives
bleaching powder.
Itconsistsofaverticalcast-irontower.Thetowerisprovided
withahopperatthetop,twoinletsnearthebase(onefor
chlorineandotherforhotair)andanexitforwastegasesnear
thetop.
Thetowerisfittedwitheightshelvesatdifferentheightseach
equippedwithrotatingrakes.Theslakedlimeisintroduced
throughthehopperanditcomesincontactwithchlorine,
whichslowlymovesupwards.Bleachingpowderiscollectedinwhichslowlymovesupwards.Bleachingpowderiscollectedin
abarrelatthebase.Thechlorineusedinthemanufactureof
bleachingpowdershouldbediluteandthetemperatureshould
bemaintainedbelow40
o
C.
TheHasenCleverplantconsistsoffourcylinderofcastiron.Eachcylinderisabout2mto3mlong.Eachcylinderis
providedwiththestirrertoensuremixingofsubstances.Thereisaninletthatispresentintheuppermostcylinderfor
Ca(OH)
2
.ThebottomcylinderhasaninletthatispresentforCl
2
andoutletatthebottomforthebleachingpowdertobe
accumulated.Eachcylinderisconnectedtotheothercylinderbymeansofpipes.
Inthisprocesstherearecastironcylindersoperatinginserieswithhydratedlimeandchlorinebeingfedcountercurrentto
eachother.Thecylindersareprovidedwithrotatingbladesandarearrangedhorizontallyoneabovetheothertherotating
bladesactbothasmixedandconveyorsoftheinsidemass.Hydratedlimeischargedatoneendofthetopmostcylinder
whilechlorineisintroducedattheotherendofbottom-mostcylinder.Withtherotationofthebladesthereisanintimate
mixingofchlorineandlimewithsimultaneousmovementofslakedlimecountercurrenttochlorinegas.Thechlorinated
limeisdischargedfromthebottomcylinderandtheun-reactedchlorineisrecoveredfromthetopcylinderandrecycled
alongwiththefreshchlorine.Thebleachingpowderdischargedisstoredincastirondrumsandwoodbarrels.Toincrease
thestoragelifeitismixedwithquicklimetoyieldtropicalbleachcontaining25%ofavailablechlorine.

27. Properties
Bleaching powder is a yellowish white powder and smells strongly of chlorine.
It is soluble in water. The lime present is always left behind as an insoluble salt. For this reason it is also called chloride of
lime.
Bleaching powder is oxidised to chlorine when exposed to air.
When bleaching powder is treated with excess of dilute acid, chlorine gas is produced.
Amount of chlorine gas produced is called available chlorine.Amount of chlorine gas produced is called available chlorine.
It is this chlorine that is responsible for the bleaching action of calcium oxy-chloride.
Uses
Bleaching powder is commonly used for bleaching clothes.
It is also used in bleaching wood pulp in the paper industry.
It is used to disinfect drinking water.
It is used in the manufacture of chloroform (CHCl
3
), an anaesthetic.
It is used as an oxidising agent.
It is used to shrink wool.

28. Bleaching Powder Plant

29. Internal View of the Bleaching Powder
Plant

30. View of the Vacuum Reaction Drum at
Bleaching Powder Plant

31. Sodium Hypochlorite Plant
Sodium hypochlorite also known as liquid bleachis produced by reacting dilute caustic soda solution with chlorine gas
under controlled conditions of temperature.
Chemical reaction
Cl
2
+ 2NaOH ---------NaOCl +NaCl +H2O

32. CalciumhypochloriteisachemicalcompoundwithformulaCa(ClO)2.Itiswidelyusedforwatertreatmentandasa
bleachingagent(bleachingpowder).Thischemicalisconsideredtoberelativelystableandhasgreateravailablechlorine
thansodiumhypochlorite(liquidbleach).
Preparation
Itismanufacturedbythecalciumprocessandthesodiumprocess.Thecalciumprocessproceedsasfollows:
2 Ca(OH)
2
+ 2 Cl
2
→ Ca(ClO)
2
+ CaCl
2
+ 2 H
2
O
Thesodiumprocessproceedsasfollows:
2 Ca(OH)
2
+ 3 Cl
2
+ 2 NaOH → Ca(ClO)
2
+ CaCl
2
+ 2 H
2
O + 2 NaCl
Bleachingpowderisactuallyamixtureofcalciumhypochlorite(Ca(ClO)
2
)andthebasicchlorideCaCl
2
,Ca(OH)
2
,H
2
O
withsomeslakedlime,Ca(OH)
2
.
Properties
Calciumhypochloriteisayellowwhitesolidwhichhasastrongsmellofchlorine.Itisnothighlysolubleinwater,andis
morepreferablyusedinsofttomedium-hardwater.Ithastwoforms:adryformandahydratedform.Thehydratedform
issafertohandle.
Calciumhypochloritereactswithcarbondioxidetoformcalciumcarbonateandreleasechlorine:Calciumhypochloritereactswithcarbondioxidetoformcalciumcarbonateandreleasechlorine:
2 Ca(ClO)
2
+ 2 CO
2
→ 2 CaCO
3
+ 2 Cl
2
+ O
2
Calciumhypochloritereactswithhydrochloricacidtoformcalciumchloride:
Ca(ClO)
2
+ 4 HCl → CaCl
2
+ 2 H
2
O + 2 Cl
2
Uses
Calciumhypochloriteisusedforthedisinfectionofdrinkingwaterorswimmingpoolwater.Itisusedasasanitizerin
outdoorswimmingpoolsincombinationwithacyanuricacidstabilizer,whichreducesthelossofchlorinedueto
ultravioletradiation.Thecalciumcontenthardensthewaterandtendstoclogupsomefilters;hence,someproducts
containingcalciumhypochloritealsocontainanti-scalingagents.
Calciumhypochloriteisalsoaningredientinbleachingpowder,usedforbleachingcottonandlinen.Itisalsousedin
bathroomcleaners,householddisinfectantsprays,mossandalgaeremovers,andweedkillers.
Inaddition,calciumhypochloritemaybeusedtomanufacturechloroform.
Safety
Calciumhypochloriteisbestkeptinacooldryplaceawayfromanyorganicmaterial.Itisknowntoundergoselfheating
andrapiddecompositionaccompaniedbythereleaseoftoxicchlorinegas.