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2.Appliedprocessesandtechniques
Inthischaptertheappliedprocessesandtechniquesarequalitativelydescribed.Chapter3coversthequantitativeaspectsofconsumptionandemissionlevels.
Thechemical,thermodynamicandkineticprinciplesarenotexplained.Forthis,referenceismadetoliteraturerelevanttothesubject[Ullmanns,1996]or[KirkOthmer,1991].
Thechloralkaliindustryproduceschlorineandcausticsolution(sodiumorpotassiumhydroxide)simultaneouslybymeansofdecompositionofasolutionofsaltinwater.Alongwiththechlorineandthecausticsolution,hydrogenisproduced.Anindustrialchloralkaliproductionunitcomprisesaseriesofoperations,structuredasshowninFigure2.1.
Inthechloralkalielectrolysisprocess,achloridesaltsolutionisdecomposedelectrolyticallybydirectcurrent.MostofthetimesodiumchlorideisusedintheprocessinwesternEuropeandlessfrequentlypotassiumchloride(about34%ofthechlorineproductioncapacity).Otherprocessessuchastheelectrolysisofhydrochloricacidortheelectrolysisofmoltensodiumchlorideareapplied,buttheseonlyaccountforabout3%oftotalchlorineproductioncapacityinEurope.
TheelectrolysisofmoltensodiumsaltswhichisappliedtoobtainsodiumandforwhichchlorineisonlyacoproductisdescribedintheBATReferencedocumentonnonferrousmetals,assodiumisanalkalimetal.
Therearethreebasicprocessesfortheelectrolyticproductionofchlorine,thenatureofthecathodereactiondependingonthespecificprocess.Thesethreeprocessesarethediaphragmcellprocess(Griesheimcell,1885),themercurycellprocess(CastnerKellnercell,1892),andthemembranecellprocess(1970).Eachprocessrepresentsadifferentmethodofkeepingthechlorineproducedattheanodeseparatefromthecausticsodaandhydrogenproduced,directlyorindirectly,atthecathode[Ullmanns,1996].
Thebasicprincipleintheelectrolysisofasodiumchloridesolutionisthefollowing:
Attheanode,chlorideionsareoxidisedandchlorine(Cl2)isformed.
At thecathode: In themercury process a sodium/mercury amalgam is formed and hydrogen (H2) and hydroxide ions (OH) are formed by thereactionofthesodiumintheamalgamwithwaterinthedenuder.Inmembraneanddiaphragmcells,waterdecomposestoformhydrogen(H2)andhydroxideions(OH)atthecathode.
Forallprocessesthedissolvingofsalt,sodiumchloride,is:
NaCl Na++Cl
Theanodereactionforallprocessesis:
2Cl(aq) Cl2(g)+2e
Thecathodereactionis:
2Na+(aq)+2H2O+2e H2(g)+2Na+(aq)+2OH(aq)
Theoverallreactionis:
2Na+(aq)+2Cl(aq)+2H2O 2Na+(aq)+2OH(aq)+Cl2(g)+H2(g)
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Figure2.1:Flowdiagramofthethreemainchloralkaliprocesses
basedon[KirkOthmer,1991],[Ullmanns,1996]
ThemaincharacteristicsofthethreeelectrolysisprocessesarepresentedinTable2.1.
Mercury Diaphragm Membrane
Causticquality High,
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Figure2.2:Simplifiedschemeofchlorineelectrolysiscells
after[Dutchreport,1998]
2.1Themercurycellprocess
ThemercurycellprocesshasbeeninuseinEuropesince1892andaccountedin1999for58%oftotalproductioninwesternEurope.AsshowninFigure2.3,themercurycellprocessinvolvestwo"cells".Intheprimaryelectrolyser(orbrinecell),purifiedandsaturatedbrinecontainingapproximately25%sodiumchlorideflowsthroughanelongatedtroughthatisslightlyinclinedfromthehorizontal.Inthebottomofthistroughashallowfilmofmercury(Hg)flowsalongthebrinecellcocurrentlywiththebrine.Closelyspacedabovethecathode,ananodeassemblyissuspended.
Electriccurrentflowingthroughthecelldecomposesthebrinepassingthroughthenarrowspacebetweentheelectrodes,liberatingchlorinegas(Cl2)attheanodeandmetallicsodium(Na)atthecathode.Thechlorinegasisaccumulatedabovetheanodeassemblyanddischargedtothepurificationprocess.
Asitisliberatedatthesurfaceofthemercurycathode,thesodiumimmediatelyformsanamalgam[KirkOthmer,1991].Theconcentrationoftheamalgamismaintainedat0.20.4%Na(byweight)sothattheamalgamflowsfreely,0.3%isthereferencefigurein[Gest93/186,1993].Theliquidamalgamflowsfromtheelectrolyticcelltoaseparatereactor,calledthedecomposerordenuder,whereitreactswithwaterinthepresenceofagraphitecatalysttoformsodiumhydroxideandhydrogengas.Thesodiumfreemercuryisfedbackintotheelectrolyserandreused.
Thereactionintheelectrolyseris:2Na++2Cl+2Hg 2NaHg+Cl2(g)
Thereactioninthedecomposeris:2NaHg+2H2O 2Na++2OH+H2(g)+2Hg
Thebrineanolyteleavingthecellissaturatedwithchlorineandmustbedechlorinatedbeforebeingreturnedtothedissolvers.
Thesodiumhydroxideisproducedfromthedenuderataconcentrationofabout50%themaximumvaluereportedis73%[Ullmanns,1996].However,industryreportsstatethatnoplantinEuropeisknowntobeoperatingabove50%.
Thedecomposermayberegardedasashortcircuitedelectricalcellinwhichthegraphitecatalystisthecathodeandsodiumamalgamtheanode.
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Foritsoperation,themercurycelldependsuponthehigheroverpotentialofhydrogenversusmercurytoachievethepreferentialreleaseofsodiumratherthanhydrogen.However,impuritiesthatcanappearonthemercurysurfacemaylackthisovervoltageprotectionandcancauselocalisedreleaseofhydrogenintothechlorine(hydrogencanformanexplosivemixture(>4%H2)inchlorineorair).Thepresenceofeventraceamountsofcertainmetals,suchasvanadium,cancausethereleaseofdangerousamountsofhydrogen.
Mercurycellsareusuallyoperatedtomaintaina2122%(byweight)concentrationofsaltinthespentbrinedischargedfromtheelectrolyser.Thiscorrespondstothedecompositionof1516%ofthesaltduringasinglepass.Furthersaltdecompositiontoalowerconcentrationinthebrinewoulddecreasebrineconductivity,withtheattendantlossofelectricalefficiency.
Aportion,orinsomecasesall,ofthedepletedbrineissubsequentlydechlorinated,resaturatedwithsolidsalt,andreturnedtothecellbrinefeed.Somefacilitiespurgesmallamountsofbrinesolutionandusenewbrineasmakeupinordertopreventthebuildupofimpurities,mainlysulphate,inthebrine.Figure2.3showsaflowdiagramofthemercurycell.
Figure2.3:Flowdiagramofmercurycelltechnology
2.1.1Themercurycathodeelectrolyseranddecomposer
Thecellismadeofanelongated,slightlyinclinedtroughandagastightcover.Thetroughismadeofsteel,anditssidesarelinedwithaprotective,nonconductivecoatingtopreventcontactwiththeanolyte,toconfinebrinecathodecontacttothemercurysurface,andtoavoidthecorrosiveactionoftheelectrolyte.Modernelectrolysersare12.5mwideand1025mlong.Asaresult,thecellareatodaycanbegreaterthan30m2.Thesizeofthecellscanbevariedoverabroadrangetogivethedesiredchlorineproductionrate.Atthedesignstage,computerprogramscanbeusedtooptimisethecellsize,numberofcells,andoptimumcurrentdensityasafunctionoftheelectricitycostandcapitalcost[Ullmanns,1996].Thesteelbaseismadeassmoothaspossibletoensuremercuryflowinanunbrokenfilm.Intheeventofabreakinthemercurysurface,causticsodawillbeformedonthebare(steel)cathode,withsimultaneousreleaseofhydrogen,whichwillmixwiththechlorine.Becausehydrogenandchlorinecanformahighlyexplosivemixture,greatcareisnecessarytopreventhydrogenformationinthecell.
Characteristicsofthecathode:Thecathodeismadebyashallowlayerofmercurywhichflowsfromoneextremityofthecelltotheotherbecauseoftheslightinclinationfromthehorizontalofthecell.
Characteristicsoftheanode:Electrolyticcellanodesweremadeofgraphiteuntilthelate1960sinwesternEuropewhenanodesoftitaniumcoatedwithrutheniumoxide(RuO2)andtitaniumoxide(TiO2)weredeveloped.TheuseofRuO2andTiO2coatedmetalanodesreducesenergyconsumptionbyabout10%andtheirlifeexpectancyishigher.Inrecentyearstherehavebeencompetitivedevelopmentsindetailedanodegeometry,allwiththeaimofimprovinggasreleaseinordertoreduceohmiclossesandincreasethehomogeneityofthebrinetoimproveanodecoatinglife.
An"endbox"isattachedtoeachendoftheelectrolyser.Theendboxincorporatescompartmentsforcollectingthechlorinegasandweirsforseparatingthemercuryandbrinestreams,washingthemercuryandpermittingtheremovalofthickmercury"butter"thatisformedbyimpurities.[KirkOthmer,1991]
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Thewholeelectrolyserisinsulatedfromthefloortopreventstraygroundcurrents.Usually,severalelectrolysersareplacedinseriesbymeansofelectricallyconnectingthecathodeofoneelectrolysertotheanodesofthenextelectrolyser.Individualcellscanbebypassedformaintenanceandreplacement.
Theelectrolyserisoperatedatatemperatureofapproximately7080C.Atthistemperature,theconductivityofthebrinesolutionandthefluidityofthemercuryarehighercomparedtooperationatambienttemperature.Thetemperaturecanbeachievedbypreheatingthesaturatedbrinewithsteamandisincreasedintheelectrolyserbytheheatofresistance.
Thedecomposeroperatesatatemperatureofapproximately90130C,whichiscausedbythechemicalreactionsinthedecomposerandtheinputofwarmamalgamfromtheelectrolyser.
2.1.2Decompositionoftheamalgam
Theamalgamisdecomposedinhorizontaldecomposers,alongsideorbeneaththecell(Figure2.4)ormoreoften,sinceca.1960,inverticaldecomposers(ordenuders),atoneendofthecell(Figure2.5).Industrialdecomposersareessentiallyshortcircuitedelectrochemicalprimarycells.Themostcommoncatalystisgraphite,usuallyactivatedbyoxidesofiron,nickelorcobaltorbycarbidesofmolybdenumortungsten.
Figure2.4:Mercurycellswithhorizontaldecomposer
[LeChlore,1996]
Themercuryprocesshastheadvantageoverdiaphragmandmembranecellsthatitproducesachlorinegaswithnearlynooxygen,anda50%causticsodasolution.However,mercurycellsoperateatahighervoltagethandiaphragmandmembranecellsand,therefore,usemoreenergy(causticsodaconcentrationexcluded).Theprocessalsorequiresapurebrinesolutionwithlittleornometalcontaminantstoavoidtheriskofexplosionthroughhydrogengenerationinthecell.Theamalgamprocessinherentlygivesrisetoenvironmentalreleasesofmercury.
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Figure.5:Mercurycellswithverticaldecomposer
[LeChlore,1996]
2.2Thediaphragmcellprocess
Thediaphragmprocesswasdeveloped in the1880s in theUSAandwas the first commercialprocessused toproducechlorineandcaustic soda frombrine.InNorthAmerica,diaphragmcellsarestill theprimarytechnology,accountingforroughly70%ofallUSAproduction.Theprocessdiffersfromthemercurycellprocessinthatallreactionstakeplacewithinonecellandthecelleffluentcontainsbothsaltandcausticsoda.Adiaphragmisemployedtoseparatethechlorineliberatedattheanode,andthehydrogenandcausticsodaproduceddirectlyatthecathode.Withoutthediaphragmtoisolatethem,the hydrogenand chlorinewould spontaneously ignite and the caustic sodaand chlorinewould react to form sodiumhypochlorite (NaClO), with furtherreactiontoproducesodiumchlorate(NaClO3)[KirkOthmer,1991].
Thediaphragmisusuallymadeofasbestosandseparatesthefeedbrine(anolyte) fromthecausticcontainingcatholyte.Purifiedbrineenters theanodecompartmentandpercolatesthroughthediaphragmintothecathodechamber.Thepercolationrateiscontrolledbymaintainingahigherliquidlevelintheanodecompartmenttoestablishapositiveandcarefullycontrolledhydrostatichead.Thepercolationrate isdeterminedasacompromisebetweena lowrate thatwould produce a desirably high concentration of caustic soda in the catholyte (which provides the cell effluent) and a high rate to limit backmigrationofhydroxylionsfromcatholytetoanolyte,whichdecreasescathodecurrentefficiency[KirkOthmer,1991].
In the diaphragm cell, saturated brine (about 25% NaCl) is decomposed to approximately 50% of its original concentration in a pass through theelectrolyserascompared toa16%decompositionof salt perpass inmercury cells.Heating causedbypassageof current through the diaphragm cellraisestheoperatingtemperatureoftheelectrolyteto8099C.
When graphite anodes were used, the diaphragm became inoperable after 90100 days due to plugging of the diaphragm by particles of graphite.Nowadays,allplants in theEuropeanUnionusemetalanodesand the lifetimeof thediaphragm isoveroneyear.Theirservice lifehasalso increasedbecause their compositions have changed. At the beginning the diaphragms were made of asbestos only and were rapidly clogged by calcium andmagnesium ions coming from the brine. Asbestos was chosen because of its good chemical stability and because it is a relatively inexpensive andabundantmaterial.Beginningintheearly1970s,asbestosdiaphragmsbegantobereplacedbydiaphragmscontaining75%asbestosand25%of fibrousfluorocarbonpolymer of high chemical resistance. These diaphragms, trade namedModified Diaphragms, are more stable. The polymer stabilises theasbestos,whichinitselflowerscellvoltageandalsoallowsfortheuseoftheexpandableanode[LeChlore,1995][Ullmanns,1996].Chrysotileasbestos("whiteasbestos")istheonlyformofasbestosusedindiaphragmcells.
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Figure2.6:Typicaldiaphragmelectrolysiscell
[USEPA,1995]
Bothdiaphragmandmembranecells for theproductionof chlorineand sodiumhydroxideare classifiedas eithermonopolar or bipolar. The designationdoes not refer to the electrochemical reactions that take place, which of course require two poles or electrodes for all cells, but to the electrolyserconstructionorassembly.Therearemanymorechloralkaliproductionfacilitieswithmonopolarcellsthanwithbipolarcells.Themonopolarelectrolyserisassembledsothattheanodesandcathodesarearrangedinparallel.Asaresultofthisconfiguration,allcellshavethesamevoltageofaboutthreetofourvoltsupto200cellscanbeconstructedinonecircuit.Bipolarelectrolysershaveunitassembliesoftheanodeofonecellunitdirectlyconnectedtothecathodeofthenextcellunit,thusminimisingintercellvoltageloss.Theseunitsareassembledinseries.
Alldiaphragmcellsproducecellliquorthatcontainsca.11%causticsodaand18%sodiumchloride.Thissolutionisevaporatedto50%NaOHbyweightatwhichpointallof thesalt,exceptaresidual1.01.5%byweight,precipitatesout.Thesaltgenerated isverypureand is typicallyused tomakemorebrine. This high quality sodium chloride is sometimes used as a raw material for an amalgam or membrane process. A flow diagram of a possibleintegratedsiteisshowninFigure2.7onpage*.
Low concentrations of oxygen in chlorine are formed by electrolytic decomposition of water and hypochlorous acid (from the reaction of chlorine withwater).
Precipitationofmagnesiumandcalciumhydroxidesonthecatholytesideofthediaphragmmayalsocreateblockingproblems.Hydrochloricacidisoftenadded to thebrine to removeCO2 itmayalsobeadded to thebrineentering theanodecompartment to reducebackmigrationofhydroxyl ionsand tosuppressformationofhypochlorousacid.
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Figure2.7:Flowdiagramofintegrationofmembraneormercuryanddiaphragmprocesses
Diaphragmcellshavetheadvantageof:
operatingatalowervoltagethanmercurycells
operatingwithlesspurebrinethanrequiredbymembranecells
Whenusingasbestosdiaphragms,thediaphragmprocessinherentlygivesrisetoenvironmentalreleasesofasbestos.
2.2.1Diaphragmwithoutasbestos
Duetothepotentialexposureofemployeestoasbestosandemissionsintheenvironment,effortsarebeingexpandedtoreplacetheasbestoswithotherdiaphragmmaterials.
Developmentofnonasbestosdiaphragmsstartedinthemiddleofthe1980sandsomecompanieshavenowsucceededinoperatingwiththem.Thebasisofthematerialusedisthesameinalldiaphragmsdevelopedfreeofasbestos,i.e.afluorocarbonpolymer,mainlyPTFE(polytetrafluoroethylene).ThedifferenceslieinthefillersusedandthewaythehydrophobicPTFEfibresaretreatedanddepositedinordertoformapermeableandhydrophilicdiaphragm(seeSection4.3.2.).
2.2.2Activatedcathodes
Manydifferenttypesofactivatedcathodiccoatingareunderdevelopment inordertoreducethepowerconsumptionof thecell.Thesehavetoberobustbecausethepowerfulwaterjetusedtoremovethediaphragmfromthecathodemeshcanadverselyaffectthecathode.
An industrial application of "integrated precathode" diaphragm has been conducted (full scale) and has been found to contribute to saving energy byreducingelectricpowerconsumptionandimprovingcurrentefficiency.Thelifetimeofthediaphragmhasalsobeenfoundtobeimprovedbyintroductionoftheprecathode.
2.3Themembranecellprocess
Inthe1970s,thedevelopmentofionexchangemembranesenabledanewtechnologytoproducechlorine:themembraneelectrolysisprocess.Thefirstionexchangemembranesweredevelopedatthebeginningofthe1970sbyDuPont(Nafion),followedbyAsahiGlass(Flemion)whichinstalledthefirstindustrialmembraneplantinJapanin1975duetothepressureofJapaneseenvironmentalregulations.NonchloralkalirelatedmercurypollutioninMinamatadrovetheauthoritiestoprohibitallmercuryprocessesandJapanwasthefirstcountrytoinstallthemembraneprocessonamassivescaleinthemid1980s.
Today,itisthemostpromisingandfastdevelopingtechniquefortheproductionofchloralkalianditwillundoubtedlyreplacetheothertwotechniquesin
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time.Thiscanbededucedfromthefactthatsince1987practically100%ofthenewchloralkaliplantsworldwideapplythemembraneprocess.Thereplacementofexistingmercuryanddiaphragmcellcapacitywithmembranecellsistakingplaceatamuchslowerratebecauseofthelonglifetimeoftheformerandbecauseofthehighcapitalcostsofreplacement.
Inthisprocess,theanodeandcathodeareseparatedbyawaterimpermeableionconductingmembrane.Brinesolutionflowsthroughtheanodecompartmentwherechlorideionsareoxidisedtochlorinegas.Thesodiumionsmigratethroughthemembranetothecathodecompartmentwhichcontainsflowingcausticsodasolution.Thedemineralizedwateraddedtothecatholytecircuitishydrolysed,releasinghydrogengasandhydroxideions.Thesodiumandhydroxideionscombinetoproducecausticsodawhichistypicallybroughttoaconcentrationof3235%byrecirculatingthesolutionbeforeitisdischargedfromthecell.Themembranepreventsthemigrationofchlorideionsfromtheanodecompartmenttothecathodecompartmenttherefore,thecausticsodasolutionproduceddoesnotcontainsaltasinthediaphragmcellprocess.Depletedbrineisdischargedfromtheanodecompartmentandresaturatedwithsalt.Ifneeded,toreachaconcentrationof50%causticsoda,thecausticliquorproducedhastobeconcentratedbyevaporation(usingsteam).
Thecathodematerialusedinmembranecellsiseitherstainlesssteelornickel.Thecathodesareoftencoatedwithacatalystthatismorestablethanthesubstrateandthatincreasessurfaceareaandreducesovervoltage.CoatingmaterialsincludeNiS,NiAl,andNiNiOmixtures,aswellasmixturesofnickelandplatinumgroupmetals.Theanodesusedaremetal.
Themembranesusedinthechloralkaliindustryarecommonlymadeofperfluorinatedpolymers.Themembranesmayhavefromoneuptothreelayers,butgenerallyconsistoftwolayers.Oneoftheselayersconsistsofperfluorinatedpolymerwithsubstitutedcarboxylicgroupsandisadjacenttothecathodicside.Theotherlayerconsistsofperfluorinatedpolymerwithsubstitutedsulphonicgroupsandisadjacenttotheanodicside.Togivethemembranemechanicalstrength,themembraneisgenerallyreinforcedwithPTFEfibres.Themembranesmustremainstablewhilebeingexposedtochlorineononesideandastrongcausticsolutionontheother.Thegeneraleconomiclifetimeofchloralkalimembranesisapproximatelythreeyears,butrangesbetween25years[EuroChlorreport,1997].
Membranecellshavetheadvantageofproducingaverypurecausticsodasolutionandofusinglesselectricitythantheotherprocesses.Inaddition,themembraneprocessdoesnotusehighlytoxicmaterialssuchasmercuryandasbestos.Disadvantagesofthemembraneprocessarethatthecausticsodaproducedmayneedtobeevaporatedtoincreaseconcentrationand,forsomeapplications,thechlorinegasproducedneedstobeprocessedtoremoveoxygen.Furthermore,thebrineenteringamembranecellmustbeofaveryhighpurity,whichoftenrequirescostlyadditionalpurificationstepspriortoelectrolysis(seeparagraphonbrinepurification).
Figure2.8:Diagramofamembraneprocess
Membranescellscanbeconfiguredeitherasmonopolarorbipolar.Asinthecaseofthediaphragmcellprocess,thebipolarcellshavelessvoltagelossbetweenthecellsthanthemonopolarcells.However,thenumberofcellsconnectedtogetherinthesamecircuitislimited.
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Figure2.9:Explodedviewofamonopolarmembraneelectrolyser
[Source:DeNora]
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Figure2.10:Viewofamembranecellroomequippedwithbipolarelectrolysers
(Source:HoechstUhde)
2.4Auxiliaryprocesses
Apart from the cells, which remain the heart of the chlorine production line, there are other processing steps or equipment, common to amalgam,diaphragmandmembranetechnologies.Theseare:
saltunloadingandstorage
brinepurificationandresaturation
chlorineprocessing
causticprocessing
hydrogenprocessing.
2.4.1Saltunloadingandstorage
Thebrineusedinthemercurycellandmembraneprocessesisnormallysaturatedwithsolidsalt,althoughsomeinstallationsusesolutionminedbrineonaoncethroughbasis(i.e.nobrinerecirculation).
Brineisgenerallyproducedbythedissolvingoffreshsaltinwaterordepletedbrinefrommercuryandmembraneprocesses.Thebasicrawmaterialisusuallysolidsalt:rocksalt,solarsalt,orvacuumevaporatedsaltfrompurifyingandevaporatingsolutionminedbrine.
Generallythesaltisstoredinasealedareaequippedwitharoof.Becauseofitshighpuritythevacuumsaltinparticularneedstobeprotected.
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2.4.2Brinepurificationandresaturation
2.4.2.1Brinepurification
Ascanbeseen inFigure2.1onpage10, thebrinepurificationprocess consistsof aprimary system formercury and diaphragm technologies and anadditionalsecondarysystem formembrane technology.Thisoperation isneeded toavoidanyundesirablecomponents (sulphateanions,cationsofCa,Mg, Ba andmetals) that can affect the electrolytic process. The quality of the rawmaterial and the brine quality requirements for each of the threetechnologiesdeterminethecomplexityofthebrinetreatmentunit.
Primarypurification
Precipitation
The initialstageofpurificationusessodiumcarbonateandsodiumhydroxide toprecipitatecalciumandmagnesium ionsascalciumcarbonate(CaCO3)andmagnesium hydroxide (Mg(OH)2). Metals (iron, titanium, molybdenum, nickel, chromium, vanadium, tungsten) may also precipitate as hydroxideduringthisoperation.Theusualwaytoavoidmetalsistospecifytheirexclusioninthepurchaseandtransportspecificationforthesalt.Sodiumsulphateiscontrolledbyaddingcalciumchloride(CaCl2)orbariumsaltstoremovesulphateanionsbyprecipitationofcalciumsulphate(CaSO4)orbariumsulphate(BaSO4).Precipitationofbariumsulphatecantakeplacesimultaneouslywith theprecipitationofcalciumcarbonateandmagnesiumhydroxide,whereastheprecipitationofcalciumsulphaterequiresaseparatevessel.
Filtration
Theprecipitatedimpuritiesareremovedbysedimentation,filtrationoracombinationofboth.Theseparatedfiltercakeisgenerallyconcentratedto5060%(althoughafigureof6080%isreportedintheliterature)solidscontentinrotarydrumvacuumfiltersorcentrifugesbeforedisposal.[Ullmanns,1996]
Thepurifiedbrineshouldcontainideally[Ullmanns,1996]:
Ca:
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(Source:AsahiGlassCO)
Thesecondarybrinepurificationconsistsofapolishfiltrationstepandbrinesofteninginanionexchangeunit:
Thesecondaryfiltrationgenerallyconsistsofcandletype,plateframeorpressureleaffilters(eitherwithorwithoutcellulosebasedprecoat)inordertosufficientlyreducethesuspensionmatterandprotecttheionexchangeresinfromdamage.
Theionexchangechelatingresintreatmentisdesignedtodecreasethealkalineearthmetalstoppblevel.Table2.2indicatesthespecificationsrequiredformetals,SO4andotherimpurities.Thesespecificationscanvaryiftheuserswanttooperateatalowcurrentdensity(
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electrolyticcell.Ifconcentratedinliquidformindownstreamprocesses,NCl3mayexplodewithdisastrousresults.
NitrogencompoundsinthebrineisthemainsourceofNCl3.Rocksalt,inparticularsolutionminedsaltusingsurfacewaters,willcontainvaryinglevelsofammoniumandnitratesalts,whereastheuseofvacuumsaltinthebrinerecyclecircuitwillgiveverylowlevelsofNCl3,exceptwhereferrocyanidesareaddedtoavoidcaking.Also,thewaterqualitymayvary,inparticularifsurfacewaterisused.Thetotalconcentrationofnitrogencompoundsinthebrineshouldbecheckedregularly.ChlorinationatapHhigherthan8.5orhypochloritetreatmentofthebrineishowever,capableofdestroyingalargeproportionoftheammoniumsaltimpurity.[Gest76/55,1990]
MethodstoremoveNCl3fromchlorineafteritisformedaredescribedinSection4.1.6.
Figure2.12:Viewofchelateresintowersinasecondarybrinepurificationsystem
(photographsuppliedbyAsahiGlassCo)
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Figure2.13:Viewofpolishingfiltersinasecondarybrinepurificationsystem
(photographsuppliedbyAsahiGlassCo)
2.4.2.2Brineresaturationanddechlorination
Mercuryandmembranesystemsusuallyoperatewithbrinerecirculationandresaturation.Thereare,however,3wastebrinemercuryplantsand1wastebrinemembraneplantoperatinginwesternEurope.
Somediaphragmcelllineshaveaoncethroughbrinecircuit,whilstothersemploybrinesaturationusingthesaltrecoveredfromthecausticevaporators.
Inrecirculationcircuits,thedepletedbrineleavingtheelectrolysersisfirstdechlorinated:
Partiallyforthemercuryprocess(leavingactivechlorineinthebrinekeepsthemercuryinionicformandreducesthepresenceofmetallicmercuryinthepurificationsludge)
Totallyforthemembraneprocess(necessaryherebecausetheactivechlorinecandamagetheionexchangeresinsofthesecondarybrinepurificationunit).
Forthispurpose,thebrineissenttoanairblownpackedcolumnorissprayedintoavacuumsystemtoextractthemajorpartofthedissolvedchlorine.
Nosuchdechlorinationtreatment isrequiredfor thediaphragmsystemsinceanychlorinepassingthroughthediaphragmreactswithcausticsoda in thecatholytecompartmenttoformhypochloriteorchlorate.
Forthemembraneprocess, there isapreliminarystageofhydrochloricacidaddition(to reachpH22.5) inorder toachievebetterchlorineextraction.Afurtherstageisalsonecessarytoeliminatethechlorinecompletelythisisdonebypassingthebrinethroughanactivatedcarbonbedorbyinjectionofareducingagent(e.g.sulphite).
Ifthesaturationismadewithimpuresalt(followedbyaprimarypurificationsteponthetotalbrineflow),thepHofthebrineisthenbroughttoanalkalinevaluewithcausticsoda,toreducethesolubilisationofimpuritiesfromthesalt.Ifthesaturationismadewithpuresalt(withsubsequentprimarypurificationonasmallpartoftheflow),thereisnoalkalisationstepatthatlevel(onlyinthepurificationphase).
Depletedbrinefromthemercuryandmembranecells,withaconcentrationof210250g/l,dependingonthetechnology,currentdensityandheatbalanceofthecell,isresaturatedbycontactwithsolidsalttoachieveasaturatedbrineconcentrationof310315g/l.
Inthecaseofdiaphragmcells,thecatholyteliquor(1012%NaOH,15%NaCl)goesdirectlytothecausticevaporatorswheresolidsaltand50%causticarerecoveredtogether.Freshbrinecanbesaturatedwithrecycledsolidsaltfromthecausticevaporatorsbeforeenteringthediaphragmelectrolysers.
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Resaturatorscanbeeitheropenorclosedvessels.
ThepHofthebrinesenttotheelectrolysersmaybeadjustedtoanacidicvalue(pH4)withhydrochloricacidinordertoprotecttheanodecoating,tokeepthe formation of chlorate at a low level and to decrease the oxygen content in the chlorine gas. Hydrochloric acid can also be added in the anodiccompartments of membrane cells to further reduce the content of oxygen in the chlorine, especially for electrolysers with older membranes (poorerperformances)The(bi)carbonatesintroducedwiththesaltaredecomposedbytheseacidadditions,producinggaseouscarbondioxide.
2.4.3Chlorineproduction,storageandhandling
Generally,before the chlorine can be used, it goes through a series of processes for cooling, cleaning, drying, compression and liquefaction. In someapplications,itcanbeusedasadrygaswithoutneedforliquefaction.Veryoccasionallyitcanbeuseddirectlyfromtheelectrolysers.Ageneral flowofchlorinefromtheelectrolyserstostorageispresentedinFigure2.14.
Figure2.14:Theflowofchlorinefromtheelectrolyserstostorage
[EuroChlorreport,1997]
Thechlorineprocessusuallytakeshot,wetcellgasandconvertsittoacold,drygas.Chlorinegasleavingtheelectrolysersisatapproximately8090Candsaturatedwithwatervapour.Italsocontainsbrinemist,impuritiessuchasN2,H2,O2,CO2andtracesofchlorinatedhydrocarbons.Electrolysersareoperatedatessentiallyatmosphericpressurewithonlyafewmilliatmospheresdifferentialpressurebetweentheanolyteandthecatholyte.
Cooling
Intheprimarycoolingprocess,thetotalvolumeofgastobehandledisreducedandalargeamountofmoistureiscondensed.Coolingisaccomplishedineitheronestagewith chilledwater or in two stageswith chilledwater only in the second stage.Care is taken to avoid excessive cooling because, ataround 10 C, chlorine can combine with water to form a solid material known as chlorine hydrate. Maintaining temperatures above 10 C prevents
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blockagesinprocessequipment.
Twomethodsaremostfrequentlyusedtocoolchlorinegas:
1. Indirectcoolingthroughatitaniumsurface(usuallyinasinglepassverticalshellandtubeexchanger).Theresultantcondensateiseitherfedbackintothebrinesystemofthemercuryprocessordechlorinatedbyevaporationinthecaseofthediaphragmprocess.Thismethodcauseslesschlorinetobecondensedorabsorbedandgenerateslesschlorinesaturatedwaterfordisposal.[BrienWhite,1995]
2. Directcontactwithwater.ThechlorinegasiscooledbypassingitdirectlyintothebottomofatowerinwhichthepackingisdividedintotwoSections,for2stagecooling.Waterissprayedintothetopandflowscountercurrenttothechlorine.Thecoolingwatershouldbefreeoftracesofammoniumsaltstoavoidtheformationofnitrogentrichloride.Thismethodhastheadvantageofbettermasstransfercharacteristicsandhigherthermalefficiency.
Closedcircuitdirectcoolingofchlorinecombinestheadvantagesofthetwomethods.Thechlorineladenwaterfromthecoolingtoweriscooledintitaniumplatecoolersandrecycled.Thesurpluscondensateistreatedexactlylikethecondensatefromindirectcooling.
Followingprimarycooling,chlorinegasisdemistedofwaterdropletsandbrineimpurities.Impuritiesareremovedmechanicallybyusingspecialfilters,orbymeansofanelectrostaticprecipitator.Chlorineisthenpassedtothedryingtowers.
Drying
Chlorinefromthecoolingsystemismoreorlesssaturatedwithwatervapour.Thewatercontentistypically13%.Thismustbereducedinordertoavoiddownstreamcorrosionandminimisetheformationofhydrates[BrienWhite,1995].
Dryingofchlorineiscarriedoutalmostexclusivelywithconcentratedsulphuricacid[Ullmanns,1996].Dryingisaccomplishedincountercurrentsulphuricacidcontacttowerswhichreducethemoisturecontenttolessthan20ppm[Stenhammar].Drychlorineleavingthetopofthedryingtowerpassesthroughhighefficiencydemisterstopreventtheentrainmentofsulphuricaciddroplets.Thespentacidusuallybecomesawasteproductorrequiresreprocessingifitisreused.Forexample,ithastobedechlorinatedbyairblowingandmaybereconcentratedbeforebeingsoldorusedforeffluenttreatment.
Compression
Afterdrying,chlorinegasmightbescrubbedwithliquidchlorineortreatedwithultravioletirradiationtoreducelevelsofnitrogentrichlorideandthenitmaybecompressedinavarietyofcompressors:
sulphuricacidliquidringcompressorsatlowpressures(~4bar)
monoormultistagecentrifugalcompressors(5barorhigher)
reciprocatingcompressors(>11bar)
screwcompressors(variouspressures)
Becauseofheatbuildup fromcompression,multistageunitswithcoolersbetweenstagesareusuallynecessary.Compressor sealsaregenerally fittedwithapressurisedpurgetoinhibitleakageofchlorinetotheatmosphere[UKGuidancenote,1993].
Toavoidnoise,whichisquiteimportantevenforlowpressures,chlorinecompressorsshouldbesoundinsulated.
Liquefaction
Liquefactioncanbeaccomplishedat different pressureand temperature levels, at ambient temperature andhighpressure (for example 18 Cand712bar),atlowtemperatureandlowpressure(forexample35Cand1bar)oranyotherintermediatecombinationoftemperatureandpressure.
Thechosenliquefactionpressureandtemperatureinfluencethechoiceofcoolingmediaandthesafetyprecautionsnecessarytooperatesafely.However,theefficiencyof liquefaction is limitedbecausehydrogen isconcentrated in theresidualgasand itsconcentrationneeds tobekeptbelowtheexplosivelimits.
Thechoiceof thecoolingmediuminacertainstageof the liquefactiondependsonthetemperatureof thechlorine.When the temperature issufficientlyhigh,watercanbeusedasan indirect coolingmedium.When the temperature is relatively low, other coolingmedia such asHCFCs orHFCs (indirectcooling),ammonia(indirectcooling)orliquidchlorine(directcooling)areused.
Thetemperatureofthechlorinegasinacertainstagedependsmainlyontheinitialtemperatureandonthepressureincreaseduringcompression.Alargepressureincreasegenerallyenableswatercooling,butimpliesanincreasedhazardrisk.Chlorinetemperaturehastobekeptwellbelowthepointwhereitreactsspontaneouslyanduncontrollablywithiron(approx.120oC).
Constructionmaterialsmustbechosentosuittheconditionsunderwhichchlorineisbeinghandled:
Wetordry
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Gasorliquid
Temperature
Pressure
In termsof safety, it is very important to avoid, during compression and liquefaction, any possibility ofmixing chlorinewith oils or greaseswhich arereactiveasregardschlorine.
Table2.3showsthepossibletradeoffbetweendifferenttypesofchlorinegasliquefaction,coolingmethodsappliedandsafetyaspects.
Liquefactionsystem
Coolingmedium
Safetyaspect
Storage
Highpressure(716bar)andhightemperatures Water Highprecautions
Lowestenergycostsbuthighmaterialcosts
Mediumpressure(26bar)andmediumtemperatures(between10and20C)
WaterHCFC/HFCorammonia
Moderateprecautions
Moderateenergyand
materialcosts
Normalpressure(~1bar)andlowtemperatures(below40C)
MainlyHCFC/HFCorammonia
Precautions1
Cryogenicstorageofliquidchlorineispossible.Highenergyandlowermaterialcosts
1.Attentionmustbepaidtotheincreasedsolubilityofothergasesatlowtemperatures,especiallycarbondioxide
Table2.3:Possibletradeoffinchlorinegasliquefaction
basedon[Ullmanns,1996],[Dutchreport,1998]
Handlingandstorage
Chlorineisliquefiedandstoredatambientorlowtemperature.Theliquidchlorinefromthebulktankcanbeusedasafeedstockforonsiteprocessesorloadedintocontainers,roadorrailtankers.Becauseofthehightoxicityofchlorine,thestorageareamustbecarefullymonitoredandspecialcaremustbetakenduringloadingoperations.
2.4.3.1Dealingwithimpurities
Chlorinegasfromtheelectrolysiscellsmaycontainimpuritiessuchasnitrogentrichloride(NCl3),bromine(Br2),halogenatedhydrocarbons(CXHYXZ),carbondioxide(CO2),oxygen(O2),nitrogen(N2)andhydrogen(H2).
Nitrogentrichloride,bromineandhalogenatedhydrocarbonspredominantlydissolveintheliquidchlorine,whereasthenoncondensablegases(CO2,O2,N2,H2)remaininthegasphaseandincreaseinconcentrationduringchlorineliquefaction.Tracesofsulphuricacid,ferricsulphate,ferricchlorideand/orcarbontetrachloridemightalsobepresentinthegasphaseafterdryingandliquefactionofchlorine.
Specialattentionshouldbepaidtothefollowingimpurities:
Water
Allmetalsareattackedbywetchlorinewiththeexceptionoftitaniumandtantalum.Titaniumcanonlybeusedinwetchlorineconditionsitspontaneouslycombustsindrychlorine.
Hydrogen
Allthreetechnologiesproducehydrogenwhichcanformanexplosivemixture(>4%H2)inchlorineorair.Light,frictionandgasdepressurisationmaybringenoughenergytoinitiatethereactionatambienttemperature.Chlorinegasisanalysedregularlytoensuretheabsenceofanexplosivemixture.
Nitrogentrichloride
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Nitrogentrichlorideisformedduringtheelectrolyticproductionofchlorine,duetosidereactionsbetweenthechlorineandvariousnitrogencompoundsinthebrinesolution.1ppmofNH3inbrineisenoughtogive>50ppmNCl3inliquidchlorine.Inplantswhichusedirectcontactwatercoolingofthechlorinegasbeforedryingandcompression,NCl3mayalsobeformedifthewaterispollutedwithnitrogencompounds[Gest76/55,1990].
Nitrogentrichlorideischaracterisedbyitsutmostinstability.ExperimentalresultsshowthataconcentrationofNCl3greaterthan3%byweightatambienttemperatureiscapableofaccelerateddecompositionwhichisstronglyexothermic.
NCl3hasahigherboilingpointthanchlorineandanyNCl3presentinthechlorinegaswillconcentrateintheliquidphaseinachlorineliquefactionprocess.AnyevaporativehandlingofliquidchlorineispotentiallydangerousduetotheselectiveconcentrationofNCl3intheliquidphase.
MethodstoremoveNCl3fromchlorinearedescribedinSection4.1.6.
Bromine
Thequantityofbrominepresentdependsonthequalityofthesaltused.Itsconcentrationisgenerallyhigherifchlorineisobtainedbyelectrolysingpotassiumchloridetoobtainpotassiumhydroxide.Bromine,likewater,canacceleratethecorrosionofthematerials.
Noncondensablegases(CO2,O2,N2,H2)
Thereareseveralwaystodealwiththenoncondensablegases,dependingonthelayoutofthechlorineliquefactionunit.Somearedescribedbelow.
DilutionwithairandproductionofweakNaOCl
Duringchlorinegascompressionandcooling,mostofthechlorinegasiscondensed.However,thenoncondensablegases(H2,CO2,O2,N2)increaseinconcentration.Bydilutingtheremainingchlorinegaswithair,theconcentrationofhydrogencanbekeptbelowtheexplosionlimit.Thisallowsadditionalliquefactionofchlorinegas.Theremaininggasesafterliquefaction(socalled"tailgas")havetobepurgedfromthesystem.Thetailgasstillcontainsasignificantamountofchlorine,andthegasisthereforenormallyledtothechlorinedestruction/absorptionunit.
Productionofhydrochloricacid
Insteadofdilutingtheremaininggasesafterpartialcondensationofthechlorinegas,thehydrogencanberemovedfromthesystembymeansofareactionwithchlorinegasinacolumn.Thisremovesvirtuallyallthehydrogenandyieldsgaseoushydrochloricacid,whichexistsharmlesslywiththechlorinegasandcanberecoveredinahydrochloricacidunit.Theremainingchlorinegascannowsafelybefurthercondensed.Thetailgaseswithsomechlorinegasandtheremainingnoncondensablegases(CO2,N2,O2)willbepassedthroughahydrochloricacidunit.ThissolutioncanbechosenifHClisasaleableproductorifitcanbeusedasafeedstockfordownstreamproduction,suchasferricchloride.
2.4.3.2Thechlorineabsorptionunit
Thepurposeofatypicalchlorineabsorptionsystemistwofold:
1. Tocontinuouslyabsorbchlorinegasarisinginstreamssuchastailgasfromliquefaction,airblownfromwastebrineorchlorinecondensatedechlorinationandalsowetanddrymaintenanceheaders.Upto5%,butnormallylessthan1%,oftheplantproductionisabsorbedinthisway.
2. Toabsorbthefullcellroomproductionduringemergencyforanadequateperiod,usuallynotlessthan15to30minutes,toenablecorrectivemeasurestobetakenortheplanttobeshutdowninasafemanner.Gravityfedheadtanksorpumpssuppliedwithbackuppowersuppliesmaybeusedtogiveincreasedreliabilityandoperationunderpowerfailureconditions.
Theabovefunctionscouldbeundertakeninseparatepurposebuiltunits,providingtheintegrityofthesystemsismaintainedbyhavingbackupscrubbers.All gaseous vents contaminated or potentially contaminated with chlorine thus pass into the atmosphere through caustic scrubbing towers containingpackingirrigatedwithcausticsoda.Heatisgeneratedbytheabsorptionreactionandtemperaturesshouldnotbeallowedtoincreaseaboveabout30Ctoavoid formation of sodium chlorate instead of sodiumhypochlorite solution. To avoid overheating during a fullscale relief, the fresh caustic soda feedsolutionshouldnotbestrongerthanabout12%byweight.Highercausticconcentrationscanbeusedprovidingadequatecooling is installed,butthere isanincreasedriskofsolidsdepositionandblockage.
Tailgasfromthechlorineliquefactionunitcontainsresidualchlorineandcarbondioxide,whichareabsorbedinthecaustic,andhydrogen,whichisdilutedwithairtolessthan4%byvolumetoavoidflammablemixtures.
Optimum design of scrubbing systemsmust include high reliability, automatic operation in emergencies, and countercurrent flow of liquid and gas toachievelowexitconcentrations.Ifsaleisnotpossible,efficientdecompositionofthesodiumhypochloriteproducedintosodiumchloridecanbeachievedusinganickelcatalyst.
2.4.4Causticproduction,storageandhandling
Sodiumhydroxide(causticsoda)isproducedinafixedratioof1.128tonnes(as100%NaOH)pertonnechlorineproduced.
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Figure2.15:Viewofcausticproductionandstorage
[OxyChem,1992]
Thecausticsodasolutionfromthethreetechnologiesistreatedinslightlydifferentwaysduetothedifferenceincompositionandconcentration.
Inthemercurycellprocess,50%causticsodaisobtaineddirectlyfromthedecomposers.Thecausticsodaisnormallypumpedthroughacooler,thenthroughamercuryremovalsystemandthentotheintermediateandfinalstoragesections.Insomecasesthecausticisheatedbeforefiltration.Themostcommonmethodforremovalofmercuryfromcausticsodaisaplate(orleaf)filterwithcarbonprecoat.Undernormaloperatingconditions,mercurycellcausticsoda(as100%NaOH)contains20100ppmofsodiumchlorideand4060gHg/kgNaOH.
Inthecaseofdiaphragmandmembranetechnologiesthecausticsodaisconcentratedbyevaporationbeforefinalstorage.
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Figure.16:Theflowtostorageofcausticsodafromthedifferenttechnologies
Basedon[OxyChem,1992]
Steamisusedasthesourceofevaporativeenergy.Thepresenceofsaltinthediaphragmcellliquorrequiresthattheevaporatorisequippedwithscraperbladesorotherdevicestodrawofftheprecipitatedsalt.Thishighqualitysodiumchloridecanthenbeusedtoenrichdepletedbrine,sometimesitisusedasarawmaterialforanamalgamormembraneprocess.Theresiduallevelofsodiumchlorideinsodiumhydroxidefromdiaphragmcellisabout1%andsodiumchlorate0.1%.Forthisreason,itisunsuitableforcertainendapplicationssuchasthemanufactureofrayon.
Saltandsodiumchlorateinthecausticsodafromdiaphragmcellscanbereducedbyammoniaextractiontoincreasemarketability,butatincreasedcost.
Thecausticsodafrommembranecellsisofhighquality,althoughthecausticsodaproduced(usuallyaround33%NaOH)needsconcentrationto50%NaOHforsomeapplications.Thesaltcontentofthemembranecellcausticsodaliesbetween20100ppm(in100%NaOH),butisonaverageslightlyhigherthanmercurycellcaustic(seeTable2.1).
Insomeplantsthecausticsodaisfurtherconcentratedtoa73%solutionandto100%assolidcausticprillsorflakes.
Somechloralkaliproductionfacilitiescancombinethecausticproductionprocessfrommercuryandmembranecellsinordertominimiseenergycosts.Itispossibletofeed33%causticfromthemembranecellstothedecomposertoproduce50%causticwithouttheneedforevaporation.
Storageandhandling
Becauseofitshighlyreactiveandcorrosiveproperties,causticsodamaycorrodecontainersandhandlingequipment.Constructionmaterialsmustbesuitedtothecausticsodahandledandstored.
Sodiumhydroxidesolutionsrequiresteamorelectricalheatingwheretemperaturescanfallbelowtheupperfreezingpoint.Frozenpipelinespresentbothsafetyandenvironmentalriskswhenattemptsaremadetounblockthem.SafetymeasuresaresetoutinChapter4.
Storagetanksmaybelinedinordertominimiseironcontaminationoftheproductortoavoidstresscorrosioncrackingthetank.Tanksareusuallyincludedinprocedurestopreventoverfloworspillageofcausticsoda.Suchproceduresincludecontainmentandmitigation.
Itshouldbenotedthatdissolvedhydrogengascanbereleasedintothevapourspaceabovetheliquidinstoragetanks.Tanksarenormallyventedfromthehighestpoint.Testingforanexplosivemixtureofhydrogeninairnormallyprecedesanymaintenanceactivityinthearea.
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2.4.5Hydrogenproduction,storageandhandling
Hydrogen isproduced ina fixed ratioof28kgper tonnechlorineproduced.Hydrogen leaving the cells is highly concentrated (>99.9%by volume) andnormallycooledtoremovewatervapour,sodiumhydroxideandsalt.Thesolutionofcondensedsaltwaterandsodiumhydroxideiseitherrecycledasbrinemakeup or treated with other waste water streams. In the mercury cell process, hydrogen has to be treated to remove mercury. Primary cooling atambienttemperatureiscarriedoutattheelectrolyser,allowingmercuryvapourtocondenseintothemainmercurycircuit.Furthercoolingtakesplaceatalaterstageusinglargeheatexchangersandcondensateissentformercuryrecovery.
Hydrogenmaybedistributedtousersusingboosterfansorfedtothemaincompressionplant.Themainhydrogencompressionplantusuallycomprisesanumberofcompressorsandagasholder (surgechamber).Thehydrogengasholder is incorporated into thesystem tominimise fluctuations in thegaspressurefromtheprimarystage.Thehydrogenproductgasstreamisalwayskeptpressurisedtoavoid ingressofair.Allelectricalequipmenttakenintothehydrogencompressionplantareamustbe"intrinsicallysafe",i.e.theequipmentwillnotproduceaspark.Areliefvalveisnormallyprovidedwithinthesystemtorelievehighpressuretoatmosphere.
Hydrogenisnormallyanalysedforoxygencontentthecompressionwillshutdownautomaticallyincriticalsituations[EuroChlorreport,1997].
The hydrogen is in general used for onsite energy production. It is burnt as a fuel, either by the company operating the chlorine plant or by anothercompany towhom ithasbeensoldasa fuel.Someorall of it canalsobeusedonsite in the caseof integrated sitesor sold toother companiesaschemicalfeedstock(productionofhydroxylamines,hydrochloricacid,hydrogenperoxide,sodiumsulphite,forexample).