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Eprints ID: 9935
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Mascarade, Jérémy and Liger, Karine and Troulay, Michèle and Joulia,
Xavier and Meyer, Xuân-Mi and Perrais, Christophe and Tosti, Silvano
On the study of catalytic membrane reactor for water detritiation:
Membrane characterization. (2013) Fusion Engineering and Design, vol.
88 (n° 6-8). pp. 844-848. ISSN 0920-3796
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On
the
study
of
catalytic
membrane
reactor
for
water
detritiation:
Membrane
characterization
Jérémy
Mascarade
a,∗,
Karine
Liger
a,
Michèle
Troulay
a,
Xavier
Joulia
b,
Xuan-Mi
Meyer
b,
Christophe
Perrais
a,
Silvano
Tosti
caCEA,DEN,DTN\STPA\LIPCCadarache,F-13108SaintPaul-lez-Durance,France bCNRS,LaboratoiredeGénieChimique,F-31030Toulouse,France
cENEA,UTFUS,C.R.ENEAFrascati,ViaE.Fermi45,00044Frascati(RM),Italy
h
i
g
h
l
i
g
h
t
s
◮CatalyticpalladiumbasedmembranereactorisstudiedforITERtritiumwastemanagement. ◮Concentrationpolarizationeffectwashighlightedbytwo-dimensionalmasstransfermodel. ◮Masstransferresistanceduetoconcentrationpolarizationisreducedbytheincreaseoffluidvelocity.
◮Concentrationpolarizationphenomenonisenhancedbythedecreaseofnon-permeablespeciescontentinthefeedstream.
Keywords: Tritiumwaste
Catalyticmembranereactor Isotopeexchange Detritiation Heavywater
a
b
s
t
r
a
c
t
Tritiumwasterecyclingisarealeconomicandecologicalissue.Generallyunderthenon-valuableQ2O
form(Q=H,DorT),wastecanbeconvertedintofuelQ2forafusionmachine(e.g.JET,ITER)byisotope
exchangereactionQ2O+H2=H2O+Q2.SuchareactioniscarriedoutoverNi-basedcatalystbedpackedin
athinwallhydrogenpermselectivemembranetube.Thiscatalyticmembranereactorcanachievehigher conversionratiosthanconventionalfixedbedreactorsbyselectiveremovalofreactionproductQ2by
themembraneaccordingtoLeChatelier’sLaw.
Thispaperpresentssomepreliminarypermeationtestsperformedonacatalyticmembranereactor. Permeabilitiesofpurehydrogenanddeuteriumaswellasthoseofbinarymixturesofhydrogen, deu-terium and nitrogen have been estimated by measuring permeation fluxes at temperatures ranging from 573to673K,andpressuredifferencesupto1.5bar.Purecomponentglobalfluxeswerelinkedto perme-ationcoefficientbymeansofSieverts’law.Thethinmembrane(150mm),madeofPd–Agalloy(23wt.%Ag),
showed good permeability and infinite selectivity toward protium and deuterium. Lower permeability valueswereobtainedwithmixturescontainingnonpermeablegaseshighlightingtheexistenceofgas phaseresistance.Thesensitivityofthisconcentrationpolarizationphenomenontothecompositionand the flow rate of the inlet was evaluated and fitted by a two-dimensional model.
1. Introduction
Conventionalgaseousstreamsdetritiationtechniquesuseatwo stepsprocessofcatalyticoxidation,toconverthydrogen contain-inggaseousspeciesintovapor,followedbyaphysicalgettersuch asadsorber[1–3]orabsorber[4].Allthesetechniqueshavethe samedrawback;theyproducegreatamounts oftritiatedwater. Admittedly,thesevolumescanbereducedeitherbyconcentration ofgaseousstreambymeansofhollowfiberorganicmembranes
[5]orbyrecyclinginthetritiumplant,butusingdensepalladium basedmembranereactorcanavoidthem.
∗ Correspondingauthor.Tel.:+33442253526;fax:+33442257287. E-mailaddress:[email protected](J.Mascarade).
Theconceptofcouplingahydrogenperm-selectivemembrane andatransitionmetal-coatedamorphoussupporthasalreadybeen studiedfor fusion applications suchasthe ImpurityProcessing moduleofJETActiveGasHandlingSystem[6]orJETsoft house-keepingdetritiation[7,8].
Thesemembranereactors,basedonacountercurrentisotopic swampingoftritiatedspecies(i.e.molecularhydrogen,waterand hydrocarbons)withprotium,aregenerallydesigned considering thatthermodynamicequilibriumofisotopeexchangereactionis reached[8].Nevertheless,toextenddesigntoolstoawiderrange ofoperations,itisnecessarytobetterunderstandthephenomena involved, in order to identify the rate limiting steps and thus provideamoreaccuratemodelingoftheprocess.
Inordertoprovidealltheparametersneededformembrane reactor scale-up, such as reaction laws and rate constants of
isotopeexchange reactions, aninstrumented pilot wasbuilt at CEA Cadarache. After some preliminary characterization tests, presentedbelow,parametricstudywillberuninordertostudy systemresponse toseveralexternal perturbationssuchas tem-perature,flowrateandcomposition.Resultswillthenbeusedto fitamodelaccountingformass,thermalandmomentumtransfer ratesaswellasreactionrates.
Inthisstudy,deuteriumhasbeenchosenastritium representa-tive.
2. Experimentaldevices
Thepilot,whoseflowsheetisrepresentedinFig.1,wasdesigned withthreemodules:atubularfixedbedreactor,apermeatoranda membranereactor.By-passsystemallowsworkingintwodifferent configurations:
-Inparallelmode,modulesarestudiedonebyone;itisalso pos-sibletoevaluateseparatelypermeationandreactionkinetics. -Inseriesmode,largeramountsofimpuritiescanbefedandimpact
ofreactionproductsontheoveralldedeuterationefficiencycan beevaluated.
Themembranes,suppliedbyENEAFrascati,consistedof commer-cialPd77Ag23tubes.Thisalloywaschosenforitsembrittlement
resistance[9]anditshighpermeability[10].Thesedense10mm diametertubeshaveanetlengthof487mmandawallthicknessof 150mm.Theyarehostedin304Lstainlesssteelshellsof650mm length,1.5mmwallthicknessand 25mmexternaldiameter.As showninFig.2,themembraneisinafinger-likeconfiguration;one endisweldedtoaflangeofthereactor’sshellwhiletheotheris closedbyaweldedcapandlinkedtotheoppositeshell’sflangeby aprestressedspringavoidingmembranedamagesduringwarping andhydrogenation[11].
Inthisconfiguration,thefeedstream,senttothemembrane lumen,is“decontaminated”throughaprotiumstream,sentinto theshellsideincounter-currentmode,thankstoisotopeexchange reaction:
Q2O+H2↔H2O+Q2 (1)
whereQstandsforprotiumordeuterium.
Non-permeablegasesarecollectedviaasmallstainlesstubeput insidethePd–Agoneandexitthemodulebytheretentatestream whereasQ2permeatesthemembraneandleavesitbyshell’soutlet.
Unlikeinthepermeator,whoselumenisempty,themembrane reactor’slumenisfilledwithaNi-basedcatalyst.Thiscatalyst con-tainstransitionmetals whichactivatehydrogenbondsandthus promotesboththeisotopeexchangereactioninthemembrane’s reactorandthehydrogenation/dehydrogenationsreactionsinthe fixedbedreactor.Thislastoneconsistsofa304Lstainlesssteel tubeof350mmlength,2mmwallthicknessand20mmexternal diameter.Itisfilledwithstackedglassballs/catalystparticles/glass ballslayersseparatedbysinteredstainlesssteelsheets.
Thereactorisbroughttoanoperating temperaturebetween 300◦Cand400◦Ctoavoidmechanicalstressduetoaandbphase
coexistenceinmembranebulk [12].Themembrane’sclosewall temperatureismonitoredbymeansofaK-typethermocouple.
Modules are connected to gas bottles and vapor generator throughpressureandmassflowcontrollers.Theseprovidegas mix-turesofdesiredcompositionsandflowrates.Inaddition,modules outlets’streamsarepressurecontrolledandtheirtemperatureand flowratesaremonitoredbyadistributedcontrolsystem(DCS).
Finally,allthemodules’streamsarelinkedtoamass spectrome-ter(MS),togettheiron-linecompositionmeasurements,allowing partialmassbalancescalculations.
Thispilotcanprocessflowratesupto300mL(STP)min−1 in
awiderangeofgasmixtures(i.e.from0to100%ofeachspecies except oxygenwhich composition must not exceed2% to stay
Fig.2. Schemeofthemembranereactor.
beyondthelowerexplosivelimitoftheternaryN2–H2–O2mixture
[7]).
Duringthepreliminarytests,onlypureH2/D2andmixturesof
N2–H2havebeenused.Thesetestshavebeencarriedoutto
mea-surethemembranepermeabilityandevaluatethemasstransfer limitationsduetoconcentrationpolarization.
3. Preliminarytests 3.1. Permeationtests
HydrogenpermeationthroughdensePd-basedmembrane fol-lowsasolution-diffusionmechanism[13].Permeationtestswere donetoassessSieverts’lawapplicability.Thisoneconsiders ther-modynamicequilibriumstatebetweengaseousH2anddissolved
protiumatmembraneinterface(i.e.kineticsofsolubilizationare widelyfasterthandiffusion).Itleadstothefollowingequilibrium constantknownasSieverts’constantKS:
KS=
p
CHPH2
(2) withCH:thedissolvedprotiumconcentration(molm−3);PH2:the
hydrogenpartialpressurenearthemembranesurface(Pa). CouplingthisexpressionwithFick’sfirstlawofprotium dif-fusioninthemembraneleadstotheRichardsonexpression[14]: JH2 = H 2·ı·
p
PH2,ret−p
PH2,perm (3) withJH2:thetransmembraneH2flux(molm−2s−1); H=KS·DH:theprotiumpermeability (molm−1s−1Pa−0.5);D
H: theprotium
diffusioncoefficientinthemembranebulk(m2s−1);ı:the
mem-branethickness(m).
Investigatedin thetemperature rangeof300–400◦Cand for
pressuredifferencebetweenlumenandshellrangingfrom0.2to 1bar,permeabilitydatawerecollectedversusthereciprocalofthe absolutetemperatureaccordingtotheArrheniuslaw(Fig.3): i= 0i ·exp
−Eai R·T (4) with 0i:thefrequencyfactor(molm−1s−1Pa−0.5);Eai:the
activa-tionenergy(Jmol−1);R:theidealgasconstant(Jmol−1K−1);T:the
absolutetemperature(K).
Fig.3.Arrheniusplot(lnfvs1/T)ofHandDpermeabilities.
Inordertoavoidmasstransferlimitationsinthefluidphase, permeationtests have beencarried out under pureH2 and D2
atmospheres.Theresults,reportedinTable1,fallintothe inter-valofvariousdatareportedintheliteratureforthesamekindof membranes.
Nowthatpermeationregimewasevaluatedinthepilot’srange of operating conditions, one can focus on the impact of non-permeablegasespresence.
3.2. Concentrationpolarization
Whenagasmixtureisfedtothemembrane’slumen,adepletion ofthepermeablecomponentQ2 isexpectednearthemembrane
Table1
Resultsofpurespeciepermeation:comparisonwithliterature. i-Specie 0 i Eai Reference (molm−1s−1Pa−0.5) (Jmol−1) H 6.12×10−8 6876 Thisstudy H 3.86×10−8 5752 [15] H 5.58×10−8 6304 [16] D 4.93×10−8 7965 Thisstudy D 2.52×10−8 6172 [15] D 3.43×10−8 6156 [16]
Fig.4. Schematicofthecomputationaldomainandboundaryconditions.
surface.Tohighlightthisphenomenon,asimpletwo-dimensional modelwasbuilttomapspeciesdistributioninthemembrane reac-tor’slumen.
3.2.1. Modeling Ifoneassumethat:
-flow regime is steady-state plug flow, the entire module is isothermal
-thelumen’spressure-dropcanbeneglected
-permeate’spressureandflowrate(shellside)remainconstant -thethermodynamicbehaviorisdescribedbyidealgasequation
ofstate
-membraneisdefectfree(infiniteselectivitytowardprotiumand deuterium)
-Axis-symmetricprofilesalongthereactor’scenterline(∂/∂=0) then,themasstransferofspeciesicanbemodeledbya convection-diffusionequation(writteninpartialpressures):
∇
−→N= 1
R·T ·
∇
−D·∇
Pi+ − →u·Pi
=0∀
i∈[H2,N2] (5)with−→N:thetotalflux(molm−2s−1); −→u :thefluidvelocity(ms−1) tuE 0;ε· ˙V /S
;ε:thecatalystbedporosity, ˙V:theactualvolume flowrate(m3s−1)S:thelumen’scrosssectionalarea(m2)D:the
isotropicbinarydiffusioncoefficientof N2–H2 mixture(m2s−1)
calculatedas[17]. D=2.66·10−7·T3/2·
"
Pret·10−5·r
2 1/MH2+1/MN2 · H 2+N2 2 2 ·˝D#
−1 (6) whereMi:themolecularweightofspeciesi(gmol−1),i:thecolli-siondiameterdeterminedfromLennard-Jonespotential(Å),˝D:a
functionofkB·T· εH2·εN2
−0.5;εi:theLennard-Jonesforcecon-stantofi-specie(J);kB:theBoltzmannconstant(JK−1).
Fig.4showsthe6boundaryconditionsneededtoresolve rela-tion(5).
3.2.2. Resultsanddiscussion
Thispartialdifferentialequationssystemwasimplementedin COMSOLMultipysics®4.2commercialcodeandresolvedby
PAR-DISOsolver.Fig.5clearlyhighlightsthedecreaseofpermeationflux withtheincreaseofnitrogenpartialpressureinthefeed.Indeed,for feedstreamscontainingmorethan10mol.%ofhydrogen,the mem-branelengthisnotsufficienttoremoveallthepermeablespecies
becausethetransmembranehydrogenpartialpressuredifference stillexistatthereactor’soutlet,involvingapermeationflux.This revealsanadditionalmasstransferresistanceinthefluidphase[18]
thatonecanwriteasanaveragepressuregradientinthegasphase (drivingforce)tofluxratio:
¯Rg=1 L ·
Z
L PH2r=r b −PH2
r=r m 1/S·
R R
SNH2·dr·d ·dz (7)AsshowninFig.6,thisresistance(dottedline)canbedecreasedby increasingfluidvelocity.
This phenomenon is due to nitrogen molecules brought to the membrane surface which generates a radial concentra-tion gradient leading to a diffusive flux (from the membrane to the fluid bulk) facing the H2 one. Increasing fluid
veloc-ity will increase nitrogen advection and smooth this gradient; it will also, by the same way, reduce this concentration polarization effect, allowing more hydrogen molecules to be broughttothemembrane surface leadingtoa permeationflux improvement.
These preliminarytests also broughtto the light the mem-braneinfiniteselectivitytowardhydrogenand,bythesametoken, verifieddefect-freemembrane assumption;indeed,notracesof nitrogenwererevealedduringpermeatestreammass spectrom-eteranalysis.
Fig. 5.Influence of H2 inlet content onaxial permeation profile 1PH2,inlet=
Fig. 6.Influence of inlet flow rate on permeation flux 1PH2,inlet=1.5bar,T=645K,yH2,inlet=50mol.%
.4. Conclusion
Aninstrumentedlab-scalepilotdedicatedtothe phenomeno-logicalstudyofwaterdedeuterationbyprotiumisotopeswamping inpalladium-silvermembranereactorwaspresented.Preliminary testsdoneonthereactor’smembraneshowitshighpermeability, inadequacywithvaluesreportedinopenliterature,andinfinite selectivitytoward hydrogenand its isotopes. Presence of non-permeable gasesrevealed a gas phase mass transfer resistance whichcouldbereducedbyincreasingfluidvelocity.Nevertheless, asitalsoshortenedthecontacttimebetweenthecatalystparticles andthegasmolecules,anoptimizationoftheoperatingconditions isneededtomaximizetheconversioncapabilitiesofthemembrane reactor.
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