June 2011, Volume 2, No.3 International Journal of Chemical and Environmental Engineering 175 Enhancement of methanol synthesis in a Water vapor-perm selective membrane reactor M. Farsia, A. Jahanmiria* aSchool of Chemical and Petroleum Engineering, Department of Chemical Engineering, Shiraz University, Shiraz, Iran * jahanmir@shirazu.ac.ir AbstractInthisworkamembranereactorisproposedforwatervaporremovalfromreactionzoneinthemethanolproductionprocessto overcome thermodynamic equilibrium limitations of the methanol synthesis reactions. A steady state heterogeneous one-dimensional mathematical model is developed for simulation of the proposed catalytic fixed bed membrane reactor. To verify the accuracy of the considered model and assumptions, simulation results of the conventional reactor is compared with the available industrial plant data. The membrane reactor benefits are the higher methanol production rate, higher quality of outlet product and consequently lower cost in the product purification stage. This configuration has enhanced the methanol yield about 5.7% than industrial reactor. Experimental proof-of-concept is needed to establish the safe operation of the proposed configuration. Keywords: Methanol, Composite membrane, Heterogeneous model, Membrane reactor. 1.Introduction Methanolasthesimplestaliphaticalcoholsisoneof themostimportant industrial petrochemical products that usedassolventandproducechemicalintermediate.Itis analternatefuelforinternalcombustionandother engines,eitherincombinationwithgasolineordirectly. Methanolisacolorlessliquid,completelymisciblewith waterthatisveryhydroscopic.Liquidmethanolhasa higherenergydensitythangasfuels.Itcanbeproduced from different sources like natural gas, coal, biomass and petroleum. Commercially, it is produced from syngas in a tubularpackedbedreactor.Aconventionaltypeof methanolreactorincludesmanytubesthathavebeen surroundedwithcirculatingboilingwaterascoolantto removetheheatofreactionsfromreactionzone.Dueto thethermodynamicequilibriumlimitationsofthe methanolsynthesisreactions,methanolconversioninthe conventionalmethanolreactorsislow.Thus,mostofthe syngasshouldbecirculatedintheprocess.Thereare severalresearchesonmethanolprocessintheliterature. Langepresentedagoodreviewofmethanolsynthesis technologies [1]. JahanmiriandEslamloueyanmodeleda conventionalmethanolreactorandshowedthatthe differencebetweenoneandtwo-dimensionalsimulation isnegligible[2].Graafetal.modeledalowpressure methanolsynthesisreactor[3].Theyshowedthat commercialsizeofthecatalystparticlesexhibit intraparticlediffusionlimitations.Kordabadiand Jahanmirioptimizedmethanolsynthesisreactorto enhanceoverallproductionatsteadystateanddynamic conditions[4].Thisoptimizationapproachenhanceda 2.9% additional yield inmethanol reactor. Shahrokhi and Baghmishehinvestigateddynamicbehaviorand controllabilityofthelowpressuremethanolsynthesis reactor[5].Toincreasethereactoryield,theydeveloped anoptimizerwhichcanbecoupledwiththecontrol system.Recently,adual-typereactorsysteminsteadofa conventionalmethanolreactorwasproposedby Rahimpour et al. [6, 7]. The dual-type methanol reactor is anadvancedtechnologyforconvertingnaturalgasto methanolatlowcostandlargequantities.Themain factorsintheproductionrateofindustrialmethanol reactorarethermodynamicequilibriumlimitationsand catalystdeactivation.Manyeffortshavebeenconsidered toincreasemethanolconversionbyremovalofproducts overapermselectivemembrane.Theapplicationof membranereactorshasattractedmuchattentioninthe recentyear[8,9].Simultaneousoccurrenceofreaction andseparationleadstolowercostoftheseparation system compared to conventional reactors. In addition, by removingsomeproductcomponentsfromreactionzone andtransferofsomereactanttothereactionzoneina membranereactor,thermodynamicequilibrium limitationscanbeovercometowardshigherconversion. Rahimpouretal.improvedmethanolproductioninan industrialmethanolreactorbyapplyingaPdbased hydrogen-permselectivemembranetotheconventional reactor[10].Inthisconfiguration,hydrogenpermeates from membrane tubes to the reaction zone.Themainobjectiveofthisworkismodelingand simulationoftheproposedmembranereactorfor methanolsynthesisandperformancecomparisonofthe proposedreactorwithaconventionalreactoratsteady Enhancement of methanol synthesis in a water vapor-perm selective membrane reactor statecondition.Theproposedreactorismodeled heterogeneouslyandnumericalsimulationutilizedto comparetheresultsofthemembranereactorwith conventionalreactoratthesameprocessconditions.The steadystatesimulationresultsshowthatthisnovel configurationintheindustrialsingletypemethanol reactor improves methanol production rate. 2.Process description 2.1 Conventional methanol reactor Figure1showstheschematicofaconventional methanolsynthesisreactor.Theconventionalmethanol reactorisbasicallyaverticalshellandtubeheat exchangerthattubesarepackedwithcatalystparticles andsurroundedbytheboilingwater.Theheatof reactionsistransferredtotheboilingwaterandsteamis produced. Figure 1, Schematic of a conventional methanol reactor Typicaloperatingconditionsofthemethanolreactor are543Kand80bar.Thebasecasespecificationsof studied reactor, catalyst and feed are tabulated in Table 1. Table 1, The base case specifications of reactor, catalyst and feedFeed compositionH265.9 CO4.6 CO29.4 N29.3 H2O0.009 CH3OH0.5 CH410.26 Total molar flow per tube [mol s-1]0.64 Inlet temperature [K]503 Inlet pressure [bar]76.98 Reactor characteristics Tube number2962 Length of reactor [m]7.022 Tube inner diameter [m]3.810-2 Tube outer diameter [m]4.310-2 Bed void fraction0.39 2.2 Membrane reactor Asasolutiontoovercomethethermodynamic limitations,methanolreactorcanbedevisedby permselectivemembranelayerforH2Oremovalthat shiftstheequilibriuminthefavorabledirection.Figure2 showstheschematicdiagramoftheproposedmembrane reactorformethanolsynthesis.Thissystemconsistsof twoconcentricpipesthattheinnertubewalliswater vapor-permselectivemembrane.Betweenthesecondand innertube,themethanolsynthesisreactionstakeplace. The second tube has been surrounded by the boiling water andtheheatofreactionsistransferredtothewaterand steam produced. Figure 2, Schematic diagram of a the proposed membrane reactor Iliutaetal.studiedH2OremovalduringDME synthesisusingacompositemembranefromagaseous mixture contains of methanol, H2, CO, CO2 and DME in a packedbedcatalyticreactor[11].Intheproposed configuration,thewalloftubesareconstructedfroma watervapor-permselectivemembrane.Partialpressure differenceofwatervaporbetweeninnertubeandand reactionzonepermitsdiffusionofwaterthroughthe composite based membrane tubes. 3.Process modeling Thereactionsofmethanolsynthesisaremainly,CO andCO2hydrogenationandreversewatergasshift reaction. CO+2H2CH3OH(1) CO2+3H2CH3OH + H2O(2) CO2+H2CO+ H2O(3) In this work, rate expressions have been selected from Graaf et al. work [12]. The rate equations combined with theequilibriumrateconstantsandprovideenough informationaboutkineticsofmethanolsynthesis.Aone dimensionalsteadystateheterogeneousmodel,basedon massandenergyconservationlaws,hasbeendeveloped forsimulationoftheproposedmembranereactor.Inthis model the following assumptions has been considered: -Radial dispersion of mass and energy is negligible.-The gas mixture is an ideal gas. -Duetohighgasvelocity,axialdiffusionofmassand heat are negligible. -Plug flow pattern in reaction tube is considered. -The chemical reactions are assumed to take place only on the catalyst surface. MeOH Water Vapor To Distillation FeedH2O permselective side Saturated water Reaction zone FeedEnergy streamMass stream 176Enhancement of methanol synthesis in a water vapor-perm selective membrane reactor Subject to these assumptions, mass and energy balances for the gas phase in the reaction zone are expressed by: ( )( ) 0 ) (1 , 2 ,, 2= + i ici w gisi gi t vgicP PAy y k c adzy F dA1t(1)( )0 ) () ( ) (101 3 2 3 221 2 2 112 22 2= + dT C F T T UADT T UADT T h adzT F dACTTpout gcg gcg sf vgcgptt(2) Leeandetal.proposedacompositemembranewith permeabilityof1.1410-7molm-2s-1Pa-1andawater-methanol selectivity of 8.4 at a permeation temperature of 250C [14]. Mass and energy balances for the solid phase are expressed by: 0 ) (2 , 2 ,= + b isigi gi t vr y y k c a p n(3)0 ) ( ) (1, 2 2= A + Nii f i bs gf vH r T T h a n p (4) Thepressuredropthroughthecatalyticbedis calculatedbasedontheErgunequation[13].Therelated equation for tubular reactors is: ( ) ( )2c23p s c322p2sAQ 1d 1.75AQ 1d 150dzdP += (5) Massandenergybalanceequationsforwatervapor permselective side (inner tube) are as follows: ( )( )1 , 2 , ,1 i i i wgiP Pdzy F d = t (6)( )( ) dT C F T T U DdzT F dCTTpout g gggp+ = 20) (1 1 2 2 1 11 1t(7) Tocompletethesimulation,auxiliarycorrelations shouldbeaddedtothemodel.Intheheterogeneous model,heatandmasstransfercoefficientsbetweengas andsolidphases,physicalpropertiesofchemicalspecies andoverallheattransfercoefficientshouldbeestimated from proper correlations. 4.Numerical solution Thegoverningequationscombinedwiththekinetic expressionsandauxiliarycorrelationsformasetof nonlinearordinarydifferentialequationsasaninitial value problem. This set of equations has been solved with 4thorder Runge-Kuttamethod.The resultsofnode kare to be used as inlet conditionsfor the next node (k+1). At theendofthisprocedureitispossibletoplotthe concentrationofcomponentsandtemperatureprofiles versus the length of reactor. 5.Result and discussion Inthissectiontheeffectofmembraneonkey parametersofmethanolreactorsuchasmethanol,CO, CO2andH2Omoleflowrateshasbeenconsidered.The modelofmethanolsynthesissideisvalidatedagainst industrialmethanolreactorfortabulateddesign specificationsinTable1[15].Thecomparisonbetween simulationresultsandplantdataforindustrialcaseis shown in Table 3. It is shown that the observed plant data has a good agreement with simulation data. Table 3, Comparison between simulation results and plant dataInletReactor Outlet CompositionSimulationPlant date CO22.0481.0551.032 CO3.1180.9780.932 H277.8775.875.12 H2O0.0081.1511.249 CH3OH0.4074.174.008 CO2andCOmoleflowrateprofilesareshownin Figure3(a)and(b).Thisfigureshowsthatinthesecond halfoftheconventionalreactor,therateofCO2 hydrogeneationreactionapproachestoitsequilibrium.It isobservedfromsynthesismethanolreactionsthatwater vaporremovalfromreactionzoneshiftsCO2 hydrogenationreactiontothehigherCO2consumption whichresultsangreaterequilibriumshiftandmethanol production.Also,Watervaporremovalshiftsreverse watergasshiftreactiontotherightsideandhigherCO canbeproduced.IncreasingCOconcentrationshiftsCO hydrogenationreactiontotherightsideandhigher methanolproductionisattainable.Thus,oneofthemain advantagesofproposedstructureishigherCO2 conversion to methanol.0 1 2 3 4 5 6 70.040.0450.050.0550.06Length (m) CO2 mole flow rate (mol s-1) Conventional reactorMembrane reactor(a) Figure 3(a), CO2 mole flow rate profiles TheoutletCO2moleflowratefromproposedand conventionalreactorforbasecaseare0.041and0.045 mols-1,respectively.TheCO2conversioninthe membranereactorhasbeenimprovedabout9%than 177Enhancement of methanol synthesis in a water vapor-perm selective membrane reactor conventionalreactor.SimultaneousoccurrenceofCO productionandconsumptionresultsasamemoleflow rateprofileforCOintheproposedandconventional reactors.0 1 2 3 4 5 6 70.010.0150.020.0250.03Length (m) CO mol flow rate (mol s-1) Conventional reactorMembrane reactor Figure 3(b), CO mole flow rate profiles InFigure4,methanolmoleflowrateinthe conventionalreactorandproposedconfigurationare shown. It is observed that the methanol mole flow rate in theproposedconfigurationishigherthanconventional reactor.Thisfigureshowsthatinthesecondhalfofthe conventionalreactor,therateofmethanolsynthesis reactionshasapproachedtoequilibriumpointandthe membranelayerinthissectionisuseful.Theoutlet methanol mole flow rate from proposed configuration and conventionalreactorforbasecaseobtainedabout0.37 and 0.35 mol s-1, respectively. 0 1 2 3 4 5 6 700.010.020.030.04Length (m) Methanol mole flow rate (mol s-1) Conventional reactorMembrane reactor Figure 4, MeOH mole flow rate profiles The water vapor mole flow rate in the reaction zone of theproposedandconventionalreactorsareshownin Figure5.AccordingtoLeChtelier'sprinciple, decreasingwatervaporconcentrationinthemethanol synthesisreactionsleadstoshifttheCO2hydrogenation and reverse water gas shift reactions to the right direction andhighermethanolisproduced.Thisfigureshowsthat thewatervapormoleflowrateinthesecondhalfofthe membranereactordecreasesduetohigherwatervapor removalthanwatervaporproduction.Whilethereisa differencebetweenwatervaporpartialpressureinthe exothermic and permeation side, vapor can penetrate from reaction zone into the permeation side through composite membrane layer. 0 1 2 3 4 5 6 700.0020.0040.0060.0080.010.0120.0140.016Length (m) H2O mol flow rate (mol s-1) Conventional reactorMembrane reactor Figure 5, Water vapor mole flow rate profiles 4. Conclusion In this study, a novel composite membrane reactor for mrthanolproductionwereproposedandmolded heterogeneouslyatsteadystatecondition.Theresultsof mathematicalsimulationforanindustrialcasewere comparedwiththeplantdataandtheaccuracyofthe modelandconsideredassumptionswasproved.The resultsofthesteadystatesimulationshowedthat proposedmembranereactorenhancesoutletmethanol moleflowrateabout5.7%thanconventionalreactor. Methanolproductioncouldbepromotedbeyond thermodynamic equilibrium by permeation of water vapor fromreactionsideasaproduct.Ingeneral,the performance of methanol reactor system improved when a membranewereusedinaconventional-typemethanol reactorandtheobtainedresultssuggestthatthis configuration could be feasible and beneficial. REFERENCES [1]J.P.Lange,Methanolsynthesis:ashortreviewoftechnology improvements, Catalysis Today, vol. 64, pp. 3-8, 2001. 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