Pd Alloy Membranes for Hydrogen Separation from Coal-Derived ...

• Requirements• High H 2 flux• High H 2 purityMembrane Requirements• Robustness and resistance to degradation by thermal cycling• Operation at the right temperature range (260-450 o C)– Above the dew point of the syngas but low enough to achieveeffective contaminant control• Tolerance to all components of coal-derived synthesis gas– Particularly to sulfur• Potential Technologies for H 2 Separation• Pressure Swing Adsorption (PSA)• Ceramic membranes• Dense ceramic membranes• Our approach is composite Pd alloy membranes• CSM carries out film deposition and characterization• TDA carries out support development, membrane testing andmodule development3 TDAR e s e a r c h

Why Pd Membranes?• Potential for perfect H 2 selectivity• The transport mechanism isunique to hydrogen• Palladium catalyzes thedissociation of molecular H 2 intoatomic H• H atom is soluble in Pd metal andtransports through a solutiondiffusionmechanism• H atoms recombine at the lowpressure side and desorb from thesurface• Potential for high flux• Flux and permeability a function ofsolubility and diffusion rate4 TDAR e s e a r c h

Why Pd Alloy Membranes?• Membranes based on pure Pd sufferfrom embrittlement and cracking due tothe α→β Pd hydride phase transition (at~300 o C)• FCC to BCC phase change causesstructural deformations in the film• Pd alloys avoids the α→β phasetransition in pure Pd• Alloying eliminates swelling, warping,cracking due to phase transitions• Eliminates the problems associatedwith thermal cyclesα phase is FCCβ phase is BCC• Some Pd alloys shows higher permeability than the pure Pd• 27% Ag, 6% Ru, 40% Cu, 5% Au alloys have shown to have muchhigher fluxes than pure Pd films• Additional benefits of alloying• Reduced cost (depending on the selected component)• Resistant to H 2 S• Dimensional stability (small degree of swelling)5 TDAR e s e a r c h

Why Pd Composite Membranes?• Pd alloys can be prepared into selfsupporting structures or can be prepared onporous supports (i.e., composite membranes)as thin films• Thin films increases the H 2flux at a givenpressure• Flux equation in Pd membranes (Way, 1996)( )J H= P Hl mp H2 , feed − p H 2, permeate• Low cost when the alloys made into thinalloy films• 5 µm PdCu (60/40) film– Pd cost = $20/ft 2 Cu Cost = 0.70/ft 2• 25 µm PdCu alloy will cost 5 times highermaterial cost and its flux will be much lower• The original idea is from the work of Uemiyaand Kikuchi, Chem. Lett., 1687, 1988• The challenge is how to make them andoperate them at high pressures1 µm thick compositePdCu membrane2 µm thickcomposite PdCumembrane6 TDAR e s e a r c h

Porous Supports for Deposition of Pd Films• Previous work at CSM focused on preparing thin Pdor Pd alloy films on ceramic supports• Pd alloy film thickness ~1-5 µm• Supports are commercially available by CoorsTek,and Pall originally developed for gas filtration• The substrates can be symmetric (constant pore size)or asymmetric (gradient in pore size)• Symmetric ceramic supports• Low cost– Symmetric Supports ~ $25/ft 2– Asymmetric Supports > $500/ft 2• Higher strength, less defectsAsymmetric SupportSymmetric Support• Membrane film isdeposited either on theinside or outside diameterof the porous supports8 TDAR e s e a r c h

Preparation of Pd Alloy Films on Steel Supports• Porous steel supports increases therobustness over the ceramic supports• It is much easier to incorporate themetal supported membranes intomodules• Welding or brazing• Elimination of ceramic/metal jointsminimize leaks• Issues need to be addressed:• Surface roughness– Thicker films are required• Inter-metallic diffusion– Cause formation of an undesiredalloy• A diffusion barrier addresses all these problems:• An oxide layer diffusion barrier deposited on steel support prior toplating prevent diffusion of Pd membrane and support• If the oxide layer diffusion barrier can be applied is in the form of smallparticles, surface roughness may be reduced9 TDAR e s e a r c h

Module Construction• The final membrane module or themembrane-WGS reactor will be similarto a shell-and-tube type heat exchangerNonporous wall ofmembrane reactorSteam SweepTubular symmetricstainless steelmembrane supportH 2PermeateHigh H 2topurity fuel cell H 23 kW ePd/Cu filmCO-rich reformateSulfur TolerantWGS CatalystCO + H 2 O = CO 2 + H 2Sulfur-richResidueN 2, CO,CO 2, H 2O,H 2S andsome H 2To be burnedwith O 2Porous Stainless Steel Tube (ACCUSEP by Pall)Steam SweepH 2PermeateH 2toTubular fuel cellmembranes7 ACCUSEP kW e tube with removable compression fittingsWeldjointNon-poroussteel tubePoroussteel tubeACCUSEP tubes weldedto the tubesheet10 TDAR e s e a r c h

• IntroductionOutline• Background on Composite Pd AlloyMembranes• Results• Sulfur Tolerant Composite PdAu Membranes• Metal Supported PdAu Membrane• Future Work11 TDAR e s e a r c h

Molecular Modeling• Molecular Dynamics Simulation is used to identify a promising Pdalloy composition that provides high flux and sulfur resistance• PdCu and PdAu alloys were selected for initial considerations• Pd-rich alloys were examined both because of their lower cost and higherperformancePolycrystalline arrangementsFocus:AuPd 3 regionSROLROSROSROLROSROLROAll crystal structure in diagramare FCCAu 50Pd 50Au 25Pd 75• Atomistic studies were performed to measure rate of H atomdiffusion through Au-Pd 3 lattice at 300-400 o C range12 TDAR e s e a r c h

Diffusion of H through PdAu Alloy• Molecular simulation results showed that H atom transport in thePd 75 Au 25 matrix is slower than it is in the Pd 60 Cu 40 at 400 o C• Both lattices are dilated to the same extent lattice to increasediffusivityPdPdPdAuAuAuPdAuPdAuPdAuTrap 1 Trap 2• Au atoms act as effective trap sites for H atom and slow thediffusion rate• Slower transport rates are correlated to higher activation energy forH diffusion13 TDAR e s e a r c h

Membrane Preparation• Several PdCu and PdAu composite membranes were developedfor testing• EDX analysis indicates that membrane consists of 87% Pd and13% Au• Small number of spots or defects in the surface of the membrane• Nitrogen leak rate measurements show that the surface defects do notpenetrate the entire thickness of the membrane15 TDAR e s e a r c h

Membrane Testing SystemThermocoupleportTest apparatusFeed InMembrane moduleShell (1.0 in)Tube (0.25 in)MembranemodulePermeateResidueThermocoupleportSub-ppm level sulfur detection capability withSievers Chemiluminescence Detector16 TDAR e s e a r c h

Baseline Performance of PdAu MembraneT=400 o C, P feed = 100 psig, P perm = 2 psig, Feed H 2 /H 2 O/CO/CO 2 Conc. = 51/21/26/2350300Hours92-97of theTestHours 140-147 of theTestHours 198-205 of theTestHours 315-331 of theTest250200150Permeate (sccm)10050H 2permeation rate=32.5 sccm/cm 2 or64 scfh/ft 2Permeation Tests, CO2 test, Air PurgeWater Gas Shift Testing (400°C,various pressures)Water Gas Shift Testing (450°C), Air Purge, H2/N2test, Water Gas Shift Testing (450°C),Water Gas Shift Temperature Testing, Flow RateTests, Pressure Tests, CO/CO2 Test H2/N2 test0• A stable membrane performance (i.e., H 2permeation and selectivity) wasobserved for over 300 hrs in the presence of CO, CO 2, H 2O and H 2mixTime17 TDAR e s e a r c h

Effect of H 2 S on PdAu MembraneT=400 o C, P feed = 100 psig, P perm = 2 psig, Feed H 2 /H 2 O/CO/CO 2 Conc. = 51/21/2/26Permeate (sccm), Permeate CO (ppm)30025020015010050ReformateCO1 ppm H 2 S inReformate5 ppm H 2 S inReformatePermeateCO21 ppm H 2 S inReformateReformate1.41.210.80.60.40.2Permeate CO 2 (%)00 1000 2000 3000 4000 5000 6000 7000 8000 9000Time (min)• With the introduction of sulfur to the syngas feed H 2permeation ratedecreased (5 ppmv sulfur caused 40% decrease)• When sulfur flow was stopped, membrane performance recovered18 TDAR e s e a r c h0

PdAu on Porous Steel SubstratesTreatment of theporous steel tubes(elimination of oils,grease etc for theapplication ofcoating)Hermatic sealing ofthe ends of theporous steel tubesApplication of anoxide diffusionbarrierDeposition of Pd film(followed withdeposition of goldand proper annealing19 TDAR e s e a r c h

Evaluation of PdAu on Porous SteelT=400 o C, P feed = 100 psig, P perm = 2 psig, Feed H 2 /H 2 O/CO/CO 2 Conc. = 51/21/2/262250.2Permeate (sccm),Permeate CO (ppmv)200175150125100755025Corresponds to a fluxof 31 scc/cm 2 min (61 scfh/ft 2 )CO averages less than 5 ppmv0.180.160.140.120.10.080.060.040.02Permate CO2 (%)000 50 100 150 200 250 300 350Time (min)• The PdAu membrane prepared on porous steel support showed improvedselectivity indicating non-selective transport is most likely due to the seals• Even with a thicker film, H 2permeation rate of this membrane matched tothat prepared on ceramic support20 TDAR e s e a r c h

Future Work• Work on reducing the thickness of the membranefilm over the porous steel supported (PSS)membranes• Evaluate performance of the PSS membranes inthe presence of H 2 S• At sulfur concentrations up to 100 ppmv• Evaluate the potential problems associated withother coal gas contaminants• Arsenic, selenium, HCl …• Module design and development• H 2 separation module• Membrane Water-Gas-Shift Reactor21 TDAR e s e a r c h

Acknowledgements• DOD SBIR Phase I (Contract No. W56HSV-06-C0077)• Contract Monitor: Mr. Dan Maslach• DOE SBIR Phase I Project• Contract Monitor: Ms. Patricia Rawls22 TDAR e s e a r c h