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Journal of Membrane Science

journal homepage: www.elsevier.com/locate/memsci

Natural gas steam reforming reaction at low temperature and pressureconditions for hydrogen production via Pd/PSS membrane reactor

Bryce Anzelmo, Jennifer Wilcox, Simona Liguori⁎

Department of Energy Resources Engineering, Stanford University, 367 Panama St. Stanford, 94305, CA, United States

A R T I C L E I N F O

Keywords:HydrogenNatural gas reformingPalladiumComposite membraneMembrane reactor

A B S T R A C T

The objective of this work is to analyze the performance of a composite palladium-based membrane reactor(MR) by performing the natural gas steam reforming reaction at low operating conditions for producing high-purity hydrogen. The MR comprises a composite membrane, having ~13 µm Pd layer deposited on a porousstainless steel support, fabricated via electroless plating and a commercial Ni-based catalyst.

The composite membrane shows infinite ideal selectivity, H2/He and H2/Ar, at trans-membrane pressuresless than 100 kPa and T=400 °C at the onset of experimental testing. The steam reforming reaction is performedat 400 °C, by varying the reaction pressures and sweep gas flow rate between 150 kPa and 300 kPa, and 0–100 mL/min, respectively. The gas hourly space velocity (GHSV) and steam-to-carbon ratio (S/C) are keptconstant at 2600 h−1 and 3.5.

The effect of CO2 as an impurity in the feed line is also analyzed at 400 °C and 150 kPa.The best performance of the Pd-based MR is obtained at 400 °C, 300 kPa and 100 mL/min of sweep-gas,

yielding a methane conversion of 84%, hydrogen recovery of 82%, and obtaining a pure hydrogen stream at thepermeate side.

The Pd/PSS MR worked for more than 700 h under differing operating conditions. As a comparison, aconventional reactor operating at the same MR conditions is compared and discussed.

1. Introduction

World power generation is largely comprised of fossil fuels use andwithin the U.S. specifically, these non-renewable sources make up 80%of energy resources, with natural gas being the largest growing fossilfuel at approximately 2% per year [1]. One potential solution toreducing CO2 emissions from these fossil sources is through research-ing alternative pathways with increased efficiency and subsequentlyfewer emissions [2–4]. As a potential energy carrier, hydrogen has theability to meet these needs with a high energy density and zero end-usegreenhouse gas emissions [5]. While it is possible to produce hydrogenfrom renewable sources, with many different methods and technologiesstill under development [6–9], hydrogen production primarily comesfrom natural gas steam reforming, where the main reaction is themethane steam reforming (SMR) [10].

The leading industrial process for natural gas reforming is char-acterized by a multi-step process in which hydrogen is produced underharsh operating conditions. The first step consists of the reformer inwhich methane and steam react at 800–1000 °C and 1.5–2.0 MPa[11,12] due to thermodynamic constraints. At the reformer outlet, the

carbon monoxide (CO) content is relatively high. Therefore, it needs tobe reduced via the WGS reaction (Eq. (2)), which takes place in tworeactors arranged in series. The products of the WGS reactors are thenseparated and purified by other energy intensive steps as PSA orcryogenic distillation.

However, as reported by several studies [13–16], it is possible toperform the SMR reaction at milder operating conditions overcomingthe thermodynamic constraints by using membrane reactor (MR)technology.

The development of the membrane reactors is an evident outcomeof the ongoing process intensification efforts. In particular, the use of aPd-based MR allows for the continuous removal of H2, owing to thecomplete selectivity of the Pd membrane towards hydrogen permea-tion, shifting the reactions, Eqs. (1)–(3), toward further productformation and, consequently, yielding an increase in conversion[17,18]. This effect is known as a “shift effect.”

CH H CO H+ O↔ +3 ΔH° =206 kJ/mol4 2 2 298 K (1)

CO H O CO H+ ↔ + ΔH° =−41 kJ/mol2 2 2 298 K (2)

CH H O CO H+2 ↔ +4 ΔH° =165kJ/mol4 2 2 2 298 K (3)

http://dx.doi.org/10.1016/j.memsci.2016.09.029Received 7 April 2016; Received in revised form 19 August 2016; Accepted 18 September 2016

⁎ Corresponding author.E-mail address: sliguori@stanford.edu (S. Liguori).

Journal of Membrane Science 522 (2017) 343–350

0376-7388/ © 2016 Elsevier B.V. All rights reserved.Available online 29 September 2016

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The capacity of palladium to absorb hydrogen was discovered byThomas Graham [19], which lead the way for the development of denseunsupported Pd membranes with an initial thickness of millimeters[20]. These membranes were characterized by low hydrogen perme-ability and high cost [18,21–24]. Over the years, tubular denseunsupported Pd-based membranes have significantly decreased in costand thickness, to around 50–60 µm, which has lead to increasedhydrogen permeability but reduced mechanical resistance [25,26].However, in the last decades composite membranes, comprised of athin Pd layer deposited onto a porous support, have been developedshowing a higher hydrogen permeability, increased mechanical resis-tance and lower cost compared to unsupported membranes [27–29].The dominant support materials used for the deposition of the metalliclayer are porous ceramic and porous stainless steel (PSS), althoughother substrates exist [30–34]. PSS supports have the undesirableattributes of broad nominal pore size and irregular surface area.Moreover, their long-term stability is negatively affected by interme-tallic diffusion of Pd into the pores of the support. Nevertheless, theirmechanical durability, thermal expansion similar to Pd, and ease ofintegration, due to their ability to be welded, make them highly suitableas a support for depositing thin Pd layers. [35,36]. In addition, severaltechniques exist as a feasible remedy for intermetallic diffusion [37–39], some of which have been developed to pre-commercial status.

The ability of the Pd-based MR to enhance the reaction perfor-mance in terms of conversion, under fixed operating conditions, isproportional to the membrane thickness. In particular, the thinner themetallic layer deposited on the porous support, the higher the hydro-gen permeation flux. This produces a more pronounced shift effect,which induces an increase in conversion. Therefore, even at lowoperating temperatures of 400 °C, significant improvement in conver-sion can be achieved using a Pd-based MR even overcoming theequilibrium conversion. For example, Kikuchi et al. [40] measured aCH4 conversion of 64% at 400 °C whereas the calculated equilibriumvalue was < 20% for a fixed bed reactor (FBR) at the same operatingconditions [41,42]. This high conversion of CH4 leads to co-benefits ofthe MR technology, i.e., a pure hydrogen stream collected on thepermeate side of the membrane with zero emissions if the purehydrogen is fed to a proton exchange membrane fuel cell (PEMFC)[43–45].

Although there is a significant amount of literature on the perfor-mance of Pd-based MRs at elevated temperatures, i.e., > 500 °C forperforming SMR experiments [13,46–52], only few experimentalworks have focused on this reaction at temperatures as low as400 °C, as shown in Table 1.

These few works are different for membrane type, membranethickness, pressure and sweep gas flow rate, and most of these workshave used thick Pd-based MR [15,53–55] and high sweep gas flow rate[15,49,53,55] for performing MSR reaction.

For example, Gallucci et al. [53] and Jørgensen et al. [15]performed SMR experiments at 400 °C using a fully-dense Pd-Ag,unsupported membrane, with thicknesses of 50 µm and 100 µm,obtaining 52% and 25% methane conversion, respectively. Although

Jørgensen et al. [15] used a higher reaction pressure, which corre-sponds to a higher hydrogen driving force through the membrane, theygained a lower conversion compared to the study of Gallucci et al. [53],due to the higher membrane thickness, which subsequently caused alower shift effect. Kikuchi et al. [40], reached a CH4 conversion of 64%at a pressure of 100 kPa operating with a composite Pd/Al2O3membrane, whereas Lin et al. [54] and Shu et al. [55], housing acomposite Pd/PSS membrane in a MR with a thickness of 20 µm,achieved 34% and 19% conversion at a pressures of 910 kPa and136 kPa, respectively. Kikuchi et al. [40] obtained a higher conversiondue to a thinner membrane thickness, realizable thanks to thecharacteristics of the pore size of the alumina support, with respectto the other two studies [54,55]. Moreover, Lin et al. [54] applied botha higher reaction pressure and sweep gas with respect to Shu et al. [55],which induced a higher hydrogen driving force and greater shift effectresulting in improved MR performance.

Tong et al. [49] used a thin Pd/PSS membrane for analyzing theperformance of the MR by carrying out the MSR reaction. However,they used a higher temperature and sweep gas flow rate with respect tothe other works and this study obtaining 84% as CH4 conversion.

In all of these studies, a hydrogen purity of nearly 100% wasobtained.

As evident from Table 1 the membrane thickness as well as thereaction pressures, and the sweep gas rate play an important role in thesystem of MR performance.

Therefore, the aim of this study is to demonstrate the potentialitiesof a thin Pd-layer supported on PSS MR to produce a high purityhydrogen stream and high methane conversion via MSR reaction at lowtemperature, pressure and sweep gas flow rate.

Moreover, a low concentration of CO2 is added to the feed stream tosimulate a common impurity present in natural gas in order toinvestigate its influence on the MR performance.

In addition, a comparison between the performance, in terms ofmethane conversion and produced hydrogen, of a MR and a FBRworking at the same operating conditions is given and discussed, andexperimental data from the open literature are reported and comparedwith the results obtained in this work.

2. Experimental

2.1. Membrane reactor details

The composite Pd-based membrane was characterized by a thin Pdlayer deposited via electroless plating on a PSS support. The Pd layercalculated gravimetrically was ~13 µm thick with a total active surfacearea of ~44 cm2. The PSS support was furnished by Mott Corporation(2300 series), with an active length of 15 cm and an outer diameter of1 cm. The membrane was fabricated in the Center for InorganicMembrane Studies at Worcester Polytechnic Institute by using asimilar procedure to Ma et al. [56–58]. This procedure consists ofsupport polishing to reduce the surface roughness, its cleaning in anultrasonic bath, support oxidation in an oven to create an intermediate

Table 1Comparison of methane conversion (ΧCH4) and H2 purity for Pd-based MRs under SMR reaction with similar operating conditions to this study.

Membrane type Thickness[μm]

Temp. [°C] Reactor feedgas

Reactionpressure [kPa]

Permeatepressure [kPa]

Sweep gas &rate [mL/min]

Active area[cm2]

S/C ΧCH4 [%] H2 purity[%]

Ref.

Dense Pd-Ag 50 400 CH4 122 110 Steam - 120 48.0 3 52 100 [53]Dense Pd-Ag 100 400 CH4 700 150 N2 - 166 18.5 2.9 25 100 [15]Pd-alumina 13 400 CH4 100 vacuum vacuum 12.6 3 64 100 [40]Pd/PSS 20 400 CH4 910 100 N2 - ≈400 60 3 34 ≈100 [54]Pd/PSS 19.8 400 CH4+5% He 136 100 None 10.7 3 19 100 [55]Pd/PSS 6 450 CH4 300 100 N2 - 500 20.0 3 84 100 [49]Pd/PSS 13 400 CH4 300 100 Ar - 100 44 3.5 84 100 This

work

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layer and avoid intermetallic diffusion, support activation and succes-sive deposition of the Pd-layer by electroless plating. The membranewas welded on both sides to a stainless steel tube with one end beingwelded closed.

A commercial Ni-based SMR catalyst supplied by Johnson MattheyInc. was used. Before packing 3 g of catalyst in the annular region ofthe MR, it was crushed and sieved to an approximate particle size of200–400 µm, in order to increase the available surface area for theSMR reaction and to avoid pressure drop in the catalytic bed. Beforethe reaction tests, the catalyst bed was activated for 2 h by flowing20 mL/min of pure hydrogen at 400 °C.

The specified working temperature of the membrane, based on theexperience of the manufacturers [59], was 400–500 °C. The MRschematic is shown in Fig. 1..

2.2. Experimental setup

A schematic showing the experimental setup used to carry out theSMR experiments is shown Fig. 2. The MR system was heated usingheating tape controlled by a voltage controller (Glas-Col) and thetemperature was monitored via a K-type thermocouple (Omega

Engineering) housed in the permeate side of the MR. Pure gases wereregulated and supplied to the MR via Aalborg GFC17 thermal massflow controllers, distilled water was supplied through an Eldex 1LMPpump, which was vaporized and mixed with gases prior to entering inthe MR, a temperature-regulated water bath (Julabo F25-EH) was usedto condense water vapor from the retentate and an Extrel Max-300LGMass Spectrometer (MS) was utilized for analyzing the composition ofdry gas species coming from both retentate and permeate sides. Beforebeginning experiments, the MS was calibrated using the standardaddition method [60] by feeding various concentrations of CH4, H2, COand CO2 mixed with Ar in order to obtain accurate molar compositionswith parts per million (ppm) precision for both retentate and permeatestreams.

The MR was heated to an operating temperature of 400 °C, with aheating ramp of 1 °C/min under an Ar atmosphere.

The effect of reaction pressure and the sweep gas on the MRperformance was analyzed. In particular, they were varied between 150and 300 kPa and 0–100 mL/min, respectively. In addition, CO2 wasadded in the feed stream with a maximum composition of 6 vol% inorder to simulate natural gas at traditional pipeline conditions. Thereaction temperature, permeate pressure, gas hourly space velocity(GHSV, defined as the ratio between the total feed volumetric flow ratesupplied to the MR at STP and catalyst volume) and steam to carbonratio (S/C) were kept constant at 400 °C, 100 kPa, 2600 h−1 and 3.5/1,respectively. Prior to the reaction experiments, the membrane wascharacterized by permeation measurements using pure gases, such asH2, He and Ar. The permeating flux of each gas through the membranewas measured using a bubble-flow meter with an average of 10experimental points recorded. The equations used for describing thepermeating characteristics of the membrane are as follows:

αPermeancePermeance

Ideal Selectivity( ) =H iH

i/2

2

(4)

where i is Ar or He.

J P p p= ( − )H H retentaten

H permeaten

, ,2 2 2 (5)

where JH2 is the hydrogen permeating flux through the membrane, P is

Feed

Permeate

Retentate

Pd/PSS Membrane

Catalyst

Sweep Thermocouple

Fig. 1. Graphical representation of the Pd-based MR.

Arg

on

H2O

Pump Pre-heating zone

Mass Spec.

Condenser

Back pressure regulator Ar standard gas Mass flow controller

Ar sweep gas

Permeate

Retentate

Composite Membrane

Retentate

Permeate

Thermocouple

Sweep Feed CatalystQuartz

Spheres

SS Tube

Membrane Reactor

Met

hane

Hyd

roge

n

Fig. 2. Flow diagram of the experimental setup.

B. Anzelmo et al. Journal of Membrane Science 522 (2017) 343–350

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the permeance, pH2is the partial pressure of hydrogen in either theretentate and permeate side of the membrane and n is the dependentfactor that relates the hydrogen partial pressure to its flux and variesbetween 0.5–1.0.

P P exp E RT= ∙ (− / )o a (6)

where Po is the pre-exponential factor, Ea apparent activation energy, Runiversal gas constant, and T is absolute temperature.

In terms of the reaction experiments, each point obtained repre-sents an average value of at least 100 measurements taken over150 min at steady-state conditions with an error variation lower than3%. Before performing the reaction, the catalytic bed was reducedthrough the addition of hydrogen at 400 °C for 2 h.

The main equations used for describing the performance of the Pd-based MR are as follows:

XQ Q

QMethane Conversion( ) =

−∙100CH

CHIN

CHOUT

CHIN4

4 4

4 (7)

where QCH IN4 is the methane molar feed rate entering the MR, andQCH OUT4 is the outlet molar flow rate.

HRQ

Q QHydrogen Recovery( ) =

+∙100H

Permeate

HRetenate

HPermeate

2

2 2 (8)

QQ

HPPHydrogen Permeate Purity ( ) = ∙100HPermeate

TotalPermeate

2

(9)

where QH Permeate2 and QH Retenate2 are the molar flow rates of H2 in thepermeate and retentate streams and QTotalPermeate is the total molar flowrate of all the permeating species within the permeate stream of themembrane. Additionally, in order to evaluate whether coke formationoccurs during reaction testing, pure H2 was supplied to the MR afterthe experiment and the retentate stream was analyzed by using the MSto check for the presence of CH4 by way of Eq. (10).

C H CH+ ↔ ΔH° =−75. 0 kJ/mol2 4 298 K (10)

The use of oxygen or air to check and eliminate the coke deposits onthe MR is another possible pathway, however, in order to avoid theformation of palladium oxides, which could dramatically damage thePd-layer and its performance, oxygen/air exposure to the membranewas avoided.

It was found that coke was not formed in any of the experimentscarried out.

Moreover, the carbon balance between inlet and outlet gaseousstreams was closed with ± 2.0% as maximum error.

The catalyst was stable throughout all the experimental tests.

3. Results and discussion

3.1. Permeation tests

In order to evaluate the permeating characteristics of the mem-brane, permeation tests of pure gases, namely H2, Ar and He wereperformed at 400 °C under varied trans-membrane pressures. Inparticular, He and Ar permeation measurements were performed toverify the presence of any defects in the palladium layer, and were thenused to calculate the ideal selectivities αH He/2 and αH Ar/2 (Eq. (4)). In

Table 2, the ideal selectivity as a function of the trans-membranepressure is reported. As shown, although the membrane exhibitedinfinite selectivity for hydrogen at small trans-membrane pressures,both ideal selectivities αH He/2 and αH Ar/2 decrease from infinity at100 kPa to 9000 and 40,000 at 200 kPa. This decreasing trend canbe attributed to the defects of the membrane, which potentially arecaused by irregularities in the support surface and/or impuritiespresent during electroless plating fabrication of the membrane [61].These defects allow for Knudsen diffusion of the gas from the retentateto permeate through the defects in the Pd layer, as it has also beenreported by Rothenberger et al. [62].

These results indicate that the composite Pd-based membrane wasnot defect-free and not fully selective to hydrogen permeation withrespect to all other gases at high pressure, although the ideal selectivitywas still high (i.e. 9000 and 40,000 at trans-membrane pressure of 200kPa). In order to establish the membrane characteristics towardhydrogen permeation, the parameters such as Po, Ea and n (Eqs. (5)–(6)) need to be estimated. Therefore, permeation measurements withpure hydrogen at different temperatures and trans-membrane pres-sures were performed.

At 400 °C, the hydrogen permeation flux as a function of thedifference in partial pressure of hydrogen to the power of n is plottedand a linear regression equation with associated R2 is used for each n asshown in Fig. 3. The best linear fit corresponding to the highestcoefficient of determination, R2, was found for n=0.6. Therefore, thetransport of hydrogen through the membrane is mainly limited by thesolution-diffusion through the bulk palladium phase. Nevertheless, adeviation from 0.5 indicates that the permeation of hydrogen may beinfluenced by a combination of other factors such as Pd surfaceimpurities, Pd surface or bulk defects, i.e., organic contaminants fromfabrication or pinholes, respectively [22,63]..

In order to evaluate Ea and Po, permeation tests with pure hydrogenwere also carried out at different temperatures and at 100 kPa. Byusing an Arrhenius relationship (Eq. (6)) between the hydrogenpermeance and the reciprocal temperature, values for Ea and Po of16.2 kJ/mol and 3.88×10−5 mol/s m2 kPa were determined, respec-tively. The Ea value is in reasonable agreement with those reported byother researchers, who also used similar membranes to the one usedfor this study. For example, Tong et al. [49] found 15.9 kJ/mol, Sanzet al. [64] 15.56 kJ/mol, Chen et al. [65] obtained different values forvarious tested membranes such as 11.8 and 17.5 kJ/mol, andMardilovich et al. [27] found an Ea value of 16.4 kJ/mol.

Table 2Ideal selectivity of H2 with respect to He and Ar under different pressures at 400 °C.

Δp [kPa] αH2/Ar αH2/He

50 ∞ ∞100 ∞ ∞150 44,000 28,000200 40,000 9000 Fig. 3. Permeation H2 flux through Pd/PSS supported membrane versus trans-mem-

brane pressure by varying ‘n’ at T=400 °C.

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3.2. Reaction testing

3.2.1. Comparison: MR vs FBRTo highlight the ability of the MR to achieve greater performance

with respect to a FBR, the SMR reaction tests were performed in bothFBR and MR configurations at the same operating conditions, with theresults reported in Table 3. The methane conversion in the MR wasgreater than that of the FBR by 57%, with a factor of two increase in thetotal amount of hydrogen produced. Furthermore, another benefits ofthe MR are the higher methane conversion with respect the equilibriumconversion, and the production of hydrogen at high purity, i.e.,approaching 100% pure. In particular, the MR produced 16.5 mL/min of pure hydrogen in the permeate stream.

3.2.2. Influence of reaction pressure and permeate sweepThe SMR experiments were performed in the composite Pd/PSS

MR packed with a Ni-based catalyst at 400 °C, at different reactionpressures varying from 100 to 300 kPa and differing sweep gas flowrates.

Fig. 4 shows the CH4 conversion vs. the reaction pressure atdifferent sweep gas flow rates. As displayed, the XCH4 (Eq. (7)) increaseswith increasing reaction pressure as well as argon sweep rate. Thisincreasing trend is caused by a growing shift effect promoted by ahigher hydrogen permeation driving force. In particular, Eq. (5) showsthat an increase in the reaction pressure results in an increased H2partial pressure in the retentate stream. This in turn causes a higher H2permeation flux through the membrane, thereby causing more hydro-gen to be removed from the reaction zone, driving the reactions (Eqs.(1)–(3)) towards further product formation and resulting in anincrease in XCH4. Furthermore, the H2 permeate partial pressure maybe lowered by introducing a sweep gas, which causes an increase in thehydrogen permeation driving force and thus an increased shift effect.The maximum XCH4 obtained was 84% at 400 °C with a sweep gas flowrate of 100 mL/min and a reaction pressure of 300 kPa. In contrast, theminimum XCH4 obtained was 68% using an argon sweep flow rate of

25 mL/min and 49% using no argon sweep under a reaction pressure of100 kPa. Additionally, by using the highest reaction pressure, i.e., 300kPa and lowering the argon sweep to 25% of the maximum rate, theXCH4 decreases only to 73.6%.

Another important parameter for evaluating the MR performance isthe hydrogen recovery HR (Eq. (8)) defined as the amount of recoveredhydrogen in the permeate stream divided by the total producedhydrogen. Fig. 5 shows HR versus the reaction pressure by varyingthe sweep gas flow rate. As HR increases with the increasing reactionpressure as well as sweep gas rate, a maximum value of 82% wasachieved under a QArSweep=100 mL/min and a reaction pressure of300 kPa. Using QArSweep=0 mL/min (no argon sweep) under a reactionpressure of 300 kPa, a HR of 36% was obtained. As previouslydiscussed, an increase in both reaction pressure and sweep gas flowrates leads to a higher hydrogen permeation driving force. The higherthe driving force, the higher the hydrogen permeation flux through themembrane. As a consequence, a greater amount of H2 is collected in thepermeate stream, thereby increasing the HR value.

In all of the experiments carried out, the hydrogen permeate purity(HPP), Eq. (9), was found to be approximately 100% indicating that, byusing a condensable sweep gas as steam, the hydrogen coming from thepermeate stream can be used directly for industrial applications as wellas a fuel cell feed.

3.2.3. Influence of CO2 in the feedTo investigate the impact of the presence of CO2 in the feed, the

SMR reaction was performed by adding 6 vol% CO2 into the feedstream to simulate a common impurity contained within natural gas.Exact specifications for the CO2 content in natural gas pipelines do notexist [66]; however, CO2 does not add heating value to natural gas andthus is minimized below 3 vol% for transportation via pipelines withinthe U.S. [67]. For this set of experiments, CO2 was important since it isa product of the SMR reaction (Eqs. (2)–(3)) and, from a thermo-dynamic perspective, an increase in product concentration leads to adecrease in conversion. This negative effect is evident in Fig. 6 whereXCH4 (Fig. 6A) as well as HR (Fig. 6B) versus different sweep gas flowrates are shown. In particular, XCH4 decreases from 67% to 61% andfrom 70% to 66% by using 25 mL/min and 50 mL/min as sweep flowrate, respectively. As a consequence, the HR decreases from 46% to41% and from 53% to 51% by using a rate of 25 mL/min and 50 mL/min. Nevertheless, the negative effect of the presence of CO2 on XCH4and HR is partially negated by using a higher sweep flow rate. Thehigher hydrogen permeation driving force, caused by the higher sweeprate, leads to an increase in hydrogen collected in the permeate stream.

Table 3Methane conversion, total H2 production and permeate H2 production at T=400 °C,preaction=175 kPa, S/C=3.5/1, GHSV =2600 h

−1, no sweep gas for MR.

Test ΧCH4 [%] Total H2 [mL/min] H2 Permeate [mL/min]

Equil 35.4 38.9 –FBR 32.1 22.4 –MR 50.4 58.1 16.5

Fig. 4. Methane conversion vs reaction pressure at various Ar permeate sweep gas forPd/PSS MR. T=400 °C, S/C=3.5/1, GHSV=2600 h−1, pperm=100 kPa.

Fig. 5. Hydrogen recovery vs reaction pressure at various Ar permeate sweep gas for Pd/PSS MR. T=400 °C, S/C=3.5/1, GHSV=2600 h−1, pperm=100 kPa.

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The increased sweep gas flow rate acts to remove more of the hydrogenwithin the permeate and maintain the trans-membrane H2 partialpressure differential, leading to an increase in XCH4. Thus, with anincrease in the permeate sweep rate (e.g., 50 mL/min), the negativeeffect due to the CO2 presence is reduced, with a 3% difference in XCH4and 2% difference in HR..

Although HPP was not 100% during these reaction experiments, itremained constant at 97% throughout all of the tests.

3.3. Membrane lifetime

The long-term stability of the Pd/PSS composite membrane is vitalfor industrial applications. For this reason, the pre- and post-reactionpermeation measurements of H2 and Ar throughout all 750 experi-mental hours were carried out to ensure adequate monitoring. Fig. 7shows the H2/Ar ideal selectivity versus the duration of the experimentat T=400 °C and Δp=50 kPa.

It is clear that the ideal selectivity drops notably within the first 250

experimental hours from nearly infinite to 600 and continues to slowlydecrease over the course of the remaining experiments. Although theprecise mechanism of this decline is not fully understood, Kumpmannet al. [68] attribute the thermodynamic instability of the grains formedby the electroless plating process to abnormal grain growth. SimilarlyBryden et al. [69] found that Pd grain growth can start as low as200 °C. Furthermore, for thin Pd composite membranes, Mardilovichet al. [27] found that large pores within the PSS support can create poregaps within the Pd layer upon heating, which were undiscovered duringroom temperature permeation testing. Lastly, intermetallic diffusionmay aid in the ideal selectivity decline, although the membrane usedhad an oxidation layer to reduce intermetallic diffusion. Table 4 showsthe MR performance during the first and last reaction experimentconducted with the Pd/PSS composite membrane. Although the idealselectivity of hydrogen with respect to argon at the end of itsexperimental life is pretty low ~30, it is important to note that noCO was detected in the permeate stream and that the HPP is 91%. Inparticular, only CH4 and CO2 were detected in the permeate side with acomposition of 8% and 1%, respectively. If the end use for the producedhydrogen is for a PEMFC the CO content in the hydrogen permeatestream of this work is < 10 ppm thus making it adequate for thisapplication [70].

4. Conclusion

The performance of a composite Pd-based PSS-supported mem-brane reactor (MR) fabricated by electroless plating was investigatedthrough a series of experiments via performing SMR reaction. Inparticular, the effect of different reaction pressures, as well as permeatesweep gas rates, on MR performance in terms of methane conversion,hydrogen recovery and hydrogen permeate purity was studied.Moreover, the effect of a common impurity, CO2, contained within

Fig. 6. CO2 effect on MR: A) Methane conversion vs argon permeate sweep rate B) Hydrogen recovery vs argon permeate sweep rate. T=400 °C, Δp=50 kPa, S/C=3.5/1,GHSV=2600 h−1, pperm=100 kPa.

Fig. 7. Long-term hydrogen/argon ideal selectivity of Pd/PSS composite membrane;T=400 °C, Δp=50 kPa.

Table 4Total SMR reaction performance life of Pd/PSS membrane; T=400 °C, preaction=150 kPa,Ar sweep=50 mL/min, GHSV=2600 h−1.

1st test Last test

Ideal selectivity ∞ 30XCH4 72% 30%HR 55% 54%HPP 100% 91%CO (permeate) 0% 0%

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natural gas has also been analyzed. A methane conversion of 84% wasreached using an Ar sweep rate of 100 mL/min, preaction=300 kPa,T=400 °C, S/C=3.5/1, GHSV=2600 h−1 and pperm=100 kPa. This highconversion allowed for a maximum hydrogen recovery of 82% whilemaintaining a permeate hydrogen purity of 100%.

The pressure and temperature conditions are significantly lowerthan the current industrial method for producing hydrogen via SMR.To further emphasize the performance of the MR, the SMR reactiontests at 400 °C were also investigated in a FBR configuration. Theoutcome shows that the methane conversion is higher by 57% in theMR configuration compared to the FBR, and produced double theamount of hydrogen. Initially, the membrane showed a near-infiniteselectivity towards hydrogen permeation at pressures less than100 kPa. After 750 experimental hours, the ideal selectivity of hydro-gen with respect to Ar dropped to 30. Nevertheless, no CO was detectedin the permeate side for all the experimental campaigns, which showsthat as the membrane slowly degraded, it had the ability to still produceCO-free hydrogen for feeding a fuel cell or for other hydrogen-sourcedindustrial applications.

Future work will consider the use of a real composition of naturalgas and the utilization of a Pd alloy that is sulfur resistant to ensure themembrane performs without degradation. The use of a Pd alloy willalso allow for better membrane stability. Moreover, the effect on theMR performance of the steam as sweep gas will be also studied and theresults will be compared with the ones obtained in this work.

Acknowledgements

The Stanford University School of Earth, Energy & EnvironmentalSciences Graduate Fellowship Program supported this work. Theauthors would like to thank Prof. Yi-Hua (Ed) Ma and Prof. IvanMardilovich as well as the Worcester Polytechnic Institute's Center forInorganic Membrane Studies for fabricating the membrane used withinthis study. In addition, the authors would like to thank Xavier Quek,David Wails, and Hugh Hamilton from Johnson Matthey for supplyingthe catalyst used and providing their insights, which have led to astronger manuscript.

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Natural gas steam reforming reaction at low temperature and pressure conditions for hydrogen production via Pd/PSS membrane reactorIntroductionExperimentalMembrane reactor detailsExperimental setup

Results and discussionPermeation testsReaction testingComparison: MR vs FBRInfluence of reaction pressure and permeate sweepInfluence of CO2 in the feed

Membrane lifetime

ConclusionAcknowledgementsReferences