Sulfur inhibition of PdCu membranes in the presence of external mass flow resistance

Journal of Membrane Science 496 (2015) 301–309

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Sulfur inhibition of PdCu membranes in the presence of external mass flow resistance Lingfang Zhao, Andreas Goldbach n, Chun Bao, Hengyong Xu n Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road 457, 116023 Dalian, China

art ic l e i nf o

a b s t r a c t

Article history: Received 16 June 2015 Received in revised form 19 August 2015 Accepted 23 August 2015 Available online 3 September 2015

Coal gasification products typically contain less than 40% H2 and less than 40 ppm H2S after desulfurization with ZnO beds at operation temperatures around 773 K. Hence, both sulfur poisoning and external mass flow resistance must play a major role in separation of such mixtures with Pd-type membranes. We have investigated the separation of 1:1 and 9:1 H2/N2 mixtures contaminated with 7–35 ppm H2S to elucidate the relative importance of these transport resistances using a ca. 5 mm thick PdCu membrane supported on a ceramic substrate. Sulfur inhibition depended strongly on H2 recovery (10– 80%) and temperature (673–773 K) in the investigated range. Sulfur poisoning of the membrane dominated H2 permeation rates at lower temperatures and in the 9:1 H2/N2 mixture especially. However, its impact declined rapidly with increasing H2 recovery in the 1:1 H2/N2 mixture. As a consequence H2 recovery was only slightly reduced from 75% to 70% at 773 K even after adding 35 ppm H2S to that mixture. This demonstrates that concentration polarization is a stronger limitation to H2 permeation than sulfur inhibition in practical separation situations where very high H2 recovery will be an economic necessity. Exposure to H2S for altogether 75 h had no lasting effect on H2 permeability of the membrane but the N2 leak rate doubled presumably due to sulfide formation at defect sites at lower temperatures. & 2015 Elsevier B.V. All rights reserved.

Keywords: Metal membranes Gas separation Concentration polarization Hydrogen recovery Sulfur tolerance

1. Introduction Coal is the most abundant and evenly distributed fossil energy carrier worldwide. Hence it will continue to play an important role in energy production in the foreseeable future despite that this comes with enormous greenhouse gas emissions. Pre-combustion carbon capture with dense Pd membranes is a promising strategy for curbing those CO2 emissions in stationary power stations because fossil fuels are converted into separate high-pressure CO2 and H2 streams prior to power generation [1]. This scheme reduces significantly compression costs associated with CO2 sequestration and burning of H2 in a gas turbine. However, coal-derived syngas contains sulfur which can easily deactivate Pd and most Pd alloy membranes even at ppm levels due to formation of sulfides [2–4]. These sulfides do not only inhibit hydrogen permeation but are also conducive to pinhole formation in the selective metal layer [4]. PdCu membranes are an exception that offer some sulfur tolerance in addition to other attractive features such as reduced noble metal costs and enhanced low-temperature membrane stability [5,6]. Another peculiarity of PdCu alloys is the existence of body-centered cubic (bcc) phases below 873 K in the 30–50% Pd stoichiometry range (alloy compositions are given in atom % n

Corresponding authors.

http://dx.doi.org/10.1016/j.memsci.2015.08.046 0376-7388/& 2015 Elsevier B.V. All rights reserved.

throughout this paper) [7,8]. Alloys with higher Pd content form face-centered cubic (fcc) lattices as Pd and most of its other alloys do. In general, hydrogen permeability decreases with increasing Cu content but alloys at the Pd-rich boundary of the bcc phase regime exhibit rather high permeabilities which are comparable to that of pure Pd [8,9]. Sulfur poisoning of metal membranes has been recently reviewed including mechanistic aspects of the metal sulfur interaction [10]. The sulfur tolerance of PdCu membranes has been often studied beginning with McKinley five decades ago [11]. He found that H2 permeability of a 25 μm thick Pd47Cu53 foil was reduced to ca. 5% at 623 K when 4.5 ppm H2S was added to H2 but permeability recovered completely after H2S removal and the foil retained its original lustre. Edlund demonstrated that H2 flux remained stable through a 50 μm thick Pd47Cu53 foil in presence of 1000 ppm H2S at 773 K [12]. The hydrogen permeance of 0.1 mm thick bcc PdCu foils dropped by up to two orders of magnitude during transient exposure to 1000 ppm H2S in experiments reported by Morreale and co-workers while that of fcc alloy foils was hardly affected between 603–1123 K [13]. However, H2 flux of such fcc Pd70Cu30 foils was rapidly reduced by more than 90% at 623 K during continuous exposure at this H2S level due to growth of a relatively thick sulfide layer on the surface [14]. Kulprathipanja et al. elaborated that inhibition of 2–10 μm thick PdCu layers increases with sulfur concentration and H2 permeation being

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completely blocked above 300 ppm H2S at 723 K [4]. They also observed micron-sized pinholes that lead to membrane failure [4]. Mundschau and co-workers reported that a Pd70Cu30 foil of unspecified thickness retained 80% of its H2 permeability at 593 K upon exposure to a 60% H2 mixture containing 20 ppm H2S even though Pd4S was detected on the membrane after this test [2]. Nishimura and co-workers coated a 300 μm thick VNi alloy membrane with a 200 nm thick Pd47Cu53 layer after which 100 ppm H2S had a marginal effect on H2 permeability above 573 K [15]. Similarly, Pomerantz and Ma furnished 10–20 μm thick Pd membranes with PdCu surface layers containing 13–28% Cu but H2 permeance dropped by 80% when those membranes were exposed to ca. 50 ppm H2S H2 at 723 K [16]. Most recently She et al. reported that H2 permeability of 45.8 μm thick fcc PdCu membranes was not affected by H2S concentrations below 39 ppm in mixtures containing 50% H2 between 673 K and 773 K [17]. These varied observations demonstrate that the degree of sulfur inhibition is a complex function of PdCu stoichiometry and structure, alloy layer thickness, sulfur concentration and exposure time, temperature and other operation parameters. Some of the weakest H2S inhibition was noted during testing in H2 mixtures [2,17] where concentration polarization [18–20] comes into play. The latter has also a negative effect on H2 permeation rates and apparent permeation rates depend on the fraction of H2 recovered through the membrane in that situation. However, those values have not been reported in aforementioned studies. Sulfur inhibition in the presence of external mass flow resistance has not been systematically investigated to the best of our knowledge. Yet, understanding their combined impact is practically very important since coal gasification products contain typically less than 40% H2 after water gas shift [2,21]. Sulfur concentration can be reduced to less than 40 ppm in such streams using conventional ZnO beds for desulfurization at usual operation temperatures of Pd-type membranes (573–773 K) [2]. Therefore we have examined sulfur inhibition as function of concentration polarization feeding 9:1 and 1:1 H2/N2 mixtures charged with 7–35 ppm H2S to a PdCu membrane. We tested in H2/N2 mixtures instead of emulating coal gasification mixtures because we wanted to avoid potential membrane inhibition effects arising from CO and CO2. We did not employ mixtures with less than 50% H2 because 500 kPa was the pressure limit of our laboratory setup and we wanted to reach practically relevant H2 recovery values ( Z75%) in our experiments.

2. Experimental 2.1 Membrane preparation The PdCu membrane was fabricated on the outside of a ceramic microfiltration membrane tube (TAMI, i.d. ¼10 mm, o.d. ¼14 mm) provided by the Energy research Centre of the Netherlands (ECN) where it had been furnished with a macroporous coating to reduce the average surface pore size to 3.4 μm [22]. Those pores were blocked with an inorganic gel prior to activation of the ceramic surface with Pd seeds using commercial solutions (OPC-50 inducer and OPC-150 cryster, Okuno Chemical Industries) to prevent ingress of Pd into the substrate pores during metal deposition as previously described [23]. Then Pd and Cu were sequentially deposited via electroless plating (ELP) employing a commercial Pd bath (PALLA TOP, Okuno) and a homemade Cu bath as reported elsewhere [8,24]. This membrane precursor was annealed at 773 K under N2 for 48 h before capping with graphite seals at ECN and afterwards further alloyed at that temperature under H2. The inorganic gel decomposed during these treatments leaving a porous residue behind in the reopened pores [23]. A 1 cm long piece of

Fig. 1. Layout of experimental setup for permeation testing with H2S.

the membrane was cut off before sealing and used for rapid alloying of the metals at 1073 K for 60 h in flowing N2 and subsequent analyses. 2.2 Permeation measurements Fig. 1 shows schematically the layout of the set-up for permeation measurements. The membrane was mounted with graphite gaskets into a stainless steel separator shell (i.d.¼25 mm) which was placed inside a furnace. Single gases or mixtures made up of H2 (purity499%) [25], N2 (99.999%) and a 507 ppm H2S/H2 mixture were fed to the membrane outside using mass flow controllers while the tube interior was kept at atmospheric pressure. The retentate line was furnished with a back pressure valve and a pressure gauge for controlling and measuring the feed pressure Pfeed. The permeate side was always kept at atmospheric pressure (Pperm ¼100 kPa) and sweep gas was not used. Permeate and retentate fluxes up to 1500 ml min  1 were measured with a soap bubble flow meter and larger ones with a TH-ZM8 electronic soap film flowmeter (Wuhan Tianhong Instrument Co. Ltd). The molar H2 fraction of retentate (CH2,ret) and permeate (CH2,perm) flows was analyzed by an online gas chromatograph (GC-8A, Shimadzu) equipped with a thermo conductivity detector (TCD). H2S content of the feed could be monitored online using a Shimadzu GC-14C gas chromatograph equipped with a flame photometric detector (FPD). First the membrane was heated at 1 K min  1 to 773 K sweeping both sides of the membrane with N2 and then annealed at that temperature under flowing H2 (ca. 150 ml min  1, Pfeed E120 kPa) to form the alloy. From time to time N2 leak rates JN2 were measured as well as H2 permeation rates JH2 in the range 573–773 K at pressure difference ΔPH2 ¼100 kPa to determine the activation energy Eact. Temperature was changed at a rate of ca. 1.5 K min  1 and held at least 40 min constant before fluxes were recorded. The pressure dependence was investigated varying Pfeed between 150 kPa to 600 kPa. Alloying was deemed complete when JH2 and Eact had become stable. Hydrogen permeation rates were also determined for a 9:1 H2/N2 mixture at Pfeed ¼200 kPa and a 1:1 H2/N2 mixture at Pfeed ¼500 kPa between 673 K and 773 K while varying the feed rate between ca. 0.076 mol m  2 s  1 and 1.2 mol m  2 s  1 (250–4000 ml min  1). Hydrogen recovery RH2 was limited to ca. 88% for the 9:1 mixture and ca.75% for the 1:1 mixture at maximum. These are the theoretical maxima under those conditions because the permeate side was kept at atmospheric pressure and no sweep gas was used. Sulfur inhibition was tested between 673 K and 773 K by adding 7 or 35 ppm H2S to the 9:1 and 1:1 H2/N2 mixtures while

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2.3 Membrane characterization PdCu layers from the rapidly alloyed cut-off sample were detached from the ceramic support for X-ray diffraction (XRD) measurements on an X′Pert PRO diffractometer (PANalytical) using Cu Kα radiation at 40 kV and 40 mA. The atomic fraction xCu (in %) of the alloy was calculated from XRD lattice constant aPdCu according to Vegard′s law [26] employing previously established lattice constants aPd and aCu for Pd and Cu [27]:

aPdCu = aPd − x Cu

aPd − a Cu = 3.887 Å − 0.00268 Å x Cu 100

(3)

Alloy composition was also determined by energy-dispersive X-ray spectroscopic (EDS) analysis on a FEI Quanta 200F scanning electron microscope (SEM) equipped with an Ametek detector. Thickness d of the PdCu layer was calculated from the area and weight of detached alloy fragments and also derived from SEM images. Fig. 2. Sequence of measurements with varying feed rate during testing with H2S in 9:1 H2/N2 mixture.

3. Results 3.1 Membrane characteristics

varying the feed rate. First measurements were carried out with 7 ppm H2S in the 9:1 mixture at 773 K starting with the highest feed rate as schematically depicted in Fig. 2. Altogether five successively lower feed rates were employed in the range JH2 ¼ 0.076– 0.76 mol m  2 s  1 (0.25–2.5 l min  1) and kept constant for 1 h each time. Next H2S was cut off, the feed rate raised back to the initial maximum value and JH2 recuperation was monitored for 3 h. Finally the membrane was kept under flowing H2 overnight. The next morning single gas JH2 and JN2 rates were determined at 773 K and ΔP¼ 100 kPa before the same sequence of measurements with H2S was repeated at 723 K and on the day after at 673 K. Note that temperature was always raised back to 773 K while keeping the membrane under H2 overnight. Proceeding in analogous manner the 9:1 mixture with 35 ppm H2S, 1:1 mixture with 7 ppm H2S, 1:1 mixture with 35 ppm H2S and again the 9:1 mixture with 7 ppm H2S were tested in that order. Feed rate was varied between 0.076–1.2 mol m  2 s  1 (0.25–4 l min  1) in experiments with the 1:1 H2/N2 mixture. After completion of the experiments with H2S the membrane was kept for 1 day under H2 at 773 K. Finally single gas H2 and N2 permeation rates were determined again as well as JH2 and RH2 for both H2/N2 mixtures as function of feed rate and temperature. We focused on RH2 and its decline in the presence of H2S in our assessment of sulfur impact on membrane performance and separation efficiency. Hydrogen recovery was calculated from measured permeate flux Jperm and measured retentate flux Jret according to the following equation:

RH2 =

Jperm × CH2, perm Jret × CH2,ret + Jperm × CH2, perm

(1)

In addition, we calculated an index Leff which describes the relative decline of RH2 after introduction of H2S, i.e.

L eff =

RH2 − RH2, H2S RH2

(2)

where RH2,H2S and RH2 denote H2 recovery values determined with and without H2S in the feed. We term this index ‘separation efficiency decline’ because it is related to the overall efficiency of the membrane separation process.

The PdCu membrane was 5.2 cm long with 22.9 cm2 separation area. The XRD pattern of the alloy layer indicated a single phase fcc structure with lattice constant a ¼3.8364 Å corresponding with ca. 19% Cu according to Vegard′s law (Eq. (3)). Large area EDS scans (600  400 μm2) yielded a little higher Cu content of ca. 24% which agreed very well with the initial estimate based on weight gain of the support tube after Pd and Cu deposition. The PdCu layer was ca. 5 μm thick according to SEM images and weight/area ratio of detached alloy pieces. Single gas H2 permeation measurements resulted in the following law for the temperature range 573–773 K after conclusion of alloy formation:

⎛ 22.4 kJ mol−1 ⎞ ⎟ JH2 = 2.83 × 10−3 mol m−2 s−1 Pa−0.69exp⎜− RT ⎝ ⎠

(

0.69 × PH0.69 2,feed − PH2,perm

)

(4)

Measurements at Pfeed up to 600 kPa yielded n ¼0.69 for the pressure exponent. However, H2 permeability Pe is most often reported for a square root pressure dependence (n ¼0.5). For ease of comparison we have calculated such values for Pfeed ¼200 kPa and Pperm ¼100 kPa arriving at 2.4  10  9 mol m  1 s  1 Pa  0.5 at 623 K and 3.3  10  9 mol m  1 s  1 Pa  0.5 at 673 K. Fig. 3 displays JH2 as function of feed rate for the 9:1 and 1:1 H2/ N2 mixtures. For comparison we included H2 fluxes in this figure that result from the aforementioned single gas permeation law (Eq. (4)) and represent upper limits for JH2 in these mixtures. We used the average of entrance and exit H2 partial pressure on the feed side based on GC analyses of the retentate, i.e.

PH2,feed =

CH2, feed + CH2,ret 2

Ptotal

(5)

for calculation of these limits with Ptotal ¼ 200 kPa for the 9:1 and Ptotal ¼ 500 kPa for the 1:1 H2/N2 mixture. The flux limits were not reached but two permeation regimes can be clearly distinguished. At high feed rates JH2 approached the single gas values especially in the 9:1 mixture. Under these circumstances H2 permeation is largely limited by intrinsic membrane permeability. At low feed rates the JH2 gap widened between calculated single gas and measured mixed feeds. This indicates that external mass transfer becomes limiting for H2 permeation. The latter regime is more interesting from a practical point of view because it

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Fig. 3. Hydrogen permeation rates as function of feed rate from (a) 9:1 and (b) 1:1 H2/N2 mixtures between 673 K and 773 K. Solid symbols indicate experimental data and open symbols indicate flux limits calculated according to Eq. (4) (lines are guides to the eye).

Fig. 4. Hydrogen recovery as function of feed rate from sulfur-free 1:1 and 9:1 H2/ N2 mixtures (lines are guide to the eye).

corresponds with high H2 recovery values as shown in Fig. 4. The displayed RH2 functions serve as baseline for our assessment of the H2S inhibition experiments presented in the next section. 3.2 Membrane inhibition by H2S Fig. 5 displays H2 fluxes with 7 or 35 ppm H2S included in 1:1 and 9:1 H2/N2 mixtures at varying Jfeed as outlined in Fig. 2. Employed feed rates can be found in Fig. 6. Fluxes began to drop immediately after H2S addition but inhibition proceeded gradually at the lower sulfur content and more than 3 h passed before JH2 became steady in the 9:1 mixture with 7 ppm H2S. In contrast JH2 reached a constant floor level within the first hour with 35 ppm H2S in the feed. Raising Jfeed affected JH2 least at 673 K due to higher sulfur inhibition at lower temperatures. That can be also seen from JH2 recuperation after cutting off H2S again. While H2 flux rose rapidly at 773 K and approached initial JH2 values within 3 h, recuperation was sluggish at 673 K with JH2 still well below initial levels after that period. Membrane recuperation was also much slower after tests with 35 ppm H2S. In Fig. 6 we compare H2 recovery with and without H2S included in H2/N2 mixtures for two example cases: (a) high H2S contamination with weak external mass transfer resistance in the 9:1 mixture and (b) low H2S contamination with strong external mass transfer resistance in the 1:1 mixture. In both cases RH2 was

significantly reduced at all temperatures upon H2S addition but the discrepancy was larger for the 9:1 mixture. However, the gap between RH2 curves from sulfur-free and contaminated mixtures became smaller at the lowest feed rates and had almost closed for the 1:1 mixture at 773 K (Fig. 6b). This convergence occurs at the highest H2 recovery values indicating that sulfur inhibition became less severe in this situation. We plotted the relative decline of RH2 due to H2S i.e. Leff as function of the initial RH2 observed for the sulfur-free mixtures in Fig. 7 to assess the inhibitive effect of H2S quantitatively. Fig. 7 reveals that the separation efficiency decline varied considerably with RH2. The overall Leff trends are consistent for both mixtures that is sulfur inhibition became stronger with increasing H2S concentration and decreasing temperature except for 7 ppm H2S in the 9:1 mixture at 723 K. There sulfur inhibition appears to be unusually weak at low RH2 values. Fig. 5a shows that steady state conditions were not reached within 1 h at the lower H2S feed level in the 9:1 mixture especially at 723 K and 773 K which resulted in an underestimation of H2S inhibition at those temperatures. This effect may have been larger at 723 K because measurements with 7 ppm H2S in the 9:1 mixture were carried out first at 723 K, and then 773 K and 673 K in a deviation from our standard sequence of temperature change. In general, the debilitating effect of H2S first became stronger with increasing RH2 as indicated by Leff before its impact was attenuated as even higher RH2 values were approached. This change in trend is linked to the two H2 permeation regimes identified above for mixtures (Fig. 3). Hydrogen sulfide concentration increased on the feed side as H2 was removed. Hence, membrane inhibition intensified with increasing RH2 because of the growing H2S level and H2 removal became more difficult. This describes the situation at low RH2 values (high feed rates) where permeation rates were still dominated by the intrinsic hydrogen permeability of the membrane. However, starting at intermediate RH2 values sulfur inhibition of the separation process abated as more and more H2 was removed despite of concomitantly rising H2S levels. This signals the crossover to H2 permeation controlled by external mass transfer resistance. The contribution of H2S to external mass transfer resistance is likely not very different from that of N2 and thus higher contaminant concentrations will have negligible impact on that resistance. Therefore the overall efficiency of the H2S involving separation process progressively converged with the efficiency of the sulfur-free one as RH2 approached maximum values. Especially the amount of H2 extracted from the 1:1 H2/N2 mixture at 773 K was only marginally reduced at the largest RH2 values after adding 7–35 ppm H2S. This suggests that sulfur

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Fig. 5. Hydrogen fluxes from 9:1 H2/N2 mixtures with (a) 7 ppm H2S and (b) 35 ppm H2S and 1:1 H2/N2 with (c) 7 ppm H2S and (d) 35 ppm H2S at varying feed rates. Employed feed rates are given in Fig. 6.

inhibition of the membrane will be much less critical at practically relevant RH2 values than insinuated by experiments carried out at low H2 recovery or in pure or marginally diluted H2 (Fig. 7a). 3.3 Membrane deactivation, recuperation and selectivity Single gas H2 permeation and N2 leak rates were determined each day prior to sulfur exposure in order to monitor the state of the membrane during experiments with H2S. Fig. 8 displays the corresponding JH2 and JN2 measured at 773 K and ΔP ¼100 kPa. It can be seen that JH2 readily reached its initial value again following

experiments with 7 ppm H2S in the feed and subsequent overnight membrane recuperation under flowing H2 (300–400 ml min  1, Pfeed ¼120 kPa) at 773 K. However, H2 permeation rates could not be fully restored though that treatment after testing with 35 ppm H2S in the 9:1 H2/N2 mixture. Therefore we allowed additional time for recuperation of the membrane permeability following that first set of tests at higher H2S concentration. Restoration of JH2 continued to proceed slowly at 773 K until we raised the H2 feed rate to 2 l min  1 and Pfeed to 200 kPa on the third day. The next morning JH2 had returned to its original level. Moving forward we continued to keep the H2 flow rate at 1.5–2 l min  1 overnight. As a

Fig. 6. Comparison of hydrogen recovery with and without H2S present in (a) 9:1 and (b) 1:1 H2/N2 mixtures (lines are guide to the eye).

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Fig. 7. Separation efficiency decline as function of H2S concentration and initial H2 recovery in (a) 9:1 and (b) 1:1 H2/N2 mixtures (lines are guide to the eye).

Fig. 8. Single gas H2 permeation and N2 leak rates at 773 K and ΔP ¼100 kPa during testing with H2S-charged H2/N2 mixtures.

testing with H2S and from which Eq. (4) had been derived. This demonstrates that the here employed sulfur levels had no lasting detrimental effects on the H2 permeability of the PdCu membrane which had been exposed to H2S for 75 h altogether. On the other hand the N2 leak rate doubled from 0.064 mmol m  2 s  1 to 0.124 mmol m  2 s  1 at 773 K and ΔPH2 ¼100 kPa in the course of these experiments (Fig. 8) reducing ideal H2/N2 selectivity αH2/N2 from 2370 to 1190 at that temperature. Thus H2S appears to be conducive to leak growth. It is interesting to note that leak rates appeared to be smaller following H2S testing. They were especially low after the first set of experiments with 35 ppm H2S and then grew considerably during the subsequent membrane treatment in H2 at 773 K (Fig. 8). This phenomenon has been previously observed and it was suggested that defects might be reduced due to sulfur segregation at grain boundaries in the PdCu alloy layer [16]. Hence membrane selectivity might be higher during separation of sulfur-contaminated H2 mixtures due to sulfur deposits at defect sites. Purity of the permeated H2 gives direct insight into the impact of H2S on separation efficiency. Fig. 10 displays permeate side H2 concentrations according to GC analyses during separation of the 1:1 H2/N2 mixtures at 773 K at the investigated H2S feed levels. Hydrogen purity depended strongly on RH2 dropping rapidly as maximum recovery levels were approached. The general shape of the curve did not change but it shifted towards lower purity with increasing sulfur contamination. Two effects contribute to this

Fig. 9. Single gas H2 permeation rates at ΔP¼ 100 kPa before and after testing with H2S.

result JH2 was always restored close to its initial value even during the second sequence of tests with 35 ppm H2S (1:1 H2/N2 mixture). After conclusion of the tests with H2S the membrane was kept another day in flowing H2 at 773 K before single gas permeation rates were tested again down to 573 K. Fig. 9 shows that H2 fluxes were practically identical with those measured before

Fig. 10. Permeate H2 purity during separation of 1:1 H2/N2 mixtures with and without H2S present at 773 K.

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shift: On one hand ideal selectivity will be reduced because H2 permeation through bulk PdCu declines sharply due to membrane surface inhibition. On the other hand the sulfur deposits may trigger defect formation and growth even though such deposits may also block some defects [16]. Presumably, membrane inhibition has a stronger influence since it grows fast with decreasing temperature. For example, at 673 K H2 purity dropped from 99.6% to 98.3% (RH2 ¼72%) in the sulfur-free mixture but from 98.4% to 97.5% (RH2 ¼43%) after addition of 35 ppm H2S to the 1:1 mixture.

4 Discussion Table 1 summarizes permeation characteristics of fcc PdCu membranes reported in the literature. Permeability of those membranes varies almost by an order of magnitude at 723–773 K but without clear correlation with alloy composition. Similarly, the spread of activation energies exceeds 10 kJ mol  1. This may be due to the differing influence of support materials such as ceramics (this work, [28]) and porous stainless steel [16,29], or the lack thereof [17]. In addition, the membranes tested by Pomerantz and Ma had asymmetric separation layers with a PdCu section with decreasing Cu content on top of a Pd section [16]. However, the permeability of the here used membrane is practically identical with that found for a ca. 5.5 mm thick Pd70Cu30/ceramic composite membrane that had been previously studied in our laboratory [28]. That earlier membrane exhibited also a very similar activation energy, i.e. Eact ¼21.3 kJ mol  1 [28]. Table 1 further shows that exposure to ca. 50 ppm H2S had a marginal effect on H2 permeability of those membranes in most cases. Ideal binary selectivity has been reported too after testing with H2S for a couple of those previous experiments. Unsurprisingly, αH2/N2 of 45.8 μm thick PdCu foils remained perfect [17] whereas αHe/N2 of a 14 μm thick Pd87Cu13 film improved even slightly [16]. Selectivity of the here tested 5 μm thick PdCu membrane remained higher after H2S experiments than in the latter case but the observed αH2/N2 decline indicates that susceptibility to sulfur induced defect growth increases with decreasing PdCu layer thickness. Product streams from coal gasification typically contain only 30–40% H2 after water gas shift. Hence external mass transfer resistance becomes very important for the overall separation efficiency of such gas streams via membranes in particular at the very high H2 recovery levels that will be necessary in a commercial context. Our experiments show that sulfur inhibition will affect this separation efficiency much less than would be expected on the basis of measurements in pure or marginally diluted H2

Fig. 11. Schematic presentation of H2S influence on separation efficiency in presence of external mass transfer resistence.

streams and at low RH2 values. Indeed, at the employed H2S concentrations the amount is marginal (5–10%) by which H2 recovery was reduced at 773 K and still minor (ca. 10–20%) at 723 K for the 1:1 H2/N2 mixture. The reason for the small impact of sulfur is concentration polarization becoming the all-dominant hydrogen transport resistance at high H2 recovery values turning permeability of the membrane and its inhibition into a minor factor. The particular shape of the separation efficiency decline curves can be readily understood by considering its evolution from the limiting cases which have been schematically depicted in Fig. 11. In Case 1 the impact of concentration polarization (characterized by RH2 dependent coefficient CP) is much larger than the impact of sulfur inhibition (characterized by RH2 dependent coefficient SI) at all RH2 values, i.e. CP 4 4 SI (or SIE 0). This case is trivial as RH2, H2S ¼RH2 and Leff ¼0 at every RH2 (Fig. 11, dashed line). In Case 2 the impact of external mass transfer resistance is negligible relative to the impact of sulfur inhibition at all RH2 values, i.e. SI o o CP (or CP E0). In this case the concentration of H2S steadily rises with increasing RH2 and in parallel membrane inhibition becomes stronger but approaches a constant level as H2S surface coverage converges towards unity. Accordingly RH2,H2S simply reflects sulfur inhibition that is RH2,H2S ¼SI  RH2 and Leff ¼ 1  SI as depicted in Fig. 11 (dash dotted curve). Case 3 describes practical situations like those encountered in the present study where the impact of sulfur inhibition is stronger at low RH2 but external mass transfer

Table 1 Properties of fcc PdCu membranes. Membrane

Pd81Cu19 Pd70Cu30 Pd84Cu16 Pd72Cu28 Pd87Cu13 Pd73Cu27 Pd70Cu30 PdCue a

d μm

5.0 5.5 5.0 14.0 14.0 11.4

22.4 21.3 14.5 24.2 17.2 15.0

45.8

16.7

H2S concentration. At 853 K. c αH2/He. d at 773 K. e Composition not mentioned. b

Eact kJ mol  1

T K

773 773 723 723 723 723 593 773

CH2Sa ppm

7–35

47.2 47.2 54 20 5  39

Pe  108 mol m  1 s  1 Pa  0.5

αH2/N2

Reference

Before

After

Before

After

0.58 0.57 1.02 0.13 0.38 0.68 0.17 0.94

0.57

2369 170b 5  170c 170c,d 220c,d

1194

1

1

0.12 0.34 0.44 0.14 0.94

180c

This work [28] [29] [16] [16] [16] [2] [17]

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gains the upper hand at high RH2 values. In this case Leff starts out at low RH2 following the Case 2 curve but eventually splits off from it and passes through a maximum as it veers towards the Case 1 baseline with increasing RH2 (Fig. 11, solid curve). Case 3 can be understood as the weighted sum of Cases 1 and 2 where the relative weights change as RH2 goes from 0% to 100%. This situation is analogous to CO inhibition in the presence of concentration polarization which has been modeled by Caravella et al. [19]. They describe the same relative trends with regard to impact on overall permeation rate albeit they did not evaluate the particular function Leff that we use in our assessment of sulfur inhibition versus external mass transfer resistance. Note that the actual shape of practical separation efficiency decline curves depends on experimental conditions such as initial H2 and H2S concentrations and pressure gradients. The Leff curves remained close to the ideal Case 2 for the 9:1 mixtures (Fig. 7a) because concentration polarization was relatively weak for the most part as the H2/N2 ratio did not drop below 1:1. However, that composition was just the starting point in the second set of experiments where the Leff curves started right at the maximum and then immediately plunged towards the Case 1 baseline with increasing RH2 (Fig. 7b). Industrial H2 recovery targets are likely even higher than the ones applied here. For example, RH2 Z90% is deemed an economic necessity for Pd membrane application in pre-combustion CO2 capture schemes for power generation from fossil fuels [1,21]. With H2 concentrations being also much smaller in coal gasification products than applied here, sulfur inhibition of the membrane could have a negligible practical impact under industrial membrane separation conditions at the investigated contamination levels. As much is implied by a recent study on separation of 50% H2 mixtures including CO2, CO, and H2O with ca. 45 μm thick PdCu membranes which concluded that H2 permeability is not affected by H2S concentrations below 39 ppm between 673 K and 773 K [17]. Thus, from a practical point of view minimization of that external mass transfer resistance will be more vital for enhancing the efficiency of separation processes with PdCu membranes than reducing sulfur inhibition of the membrane. Condensation of steam upstream of the membrane would alleviate the impact of concentration polarization and also increase the driving force for membrane permeation i.e. H2 partial pressure considerably. That would render the separation process more efficient and allow a reduction of membrane area. Our tests with the 1:1 H2/N2 mixture actually reflect sulfur inhibition in dry coal gasification mixtures since those will contain 50–60% H2 depending on efficiency of the water gas shift stage [21,30]. Accordingly sulfur inhibition of the PdCu membrane would still remain a secondary transport limitation to external mass transfer resistance under those circumstances. Moreover, dry reformate separation means exposure of the membrane′s downstream section to highly concentrated CO2 (4 90%) at intustrially interesting H2 recovery levels [21]. Membrane deactivation has been observed for pure Pd membranes under such conditions due to coke formation [1,31]. It is unclear whether PdCu membranes will be more resistant to coking in dry coal syngas mixtures at the practically necessary high operation pressures and H2 recovery rates. Of course practical irrelevance of sulfur inhibition does not eliminate the need for sulfur tolerance of membranes operated in sour H2 streams. The decline of membrane selectivity during H2S exposure of the here used PdCu membrane hints at stronger interactions between membrane and contaminant than mere physical adsorption. Indeed Pd4S, Cu2S and to a lesser extent Pd13Cu3S7 have been observed on PdCu membranes after H2S exposure giving rise to sulfide nodules on the membrane surface [2,32]. Sulfide formation depends on the H2S/H2 ratio. In our experiments with 35 ppm H2S this ratio increased from 3.9  10  5 to

1.0  10  4 in the 9:1 H2/N2 mixture after extraction of 62% H2 but ranged from 7  10  5 to 2.3  10  4 in feed and retentate of the 1:1 H2/N2 mixture after extraction of 70% H2. Based on thermodynamic data Iyoha et al. calculated H2S/H2 thresholds above which Cu2S and Pd4S form on Pd70Cu30 [32], a stoichiometry close to our membrane. Assuming an ideal Raoultian alloy solution they obtained minimum H2S/H2 ratios for Cu2S that far exceeded our experimental values precluding Cu2S crystallization in the present case. However, for Pd4S formation they calculated H2S/H2 thresholds of 2.6  10  6, 5.8  10  5 and 3.6  10  4 at 673 K, 763 K and 833 K [32], respectively, well below the highest ratios in our experiments. Experimental tests [32] showed that these thresholds were somewhat underestimated because PdCu solutions are nonideal with metal activities lower than in the assumed ideal case. Still, Pd4S was experimentally observed after exposure to H2S/H2 ¼1.1  10  4 at 673 K [32]. Therefore sulfidation should have occurred in some of our experiments at lower temperatures. It has been previously proposed [16] that sulfur deposits form at grain boundaries. It is likely that such deposits cause mechanical stress due to incommensurate PdCu and Pd4S crystal lattices which may have triggered the growth of defects. This does not necessarily lead to an immediate enhancement of leak flows as indicated by the high purity of the permeated H2 because the deposits can plug those defects [16]. However, at least parts of those deposits have been probably removed during the frequent membrane recuperation in H2 at 773 K. In fact, the full restoration of H2 permeation properties suggests that H2S induced morphological changes had been largely reversed after the final recuperation treatment. Still, sulfide deposits and their subsequent removal may have caused the reduction of the PdCu membrane′s ideal separation factor. The above considerations further show that contamination conditions were much harsher in the 1:1 than the 9:1 H2/N2 mixture at the same H2S feed concentrations. Thus caution needs to be exerted also with regard to contamination level when extrapolating H2S test results from pure or marginally diluted H2 to technical mixtures. Moreover, the H2S/H2 threshold increases rapidly with increasing temperature emphasizing that sulfur poisoning of PdCu membranes is a low-temperature issue. According to Iyoha's thermodynamic evaluation and experiments [32] it is likely that 773 K or slightly higher temperatures will be sufficient to avoid sulfidation of the here used PdCu membrane in the investigated H2S contamination range. Thus, sulfidation should not impact practical separation processes with PdCu membranes at 773 K after prior desulfurization of H2 mixtures with conventional ZnO beds.

5. Conclusions Permeation studies in 1:1 and 9:1 H2/N2 mixtures with a ca. 5 mm thick, supported PdCu membrane show that the influence of 7–35 ppm H2S on H2 separation efficiency depends strongly on H2 recovery and temperature in the range 673–773 K. Sulfur inhibition of the membrane became weaker at higher temperature but stronger with increasing H2 recovery due to concomitant concentration of H2S in the retentate. On the other hand external mass flow resistance became also more important with increasing H2 recovery. Especially in the 1:1 H2/N2 mixture the influence of H2S on overall separation efficiency was significantly attenuated at the highest investigated H2 recovery values around 80%. This shows that concentration polarization will remain a stronger limitation to H2 permeation than sulfur inhibition of PdCu membranes in practical separation situations. Therefore even 35 ppm H2S resulted only in a marginal reduction (  10%) of the highest H2 extraction level at 773 K when compared to separation of the sulfur-

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free 1:1 H2/N2 mixture. Single gas H2 permeation rates were significantly reduced following testing with 35 ppm H2S at the lower temperatures but they could be fully restored through operation of the membrane under H2 at 773 K. However, the N2 leak rate doubled during the experiments with H2S. This might have been caused by sulfidation at defect sites as H2S/H2 ratios exceeded the threshold for Pd4S formation at lower temperatures and high H2 recovery values in our experiments according to thermodynamic evaluation in the literature [32]. The H2S/H2 threshold should be sufficiently high at 773 K or slightly above to avoid sulfidation of PdCu membranes at H2S levels that remain typically after desulfurization with ZnO beds at those temperatures. In conclusion, the detrimental effects of readily attainable practical H2S levels largely disappear when raising the operation temperature of PdCu membranes to 773 K. At that temperature membrane inhibition by H2S may be practically irrelevant in industrial separation of H2 from coal gasification product streams which typically contain less than 40% H2 and where H2 extraction may need to exceed 80% to render membrane application economically viable. This assessment may change if external mass transfer resistance could be effectively reduced for such membrane separation processes.

Acknowledgment Financial support by the European Union through FP7 project CACHET II (Contract no. 241342, http://www.cachet2.eu) and the Chinese Academy of Sciences through the External Cooperation Program (Helmholtz-CAS Joint Research Group on Integrated Catalytic Technologies for Efficient Hydrogen Production, Grant no. GJHZ1304) is gratefully acknowledged. We are also indebted to the ECN and Dr. F. van Berkel for providing ceramic support membranes and seals.

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