Article pubs.acs.org/EF
Enhancement of the Long-Term Permeance, Selectivity Stability, and Recoverability of Pd−Au Membranes in Coal Derived Syngas Atmospheres Federico Guazzone,† Jacopo Catalano,†,‡ Ivan P. Mardilovich,† Tony Wu,§ Robert C. Lambrecht,§ Subhash Datta,§ Jay Kniep,∥ Saurabh Pande,∥ Nikolaos K. Kazantzis,† and Yi Hua Ma*,† †
Center for Inorganic Membrane Studies (CIMS), Worcester Polytechnic Institute, Chemical Engineering Department, 100 Institute Road, Worcester, Massachusetts 01609, United States ‡ Department of Engineering, Finlandsgade 22, 8200 Aarhus, Denmark § National Carbon Capture Center, Southern Company, P.O. Box 1069, Wilsonville, Alabama 35186, United States ∥ Membrane Technology and Research, Inc., 1360 Willow Road, Menlo Park, California 94025, United States ABSTRACT: Two new large scale (200 cm2) composite Pd and Pd−Au membranes were prepared and tested in an actual coal derived, but desulfurized, syngas in order to further quantify permeance loss due to species other than sulfur and to determine the nature of the contaminants. As in our previous work, membranes were tested at the National Carbon Capture Center (NCCC) in Wilsonville, Alabama. Before the syngas test, the Pd and Pd−Au membranes had thicknesses of 7 and 6.6 μm, H2 permeances at 450 °C of 17.7 and 29.2 N m3 m−2 h−1 bar−0.5, and H2/He selectivities higher than 2700 and 160 000, respectively. The two membranes produced H2 at an exceptionally high purity level of 99.8−99.9%. The selectivity of the Pd−Au membrane was stable for over 473 h in an actual syngas atmosphere at 450 °C and 12.6 bar demonstrating the high robustness and suitability of these membranes in industrial environments. However, as seen in our previous study, the two membranes showed a decrease in H2 permeance upon syngas introduction (ranging from 40 to 50%), indicating the presence of a fast surface poisoning process. The XPS analysis of a Pd coupon attached to the Pd membranes, and therefore exposed to the same syngas as the Pd membranes, revealed the presence of Mg, Na, Hg, O, C, and S on its surface. Furthermore, depth profile analysis revealed the presence of C at a concentration level of 4 atom % at a depth of 1.1 μm. Tests in pure H2 atmosphere at 450 °C after syngas exposure resulted in a moderate permeance recoverability for the pure Pd membrane (77%), while an outstanding 100% recoverability was achieved with the new Pd−Au alloy. Compared to our previous study, the Pd−Au membrane had a thicker Au layer on its surface (approximately 0.5 μm instead of 0.2 μm) that was intended to better mitigate surface poisoning by syngas contaminants. The average Au content of the layer with the Au gradient (2.2 μm in thickness) was determined by XRD and equaled 23 atom %.
1. INTRODUCTION A very large percentage (96%) of hydrogen, of which 87% is utilized for ammonia synthesis and petroleum processing,1 is produced by the steam reforming of natural gas and partial oxidation of other fossil fuels followed by the water gas shift reaction (for CO conversion) in two separate reactors coupled with purification via pressure swing adsorption (PSA) to meet the necessary purity grades.2 Both academic and industrial researchers have been focusing their efforts on the development of catalytic membrane reactors (CMRs) for the production of H23−11 in order to provide fuel for the future H2 economy.12 The methane steam reforming (MSR) and the water gas shift (WGS) reaction have been studied in membrane reactors equipped with permeable Pd based membranes since the removal of H2 from the reactant media leads to higher conversions, high purity H2, high pressure CO2, and significant enhancement in energy efficiencies. Dense metallic Pd-based membranes have been preferred due to their theoretical infinite selectivity toward H2 as well as their high stability in steam at the high operating temperatures of 400−500 °C in CMRs.13 Pd alloy membranes, and especially Pd−Cu,14−18 Pd−Au,14,18−21 and even Pd−Au−Pt22 alloys, are being thoroughly investigated © 2013 American Chemical Society
since they have shown higher permeances than pure Pd and a significant degree of resistance in atmospheres containing H2S. The resistance toward H2S is the main reason Guazzone et al.23 tested composite Pd and Pd−Au membranes (Au was only present in the 1−2-μm-thick top layer) in a stream of actual desulfurized syngas derived from coal at the National Carbon Capture Center (NCCC) (Wilsonville, AL), and they achieved (1) robustness under industrial operating conditions for over 200 h at 450 °C and (2) the production of H2 with a purity as high as 99.89% (450 °C, 12.6 bar and [H2] ≅ 35 mol %) for over 200 h. To the best of our knowledge, both results represent potentially ground breaking research findings not previously reported to the membrane community and in the pertinent literature. However, all membranes experienced a H2 permeance decline upon syngas exposure, and the nature of the Special Issue: Accelerating Fossil Energy Technology Development through Integrated Computation and Experiment Received: December 20, 2012 Revised: February 28, 2013 Published: March 7, 2013 4150
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impurities39 as well as cyclohexene.40 Therefore, it became necessary to also study the Pd-alloy bulk (by XPS and XRD techniques) of new membranes exposed to syngas in order to explain the significant (50−60%) initial decrease in the H2 permeance observed. The various experimental tests reported in this new study represent an attempt to address the challenges reported in our previous work,23 and therefore, the objectives of the present study were (1) to demonstrate the stability of composite Pd and Pd−Au membranes for a longer period of time in an actual syngas atmosphere and quantify any observed H2 permeance loss, (2) to study and characterize the effect of the [H2O]/ [CO] ratio on coking and H2 permeance stability, (3) to test Pd−Au membranes with a higher Au content at the surface to determine if the possible sub-parts-per-million levels of H2S were indeed detrimental to the H2 permeance in the previous syngas tests, (4) to investigate the nature of the contaminant(s) leading to H2 permeance decline by XPS, and (5) to study the recoverability of the new Pd−Au membrane with a higher Au content in pure H2 at 450 °C. Therefore, the second campaign of tests at NCCC was performed with both a pure Pd membrane as well as a Pd−Au membrane with a higher Au content on its surface. The H2 permeance and selectivities of the two replated membranes were determined at WPI before the syngas test. The syngas was shifted by the implementation of a WGS reactor in the syngas cleaning line leading to an [H2O]/[CO] ratio of approximately five. Furthermore, during the syngas test, small coupons plated with Pd were inserted in the permeation module for XPS/XRD analysis and contaminants determination. The present paper is organized as follows: In section 2, the experimental setup and procedures are described. The paper’s main results and findings are presented and discussed in section 3, followed by a few concluding remarks that are provided in section 4.
contaminant(s) leading to the permeance decline was not determined. The effects of a single or a binary mixture of the gases constituting the WGS feedstock have been studied by numerous researchers with a certain degree of discrepancy in their respective findings due to the inevitable difficulty in isolating mass transfer limitation effects in the gas phase, H2 depletion along the membrane, surface poisoning, as well as the associated mass transfer limitations in the porous supports. However, it is widely accepted that CO inhibits H2 flux by competitive adsorption at low temperatures,24−26 but it is commonly recognized that its inhibiting effect vanishes at temperatures higher than 450 °C.27,28 The effect of CO2 on H2 permeance is essentially negligible,24,27 even though CO2 alone may lead to deactivation by “the growth of nanoscopic carbon deposits on the membrane.”29 The effect of H2O on H2 permeance is believed to be negative by competitive adsorption at low temperatures, though beneficial for cleaning the surface of Pd membranes from C deposits and other impurities when used alone30 and negligible at temperatures higher than 358 °C27 and 450 °C31 (for H2O in the 9.5−12% range). Finally, CH 4 is believed to have a negligible impact on H 2 permeance.27,32 In order to increase the degree of complexity, even though we may quantify and understand the effect of each gas on the H2 permeance, our understanding of the combined effect of CO, CO2, H2O, and H2 on the permeance of H2 is currently limited due to all the reactions that may occur on the Pd surface. Indeed, CO may lead to C formation on the surface that may further react with H2 to produce CH4. It may also react with H2O to produce CO and H2 or still it may react with CO2 to produce CO. Moreover, H2O may dissociate and leave oxygen33 on the surface that may further react with other species. It is then important to study WGS mixtures in a more integrated way. Li et al.34 measured a drop of 20% when exposing Pd membranes at 400 °C to a WGS atmosphere containing 60.1% H2, 19.2% CO2, 15.4% H2O, 4% CO, and 1.2% CH4. Peters at al.18 measured an 8.5−12% H2 flux decline when exposing at high velocity rates (10−15% H2 recovery values) a Pd−Au membrane at 450 °C to a WGS atmosphere containing 62% H2, 14% CO2, 20% H2O, and 4% CO and attributed the low decrease to mainly competitive adsorption of CO since they found experimentally that H2 depletion and mass transfer within the gas phase to have a very low effect on H2 flux at the specified H2 recovery rates. Since it appears that the effect of CO2, CO, CH4, and H2O on the H2 permeance at 450 °C could be considered relatively mild18 (at maximum 10%) and since the membranes at NCCC were tested under operating conditions at which H2 depletion and mass transfer within the gas phase could also be considered as quite low, Guazzone et al.23 concluded that the significant reduction in the observed H2 permeance decline (50−60%) could have been due to trace amounts of H2S (less than 1 ppm) still present in the syngas or to another (other) known35 or even unknown contaminant(s) among the multitude present in actual syngas derived from coal. The formation of coking seen on all membranes was also thought to have some detrimental effect on the H2 flux; however, the extent of such poisoning was difficult to assess. Moreover, it was not possible to totally recover the poisoned membranes even after 100 h in a stream of pure H2 at 450 °C. It is known that the H2 permeance may be inhibited by a Pd uptake of impurities such as C, as it is already known under given sets of conditions such as the presence of CO,25,36 CH4,37 CH2CH2,38 and hydrocarbon
2. EXPERIMENTAL SETUP AND PROCEDURES 2.1. Membrane Preparation and Membrane Microstructure Characterization. The composite Pd based membranes were prepared on porous stainless steel (PSS) supports, 316L, 0.2 and 0.5 μm media grade purchased from Mott Metallurgical, Burlington, Connecticut. The porous supports were 1 in. o.d., 10 in. long (200 cm2 permeable surface area) welded to a 1 in. o.d., 3 in. long 316L nonporous tube capped at one end (membrane bottom part), and to a 1 in. o.d., 14 in. long 316L nonporous tube at the other end (membrane upper part). After an initial cleaning step with acetone in an ultrasonic bath for grease removal, all supports were oxidized in the air at 500−600 °C for 12 h41 and graded with preactivated Al2O3 particles cemented with Pd.42,43 The surface activation was performed using the SnCl2−PdCl2 activation procedure, and a dense layer of pure Pd was deposited on the graded support by the electroless plating technique.44 The thickness of the dense Pd layer was estimated by gravimetric methods and scanning electron microscopy (SEM). As described in our previous work,23 a two-step procedure was followed in order to obtain highly selective membranes. After the first cycle of palladium plating, the membranes were annealed and tested45 at temperatures between 350 and 525 °C over a minimum period of 48 h in a H2 atmosphere to check their mechanical integrity. The results after the first plating of M-03 were reported in our previous work.23 The H2 permeance and H2/He selectivity at 450 °C for M-07 were 44 N m3 m−2 h−1 bar−0.5 and 936, respectively, after the first plating. As mentioned in our previous work, M-03 was replated with Pd (second plating), plated with Au, and tested at NCCC.23 After the first syngas test, the surface of M-03 was slightly polished46 and replated for a third time with approximately 2 μm of Pd. Since the Au layer plated on M-03 was only around 0.16 μm,23 it is reasonable to assume that the mechanical treatment performed after the first NCCC test (2.2 μm 4151
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and require temperatures higher than 500 °C and annealing times longer than 500 h.47 After the characterization at WPI, the membranes were tested in syngas at NCCC and sent back to WPI for H 2 recovery characterization and further microstructure analysis. The H 2 permeance recovery was performed in pure H2 at a ΔP of 1 bar (retentate pressure of 2 bar, tube side pressure of 1 bar) and at temperatures ranging from 400 to 500 °C. No rising water test was performed on either membrane due to the low level of the He leak after the NCCC test. After the H2 recovery test, the membranes were cut, and their surface and cross sections were analyzed with SEM-EDX to determine their thickness and elemental composition. SEM-EDX characterization was performed using an Amray 1610 Turbo Scanning Electron Microscope (SEM) coupled with Energy Dispersion X-ray (EDX) capabilities (Princeton Gamma-Tech Instruments Inc., Rocky Hill, NJ). The EDX detector was equipped with a beryllium window allowing the detection of light elements. Atom and weight concentration profiles were determined with the Spirit software (Princeton Gamma-Tech Instruments Inc., Rocky Hill, NJ). Elemental composition (line-scans) was performed at an acceleration potential of 10, 15, or 20 kV; a 39 mm working distance; a 33° tilt; and a 5−10 min counting time. The spatial resolution increased as the accelerating voltage was decreased and equaled 0.3 μm at 15 kV. Elemental composition analysis was performed at 15 kV. The Au gradient on M-07 was characterized by EDX in order to determine the depth up to which Au diffused into the Pd layer; however the average Au content of the layer with the gradient was determined by XRD (Vegard’s law) for better accuracy. Half of the sample drawn from M-07 for the XRD analysis was plated with a thin layer of Au that worked as an internal standard. X-ray data were collected on a Rigaku Geigerflex diffractometer using a Cu Kα (λ = 1.5418 Å) radiation source, a curved crystal monochromator for β line removal, 1° soller/divergence slits, and a 0.3 receiving slit. All scans were performed with a minimum 0.01° angle step and enough counting time to achieve a high signal to background ratio for all reflections. Peak fitting was performed on Jade 8 software with a level background function and a pseudo-Voigt peak profile accounting for Kα1 and Kα2 radiations. XPS analysis was performed by Intertek Analytical Sciences Americas, Allentown, Pennsylvania. The X-ray photoelectron spectroscopy experiments were performed on a PHI 5000VersaProbe Spectrometer equipped with Multiple Channels Plates (MCD) and a focused Al monochromatic X-ray source. The low resolution survey scan was performed at 117.4 eV pass energy, 1.000 eV/step, and a 50 ms/step dwell time. The high resolution multiplex scans for the surface composition and the profile were performed at 29.35 eV pass energy, 0.100 eV/step, and a 100 ms/step dwell time. The analysis area was 200 μm in diameter with a takeoff angle of 45°. The data were collected with CasaXPS using transmission function corrected Area Sensitivity Factors (ASF). 2.2. Coal Gasification, Syngas Desulfurization, Syngas H2 Enrichment, Permeation Skid, and Membrane Testing Procedure. The TRIGTM coal gasifier and system operation were described in a previous publication23 and in the Southern Company Services Inc.’s final report.49 The most significant change compared to the previous work23 is that at the syngas conditioning unit, the syngas was first introduced into a WGS reactor in order to reduce the CO concentration levels to approximately 1%. After the WGS reactor, the syngas was desulfurized as in our previous work.23 The reader is referred to Guazzone et al.23 for details on ZnO beds, the COS bed, H2S, PAHs, and typical concentration values of other metal impurities, as well as a technical description of the instrumentation used, H2 enrichment management, and finally membrane testing operations. In run R07 (this work), a Pd coated 316L PSS coupon, with an area of 1 cm2, was attached to the membrane in order to be analyzed by XPS and determine the contaminants that might bond to a Pd surface exposed to the same syngas as the membranes. The description of the WPI-MTR skid and the testing procedures of Pd based composite membranes were also provided in our previous work.23 As previously stated, the main parameter change between runs
of Pd were removed) led to a membrane containing no Au. Furthermore, the additional 2 μm of Pd plated after the polishing step ensures that the surface of M-03 consisted only of Pd. The final membrane was named M-03b, and the results of its second test at NCCC are part of this work. After the first test in H2, M-07 was slightly polished, replated with 2.1 μm of Pd, and further plated with 0.5 μm of Au to achieve a higher Au content than that of M-01 and M03, which were only plated with 0.2 μm of Au.23 Au was deposited using the electroplating method.23 As described in our previous work, the weld nuggets of M-03b and M-07 were plated with a thick layer of Au to prevent any leak that may originate in the welds.23 The permeance of the two replated membranes (M-03b and M-07) was measured at Worcester Polytechnic Institute (WPI) before the NCCC test. The H2 permeances as a function of time measured at WPI for membranes M-03b and M-07 are respectively shown in Figure 1a and b along with the corresponding temperature profiles. Table 1
Figure 1. H2 permeance and temperature as a function of time measured at WPI for (a) M-03b and (b) M-07.
Table 1. Characteristics of Composite Pd and Pd−Au Membranes Tested at NCCC
membrane # composition M-03b M-07
7 μm Pd 6.1 μm Pd + 0.5 μm Au
H2 permeance at 450 °C N m3 m−2 h−1 bar−0.5
He leak at 450 °C N cm3 min−1 bar−1
H2/He selectivity at 450 °C ΔP = 1 bar
17.7 29.2
0.9 160 000
lists the thickness, H2 permeance at 450 °C, and H2/He selectivities at 450 °C of both membranes. For clarification purposes, it is important to mention that M-07 was not alloyed before the test at WPI. The test shown in Figure 1b was in fact the alloying of the bimetal Pd−Au structure, and as can be easily inferred, the permeance increased from an initial value of 5 to 30 N m3 m−2 h−1 bar−0.5 after 72.5 h at 450 °C. Even though it appeared that the permeance reached a stable value, there is no further confirmation of the attainment of a uniform Pd−Au alloy.47 In fact, by applying only a 0.5 μm of a Au layer the formation of a uniform alloy was not sought but rather the formation of a Au gradient in the top 1−2 μm of the membrane. Such thin Au gradients at the surface of Pd membranes have been shown to reach a “quasiequilibrium” state, because after 100−200 h at 450 °C, the driving force for further inter-diffusion becomes quite weak at 450 °C.48 It should be pointed out that uniform Pd−Au alloys are difficult to form 4152
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R06 and R07 was the [H2O]/[CO] ratio, which was increased from an approximate value of 0.9 in run R06 to an approximate value of 5 in run R07. The average concentration of all measured components is listed in Table 2 along with the operating conditions. Due to the shift
Table 2. Average Wet Composition (mol %) of the H2 Enriched Desulfurized Syngas Stream Fed to the Membrane Module and Operating Conditions at NCCC for Run R07 compound
mol %
H2 (%) N2 (%) CO2 (%) CO (%) H2O (%) CH4 (%) H2S (ppm) impurities temperature (°C) pressure (barg) feed flow (kg/h)]
35 47.5 10.5 1 5 1
