Hydrogen flux through the membrane based on the Pd–In–Ru foil

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Hydrogen flux through the membrane based on the PdeIneRu foil L.P. Didenko*, V.I. Savchenko, L.A. Sementsova, L.A. Bikov Institute of Problems of Chemical Physics, Russian Academy of Sciences, Moscow oblast, Chernogolovka, Semenov Prospect, 1, 142432, Russia

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abstract

Article history:

The hydrogen flux through the foil (thickness 30 mm) with the composition Pde6.0 wt % In

Received 6 July 2015

e0.5 wt % Ru in the temperature range from 593 to 823 K is studied. The permeability of Pd

Received in revised form

eIneRu foil at 823 K is 3 times higher than the foil of the Pde23 wt % Ag alloy. The H2 flux in

25 October 2015

the PdeIneRu foil is controlled by the diffusion of atomic hydrogen in the membrane bulk.

Accepted 26 October 2015

The apparent activation energy of the hydrogen permeance is 18.7 kJ/mol. The influence of

Available online xxx

impurities (CO, N2, Ar) on the hydrogen flux is studied using both a sweep gas (N2) and transmembrane pressure. An insignificant negative effect of CO is observed in experiments

Keywords:

using a sweep gas. When the transmembrane pressure is used, CO exerts a negative effect

Hydrogen flux

on the H2 flux, and this influence decreases with increasing temperature. The negative

Palladium alloy foil

effect of CO decreases with the simultaneous use of transmembrane pressure and sweep

H2 separation

gas. Contact of the foil with pure CO results in an insignificant formation of carbon deposits

CO negative effect

on the membrane surface, which exerts almost no effect on the H2 flux. No carbon deposits

Carbon deposits

are formed on contact of the foil with the mixture H2e22.5% CO due to CO methanation with the formation of CH4 and H2O. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Demand in high-purity (99.9999 vol %) hydrogen is being increased sharply due to the fast development of hydrogen power engineering [1]. Since fuel cells are susceptible to contamination by even trace levels of impurity gases such as CO, CO2, H2S, and SO2, efficient technologies for the purification of hydrogen-containing gases from these impurities are necessary. Among existing methods for hydrogen purification, membrane processes are characterized by lowest operating and capital outlays.

The most promising membrane material is palladium (Pd) with a high H2 permeation, 100% H2 selectivity, and the catalytic surface on which hydrogen molecules dissociate to atoms with a high rate [2]. Palladium membranes can function at high temperatures (573e973 K), which makes it possible to combine the production of hydrogen-containing mixtures and hydrogen purification by incorporating the membranes into catalytic membrane reactors. A drawback of the palladium material is its embrittlement in a hydrogen medium because of the a / b phase transition of palladium hydride that occurs below the critical temperature (566 K) and pressure (2 МPa) [3]. Another problem is a high cost of palladium. These problems are solved by using Pd alloys with other metals [4]. The PdeAg

* Corresponding author. Tel.: þ7 9166198843. E-mail address: [email protected] (L.P. Didenko). http://dx.doi.org/10.1016/j.ijhydene.2015.10.107 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Didenko LP, et al., Hydrogen flux through the membrane based on the PdeIneRu foil, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.107

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alloys are studied most widely [5,6]. Since Ag is cheaper than Pd, its addition to the alloy makes it possible to reduce the cost of the membrane material. In addition, permeation of H2 increases significantly. However, a drawback of PdeAg alloys is irreversible poisoning in gas mixtures containing even small amounts of hydrogen sulfide. Therefore, it seems interesting to study palladium alloys with other alloying additives. The use of cheaper (than Ag) alloying additives can reduce the cost of palladium alloys. Also, these alloys are more permeable, as a rule. To decrease the cost and to enhance hydrogen permeation with the simultaneous retention of mechanical strength, composite palladium membranes are obtained using different techniques of supporting palladium layers (thickness 5e20 mm) on metallic or porous ceramic supports [7,8]. In spite of the fact that the methods for the preparation of palladium layers without defects are often complicated, at present there are many papers reporting supported membranes with very high H2 permeation and selectivity. In spite of lower hydrogen permeation, dense palladium membranes (foils) are worth of further development, since they have 100% hydrogen selectivity, are simple in manufacturing, and are readily attached to structural units of a steel reactor. Palladium forms a wide range of solid solutions with many metals. Such palladium-based solid solutions are formed with 10e30 wt % of refractory metals (niobium, molybdenum, ruthenium, tantalum, tungsten, rhenium, vanadium, etc.) and low-melting metals (lithium, magnesium, indium, lead, tin, bismuth, etc.) [9]. An interesting feature of palladium is the existence of wide solid solution regions (around 10e15 wt %) with all rare-earth metals except lanthanum and neodymium, for which the regions are around 2 wt % [9]. Some alloys containing no Ag exceed Pd and PdeAg alloy in characteristics. For example, the PdeCu alloy is highly resistant to poisoning with hydrogen sulfide [10]. Permeability of PdeCu alloy depends on the Cu content. It was found that initial additives of copper reduce the permeability, but at copper contents about 40 percent a sharp maximum is observed. This maximum is associated with the ordered body centered cubic b-phase, which is formed in the palladium-copper system. The ordered b-Pd-Cu phase was found to have the highest room temperature diffusivity of any metal-hydrogen system, i.e., D25  C ¼ 2$105 cm2/s compared with a value of 2$107 cm2/ s for pure palladium. Unfortunately, although the diffusion coefficient is extremely high, the hydrogen solubility is reduced to a low value on alloying with copper. As a consequence, the permeability passes through a peak but without attaining values much higher than that of unalloyed palladium [10]. The PdeAu membranes are of interest, because the presence of gold reduces the embrittlement problem resulting from the hydride phase transition and improves resistance to corrosive degradation by sulfur compounds while giving rise to higher hydrogen permeability than that of pure Pd up to 15% Au content [11]. The PdeRu membranes fabricated by electroless co-deposition was studied in Ref. [12] and it was shown that Pd and PdeRu alloys with up to 10 wt % Ru had similar values of hydrogen permeance, whereas the membrane hardness of a ruthenium alloy is 80% higher than that of pure palladium. The hydrogen permeance of the composite

membrane PdeAgeRu/a-Al2O3 is much higher than those of Pd/a-Al2O3 and PdeAg/a-Al2O3 membranes obtained by a similar method [13]. Membrane palladium alloys containing no silver and design of membrane units for the extraction of high-purity hydrogen from gas mixtures are being developed at the Baikov Institute of Metallurgy and Materials Science (Russian Academy of Sciences, Moscow) [9]. Palladium alloys of high plasticity are required, when they are used in the form of thin foils and tubes. The search for efficient alloys was restricted by palladium-based solid solutions, because these are the only materials which can demonstrate all the properties required. Therefore, the technical characteristics of palladium alloys of various compositions were studied. The choice of the palladiume6.0 wt % indiume0.5 wt % ruthenium alloy optimized in composition is due to a complex of technological characteristics: high plasticity and strength during operation at temperatures of 573e973 K, low expansion when saturated with hydrogen, good corrosion resistance in certain media, and high hydrogen permeability [14,15]. The H2 flux in a foil of this alloy was studied in detail in this work. The purpose of the present study is to investigate the influence of the operation parameters on the separation performance of a membrane unit based on a foil 30 mm thick of the PdeIneRu alloy. In addition, the influence of Ar, N2, and CO impurities on the separation of hydrogen-containing mixtures was studied with the variation of the temperature, impurity concentration, and gas flow rate.

Experimental Preparation of the PdeIneRu foil The alloy with the optimal composition palladiume6.0 wt % indiume0.5 wt % ruthenium was obtained at the Baikov Institute of Metallurgy and Materials Science (Russian Academy of Sciences). This alloy was found to have an optimum combination of strength, plasticity, hydrogen permeability, and corrosion resistance. The addition of 0.5 wt % ruthenium to the palladiume6.0 wt % indium matrix increased the strength of the alloy and stabilized its operation in a hydrogen-containing atmosphere without any changes to the composition of its surface, which were observed in the Pde6.0 wt % In alloy. To exclude the formation of impurity inclusions and defects, the purity of the used materials was not lower than 99.95 wt %. The alloy was melted in a protective atmosphere in an electric-arc furnace. The characteristics of the obtained alloy were described in Ref. [9]. A foil of the indicated alloy was prepared by cold rolling on a four-high rolling mill with intermediate vacuum annealings. The obtained foil did not require additional purification prior to experiments. The average thickness of the foil obtained through SEM cross-section images was approximately 30 mm.

Measurement of the hydrogen flux The Н2 flux through the membrane was studied in a measuring cell (MC), whose scheme is presented in Fig. 1.

Please cite this article in press as: Didenko LP, et al., Hydrogen flux through the membrane based on the PdeIneRu foil, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.107

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state flux was achieved. The H2 content in the reaction products was determined on a column packed with molecular sieves 13 (2 mm  2 m, 50  C, argon as a carrier gas). The hydrocarbon composition of the products was determined on an HP-Al/KCl column (0.5 mm  30 m, 80  C, helium as a carrier gas). The absolute calibration method was used for the calculation of the content of the products. The analysis accuracy was 99.2%. An MI 1201 V mass spectrometer was used to determine the content of CO and CO2 (electron impact as a method for ionization of gases coming to the ion source, sensitivity of the method 103 wt %, dynamic range 106).

Results and discussion Study of the H2 flux through the membrane

Fig. 1 e Scheme of the measuring cell: 1epermeate side; 2emembrane unit; 3eretentate side; and 4egas inlet and outlet.

The main unit of the MC is a hydrogen-permeated disc (2) 56 mm in diameter made of the studied foil. The disc was fixed between two stainless steel grids of fine netting and placed between the chamber of gas mixture supply to the retentate side (3) and the chamber of hydrogen outlet, permeate side (1), the height of each side is 3 mm; (4) are jets for gas supply and withdrawal. Graflex was used as a material for unit sealing. The effective surface area of the membrane is 15.2 cm2. The gas flow came to the retentate side on the membrane surface through holes arranged at the periphery and was radially distributed over the surface, and the flow depleted in hydrogen was withdrawn through the central hole. In experiments with the sweep gas, which was supplied to the permeate side, the flows of the retentate and permeate were countercurrent. The MC was heated with an electric furnace. Chromelealumel thermocouples were used for temperature control on the membrane and in the furnace. The contact of the membrane unit with hydrogen at temperatures below 573 K was avoided to prevent embrittlement of the palladium foil. The hydrogen flux through the foil was studied in the temperature range from 593 to 823 K both using the sweep gas (N2) and without it. In the latter case, the transmembrane pressure was the driving force of hydrogen transfer. The mixture pressure at the retentate side was maintained using a pressure controller, and the atmospheric pressure was kept at the permeate side. The inhibition effect of CO, Ar, and N2 on the hydrogen flux through the membrane was studied by the variation of the temperature, impurity concentration, gas flow rate, and time. Three mixtures were used: H2/N2, H2/Ar, and H2/CO. The purity of H2, N2, and Ar was 99.95%, while that of CO was 99.9%. The H2 flux was measured with a soap-film flow meter, and the composition of the permeate gas was monitored on-line with a Kristall 5000 gas chromatograph equipped with TCD and FID detectors. The H2 flux was measured until a steady-

A membrane as a disc was used in the MC design. In terms of ease of manufacturing, service characteristics, efficiency, and ease of repairing, it can be demonstrated that disc membranes in the form of foils are promising for commercial application [15,16]. Their advantage over tubular foil membranes from palladium alloys is a lower palladium amount. This is due to the use of the cold rolling method that provides thin foils 10e100 mm thick. The method for preparation of condensed foils several mm thick from palladium alloys by magnetron sputtering is being developed at present [17]. At the same time, tubular foil membranes from palladium alloys for the retention of mechanical strength should have a thickness of 100 mm and more, which requires a larger amount of palladium. The thermal stability of the membrane unit was studied at T ¼ 823 K and a constant transmembrane pressure of 176.5 kPa. Heating to the required temperature was carried out in an N2 atmosphere, and then H2 was fed. The H2 flux was measured at an interval of 5 h during 500 h. The measurement results showed that the H2 flux varied within 10%, indicating good thermal stability of the membrane under chosen conditions. Pure gas permeation tests have been performed also with nitrogen under the same conditions. No permeation could be measured in the permeate side, which indicates the absence of defects in both the palladium foil and Graflex seals. The H2 flux through the PdeIneRu foil was measured at 823 K under different transmembrane pressures. The studies were carried out with increasing H2 pressure at the retentate side, whereas the permeate side was kept under atmospheric pressure, and no sweep gas was used. Hydrogen permeation through palladium or palladium alloy membranes proceeds via the “solutionediffusion” mechanism. The general expression for the hydrogen flux, J (mol m2 s1), through a membrane may be written according to the Sieverts law as:
P J ¼ $ Pn1  Pn2 t

(1)

where P is the permeability, t is the membrane thickness, and p1 and p2 are the hydrogen partial pressures in the gas phase at the high- and low-pressure sides of the membrane, respectively. The value of n depends on the rate-limiting steps in the permeation process. When bulk diffusion in the membrane is

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the rate-limiting step, the value of n is close to 0.5, because the diffusion rate is proportional to the concentration of hydrogen atoms on opposite sides of the metal surface and this hydrogen concentration is proportional to the square root of the hydrogen pressure [18,19]. At a sufficiently small thickness of the membrane, surface reactions or gas-phase diffusion may be rate-controlling processes of permeation [18,19]. It is experimentally shown that the bulk diffusion of atomic hydrogen in palladium is the rate-determining step for membranes with a thickness of several mm and more when hydrogen is used as a feed gas [19,20]. However, in some cases, n-values differed from 0.5 are reported for thick membranes, which can be attributed to thermomechanical properties of the membrane and to defects through which a substantial portion of the hydrogen permeates [20,21]. Therefore, to conclude about the nature of the ratedetermining step from the exponent, this information should be coupled and analyzed together with the overall activation energy of the permeance [22]. A low value of the activation energy (<~30 kJ/mol) indicates that the surface phenomena of dissociative adsorption and recombinative desorption do not provide an influence on the permeation process since they are characterized by a higher activation energy (~54e146 kJ/mol) [23]. Taking into account this fact, in this work we studied the dependence of the H2 flux on the transmembrane pressure and also calculated the apparent activation energy of permeance. Since in a range of H2 flow rate of 300e600 cm3/min the H2 flux remains unchanged, measurements were carried out in this flow rate range. It is seen from Fig. 2 that the H2 flux through the studied PdeIneRu foil increased linearly with the square root of the pressure ðp1 1=2  p2 1=2 Þ, which can indicate that the hydrogen permeation in the foil is controlled by the diffusion of atomic hydrogen. Then the temperature dependence of the hydrogen permeance in the range from 573 to 773 K was obtained and presented in the Arrhenius coordinates in Fig. 3. The apparent

Fig. 3 e Arrhenius plots of the hydrogen permeance for the PdeIneRu membrane in the temperature range from 573 to 773 K.

activation energy was estimated from this plot as 18.7 kJ/mol with the correlation coefficient R2 ¼ 0.99. The obtained value is within the range of Eа values characteristic of the control of permeance by bulk diffusion of atomic hydrogen in palladium. Thus, the results obtained indicate that the permeance is controlled by diffusion of atomic hydrogen and the palladium foil and Graflex seals have no defects. These facts are also confirmed by the preliminary measurements described above. In the opposite case, Knudsen diffusion and viscous flow through Graflex seals would lead to higher n-values [20,21]. The results presented also show that the radial distribution of the gas flux over the membrane surface in MC is satisfactorily described by the Sieverts law under the chosen conditions. A similar dependence for the hydrogen flux through the Pde23 wt % Ag foil 30 mm thick is presented for comparison in Fig. 2. It can be seen that the H2 flux through the PdeIneRu foil at 823 K is 3 times higher than that through the foil of the PdeAg alloy. It is known that some tertiary palladium alloys have high H2 permeance. As shown in Table 1 [24,25], permeance of such

Table 1 e Higher hydrogen permeance of various palladium-alloys compared with pure palladium. Pd-alloy Pd PdeY PdeY PdeAg PdeCe PdeCu PdeAu PdeRueIn PdeAgeRu PdeAgeRu

Fig. 2 e Hydrogen flux through the membranes as a function of the difference in the square root of the hydrogen partial pressures at 823 K H2 flow rate 300 cm3/ min.

a

b

Wt % of alloy Average bond Permeance ratio metal distancea,b/nm Pd-alloy/Pd 0 6.6 10 23 7.7 10 5 0.5, 6 30, 2 19,1

0.275 0.281 0.284 0.278 0.280 0.272 0.275 0.278 0.279 0.278

1.0 3.5 3.8 1.7 1.6 0.48 1.1 2.8 2 2.6

Bond distance of each metal: Pd (0.275), Y (0.355), Ag (0.289), Ce (0.365), Cu (0.256), Au (0.288), Ru (0.265), In (0.325). Average bond distance: i bond distance of Mi Xi, Mi: metal, Xi: mole fraction.

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alloys as PdeIneRu, PdeAgeRu, and PdeAgeRh is higher than that of Pd and also of PdeAg alloy. This can be due to their higher H2 solubility. In the literature, the product of diffusivity and solubility is proportional to the H2 permeance in Pd alloys [26]. Rare-earth palladium alloys such as PdeY or PdeCe can achieve a higher H2 solubility because the rare-earth elements are 30% larger than Pd in atomic size, thus increasing the hydrogen permeation rate, even although the diffusion coefficients in the alloys are relatively smaller than that of pure Pd [26]. In the case of PdeAg alloys, the H2 solubility increases with Ag content up to the 20e40 wt % of Ag, while the H2 diffusivity decreases with increasing Ag content. The simultaneous changes in solubility and diffusivity result in a 1.7 times higher H2 permeability than pure Pd at 23 wt % of Ag and at 623 K [27]. The estimated solubility for PdeAg (30 wt % Ag) and for PdeAu (20 wt % Au) are 10 times and 12 times higher, respectively, than that of pure Pd at 456 K, while the solubility of PdeCu (20 wt % Cu) is 5 times lower than that of pure Pd, which corresponds to the permeability behavior. Taking into account these data and the fact that the size of the In atom is larger than that of Pd, it can be assumed that the high hydrogen permeability of the studied PdeIneRu alloy is determined by a combination of Н2 solubility and diffusivity.

Inhibition effects of CO, Ar, and N2 Traditionally, H2 is produced by steam reforming (SR) of hydrocarbons such as methane, naphtha, or methanol. On an industrial scale, most H2 is currently produced by SR of natural gas. These reactions lead to H2eenriched gas mixture containing carbon oxides and other by-products as well as unreacted reagents. These components can affect the membrane separation of H2, in particular, the H2 flux through the membrane. It is shown [28] that CO exerts the highest effect. It is well known that the presence of other gases along with hydrogen affects the permeation of H2 not only by means of the dilute effect but other mechanisms are also involved. It is reported [29] that the adsorbed CO molecules block the available hydrogen adsorption sites on the membrane surface and, hence, quench H2 permeation or (and) the presence of CO increases the activation barriers associated with some surface steps of the hydrogen permeation mechanism. In order to investigate possible surface and dilution effects on the hydrogen permeation through the PdeIneRu foil, mixtures of H2 with CO, N2, and Ar were used. The measurements were carried out both using the sweep gas and without it. Prior to measurements using the sweep gas (N2), we studied the influence of its flow rate on the H2 flux. The supply of the sweep gas decreases the partial hydrogen pressure at the permeate side of the MC to form a driving force for the hydrogen flux through the membrane. It was found in experiments with pure H2 (flow rate 300 cm3/min) at 823 K that the hydrogen flux through the PdeIneRu foil increased with an increase in the sweep gas flow rate in the range from 20 to 160 cm3/min and remained unchanged in a range of sweep gas flow rates of 160e250 cm3/min, which can be explained by the limiting possible H2 withdrawal through the membrane under the chosen conditions. Taking this into account, the experiments described below were carried out at the constant flow rate of the sweep gas equal to 180 cm3/min.

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Fig. 4 e Dependence of the hydrogen flux on the H2 content in H2/CO, H2/N2, and H2/Ar mixtures, nitrogen is a sweep gas (180 cm3/min). Flow rate of the mixture (cm3/min): 1e40; 2e120; 3e200; 4e400.

The hydrogen flux vs. hydrogen percentage in the feed mixtures H2eCO, H2eN2, and H2eAr is shown in Fig. 4. The measurements were carried out at 593 and 773 K and flow rates of the mixture of 40, 120, 200, and 400 cm3/min. The presented plots show that the hydrogen flux increases with an increase in the flow rate of the mixture from 40 to 200 cm3/min and remains unchanged with the further increase in the flow rate. An insignificant negative effect of CO on the hydrogen flux is observed only at high flow rates of the mixture (curves 3, 4), and the effect of CO decreases with temperature. At lower flow rates of the raw materials, the hydrogen flux increases with increasing H2 content in the mixture, i.e., with decreasing hydrogen dilution, and the plots for CO, N2, and Ar coincide. In other words, for CO, N2, and Ar the possible surface effects are the same or are absent at all. The situation changes in experiments without using a sweep gas, when the transmembrane pressure is a driving force for the Н2 flux through the membrane. Fig. 5 shows the results of measuring the hydrogen flux through the membrane from binary mixtures of hydrogen with different contents of CO, Ar, and N2 at temperatures of 593 and 773 K. The

Fig. 5 e Hydrogen flux vs. transmembrane pressure for CO (20.6%)/H2 (1), N2 (19.9%)/H2 (2), Ar (19.6%)/H2 (3), CO (5.6%)/ H2 (4), and N2 (4.6%)/H2 (5). Flow rate 300 cm3/min.

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measurements were carried out at a flow rate of the mixture of 300 cm3/min. It was found that an increase in the flow rate to 600 cm3/min exerted no effect on the dependences presented in Fig. 5. Binary hydrogen-containing mixtures with an impurity content about 6 and 20% were chosen for the study. It follows from Fig. 5 that CO exerts a negative effect on the H2 flux. The negative influence of CO decreases with increasing temperature, and at T ¼ 773 K the hydrogen flux trend for the CO (5.6%)/H2 mixture was found to be close to that of N2 (4.6%)/H2. The negative influence of CO could be interpreted admitting that strong interactions between CO and Pd atoms (surface effect) exist in addition to the dilution effect. The surface effect is temperature-dependent. Specifically, high temperatures determine the low surface effect of CO. This fact is well known in the literature for the palladium membranes [30,31]. Unlike mixtures with CO, the dependences for mixtures with N2 and Ar coincide (curves 2, 3), which suggests that these gases exert no negative effect on the hydrogen flux. It should be mentioned that the negative influence of CO on the H2 flux was found only in the experiments where the difference between the square roots of the hydrogen partial pressure on the feed and permeate sides of the membrane is the driving force for hydrogen permeation. At the same time, as shown above in Fig. 4, only an insignificant negative effect of CO is observed when using sweep gas. It is reported that, in the course of H2 separation from the multicomponent systems, components (or one component) of the mixture can be accumulated, preventing H2 access to the membrane surface because of a depletion of H2 in the boundary layer near the membrane surface. This so-called concentration polarization has a serious adverse effect on the performance of a membrane-based separation unit [31,32] and is one of the main reasons for the negative CO effect on the H2 flux. The insignificant negative effect of CO in the above described experiments using the sweep gas suggests that concentration polarization of the mixture components occurs near the membrane surface only to a small extent under the experimental conditions. Moreover, we assume a decrease in concentration polarization in experiments with a mixture CO (20.6%)/H2 with the simultaneous use of transmembrane pressure and sweep gas for H2 transport through the membrane. The values of hydrogen fluxes for H2 withdrawal from mixtures CO (20.6%)/H2 (mixture 1) and N2 (19.9)/H2 (mixture 2) are presented in

Table 2. The withdrawal of H2 was carried out at constant values of transmembrane pressure (69 kPa) and sweep gas flow rate (180 cm3/min). The results presented in Table 2 lead to two main conclusions. First, a comparison of items 1 and 3 shows that for the simultaneous use of transmembrane pressure and sweep gas the hydrogen flux through the membranes of mixtures 1 and 2 at Т ¼ 573 K increases by 6.2 and 3.7 times, respectively, compared to the experiments where only transmembrane pressure is used. With increasing temperature to 773 K, the increase in the hydrogen flux for mixture 1 is 4.5 times, whereas that for mixture 2 remains unchanged being 3.6 (items 2, 4). Second, the negative effect of CO decreases considerably for the simultaneous use of transmembrane pressure and sweep gas. In fact, in the absence of sweep gas due to the negative effect of CO, the hydrogen flux from mixture 1 at temperatures 573 and 773 K decreases by 2.06 and 1.41 times, respectively, compared to mixture 2 (items 1, 2). At the same time, when using sweep gas these parameters are 1.22 and 1.13 (items 3, 4). This means that for the simultaneous use of transmembrane pressure and sweep gas the temperature exerts a less effect on the hydrogen flux from a mixture with CO, which indicates a decrease in the activation barrier in the mechanism of H2 transport. On the whole, these results show that the driving force for H2 transport through the membrane increases for the simultaneous use of transmembrane pressure and sweep gas because of a decrease in the partial pressure of H2 in the permeate side, due to which the influence of the effect of concentration polarization of CO near the membrane surface decreases.

Carbon deposit formation on the PdeIneRu foil from CO and H2eCO mixtures Another type of the CO inhibition effect can be the reaction of CO on the PdeIneRu foil surface at high temperatures with the formation of carbon deposits. The diffusion of deposited carbonaceous impurities into the bulk phase of the membrane leads to the expansion of metal lattice, due to which the lattice microstructure changes and defects appear [33]. In addition, the incorporation of carbon into the metal lattice could reduce hydrogen solubility, decreasing the hydrogen permeability of the membrane. The published data show a tendency for carbon deposits formation increasing with temperature and the CO concentration in the retentate [34]. When pure CO or its

Table 2 e Hydrogen flux through the membrane with the simultaneous use of transmembrane pressure (69 kPa) and sweep gas (180 cm3/min). Flow rate of the mixture 300 cm3/min. Item no.

T, K

Hydrogen flux  100 (mol m2s1) CO (20.6%)/H2 (1)

Without sweep gas 1

573

Hydrogen flux(2)/hydrogen flux(1)

N2 (19.9%)/H2 (2)

0.91

2.06 1.88

2

773

1.96

1.41 2.77

With sweep gas 3 4

573 773

5.65 8.91

6.91 10.11

1.22 1.13

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mixtures with H2 are fed to the membrane, such reactions as the Boudouard reaction (2) and methanation (3) can occur: 2CO4C þ CO2

(2)

CO þ 3H2 4CH4 þ H2 O

(3)

These reactions can influence on the membrane separation process. Both the foil material and walls of the stainless steel membrane unit can be catalytically active in these reactions. In order to investigate the occurrence of reactions (2) and (3) in the studied system, both pure CO and the H2eCO mixture were used. In the first series of experiments, the PdeIneRu membrane was treated with pure CO at Т ¼ 673 K and 773 K for 5 h. The composition of the tail gas ejecting from the MC was analyzed by GC and mass spectrometry. The analysis results show that the tail gas contained CO2 along with CO, indicating that CO disproportionates via reaction (2). The CO2 content at 673 K is about 0.5%, while at 773 K it is 0.4e0.6%. Note that there was about 0.3% CO2 detected in a blind experiment at 773 K in the absence of the PdeIneRu membrane. Thus, both the PdeIneRu foil and, to a lower extent, reactor walls exhibit catalytic activity in reaction (2). After the experiment at 773 K, the system was flushed with N2 until no more CO and CO2 were detected. Then the membrane was exposed for 15 min to air, and CO and CO2 were detected in the reactor effluent indicating the presence of carbon deposits. In order to check their influence on the H2 flux, the measurements were carried out after the treatment of the membrane with pure CO at 773 K for 6 h. The results of the measurements showed a decrease in the H2 flux by 5% compared to the initial value. Thus, the carbon deposits formed due to CO disproportionation exert an insignificant effect on the H2 permeability of the membrane. The kinetic study of CO disproportionation on the surface of a series membrane palladium alloys suggested [35] that at T > 573 K the major portion of carbon deposits desorbs from the surface to the gas flow as a mixture of Cn molecules and only a minor portion of heavier molecules deposits on the surface. In the second series of experiments, a hydrogen mixture containing 22.5 vol.% CO was passed above the membrane at 673 and 773 K for 5 h. An analysis of the reactor effluent showed the formation of both CH4 and CO2 at 673 K, while only CH4 is formed at 773 K. As in the first series of experiments, after the treatment with the H2eCO mixture at 673 and 773 K, the MC was flushed with nitrogen to the complete disappearance of CO and H2 from the effluent, and then air was passed for 15 min at the same temperatures. After the experiments at 673 K, CO and CO2 are observed in the effluent, whereas no CO and CO2 were observed after the experiment at 773 K. These results show that passing the H2eCO mixture above the membrane at T ¼ 673 K is accompanied by both CO disproportionation (2), leading to the formation of carbon deposits and CO2, and methanation (3), which can be presented as the following steps: CO þ H2 4C þ H2 O

(4)

C þ 2H2 4CH4

(5)

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It is known that Boudouard equilibrium (2) shifts to the left with increasing temperature, which reduces the probability for carbon deposits formation. At the same time, equilibrium (5) shifts to the right with temperature. Taking into account this fact, we can assume that at 773 K the contribution of CO disproportionation (2) is insignificant and the carbon deposits formed in reaction (4) are transformed into CH4 upon the interaction with H2 via reaction (5). If this conclusion is valid, the permeability of the membrane should not change upon the treatment with the H2eCO mixture at 773 K. This was confirmed in the experiment in which a hydrogen mixture containing 22.5% CO was passed above the PdeIneRu membrane at 773 K for 6 h. The measurements showed that the decrease in the H2 flux was 3%, which is within the measurement experimental error. It can be concluded that the contact of pure CO with the membrane unit based on the PdeIneRu foil at 673 and 773 K results in an insignificant formation of carbon deposits on the foil surface and walls of the stainless steel membrane unit, which exerts almost no effect on the H2 permeability of the membrane. When the membrane interacts with the H2e22.5% CO mixture at 773 K, no carbon deposits are formed, since methanation occurs predominantly.

Conclusions The results of the present study show that the foil of the Pde6.0 wt % Ine0.5 wt % Ru alloy is a good material for membrane technology of preparing high-purity hydrogen. The permeability of PdeIneRu foil at 823 K three times exceeds the corresponding parameter for the traditionally used foil of the Pde23 wt % Ag alloy. The study of the dependence of the H2 flux on the difference of the hydrogen pressures in the gas phase at the retentate and permeate sides of the MC showed that the process was controlled by the bulk diffusion of atomic hydrogen. A low value of the apparent activation energy of permeation equal to 18.7 kJ/mol also indicates that the surface phenomena of dissociative adsorption and recombinative desorption do not influence on the permeation process. The influence of CO on the H2 flux through the studied membrane depends on the nature of the driving force used for H2 withdrawal. An insignificant negative effect of CO was found in experiments with sweep gas supply at the permeate side of the MC. In this case, the decrease in the H2 flux with increasing the impurity content in hydrogen is caused by the dilution of the mixture only and the effects of CO, N2, and Ar are equal. In the case of using the transmembrane pressure on the membrane, CO exerts a negative effect on the H2 flux. This negative influence decreases with temperature, and at T ¼ 773 K the rates of H2 withdrawal from mixtures CO(5.6%)/H2 and N2(4.6%)/H2 are close. It was assumed that a reason for the negative effect of CO can be its accumulation near the membrane surface (concentration polarization), which favors the adsorption of CO molecules and blocking surface adsorption sites accessible for H2. The negative effect of CO on hydrogen flux can be decreased using simultaneously transmembrane pressure and sweep gas. Of course, it is difficult to use a sweep gas in practice because of hydrogen dilution and a problem of its subsequent separation.

Please cite this article in press as: Didenko LP, et al., Hydrogen flux through the membrane based on the PdeIneRu foil, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.107

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Taking into account the positive effect of the sweep gas on the parameters of the process (an increase in the hydrogen flux, a decrease in the negative effect of concentration polarization), is seems reasonable to study the separation problem. For example, water vapor can be used as a sweep gas, which would simplify the subsequent hydrogen separation. An insignificant decrease in the H2 flux through the membrane due to carbon deposits formation is observed only on contact with pure CO in a temperature range of 673e773 K. At the same temperatures, the contact of the membrane with the H2eCO mixture does not result in the formation of carbon deposits at all because of the predominant occurrence of methanation to form CH4 and H2O. The optimal temperature for H2 separation from CO-containing mixtures is 773 K, since the inhibition of the separation process because of the reversible adsorption of CO molecules becomes lower, and the efficiency of the catalytic reaction on the membrane surface resulting in carbon deposits formation is insignificant.

Acknowledgments This work was financially supported by the Russian Foundation for Basic Research, project no. 13-03-12419.

references

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Please cite this article in press as: Didenko LP, et al., Hydrogen flux through the membrane based on the PdeIneRu foil, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.10.107