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Effect of poisoning on metallic membranes
7 Effect of poisoning on metallic membranes Xiao-Liang Zhang College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, Jiangxi, China
Introduction In the last few decades, dense metallic membranes such as palladium-based membranes have been successfully investigated for membrane reactors using water gas shift (WGS), steam reforming of hydrocarbons, and ammonia decomposition reactions to generate ultrahigh purity hydrogen for proton exchange membrane fuel cell systems and to increase conversion of the feed such as CH4 and CO [1e6]. There are different levels of various impurities such as CH4, steam, CO and CO2, NH3, H2S, etc. along with hydrogen from the effluent of the catalytic membrane reformers. However, these nonhydrogen species would have possible deterioration and poisoning effects on hydrogen permeation through Pdbased membranes in membrane reformers [1,4e6]. What about the poisoning effect on hydrogen permeation through metallic membranes? In this chapter, we will critically review the experimental and theoretical studies of nonhydrogen species for Pd-based membranes, focusing particularly on the effect of potential gaseous contaminants. The poisoning effect of the main nonhydrogen species such as H2S, CO, and NH3 on hydrogen permeation are subsequently discussed in detail, which are the main obstacles blocking membrane commercial application. Finally, we will give an overview of the relevant development strategies for improved chemical stability of Pd-based membranes.
The influence of nonhydrogen species Generally, the nonhydrogen species always coexist with hydrogen in catalytic Pd-based membrane reactors for the generation of
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hydrogen. However, the nonhydrogen components will to some extent affect the hydrogen permeation behavior and hydrogen permeation performance of Pd-based membranes, and will even destroy the structure of metallic membranes by forming Pd compounds such as palladium sulfide (PdSx), palladium carbide (PdCx), and other irreversible toxic effects on membrane surfaces. Table 7.1 summarizes the potential effect of the coexisting gases of H2ex% X mixtures (X ¼ CH4, CO2, CO, and steam) on hydrogen permeation through Pd-based membranes [7e21]. Although there are differences in membrane supports, thicknesses, and even preparation methods, the potential influence of nonhydrogen components on the H2 permeation behavior and permeation performance through these Pd-based membranes was classified in five issues: dilution effect, concentration polarization (external diffusion resistance), competitive adsorption, irreversible poisoning effect, and surface catalytic effect.
Dilution effect In H2eX mixtures, the addition of coexisting nonhydrogen components will dilute the hydrogen concentration and then decrease the hydrogen partial pressure in the feed side, thus decreasing the driving force of hydrogen transport through Pd-based membranes. As a result, the hydrogen permeation flux or hydrogen permeability of Pd-based membranes seems to be reduced by the dilution effect, while not poisoning the membrane surface [9,15,18,20]. Li et al. [18] investigated the influence of N2 gas with different concentrations on hydrogen permeation through a 10 mm-thick Pd/PSS membrane at 380 C. Hydrogen permeability in H2eN2 mixtures was obviously lower than that of pure H2 atmospheres. Nitrogen was regarded as an inert gas, with which it was difficult to perform chemical adsorption on the Pd membrane surface. Compared to pure hydrogen permeation, the presence of nitrogen was diluted by hydrogen concentrations, reducing the hydrogen partial pressure on the feed side. Thus it decreased the driving force of hydrogen transport and decreased hydrogen permeation flux. Their experimental results were in good agreement with simulated assumption. Gallucci et al. [15] also studied hydrogen permeation with different concentrations of nonhydrogen components through a 60 mm-thick foil Pd membrane at 250e350 C. It was also found that N2, Ar, and CO2 had only a dilution effect on hydrogen permeation, meanwhile CO had serious inhibition for hydrogen permeation performance through the Pd membranes.
Table 7.1 The potential influence of nonhydrogen species of H2ex% X mixtures on hydrogen permeation through Pd-based membranes Membrane Shape t (mm) H2ex% X mixtures T (o C)
Influence
Reason
References
Pd Pd PdAg/Al2O3 PdAg Pd
Disk Disk Tube Disk Disk
700 50 20 20 700
10% CH4 50% CH4 1%, 10%, 25% CH4 0%e6.8% CH4 50% CO2
Tube
5e6
1%e8% CO2
Pd, PdAg PdAg/Al2O3 Pd/Ni-ps PdAg Pd Pd
Disk Tube Disk Disk Tube Disk
0.5e1 20 2.5 20 60 700
20% CO2 1%, 10%, 25% CO2 28.3% CO2 0%e27.9% CO2 0%e70% CO2 10%, 50% CO
Pd PdRuIn Pd/PSS PdAg/Al2O3 Pd PdAg/Al2O3 PdAg PdAg Pd Pd/Al2O3 Pd/PSS PdAg/Al2O3 Pd PdAg PdAg
Disk Tube Tube Tube NA Tube Disk Disk Tube Tube Tube Tube Disk Disk Disk
700 200 10 5e6 100 20 1.6 20 60 2e4 10 5e6 0.9 1.6 20
10% CO 33.2% CO 1.7%e17% CO 1%e7% CO 0%e6% CO 1%, 10%, 25% CO 20% CO 0%e4.6% CO 0%e70% CO 20%e33.3% CO 1%e13% H2O 1%e7% H2O 20% H2O 20% H2O 0%e9.5% H2O
Comp. adsorp. / Dilution effect / Comp. adsorp. / Comp. adsorp. / Surface effect Dilution effect NA / Dilution effect Comp. adsorp. / Comp. adsorp. Comp. adsorp. Comp. adsorp. Comp. adsorp. Comp. adsorp. Comp. adsorp. Comp. adsorp. / Dilution effect þ surface effect Comp. adsorp. þC deposition Comp. adsorp. Comp. adsorp. Comp. adsorp. Dilution effect /
[7] [8] [9] [10] [11]
PdAg/Al2O3
Reduced No Reduced No Reduced No Reduced No Reduced Reduced Reduced No Reduced Reduced No Reduced Reduced >5%, reduced Reduced Reduced Reduced Reduced No Reduced Reduced Reduced Reduced Reduced Reduced No
250e450 350e600 300e500 500e600 150e200 200e450 275 >325 350e450 300e500 400 500e600 250 150e250 250e450 250e450 220e280 380 275e450 150e200 300e500 255 500e600 250 320e500 380 275e350 350e450 255 500e600
Comp. adsorp., Competitive adsorption; Ni-ps, Ni porous support; PSS, porous stainless steel; t, thickness of membrane.
[12] [13] [9] [14] [10] [15] [11] [16] [17] [18] [12] [19] [9] [20] [10] [15] [21] [18] [12] [13] [20] [10]
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Moreover, the hydrogen permeation performance was restored when the H2eN2 mixtures switched to pure hydrogen treatment [18]. The dilution effect was just a reduction of the apparent permeability, not a change in hydrogen permeation behavior, including the hydrogen permeation mechanism (see also the discussion in Chapter 8).
Concentration polarization and external diffusion resistance During the hydrogen separation process with Pd-based membrane reactors in coating hydrogen mixtures, the relative concentration of hydrogen in the feed side was decreasing, meanwhile that of nonhydrogen species was increasing due to most of the hydrogen in the mixture feed continuously permeating through the Pd membrane. Therefore there was an accumulation of nonpermeating gases in the boundary layer adjacent to the membrane surface [22]. This would cause a concentration gradient to build up in the boundary layer and prevent hydrogen dissociative adsorption on the membrane surface. Thus the hydrogen concentration adjacent to the membrane surface was lower than that of bulk gas. This would in turn result in a significant decrease in hydrogen partial pressure difference through the Pd membrane and thus decrease hydrogen-permeated flux. This blockage effect on hydrogen permeation was well known as concentration polarization or external mass transfer resistance [12,17,23,24]. Hara et al. [17] reported hydrogen permeation performance with a 0.2 mm-thick Pd91Ru6In3 alloy membrane within the temperature range of 220e300 C and a total pressure of 2.0 bar. The apparent hydrogen permeability in the H2-Ar (2/1, mol%) mixtures was far lower than that of pure hydrogen. Due to the concentration polarization effect by the existence of Ar radial diffusion in the mixtures, the H2 concentration on the membrane surface was obviously lower than that in the bulk phase, which would greatly reduce the hydrogen permeability of the membrane. Two thin (2 mm) Pd membranes with different hydrogen permeance ranging widely from 13.8 to 44.3 m3 m2 h1 bar1 were evaluated for hydrogen permeation behavior at 350e500 C and under pressures up to 5 bar. Zhang et al. [23] found that the existence of concentration polarization caused a loss of hydrogen performance and hydrogen yield in an H2e25% N2 mixture system. The operation parameters such as pressure, temperature, feed gas flow rate, and membrane permeability played a role in the influence of the concentration polarization effect. Concentration
Chapter 7 Effect of poisoning on metallic membranes
polarization resulted in a loss in effective surface area of the Pd membrane and thus a decrease in hydrogen separation efficiency of the NH3 cracker generation system. Hou et al. [12] and Mori et al. [24] separated hydrogen with PdeAg alloy composite membranes in H2eN2 mixtures. They also found that N2 gas had a severe concentration polarization effect on hydrogen permeation under operation conditions. In addition, for a Pd-based membrane with a certain surface area, if the amount of hydrogen in the mixture was not sufficient to completely occupy the entire Pd membrane surface for H2 adsorption and dissociation, it would inevitably increase the external diffusion resistance, which was similar to the concentration polarization effect and also reduced the hydrogen permeability of this membrane [24]. To weaken or eliminate the negative influence of concentration polarization (or external diffusion resistance) through Pdbased membranes within a limited effective surface area, increasing feed gas flow rate and total operation pressure may be the effective approaches under operating conditions [23]. We also investigated the concentration polarization effect of coexisting gases on hydrogen permeation through a thin Pd/ Al2O3 tubular membrane by means of comparisons between experimental and simulation data at temperatures of 350e500 C [4]. Fig. 7.1 shows the hydrogen flux as a function of feed flow rate in a pure hydrogen atmosphere and H2e10% X (X ¼ CH4, CO2, and steam) mixtures at 500 C. In the case of pure hydrogen atmosphere, as seen in Fig. 7.1A, the hydrogen flux increased initially with the feed flow rate and then approached a constant value when the feed flow rate of pure hydrogen was above 250 mL min1. Similarly, hydrogen flux also increased initially with the feed flow rate of H2eX mixtures and then increased steadily at the feed flow rate above 375 mL min1 in the mixtures’ feed. This might be related to the influence of external mass transfer resistance on the Pd membrane. A simple isothermic plug flow model based on Wang’s work [25] was developed to evaluate hydrogen permeation behavior of a Pd membrane in the foregoing H2eX mixtures’ permeation experiments. It was assumed that the nonhydrogen species (X) was inert gas with no chemisorption on the Pd membrane surface and did not diffuse through the Pd composite membrane [4]. Fig. 7.2 demonstrates the simulated profiles of hydrogen in the shell side along the dimensionless length of the Pd tubular membrane at 500 C. The value of n(Hshell )/n(Hin 2 2 ) is the ratio of hydrogen molar amount in the shell side to that in the inlet of the feed, which is the evaluation factor for the degree of external
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(A)
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30
(B)
2.0
30
H flux
pure H
H -10% X mixture, P
... P
= 2.0 bar
1.5
1.0
2
H2
4
H2
P (bar)
20
H2
10
O
2
20
10
0.5
0
0
100
200
300
400
500
600
0.0 700
0
Feed flow rate (ml/min)
0
100
200
300
400
500
600
700
Feed flow rate (ml/min)
Figure 7.1 The influence of feed flow rate on the hydrogen permeated flux through a thin Pd/Al2O3 composite membrane in the case of (A) pure hydrogen and (B) H2e10% X mixtures at 500 C. Unpublished results from X.L. Zhang, report Preparation and Hydrogen Permeation of Pd-Based Membranes and Gas Mixtures Separation Process in the Membrane Reactors (Ph. D. Dissertation). (in Chinese), Chinese Academy of Sciences of China, 2008
(A)
(B)
Figure 7.2 The simulated hydrogen profiles in the shell side along the dimensionless length of the Pd tubular membrane in the case of (A) pure hydrogen and (B) H2e10% X mixtures fed with different feed flow rates at 500 C. Unpublished results from X.L. Zhang, report Preparation and Hydrogen Permeation of Pd-Based Membranes and Gas Mixtures Separation Process in the Membrane Reactors, (Ph.D. Dissertation). (in Chinese), Chinese Academy of Sciences of China, 2008
mass transfer resistance on hydrogen permeation through the Pd membrane. If the ratio profile is parallel to the X axes of the dimensionless length or very close to zero at the tubular membrane outlet, it means that nearly all the hydrogen in the feed diffuses through the Pd membrane to the permeate side and there are still some vacant sites for hydrogen dissociative adsorption
Chapter 7 Effect of poisoning on metallic membranes
on the membrane surface at the corresponding operating conditions. Consequently, the accumulation of nonhydrogen species on the Pd membrane surface would decrease the hydrogen permeated flux. Compared with the pure hydrogen profile (Fig. 7.2A), as shown in Fig. 7.2B, the hydrogen profile was parallel to the X axes of the dimensionless length and very close to zero at the outlet, when the feed flow rate was below 375 mL min1 for H2e10% X mixtures feed. These simulated results were consistent with the foregoing experimental results (Fig. 7.1). It was confirmed that the external mass transfer resistance occurred only in the case of lower feed flow rate, which would influence hydrogen permeation behavior through the thin Pd tubular composite membrane. Therefore increasing feed flow rate will eliminate the negative effect of concentration polarization and external diffusion resistance on hydrogen permeation through Pd-based membranes.
Competitive adsorption Further studies have demonstrated that the strong adsorption phenomena would occur when the coexisting nonhydrogen species such as CO, steam, and NH3 contacted with the metallic membrane surface. This would decrease the amount of adsorbed hydrogen by occupying the coverage sites of hydrogen dissociative adsorption onto the membrane surface. Hence the competitive adsorption of the nonhydrogen species would decrease the effective area for the hydrogen dissociation process, thereby greatly decreasing the hydrogen permeation flux. Moreover, this inhibition effect of competitive adsorption is reversible on hydrogen permeation, and hydrogen permeability of metallic membranes can be restored to pristine values after stopping nonhydrogen species addition and switching to pure hydrogen treatment [7,11,12,16e21,26e30]. Chen et al. [7] studied the hydrogen permeation behavior of H2e10% CH4 and H2e10% C2H4 binary mixtures in Pd membrane and PdeAg alloy membrane reactors at 250e450 C. For the H2eCH4 mixtures system, the decreasing trend of hydrogen permeability through the Pd membrane decreased with increasing temperatures, while the declining trend increased with increasing temperatures through PdeAg alloy membranes. However, for H2eC2H4 mixtures, the hydrogen declining tendency was similar through both Pd membrane and PdeAg alloy membrane, which decreased with increasing temperatures. This was attributed to the competitive adsorption of CH4 and C2H4 with hydrogen on the membrane surface, which reduced the
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effective active sites of hydrogen adsorption and dissociation processes, thus reducing the effective hydrogen permeation area through the metallic membrane. Consequently, the competitive adsorption of the nonhydrogen species (CH4 and C2H4) decreased the hydrogen permeability [7]. Li et al. [18] also studied the influence of CO and steam with different concentrations on hydrogen permeation through a 10 mm-thick Pd/PSS membrane at 380 C (see also Table 7.1). As shown in Fig. 7.3, the influence of CO on hydrogen permeation was as low as that of N2 when the CO concentration was lower than 5%. However, the inhibition effect on hydrogen permeation increased when CO concentration in H2eCO mixtures was over 5%. The adsorption capacity of CO on the metallic membrane surface is much higher than that of H2 [16,19,26]. Thus there is strong competitive adsorption between CO and H2 on the membrane surface. This capacity of competitive adsorption will increase with increasing CO concentration in H2eCO mixtures. Therefore CO occupied the active site of hydrogen adsorption and dissociation, reducing the effective activity of hydrogen permeation. It would also reduce H2 permeability and change the ratedetermining step for hydrogen permeation from the solid-phase diffusion step to the surface process through the thin dense metallic composite membranes.
Figure 7.3 The hydrogen permeation rate as a function of feed mixture flow with various mixtures. Reprinted from A. Li, W. Liang, R. Hughes, The effect of carbon monoxide and steam on the hydrogen permeability of a Pd/stainless steel membrane, J. Membr. Sci. 165 (2000) 135e141, copyright with permission from Elsevier.
Chapter 7 Effect of poisoning on metallic membranes
Additionally, Li et al. [18] compared the competitive adsorption effect of both water vapor (steam) and CO on hydrogen permeation. As described in Fig. 7.3, the inhibition effect of steam was obviously more serious than that of CO on the Pd membrane surface. Hou et al. [12] examined the influence of CO, CO2, and steam on hydrogen permeation through a 5e6 mm-thick PdeAg alloy membrane at a temperature range of 275e450 C and a pressure of 2 bar. Their experimental results were similar to that of Li et al. [18], which established the inhibited order of competitive adsorption as steam > CO > CO2. The nonhydrogen species of CO2 had the lowest influence on hydrogen permeation, and had no inhibition effect even when the feed temperature was over 325 C. On the contrary, hydrogen permeability would decrease by about 60% even if the steam concentration was only 1% in H2e steam binary mixtures. This was ascribed to the strong competitive adsorption of steam on the membrane surface [12]. Furthermore, unsaturated alkanes such as C2H4 and C3H6 from the hydrocarbon dehydrogenation reactions and NH3 in the catalytic membrane reactors also had a negative effect of competitive adsorption on hydrogen permeation [27e30]. These small molecules would deactivate the adsorption and dissociation activity of hydrogen molecules on the surface of the membrane, thus significantly weakening hydrogen permeability through the metallic membrane and then leading to the degradation of membrane separation efficiency and catalytic performance in the Pdbased membrane reactors.
Irreversible poisoning effect Contaminants such as cokes (carbon deposition), sulfide, chloride, and other species produced from Pd-based membrane catalytic reactions would perform irreversible chemical adsorption on the membrane surface and then permanently poison the Pd surface by forming Pd compounds such as PdSx and PdCx. As a result, this would decrease hydrogen permeation performance and even destroy the phase structure of Pd-based membranes [2,7,21,24,25,31]. It should be noted that the poisoning effect is permanently irreversible, and should be avoided as far as possible in catalytic membrane reactors. At high temperature, hydrocarbons always contact with Pdbased membranes for a long time and are easily adsorbed on the membrane surface to form cokes via dehydrogenation reaction. Cokes formation not only decreases the effective separation area of the hydrogen permeable membrane, which would affect the adsorption, dissociation, and desorption of hydrogen on the
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membrane surface, but also gradually penetrate into the Pd phase lattice to form PdCx alloy, which would seriously affect hydrogen diffusion in the Pd lattice and even destroy the phase structure of the Pd membrane [21,32e37]. Raich et al. [32] studied the isobutane dehydrogenation reaction in a Pd membrane reactor. The membrane surface morphology changed from scanning electron microscope images by comparing the reaction before and after. Carbon species was present on the surface of the Pd membrane determined by auger spectrum. Zadai [33] and Roshan [34] found that C3H8 would adsorb and dissociate on the surface of the Pd membrane, resulting in CeC bonds cracking to deposit carbon (cokes). Carbon atoms would be dissolved in the Pd lattice, resulting in a reduction of hydrogen permeability of the Pd membrane. Li et al. [21] also found that, for H2eCO mixtures, the CO disproportionate reaction (2CO ¼ C þ CO2) would occur on the Pd surface above 450 C and thus form PdC species. Ziemecki et al. [35,36] proposed the formation and diffusion mechanism of carbon species on the surface of a Pd membrane: the coexisting gases containing carbon such as CO, C2H2, C2H4, etc. first occurred by chemical adsorption on the Pd membrane surface within medium temperature ranges, and then generated a decomposition reaction to produce carbon species on the membrane surface. These carbon species would penetrate into the metal body phase and dissolve in the metal lattice with PdC0.15 alloy mode, thus eventually resulting in the loss of hydrogen permeability for the Pd membrane. The diffusion of these carbon species in the Pd lattice could lead to hydrogen embrittlement and even membrane deformation [37]. Sulfur, which is found in many chemical materials and petroleum products, is one of the most fatal contaminants for Pd membranes [38e41]. Specifically, H2S has an irreversible poisoning effect on the Pd membrane to form PdS. This would reduce hydrogen permeability by 1% through the Pd membrane being treated with 1 ppm H2S [39]. Sulfides perform the strongest adsorption on metallic surfaces to decrease the electron density of Pd Fermi energy, resulting in a decrease of 4d band electrons [2]. So, PdSx compounds were formed on the surface of the Pd membrane, which would accelerate the complete loss of hydrogen permeation properties through pure Pd membranes. Dense Pt and Ru membranes and PdeCu and PdeAu alloy membranes can resist the poisoning influence of H2S with a certain concentration and keep hydrogen permeation performance and behaviors for these metallic membranes [2,38e41].
Chapter 7 Effect of poisoning on metallic membranes
Actually, the coexisting gases of CO, CO2, and steam also had a negative inhibition effect on hydrogen permeation, even showing the poisoning effect on the Pd membrane surface under certain temperature and concentration conditions.
Surface catalytic effect On the metallic surface, hydrogen molecules have a strong ability for active dissociation adsorption. Thus during the hydrogen permeable separation processes, the nonhydrogen species and hydrogen molecules could perform a potential surface catalytic reaction with Pd and Pd alloys as catalysts under certain temperature and pressure conditions [8,11,13,21,42e44]. For example, decomposition reaction under high temperature, WGS reaction, reverse WGS reaction (rWGS), and methanation reaction (Eqs. 7.1e7.3) would occur under appropriate operating conditions. CO2 þ H2 ¼ CO þ H2 O
(7.1)
CO2 þ 4 H2 ¼ CH4 þ 2 H2 O
(7.2)
CO þ 3H2 ¼ CH4 þ H2 O
(7.3)
The surface catalytic effect exacerbates the complexity of H2eX mixtures through metallic membranes. The nonhydrogen species (X) and the catalytically generated species would change the surface composition and even body structure of metallic membranes due to the interaction among these gases. Thus it would partly decrease the capacities of hydrogen permeation through metallic membranes [8,11,13,21,42e44]. Jung et al. [8] studied the separation behavior of H2e50% C3H6 mixtures through a 50 mm Pd membrane at 350e600 C. Hydrogen permeability was reduced with operating time and this tendency became more seriously with increasing temperature. It was found that carbon compound was deposited on the membrane surface, which might be due to the C3H6 cracking reaction. The cracking rate of C3H6 increased with increasing temperatures. Thus hydrogen permeability decreased with the surface catalytic effect via the formation of PdeCeH species on the membrane surface. Moreover, they also investigated the influence of steam addition for H2eC3H6 mixtures at 600 C [8]. This demonstrated double contradictory influences of steam on hydrogen permeation: (1) the competitive adsorption of steam would reduce the dissociation adsorption activity of hydrogen molecules, thus reducing hydrogen permeability, and (2) steam would decarbonize with
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cokes on the membrane surface, thus removing carbon deposition and improving H2 permeability to some extent [13]. Cheng et al. [44] investigated separation performance through Pd and PdeAg alloy membrane reactors using polynary mixtures (town gas: 49% H2, 28.5% CH4, 19.5% CO2, and 3% CO). CO and H2O were formed via rWGS reaction by CO2 and H2 at 450 C, which would reduce H2 partial pressure in the mixtures, thus decreasing hydrogen permeability. Similar results had been observed by Kulprathipanja et al. [42]. WGS mixtures also revealed the surface catalytic effect of the hydrogen permeation properties for Pd and PdeCu alloy membranes. Wang et al. [43] examined the surface effect with H2e50% N2 mixtures through a Pd membrane for long-term operation over 3000 h. It was an interesting phenomenon. In a relatively short period, the coexisting N2 had no obvious influence on hydrogen performance. However, hydrogen permeance would decrease in comparison with the original value when the Pd membrane was exposed to nitrogen for a long time within the temperature range of 400e450 C. They regarded that the blocking effect was due to the formed nitrogen-containing species (NHx, see also Fig. 7.4), which was responsible for membrane deactivation in the suggested deactivation mechanism. Fortunately, the deactivated Pd membrane could be regenerated via pure hydrogen treatment at a high temperature of 500 C for a period of time. The NHx species could react to form ammonia and desorb from the membrane surface to release the blocked effective area.
Figure 7.4 Modeling of formation and occupation of NHx (x ¼ 0e3) on a Pd surface causing the deactivation of Pd membranes after pure nitrogen treatment or long-term exposure to the equimolar mixture of H2/N2 under certain conditions. Reprinted from W.P. Wang, X.L. Pan, X.L. Zhang, W.S. Yang, G.X. Xiong, The effect of co-existing nitrogen on hydrogen permeation through thin Pd composite membranes, Separ. Purif. Technol. 54 (2007) 262e271, copyright with permission from Elsevier.
Chapter 7 Effect of poisoning on metallic membranes
Unfortunately, limited attention was given to the surface catalytic effect on hydrogen permeation through Pd and Pd alloy membranes [4,21]. Moreover, few reasonable theory models considering the influence of dilution effect and external mass transfer resistance were developed to evaluate the possible influence of coexisting gases on hydrogen permeation at various operating conditions. This should create a new mathematical model based on the foregoing scientific cognition, i.e., simultaneously considering the influence of dilution effect, concentration polarization (external diffusion resistance), competitive adsorption, and hydrogen permeation mechanism to investigate the possible degree of poisoning effect through metallic membranes under operating conditions.
The poisoning effect of typical gases and development strategies This section does not intend to provide an exhaustive review, but rather provide recent innovations about typical gases such as H2S, CO, and NH3 in the literature up to 2018. H2S exposure on the pure Pd membrane surface resulted in a reduction of hydrogen permeance due to (1) surface site blocking by the dissociative adsorption of H2S, and (2) bulk sulfidation of Pd with the formation of Pd4S [45e50]. Consequently, a pure Pd membrane lattice structure would be destroyed resulting in failure to separate hydrogen from the H2eX mixtures. In contrast, Pd alloy membranes (e.g., PdeCu and PdeAu) were observed to have good sulfur resistance [45e50]. Chen et al. [45] reported that a PdeAu alloy membrane exhibited resistance to H2S exposure without significant hydrogen permeance decline and structural change. However, as shown in Fig. 7.5, after 1000 ppm H2S exposure on the Pd47Cu53 (mol%) foil membrane at 350 C, the hydrogen flux rapidly reduced to an undetectable level within 5 min [46]. This was attributed to a thin PdeCueS terminal layer that rapidly formed at the top surface of the alloy foil, which is either inactive for hydrogen dissociation or impermeable to hydrogen atoms. So, the body-centered cubic-PdeCu alloy membranes are easily deactivated in the presence of H2S. Similarly, hydrogen permeation through a Pd-based membrane was inhibited under exposure to trace amounts of CO and NH3 [51,52]. Peters et al. [52] investigated the hydrogen flux inhibition phenomenon through a 10 mm-thick Pd77Ag23 (face-centered cubic) membrane under exposure to different NH3 concentrations (10e500 ppm) at 300e450 C. As shown in Fig. 7.6, no inhibition
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Figure 7.5 Hydrogen flux through 25 mm Pd (solid line) and Pd47Cu53 (wavy dashed line) membranes (T ¼ 350 C, P ¼ 310 kPa). A 90% H2e10% He mixture was exposed to the membrane surface during the first portion of the test and then 1000 ppm H2S/10% He/balance H2 was fed to the membrane surface at 6 h. Reprinted from C.P. OBrien, B.H. Howard, J.B. Miller, B.D. Morreale, A.J. Gellman, Inhibition of hydrogen transport through Pd and Pd47Cu53 membranes by H2S at 350 C, J. Membr. Sci. 349 (2010) 380e384, copyright with permission from Elsevier.
Figure 7.6 Hydrogen flux as a function of process time during the stepwise rampdown of the operating temperature from 450 to 300 C in the absence and presence of 200 ppm NH3 through a Pd77Ag23 membrane (feed: 90% H2 in N2, and Ar sweep, feed, and permeate side at atmospheric pressure). Reprinted from T.A. Peters, J.M. Polfus, M. Stange, P. Veenstra, A. Nijmeijer, R. Bredesen, H2 flux inhibition and stability of Pd-Ag membranes under exposure to trace amounts of NH3, Fuel Process. Technol. 152 (2016) 259e265, copyright with permission from Elsevier.
Chapter 7 Effect of poisoning on metallic membranes
of hydrogen flux was found in the presence of 200e500 ppm NH3 at 450 C. Meanwhile, gradual hydrogen flux decline and loss of membrane stability were observed with decreasing operating temperatures from 450 to 300 C even in the presence of 200 ppm NH3. About 36% hydrogen flux was reduced at 300 C compared to pure hydrogen permeated flux without NH3 under the same operating conditions. However, as seen in Fig. 7.6, hydrogen flux could be quickly recovered to the pristine value when the membrane was treated at 450 C after NH3 exposure. A similar phenomenon was observed for a pure Pd membrane in H2eN2 mixtures under different operating temperatures [43]. The nonhydrogen species of nitrogen and consequently the formation of NHx species on the Pd membrane surface could affect the hydrogen flux [43]. However, hydrogen flux decline could not be associated with the simple competitive adsorption of NH3 with H2 on a PdeAg alloy membrane surface, which was indicated by the authors’ firstprinciples calculations [52]. They considered that the influence of NH3 was more complex, which was still uncertain. It might be related to surface segregation and microstructural changes for H2 dissociation kinetics and incorporation or changes in the Pd77Ag23 membrane. This still requires further investigation of the inherent mechanism. To improve the thermal and chemical stability of metallic membranes, especially chemical stability for resistance to coexisting nonhydrogen species, some effective strategies had been developed [53e57]: (1) precise construction of metallic composite membranes with fine microstructures, including anchorage of the Pd layer to the porous support, and incorporation of appropriate interlayers such as a ceramic or metallic layer to avoid interdiffusion, minimize the shear stress, and act as a protective layer. For example, Yu et al. [54] proposed to prepare a thin and compact NaA zeolite layer as a protective armor on a Pd membrane. The zeolite protective layer could effectively restrain hydrogen permeance reduction and improve the chemical stability of the Pd membrane under the coexisting ethanol/water and tetrahydrofuran/ N2 atmosphere. (2) Surface coatings onto the porous supports to constrain the thermal expansion of metallic membranes. (3) Metallic alloy membranes, including polynary alloys (e.g., Pde CueM alloys), were good approaches to preventing phase transition and also enhancing chemical stability toward their commercial applications.
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Conclusions and future trends The presence of coexisting nonhydrogen species such as CO, NH3, trace levels of H2S, and cokes generated from H2eX mixtures feed due to surface catalytic effects can inhibit hydrogen permeation through metallic membranes. Under operating conditions, this inhibition effect will be irreversible and have a poisoning influence on hydrogen permeation by forming the strongest adsorption species or Pd compounds such as PdSx and PdCx. It is an effective strategy for precise construction of metallic composite membranes with fine microstructures, including anchorage of the Pd layer to the porous support and incorporation of appropriate interlayers or surface coatings. Metallic alloy membranes, including polynary alloys, are also ideal approaches to enhance chemical stability toward their commercial applications in the future.
List of acronyms Ni-ps PSS rWGS WGS
Ni porous support Porous stainless steel Reverse water gas shift Water gas shift
References [1] S.N. Paglieri, J.D. Way, Innovation in palladium membrane research, Separ. Purif. Methods 31 (2002) 1e169. [2] H.Y. Gao, Y.S. Lin, Y.D. Li, B.Q. Zhang, Chemical stability and its improvement of palladium- based metallic membranes, Ind. Eng. Chem. Res. 43 (2004) 6920e6930. [3] R. Dittmeyer, V. Höllein, K. Daub, Membrane reactors for hydrogenation and dehydrogenation processes based on supported palladium, J. Mol. Catal. A Chem. 173 (2001) 135e184. [4] X.L. Zhang, Preparation and Hydrogen Permeation of Pd-Based Membranes and Gas Mixtures Separation Process in the Membrane Reactors (Ph.D. Dissertation). (in Chinese), Chinese Academy of Sciences of China, 2008. [5] A.A. Plazaola, D.A.P. Tanaka, M.V.S. Annaland, F. Gallucci, Recent advances in Pd-based membranes for membrane reactors, Molecules 22 (2017) 51, https://doi.org/10.3390/molecules22010051. [6] D. Alique, D. Martinez-Diaz, R. Sanz, J.A. Calles, Review of supported pdbased membranes preparation by electroless plating for ultra-pure hydrogen production, Membranes 8 (2018) 5, https://doi.org/10.3390/ membranes8010005. [7] F.L. Chen, Y. Kinari, F. Sakamoto, Y. Nakayama, Y. Sakamoto, Hydrogen permeation through palladium-based alloy membranes in mixtures of 10% methane and ethylene in the hydrogen, Int. J. Hydrogen Energy 21 (1996) 555e561.
Chapter 7 Effect of poisoning on metallic membranes
[8] S.H. Jung, K. Kusakabe, S. Morooka, S. Kim, Effects of co-existing hydrocarbons on hydrogen permeation through a palladium membrane, J. Membr. Sci. 170 (2000) 53e60. [9] L. Yang, Z. Zhang, B. Yao, X. Gao, H. Sakai, T. Takahashi, Hydrogen permeance and surface states of Pd-Ag/ceramic composite membranes, AIChE J. 52 (2006) 2783e2791. [10] A. Unemoto, A. Kaimai, K. Sato, T. Otake, K. Yashiro, J. Mizusaki, et al., The effect of co-existing gases from the process of steam reforming reaction on hydrogen permeability of palladium alloy membrane at high temperatures, Int. J. Hydrogen Energy 32 (2007) 2881e2887. [11] M. Amano, C. Nishimura, M. Komaki, Effects of high concentration CO and CO2 on hydrogen permeation through the palladium membrane, Mater. Trans., JIM 31 (1990) 404e408. [12] K. Hou, R. Hughes, The effect of external mass transfer, competitive adsorption and coking on hydrogen permeation through thin Pd/Ag membranes, J. Membr. Sci. 206 (2002) 119e130. [13] F.C. Gielens, R.J.J. Knibbeler, P.F.J. Duysinx, H.D. Tong, M.A.G. Vorstman, J.T.F. Keurentjes, Influence of steam and carbon dioxide on the hydrogen flux through thin Pd/Ag and Pd membranes, J. Membr. Sci. 279 (2006) 176e185. [14] Y. Zhang, J. Gwak, Y. Murakoshi, T. Ikehara, R. Maeda, C. Nishimura, Hydrogen permeation characteristics of thin Pd membrane prepared by microfabrication technology, J. Membr. Sci. 277 (2006) 203e209. [15] F. Gallucci, F. Chiaravalloti, S. Tosti, E. Drioli, A. Basile, The effect of mixture gas on hydrogen permeation through a palladium membrane: experimental study and theoretical approach, Int. J. Hydrogen Energy 32 (2007) 1837e1845. [16] Y. Sakamoto, F.L. Chen, Y. Kinari, F. Sakamoto, Effect of carbon monoxide on hydrogen permeation in some palladium-based alloy membranes, Int. J. Hydrogen Energy 210 (1996) 1017e1024. [17] S. Hara, K. Sakaki, N. Itoh, Decline in hydrogen permeation due to concentration polarization and CO hindrance in a palladium membrane reactor, Ind. Eng. Chem. Res. 38 (1999) 4913e4918. [18] A. Li, W. Liang, R. Hughes, The effect of carbon monoxide and steam on the hydrogen permeability of a Pd/stainless steel membrane, J. Membr. Sci. 165 (2000) 135e141. [19] D. Wang, T.B. Flanagan, K.L. Shanahan, Permeation of hydrogen through pre-oxidized Pd membranes in the presence and absence of CO, J. Alloy. Comp. 372 (2004) 158e164. [20] B. Arstad, H. Venvik, H. Klette, J.C. Walmsley, W.M. Tucho, R. Holmestad, et al., Studies of self-supported 1.6 mm Pd/23 wt.% Ag membranes during and after hydrogen production in a catalytic membrane reactor, Catal. Today 118 (2006) 63e72. [21] H. Li, A. Goldbach, W.Z. Li, H.Y. Xu, PdC formation in ultra-thin Pd membranes during separation of H2/CO mixtures, J. Membr. Sci. 299 (2007) 130e137. [22] R.G. Musket, Effects of contamination on the interaction of hydrogen gas with palladium: a review, J. Less Common. Met. 45 (1976) 173e183. [23] J. Zhang, D. Liu, M. He, H. Xu, W. Li, Experimental and simulation studies on concentration polarization in H2 enrichment by highly permeable and selective Pd membranes, J. Membr. Sci. 274 (2006) 83e91. [24] M. Mori, T. Nakamura, K. Noda, O. Sakai, A. Takahashi, N. Ogawa, et al., Reactor configuration and concentration polarization in methane steam
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Chapter 7 Effect of poisoning on metallic membranes
[41] B.D. Morreale, M.V. Ciocco, B.H. Howard, R.P. Killmeyer, A.V. Cugini, R.M. Enick, Effect of hydrogen-sulfide on the hydrogen permeance of palladium-copper alloys at elevated temperatures, J. Membr. Sci. 241 (2004) 219e224. [42] A. Kulprathipanja, G. Alptekin, J. Falconer, J.D. Way, Effects of water gas shift gases on Pd-Cu alloy membrane surface morphology and separation properties, Ind. Eng. Chem. Res. 43 (2004) 4188e4198. [43] W.P. Wang, X.L. Pan, X.L. Zhang, W.S. Yang, G.X. Xiong, The effect of coexisting nitrogen on hydrogen permeation through thin Pd composite membranes, Separ. Purif. Technol. 54 (2007) 262e271. [44] Y.S. Cheng, M.A. Peña, J.L. Fierro, D.C.W. Hui, K.L. Yeung, Performance of alumina,zeolite, palladium, Pd-Ag alloy membranes for hydrogen separation from towngas mixture, J. Membr. Sci. 204 (2002) 329e340. [45] C.-H. Chen, Y.H. Ma, The effect of H2S on the performance of Pd and Pd/ Au composite membrane, J. Membr. Sci. 362 (2010) 535e544. [46] C.P. O’Brien, B.H. Howard, J.B. Miller, B.D. Morreale, A.J. Gellman, Inhibition of hydrogen transport through Pd and Pd47Cu53 membranes by H2S at 350 C, J. Membr. Sci. 349 (2010) 380e384. [47] T.A. Peters, T. Kaleta, M. Stange, R. Bredesen, Hydrogen transport through a selection of thin Pd-alloy membranes: membrane stability, H2S inhibition, and flux recovery in hydrogen and simulated WGS mixtures, Catal. Today 193 (2012) 8e19. [48] H. Gharibi, M. Saadatinasab, A. Zolfaghari, Hydrogen permeability and sulfur tolerance of a novel dual membrane of PdAg/PdCu layers deposited on porous stainless steel, J. Membr. Sci. 447 (2013) 355e361. [49] Y.H. Ma, C.-H. Chen, J. Catalano, F. Guazzone, E.A. Payzant, Synthesis, annealing, and performances of PdeAu asymmetric composite membranes for hydrogen purification, Ind. Eng. Chem. Res. 52 (2013) 8732e8744. [50] S.J. Khatib, S. Yun, S.T. Oyama, Sulfur resistant Pd and Pd alloy membranes by phosphidation, J. Membr. Sci. 455 (2014) 283e293. [51] C.P. O’Brien, I.C. Lee, CO poisoning and CO hydrogenation on the surface of Pd hydrogen separation membranes, J. Phys. Chem. C 121 (2017) 16864e16871. [52] T.A. Peters, J.M. Polfus, M. Stange, P. Veenstra, A. Nijmeijer, R. Bredesen, H2 flux inhibition and stability of Pd-Ag membranes under exposure to trace amounts of NH3, Fuel Process. Technol. 152 (2016) 259e265. [53] H. Li, A. Caravella, H.Y. Xu, Recent progress in Pd-based composite membranes, J. Mater. Chem. A 4 (2016) 14069e14094. [54] J. Yu, J. Zhang, C. Bao, Z. Zhang, H. Li, H. Xu, Controllable growth of defectfree zeolite protective layer on the surface of Pd membrane for chemical stability enhancement, Microporous Mesoporous Mater. 244 (2017) 119e126. [55] S. Nayebossadri, J.D. Speight, D. Book, Pd-Cu-M (M ¼ Y, Ti, Zr, V, Nb, and Ni) alloys for the hydrogen separation membrane, ACS Appl. Mater. Interfaces 9 (2017) 2650e2661. [56] J. Tosques, B.H. Guerreiro, M.H. Martin, L. Roue, D. Guay, Hydrogen solubility of bcc PdCu and PdCuAg alloys prepared by mechanical alloying, J. Alloy. Comp. 698 (2017) 725e730. [57] N.A. Al-Mufachi, R. Steinberger-Wilckens, X-ray diffraction study on the effects of hydrogen on Pd60Cu40 wt% foil membranes, J. Membr. Sci. 545 (2018) 266e274.
187
Introduction In the last few decades, dense metallic membranes such as palladium-based membranes have been successfully investigated for membrane reactors using water gas shift (WGS), steam reforming of hydrocarbons, and ammonia decomposition reactions to generate ultrahigh purity hydrogen for proton exchange membrane fuel cell systems and to increase conversion of the feed such as CH4 and CO [1e6]. There are different levels of various impurities such as CH4, steam, CO and CO2, NH3, H2S, etc. along with hydrogen from the effluent of the catalytic membrane reformers. However, these nonhydrogen species would have possible deterioration and poisoning effects on hydrogen permeation through Pdbased membranes in membrane reformers [1,4e6]. What about the poisoning effect on hydrogen permeation through metallic membranes? In this chapter, we will critically review the experimental and theoretical studies of nonhydrogen species for Pd-based membranes, focusing particularly on the effect of potential gaseous contaminants. The poisoning effect of the main nonhydrogen species such as H2S, CO, and NH3 on hydrogen permeation are subsequently discussed in detail, which are the main obstacles blocking membrane commercial application. Finally, we will give an overview of the relevant development strategies for improved chemical stability of Pd-based membranes.
The influence of nonhydrogen species Generally, the nonhydrogen species always coexist with hydrogen in catalytic Pd-based membrane reactors for the generation of
Current Trends and Future Developments on (Bio-) Membranes. https://doi.org/10.1016/B978-0-12-818332-8.00007-7 © 2020 Elsevier Inc. All rights reserved.
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hydrogen. However, the nonhydrogen components will to some extent affect the hydrogen permeation behavior and hydrogen permeation performance of Pd-based membranes, and will even destroy the structure of metallic membranes by forming Pd compounds such as palladium sulfide (PdSx), palladium carbide (PdCx), and other irreversible toxic effects on membrane surfaces. Table 7.1 summarizes the potential effect of the coexisting gases of H2ex% X mixtures (X ¼ CH4, CO2, CO, and steam) on hydrogen permeation through Pd-based membranes [7e21]. Although there are differences in membrane supports, thicknesses, and even preparation methods, the potential influence of nonhydrogen components on the H2 permeation behavior and permeation performance through these Pd-based membranes was classified in five issues: dilution effect, concentration polarization (external diffusion resistance), competitive adsorption, irreversible poisoning effect, and surface catalytic effect.
Dilution effect In H2eX mixtures, the addition of coexisting nonhydrogen components will dilute the hydrogen concentration and then decrease the hydrogen partial pressure in the feed side, thus decreasing the driving force of hydrogen transport through Pd-based membranes. As a result, the hydrogen permeation flux or hydrogen permeability of Pd-based membranes seems to be reduced by the dilution effect, while not poisoning the membrane surface [9,15,18,20]. Li et al. [18] investigated the influence of N2 gas with different concentrations on hydrogen permeation through a 10 mm-thick Pd/PSS membrane at 380 C. Hydrogen permeability in H2eN2 mixtures was obviously lower than that of pure H2 atmospheres. Nitrogen was regarded as an inert gas, with which it was difficult to perform chemical adsorption on the Pd membrane surface. Compared to pure hydrogen permeation, the presence of nitrogen was diluted by hydrogen concentrations, reducing the hydrogen partial pressure on the feed side. Thus it decreased the driving force of hydrogen transport and decreased hydrogen permeation flux. Their experimental results were in good agreement with simulated assumption. Gallucci et al. [15] also studied hydrogen permeation with different concentrations of nonhydrogen components through a 60 mm-thick foil Pd membrane at 250e350 C. It was also found that N2, Ar, and CO2 had only a dilution effect on hydrogen permeation, meanwhile CO had serious inhibition for hydrogen permeation performance through the Pd membranes.
Table 7.1 The potential influence of nonhydrogen species of H2ex% X mixtures on hydrogen permeation through Pd-based membranes Membrane Shape t (mm) H2ex% X mixtures T (o C)
Influence
Reason
References
Pd Pd PdAg/Al2O3 PdAg Pd
Disk Disk Tube Disk Disk
700 50 20 20 700
10% CH4 50% CH4 1%, 10%, 25% CH4 0%e6.8% CH4 50% CO2
Tube
5e6
1%e8% CO2
Pd, PdAg PdAg/Al2O3 Pd/Ni-ps PdAg Pd Pd
Disk Tube Disk Disk Tube Disk
0.5e1 20 2.5 20 60 700
20% CO2 1%, 10%, 25% CO2 28.3% CO2 0%e27.9% CO2 0%e70% CO2 10%, 50% CO
Pd PdRuIn Pd/PSS PdAg/Al2O3 Pd PdAg/Al2O3 PdAg PdAg Pd Pd/Al2O3 Pd/PSS PdAg/Al2O3 Pd PdAg PdAg
Disk Tube Tube Tube NA Tube Disk Disk Tube Tube Tube Tube Disk Disk Disk
700 200 10 5e6 100 20 1.6 20 60 2e4 10 5e6 0.9 1.6 20
10% CO 33.2% CO 1.7%e17% CO 1%e7% CO 0%e6% CO 1%, 10%, 25% CO 20% CO 0%e4.6% CO 0%e70% CO 20%e33.3% CO 1%e13% H2O 1%e7% H2O 20% H2O 20% H2O 0%e9.5% H2O
Comp. adsorp. / Dilution effect / Comp. adsorp. / Comp. adsorp. / Surface effect Dilution effect NA / Dilution effect Comp. adsorp. / Comp. adsorp. Comp. adsorp. Comp. adsorp. Comp. adsorp. Comp. adsorp. Comp. adsorp. Comp. adsorp. / Dilution effect þ surface effect Comp. adsorp. þC deposition Comp. adsorp. Comp. adsorp. Comp. adsorp. Dilution effect /
[7] [8] [9] [10] [11]
PdAg/Al2O3
Reduced No Reduced No Reduced No Reduced No Reduced Reduced Reduced No Reduced Reduced No Reduced Reduced >5%, reduced Reduced Reduced Reduced Reduced No Reduced Reduced Reduced Reduced Reduced Reduced No
250e450 350e600 300e500 500e600 150e200 200e450 275 >325 350e450 300e500 400 500e600 250 150e250 250e450 250e450 220e280 380 275e450 150e200 300e500 255 500e600 250 320e500 380 275e350 350e450 255 500e600
Comp. adsorp., Competitive adsorption; Ni-ps, Ni porous support; PSS, porous stainless steel; t, thickness of membrane.
[12] [13] [9] [14] [10] [15] [11] [16] [17] [18] [12] [19] [9] [20] [10] [15] [21] [18] [12] [13] [20] [10]
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Moreover, the hydrogen permeation performance was restored when the H2eN2 mixtures switched to pure hydrogen treatment [18]. The dilution effect was just a reduction of the apparent permeability, not a change in hydrogen permeation behavior, including the hydrogen permeation mechanism (see also the discussion in Chapter 8).
Concentration polarization and external diffusion resistance During the hydrogen separation process with Pd-based membrane reactors in coating hydrogen mixtures, the relative concentration of hydrogen in the feed side was decreasing, meanwhile that of nonhydrogen species was increasing due to most of the hydrogen in the mixture feed continuously permeating through the Pd membrane. Therefore there was an accumulation of nonpermeating gases in the boundary layer adjacent to the membrane surface [22]. This would cause a concentration gradient to build up in the boundary layer and prevent hydrogen dissociative adsorption on the membrane surface. Thus the hydrogen concentration adjacent to the membrane surface was lower than that of bulk gas. This would in turn result in a significant decrease in hydrogen partial pressure difference through the Pd membrane and thus decrease hydrogen-permeated flux. This blockage effect on hydrogen permeation was well known as concentration polarization or external mass transfer resistance [12,17,23,24]. Hara et al. [17] reported hydrogen permeation performance with a 0.2 mm-thick Pd91Ru6In3 alloy membrane within the temperature range of 220e300 C and a total pressure of 2.0 bar. The apparent hydrogen permeability in the H2-Ar (2/1, mol%) mixtures was far lower than that of pure hydrogen. Due to the concentration polarization effect by the existence of Ar radial diffusion in the mixtures, the H2 concentration on the membrane surface was obviously lower than that in the bulk phase, which would greatly reduce the hydrogen permeability of the membrane. Two thin (2 mm) Pd membranes with different hydrogen permeance ranging widely from 13.8 to 44.3 m3 m2 h1 bar1 were evaluated for hydrogen permeation behavior at 350e500 C and under pressures up to 5 bar. Zhang et al. [23] found that the existence of concentration polarization caused a loss of hydrogen performance and hydrogen yield in an H2e25% N2 mixture system. The operation parameters such as pressure, temperature, feed gas flow rate, and membrane permeability played a role in the influence of the concentration polarization effect. Concentration
Chapter 7 Effect of poisoning on metallic membranes
polarization resulted in a loss in effective surface area of the Pd membrane and thus a decrease in hydrogen separation efficiency of the NH3 cracker generation system. Hou et al. [12] and Mori et al. [24] separated hydrogen with PdeAg alloy composite membranes in H2eN2 mixtures. They also found that N2 gas had a severe concentration polarization effect on hydrogen permeation under operation conditions. In addition, for a Pd-based membrane with a certain surface area, if the amount of hydrogen in the mixture was not sufficient to completely occupy the entire Pd membrane surface for H2 adsorption and dissociation, it would inevitably increase the external diffusion resistance, which was similar to the concentration polarization effect and also reduced the hydrogen permeability of this membrane [24]. To weaken or eliminate the negative influence of concentration polarization (or external diffusion resistance) through Pdbased membranes within a limited effective surface area, increasing feed gas flow rate and total operation pressure may be the effective approaches under operating conditions [23]. We also investigated the concentration polarization effect of coexisting gases on hydrogen permeation through a thin Pd/ Al2O3 tubular membrane by means of comparisons between experimental and simulation data at temperatures of 350e500 C [4]. Fig. 7.1 shows the hydrogen flux as a function of feed flow rate in a pure hydrogen atmosphere and H2e10% X (X ¼ CH4, CO2, and steam) mixtures at 500 C. In the case of pure hydrogen atmosphere, as seen in Fig. 7.1A, the hydrogen flux increased initially with the feed flow rate and then approached a constant value when the feed flow rate of pure hydrogen was above 250 mL min1. Similarly, hydrogen flux also increased initially with the feed flow rate of H2eX mixtures and then increased steadily at the feed flow rate above 375 mL min1 in the mixtures’ feed. This might be related to the influence of external mass transfer resistance on the Pd membrane. A simple isothermic plug flow model based on Wang’s work [25] was developed to evaluate hydrogen permeation behavior of a Pd membrane in the foregoing H2eX mixtures’ permeation experiments. It was assumed that the nonhydrogen species (X) was inert gas with no chemisorption on the Pd membrane surface and did not diffuse through the Pd composite membrane [4]. Fig. 7.2 demonstrates the simulated profiles of hydrogen in the shell side along the dimensionless length of the Pd tubular membrane at 500 C. The value of n(Hshell )/n(Hin 2 2 ) is the ratio of hydrogen molar amount in the shell side to that in the inlet of the feed, which is the evaluation factor for the degree of external
173
174
(A)
Chapter 7 Effect of poisoning on metallic membranes
30
(B)
2.0
30
H flux
pure H
H -10% X mixture, P
... P
= 2.0 bar
1.5
1.0
2
H2
4
H2
P (bar)
20
H2
10
O
2
20
10
0.5
0
0
100
200
300
400
500
600
0.0 700
0
Feed flow rate (ml/min)
0
100
200
300
400
500
600
700
Feed flow rate (ml/min)
Figure 7.1 The influence of feed flow rate on the hydrogen permeated flux through a thin Pd/Al2O3 composite membrane in the case of (A) pure hydrogen and (B) H2e10% X mixtures at 500 C. Unpublished results from X.L. Zhang, report Preparation and Hydrogen Permeation of Pd-Based Membranes and Gas Mixtures Separation Process in the Membrane Reactors (Ph. D. Dissertation). (in Chinese), Chinese Academy of Sciences of China, 2008
(A)
(B)
Figure 7.2 The simulated hydrogen profiles in the shell side along the dimensionless length of the Pd tubular membrane in the case of (A) pure hydrogen and (B) H2e10% X mixtures fed with different feed flow rates at 500 C. Unpublished results from X.L. Zhang, report Preparation and Hydrogen Permeation of Pd-Based Membranes and Gas Mixtures Separation Process in the Membrane Reactors, (Ph.D. Dissertation). (in Chinese), Chinese Academy of Sciences of China, 2008
mass transfer resistance on hydrogen permeation through the Pd membrane. If the ratio profile is parallel to the X axes of the dimensionless length or very close to zero at the tubular membrane outlet, it means that nearly all the hydrogen in the feed diffuses through the Pd membrane to the permeate side and there are still some vacant sites for hydrogen dissociative adsorption
Chapter 7 Effect of poisoning on metallic membranes
on the membrane surface at the corresponding operating conditions. Consequently, the accumulation of nonhydrogen species on the Pd membrane surface would decrease the hydrogen permeated flux. Compared with the pure hydrogen profile (Fig. 7.2A), as shown in Fig. 7.2B, the hydrogen profile was parallel to the X axes of the dimensionless length and very close to zero at the outlet, when the feed flow rate was below 375 mL min1 for H2e10% X mixtures feed. These simulated results were consistent with the foregoing experimental results (Fig. 7.1). It was confirmed that the external mass transfer resistance occurred only in the case of lower feed flow rate, which would influence hydrogen permeation behavior through the thin Pd tubular composite membrane. Therefore increasing feed flow rate will eliminate the negative effect of concentration polarization and external diffusion resistance on hydrogen permeation through Pd-based membranes.
Competitive adsorption Further studies have demonstrated that the strong adsorption phenomena would occur when the coexisting nonhydrogen species such as CO, steam, and NH3 contacted with the metallic membrane surface. This would decrease the amount of adsorbed hydrogen by occupying the coverage sites of hydrogen dissociative adsorption onto the membrane surface. Hence the competitive adsorption of the nonhydrogen species would decrease the effective area for the hydrogen dissociation process, thereby greatly decreasing the hydrogen permeation flux. Moreover, this inhibition effect of competitive adsorption is reversible on hydrogen permeation, and hydrogen permeability of metallic membranes can be restored to pristine values after stopping nonhydrogen species addition and switching to pure hydrogen treatment [7,11,12,16e21,26e30]. Chen et al. [7] studied the hydrogen permeation behavior of H2e10% CH4 and H2e10% C2H4 binary mixtures in Pd membrane and PdeAg alloy membrane reactors at 250e450 C. For the H2eCH4 mixtures system, the decreasing trend of hydrogen permeability through the Pd membrane decreased with increasing temperatures, while the declining trend increased with increasing temperatures through PdeAg alloy membranes. However, for H2eC2H4 mixtures, the hydrogen declining tendency was similar through both Pd membrane and PdeAg alloy membrane, which decreased with increasing temperatures. This was attributed to the competitive adsorption of CH4 and C2H4 with hydrogen on the membrane surface, which reduced the
175
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Chapter 7 Effect of poisoning on metallic membranes
effective active sites of hydrogen adsorption and dissociation processes, thus reducing the effective hydrogen permeation area through the metallic membrane. Consequently, the competitive adsorption of the nonhydrogen species (CH4 and C2H4) decreased the hydrogen permeability [7]. Li et al. [18] also studied the influence of CO and steam with different concentrations on hydrogen permeation through a 10 mm-thick Pd/PSS membrane at 380 C (see also Table 7.1). As shown in Fig. 7.3, the influence of CO on hydrogen permeation was as low as that of N2 when the CO concentration was lower than 5%. However, the inhibition effect on hydrogen permeation increased when CO concentration in H2eCO mixtures was over 5%. The adsorption capacity of CO on the metallic membrane surface is much higher than that of H2 [16,19,26]. Thus there is strong competitive adsorption between CO and H2 on the membrane surface. This capacity of competitive adsorption will increase with increasing CO concentration in H2eCO mixtures. Therefore CO occupied the active site of hydrogen adsorption and dissociation, reducing the effective activity of hydrogen permeation. It would also reduce H2 permeability and change the ratedetermining step for hydrogen permeation from the solid-phase diffusion step to the surface process through the thin dense metallic composite membranes.
Figure 7.3 The hydrogen permeation rate as a function of feed mixture flow with various mixtures. Reprinted from A. Li, W. Liang, R. Hughes, The effect of carbon monoxide and steam on the hydrogen permeability of a Pd/stainless steel membrane, J. Membr. Sci. 165 (2000) 135e141, copyright with permission from Elsevier.
Chapter 7 Effect of poisoning on metallic membranes
Additionally, Li et al. [18] compared the competitive adsorption effect of both water vapor (steam) and CO on hydrogen permeation. As described in Fig. 7.3, the inhibition effect of steam was obviously more serious than that of CO on the Pd membrane surface. Hou et al. [12] examined the influence of CO, CO2, and steam on hydrogen permeation through a 5e6 mm-thick PdeAg alloy membrane at a temperature range of 275e450 C and a pressure of 2 bar. Their experimental results were similar to that of Li et al. [18], which established the inhibited order of competitive adsorption as steam > CO > CO2. The nonhydrogen species of CO2 had the lowest influence on hydrogen permeation, and had no inhibition effect even when the feed temperature was over 325 C. On the contrary, hydrogen permeability would decrease by about 60% even if the steam concentration was only 1% in H2e steam binary mixtures. This was ascribed to the strong competitive adsorption of steam on the membrane surface [12]. Furthermore, unsaturated alkanes such as C2H4 and C3H6 from the hydrocarbon dehydrogenation reactions and NH3 in the catalytic membrane reactors also had a negative effect of competitive adsorption on hydrogen permeation [27e30]. These small molecules would deactivate the adsorption and dissociation activity of hydrogen molecules on the surface of the membrane, thus significantly weakening hydrogen permeability through the metallic membrane and then leading to the degradation of membrane separation efficiency and catalytic performance in the Pdbased membrane reactors.
Irreversible poisoning effect Contaminants such as cokes (carbon deposition), sulfide, chloride, and other species produced from Pd-based membrane catalytic reactions would perform irreversible chemical adsorption on the membrane surface and then permanently poison the Pd surface by forming Pd compounds such as PdSx and PdCx. As a result, this would decrease hydrogen permeation performance and even destroy the phase structure of Pd-based membranes [2,7,21,24,25,31]. It should be noted that the poisoning effect is permanently irreversible, and should be avoided as far as possible in catalytic membrane reactors. At high temperature, hydrocarbons always contact with Pdbased membranes for a long time and are easily adsorbed on the membrane surface to form cokes via dehydrogenation reaction. Cokes formation not only decreases the effective separation area of the hydrogen permeable membrane, which would affect the adsorption, dissociation, and desorption of hydrogen on the
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membrane surface, but also gradually penetrate into the Pd phase lattice to form PdCx alloy, which would seriously affect hydrogen diffusion in the Pd lattice and even destroy the phase structure of the Pd membrane [21,32e37]. Raich et al. [32] studied the isobutane dehydrogenation reaction in a Pd membrane reactor. The membrane surface morphology changed from scanning electron microscope images by comparing the reaction before and after. Carbon species was present on the surface of the Pd membrane determined by auger spectrum. Zadai [33] and Roshan [34] found that C3H8 would adsorb and dissociate on the surface of the Pd membrane, resulting in CeC bonds cracking to deposit carbon (cokes). Carbon atoms would be dissolved in the Pd lattice, resulting in a reduction of hydrogen permeability of the Pd membrane. Li et al. [21] also found that, for H2eCO mixtures, the CO disproportionate reaction (2CO ¼ C þ CO2) would occur on the Pd surface above 450 C and thus form PdC species. Ziemecki et al. [35,36] proposed the formation and diffusion mechanism of carbon species on the surface of a Pd membrane: the coexisting gases containing carbon such as CO, C2H2, C2H4, etc. first occurred by chemical adsorption on the Pd membrane surface within medium temperature ranges, and then generated a decomposition reaction to produce carbon species on the membrane surface. These carbon species would penetrate into the metal body phase and dissolve in the metal lattice with PdC0.15 alloy mode, thus eventually resulting in the loss of hydrogen permeability for the Pd membrane. The diffusion of these carbon species in the Pd lattice could lead to hydrogen embrittlement and even membrane deformation [37]. Sulfur, which is found in many chemical materials and petroleum products, is one of the most fatal contaminants for Pd membranes [38e41]. Specifically, H2S has an irreversible poisoning effect on the Pd membrane to form PdS. This would reduce hydrogen permeability by 1% through the Pd membrane being treated with 1 ppm H2S [39]. Sulfides perform the strongest adsorption on metallic surfaces to decrease the electron density of Pd Fermi energy, resulting in a decrease of 4d band electrons [2]. So, PdSx compounds were formed on the surface of the Pd membrane, which would accelerate the complete loss of hydrogen permeation properties through pure Pd membranes. Dense Pt and Ru membranes and PdeCu and PdeAu alloy membranes can resist the poisoning influence of H2S with a certain concentration and keep hydrogen permeation performance and behaviors for these metallic membranes [2,38e41].
Chapter 7 Effect of poisoning on metallic membranes
Actually, the coexisting gases of CO, CO2, and steam also had a negative inhibition effect on hydrogen permeation, even showing the poisoning effect on the Pd membrane surface under certain temperature and concentration conditions.
Surface catalytic effect On the metallic surface, hydrogen molecules have a strong ability for active dissociation adsorption. Thus during the hydrogen permeable separation processes, the nonhydrogen species and hydrogen molecules could perform a potential surface catalytic reaction with Pd and Pd alloys as catalysts under certain temperature and pressure conditions [8,11,13,21,42e44]. For example, decomposition reaction under high temperature, WGS reaction, reverse WGS reaction (rWGS), and methanation reaction (Eqs. 7.1e7.3) would occur under appropriate operating conditions. CO2 þ H2 ¼ CO þ H2 O
(7.1)
CO2 þ 4 H2 ¼ CH4 þ 2 H2 O
(7.2)
CO þ 3H2 ¼ CH4 þ H2 O
(7.3)
The surface catalytic effect exacerbates the complexity of H2eX mixtures through metallic membranes. The nonhydrogen species (X) and the catalytically generated species would change the surface composition and even body structure of metallic membranes due to the interaction among these gases. Thus it would partly decrease the capacities of hydrogen permeation through metallic membranes [8,11,13,21,42e44]. Jung et al. [8] studied the separation behavior of H2e50% C3H6 mixtures through a 50 mm Pd membrane at 350e600 C. Hydrogen permeability was reduced with operating time and this tendency became more seriously with increasing temperature. It was found that carbon compound was deposited on the membrane surface, which might be due to the C3H6 cracking reaction. The cracking rate of C3H6 increased with increasing temperatures. Thus hydrogen permeability decreased with the surface catalytic effect via the formation of PdeCeH species on the membrane surface. Moreover, they also investigated the influence of steam addition for H2eC3H6 mixtures at 600 C [8]. This demonstrated double contradictory influences of steam on hydrogen permeation: (1) the competitive adsorption of steam would reduce the dissociation adsorption activity of hydrogen molecules, thus reducing hydrogen permeability, and (2) steam would decarbonize with
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cokes on the membrane surface, thus removing carbon deposition and improving H2 permeability to some extent [13]. Cheng et al. [44] investigated separation performance through Pd and PdeAg alloy membrane reactors using polynary mixtures (town gas: 49% H2, 28.5% CH4, 19.5% CO2, and 3% CO). CO and H2O were formed via rWGS reaction by CO2 and H2 at 450 C, which would reduce H2 partial pressure in the mixtures, thus decreasing hydrogen permeability. Similar results had been observed by Kulprathipanja et al. [42]. WGS mixtures also revealed the surface catalytic effect of the hydrogen permeation properties for Pd and PdeCu alloy membranes. Wang et al. [43] examined the surface effect with H2e50% N2 mixtures through a Pd membrane for long-term operation over 3000 h. It was an interesting phenomenon. In a relatively short period, the coexisting N2 had no obvious influence on hydrogen performance. However, hydrogen permeance would decrease in comparison with the original value when the Pd membrane was exposed to nitrogen for a long time within the temperature range of 400e450 C. They regarded that the blocking effect was due to the formed nitrogen-containing species (NHx, see also Fig. 7.4), which was responsible for membrane deactivation in the suggested deactivation mechanism. Fortunately, the deactivated Pd membrane could be regenerated via pure hydrogen treatment at a high temperature of 500 C for a period of time. The NHx species could react to form ammonia and desorb from the membrane surface to release the blocked effective area.
Figure 7.4 Modeling of formation and occupation of NHx (x ¼ 0e3) on a Pd surface causing the deactivation of Pd membranes after pure nitrogen treatment or long-term exposure to the equimolar mixture of H2/N2 under certain conditions. Reprinted from W.P. Wang, X.L. Pan, X.L. Zhang, W.S. Yang, G.X. Xiong, The effect of co-existing nitrogen on hydrogen permeation through thin Pd composite membranes, Separ. Purif. Technol. 54 (2007) 262e271, copyright with permission from Elsevier.
Chapter 7 Effect of poisoning on metallic membranes
Unfortunately, limited attention was given to the surface catalytic effect on hydrogen permeation through Pd and Pd alloy membranes [4,21]. Moreover, few reasonable theory models considering the influence of dilution effect and external mass transfer resistance were developed to evaluate the possible influence of coexisting gases on hydrogen permeation at various operating conditions. This should create a new mathematical model based on the foregoing scientific cognition, i.e., simultaneously considering the influence of dilution effect, concentration polarization (external diffusion resistance), competitive adsorption, and hydrogen permeation mechanism to investigate the possible degree of poisoning effect through metallic membranes under operating conditions.
The poisoning effect of typical gases and development strategies This section does not intend to provide an exhaustive review, but rather provide recent innovations about typical gases such as H2S, CO, and NH3 in the literature up to 2018. H2S exposure on the pure Pd membrane surface resulted in a reduction of hydrogen permeance due to (1) surface site blocking by the dissociative adsorption of H2S, and (2) bulk sulfidation of Pd with the formation of Pd4S [45e50]. Consequently, a pure Pd membrane lattice structure would be destroyed resulting in failure to separate hydrogen from the H2eX mixtures. In contrast, Pd alloy membranes (e.g., PdeCu and PdeAu) were observed to have good sulfur resistance [45e50]. Chen et al. [45] reported that a PdeAu alloy membrane exhibited resistance to H2S exposure without significant hydrogen permeance decline and structural change. However, as shown in Fig. 7.5, after 1000 ppm H2S exposure on the Pd47Cu53 (mol%) foil membrane at 350 C, the hydrogen flux rapidly reduced to an undetectable level within 5 min [46]. This was attributed to a thin PdeCueS terminal layer that rapidly formed at the top surface of the alloy foil, which is either inactive for hydrogen dissociation or impermeable to hydrogen atoms. So, the body-centered cubic-PdeCu alloy membranes are easily deactivated in the presence of H2S. Similarly, hydrogen permeation through a Pd-based membrane was inhibited under exposure to trace amounts of CO and NH3 [51,52]. Peters et al. [52] investigated the hydrogen flux inhibition phenomenon through a 10 mm-thick Pd77Ag23 (face-centered cubic) membrane under exposure to different NH3 concentrations (10e500 ppm) at 300e450 C. As shown in Fig. 7.6, no inhibition
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Figure 7.5 Hydrogen flux through 25 mm Pd (solid line) and Pd47Cu53 (wavy dashed line) membranes (T ¼ 350 C, P ¼ 310 kPa). A 90% H2e10% He mixture was exposed to the membrane surface during the first portion of the test and then 1000 ppm H2S/10% He/balance H2 was fed to the membrane surface at 6 h. Reprinted from C.P. OBrien, B.H. Howard, J.B. Miller, B.D. Morreale, A.J. Gellman, Inhibition of hydrogen transport through Pd and Pd47Cu53 membranes by H2S at 350 C, J. Membr. Sci. 349 (2010) 380e384, copyright with permission from Elsevier.
Figure 7.6 Hydrogen flux as a function of process time during the stepwise rampdown of the operating temperature from 450 to 300 C in the absence and presence of 200 ppm NH3 through a Pd77Ag23 membrane (feed: 90% H2 in N2, and Ar sweep, feed, and permeate side at atmospheric pressure). Reprinted from T.A. Peters, J.M. Polfus, M. Stange, P. Veenstra, A. Nijmeijer, R. Bredesen, H2 flux inhibition and stability of Pd-Ag membranes under exposure to trace amounts of NH3, Fuel Process. Technol. 152 (2016) 259e265, copyright with permission from Elsevier.
Chapter 7 Effect of poisoning on metallic membranes
of hydrogen flux was found in the presence of 200e500 ppm NH3 at 450 C. Meanwhile, gradual hydrogen flux decline and loss of membrane stability were observed with decreasing operating temperatures from 450 to 300 C even in the presence of 200 ppm NH3. About 36% hydrogen flux was reduced at 300 C compared to pure hydrogen permeated flux without NH3 under the same operating conditions. However, as seen in Fig. 7.6, hydrogen flux could be quickly recovered to the pristine value when the membrane was treated at 450 C after NH3 exposure. A similar phenomenon was observed for a pure Pd membrane in H2eN2 mixtures under different operating temperatures [43]. The nonhydrogen species of nitrogen and consequently the formation of NHx species on the Pd membrane surface could affect the hydrogen flux [43]. However, hydrogen flux decline could not be associated with the simple competitive adsorption of NH3 with H2 on a PdeAg alloy membrane surface, which was indicated by the authors’ firstprinciples calculations [52]. They considered that the influence of NH3 was more complex, which was still uncertain. It might be related to surface segregation and microstructural changes for H2 dissociation kinetics and incorporation or changes in the Pd77Ag23 membrane. This still requires further investigation of the inherent mechanism. To improve the thermal and chemical stability of metallic membranes, especially chemical stability for resistance to coexisting nonhydrogen species, some effective strategies had been developed [53e57]: (1) precise construction of metallic composite membranes with fine microstructures, including anchorage of the Pd layer to the porous support, and incorporation of appropriate interlayers such as a ceramic or metallic layer to avoid interdiffusion, minimize the shear stress, and act as a protective layer. For example, Yu et al. [54] proposed to prepare a thin and compact NaA zeolite layer as a protective armor on a Pd membrane. The zeolite protective layer could effectively restrain hydrogen permeance reduction and improve the chemical stability of the Pd membrane under the coexisting ethanol/water and tetrahydrofuran/ N2 atmosphere. (2) Surface coatings onto the porous supports to constrain the thermal expansion of metallic membranes. (3) Metallic alloy membranes, including polynary alloys (e.g., Pde CueM alloys), were good approaches to preventing phase transition and also enhancing chemical stability toward their commercial applications.
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Conclusions and future trends The presence of coexisting nonhydrogen species such as CO, NH3, trace levels of H2S, and cokes generated from H2eX mixtures feed due to surface catalytic effects can inhibit hydrogen permeation through metallic membranes. Under operating conditions, this inhibition effect will be irreversible and have a poisoning influence on hydrogen permeation by forming the strongest adsorption species or Pd compounds such as PdSx and PdCx. It is an effective strategy for precise construction of metallic composite membranes with fine microstructures, including anchorage of the Pd layer to the porous support and incorporation of appropriate interlayers or surface coatings. Metallic alloy membranes, including polynary alloys, are also ideal approaches to enhance chemical stability toward their commercial applications in the future.
List of acronyms Ni-ps PSS rWGS WGS
Ni porous support Porous stainless steel Reverse water gas shift Water gas shift
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