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Experimental Study of the Methane Steam Reforming Reaction in a Dense Pd/Ag Membrane Reactor Fausto Gallucci, Luca Paturzo, Angelo Fama` , and Angelo Basile* Institute on Membrane Technology, ITM-CNR, c/o University of Calabria, Via P. Bucci, cubo 17/C, I-87030 Rende (CS), Italy
The reaction of methane steam reforming was carried out in both a traditional reactor (TR) and a membrane reactor (MR). The membrane consisted of a 50-µm-thick pinhole-free palladium/ silver alloy. To investigate the enhancement of the methane conversion in the steam reforming reaction in an MR, the effects of various kinds of sweep gas (hydrogen, nitrogen, carbon monoxide, air, oxygen, and steam), as well as no sweep gas, were analyzed as a function of temperature at different molar flow rates of the sweep gas used. The dependence of the methane conversion on increasing H2O/CH4 feed ratio was also studied. Experimental results in terms of methane conversion for the MR were compared with those obtained using a TR and literature data. 1. Introduction In conventional technology, the steam reforming of methane, an important industrial process, is carried out using multitubular fixed-bed reactors to produce syngas (a mixture of hydrogen and carbon monoxide). Because of the endothermicity of the reaction, high temperatures favor the process. In fact, 80% conversion of methane at temperatures up to 850 °C is commonly achieved with H2O/CH4 feed ratios in the range 3-4. To improve the energy efficiency of the process, high pressures (ranging from 1 to 4 MPa) are also used. However, as reported in the recent literature,1-7 it is possible to mitigate some of the drastic operating conditions by using membrane reactors (MRs). Considering, for example, the pressure as an operating parameter, it can be observed in Table 1 that, apart from the work of Jarosch et al.,7 the pressure used in MRs ranges from 1 to 1.36 bar. Other operating parameters (e.g., membrane thickness) characteristic of the steam reforming reaction carried out in MRs are also reported in this table. The same consideration could be made concerning temperature. From recent literature data, the experimental results in terms of methane conversion versus temperature are summarized in Figure 1, where the methane conversion is plotted as a function of the reaction temperature. For the purpose of comparison, data from both traditional reactors (TRs) and MRs are reported. It is clear that, at each temperature in the range of 300-600 °C considered, MRs give higher methane conversions than TRs. Reported in the same figure are two thermodynamic equilibrium curves corresponding to p ) 1.1 bar and p ) 1.36 bar. In this way, it is also easy to observe that the methane conversions for TRs do not exceed the thermodynamic equilibrium conversion value. In contrast, MRs exhibit methane conversion values higher than the corresponding thermodynamic equilibrium. However, this fact should not be considered as a rigid rule. In fact, depending on the * To whom correspondence should be addressed. Tel.: (+39) 0984 492011. Fax: (+39) 0984 402103. E-mail: a.basile@ itm.cnr.it.
Figure 1. Methane conversion versus temperature: methane steam reforming in the literature.
particular experimental conditions, even if hydrogen removal from the reacting zone is perfect (i.e., hydrogen membrane selectivity is infinite), the methane conversion for MRs might also be lower than that for TRs (see, for example, Figure 7 below and its comments). Nevertheless, despite these extensive studies on the methane steam reforming reaction using MRs, to better understand the influence of some parameters on methane conversion, a deeper investigation is needed. In this work, one of the main variables, from an experimental point of view, influencing the performance of an MR was investigated. In particular, we focused our attention on the influence on the methane conversion of both the sweep gas flow rate and different kinds of sweep gas, at various temperatures, used in the shell to extract hydrogen from the outer surface of the tubular membrane.
10.1021/ie030485a CCC: $27.50 © 2004 American Chemical Society Published on Web 01/16/2004
Ind. Eng. Chem. Res., Vol. 43, No. 4, 2004 929 Table 1. Methane Steam Reforming in Membrane Reactors: Operating Conditions of Some Papers Reported in the Literature catalyst author
p (bar)
H2O/CH4
Kikuchi et al. 1 Chai et al. 2 Madia et al. 3 Barbieri et al. 4 Shu et al. 5 Nam et al. 6 Jarosch et al. 7
1 1.15-1.2 1.01 1.01 1.36 1 22
3 3 3 4 3 4 2.2
membrane
type
weight (g)
type
thickness (µm)
Ni-based Ru/Al2O3 Ni/Al2O3 Ni/Al2O3 Ni/Al2O3 Ni Ni/Al2O3
6.5 17.3 7 11 3.1 -
Pd composite Ru/Al2O3 Pd/Ag-Al2O3 Pd/Al2O3 Pd/Ag-porous SS Pd/Ru Pd/porous Inconel
13 15 7.5 0.1 10.3 100 30-100
Figure 2. Sievert plot for the dense Pd/Ag membrane, T ) 300 °C.
2. Description of the Process 2.1. Experiment. 2.1.1. Traditional and Membrane Reactors. The TR used consisted of a stainless steel tube with a length of 25 cm and an i.d. of 0.67 cm; the reaction zone was 15 cm, and the volume of the reactor suitable for packing the catalyst was 5.3 cm3. The MR consisted of a tubular stainless steel module with a length of 20 cm and an i.d. of 2 cm that contained a pinhole-free palladium/silver membrane. This membrane was permeable only to hydrogen and had a thickness of 50 µm, and o.d. of 1.02 cm, and a length of 15 cm, and it contained an internal movable ceramic support. The lumen volume of the Pd/Ag membrane was 5.3 cm3. Four graphite O-rings (99.53% C and 0.47% S) furnished by Gee Graphite Ltd. (Mirfield, West Yorkshire, U.K.), 2.8 g each, were used to ensure that the permeate and lumen streams did not mix with each other in the membrane module. The Pd/Ag membrane was produced by a lamination technique at E.N.E.A. Laboratories (Frascati, Italy); details of this technique were presented elsewhere by Tosti et al.8 Palladium alloy foil was cold-rolled by a two-high laboratory mill to reduce its thickness and to obtain the required characteristics of mechanical strength and surface finishing. The rolling process was operated starting from a commercial palladium/silver foil, 127 µm thick. This metal sheet was rolled to a thickness of 50 µm in five steps. Figures 2 and 3 show that both the Sievert and Arrhenius laws are followed. The values of apparent activation energy and preexponential factor are 29.16 kJ/mol and 1.12 × 10-5 mol‚m/(s‚m2‚Pa0.5), respectively. Table 2 shows that these values are compatible with experimental results reported in the literature. It should be noted that the Pd/Ag membrane shows infinite selectivity to H2 over other gases.
Figure 3. Arrhenius plot for the dense Pd/Ag membrane, plumen - pshell)1.2 bar. Table 2. Apparent Activation Energy and Preexponential Factor Compared with the Literature Ea (kJ/mol)
A0 [10-5 mol‚m/(s‚m2‚Pa0.5)]
ref
29.16 29.73 15.70 15.50 12.48 18.45 48.50
1.12 7.71 2.19 2.54 0.38 1.02 9.33
this work Basile et al.12 Koffler et al.13 Balovnev14 Itoh and Xu15 Itoh et al.16 Tosti et al.17
2.1.2. Experimental Details. The reaction system was studied under steady-state conditions using an experimental apparatus. A simplified schematic diagram of this apparatus is presented in Figure 4. The reactor (TR or MR) was placed in a temperaturecontrolled PID oven. The temperature was in the range between 300 and 500 °C. Reactants with a purity percentage of >99.95% were fed by means of mass-flow controllers (Brooks Instruments 5850S) driven by computer software furnished by Lira (Milan, Italy) used for all the experiments. Three different methane feed flow rates were used: 7.29, 11.06, and 17.80 mL/min. Three different H2O/CH4 feed gas ratios were also considered: 3.00, 5.77, and 8.74. The reactor pressure was held at 1.22 bar by means of a regulating-valve system placed at the outlet side. The permeate pressure was always 1.1 bar. The same apparatus was used for permeation tests. The flow rate of the outlet streams were measured by means of bubble flowmeters, and their compositions were detected using a temperature-programmed HP 6890 gas chromatograph (GC) with a TCD detector at 250 °C with He as the carrier gas. The GC was equipped with three packed columns, namely, Porapak R 50/80 (8 ft × 1/8 in.) and Carboxen 1000 (15 ft × 1/8 in.) connected in series, and molecular sieve 5Å (6 ft × 1/8
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Figure 4. Scheme of the laboratory plant used for both permeation and reaction tests.
in.), and a 10-way valve was used to optimize the total time of the analysis, which was about 5.1 min. This apparatus was driven by software furnished by HewlettPackard. To detect the outlet stream composition in terms of the permeate gases, two TCD detectors were simultaneously used for both the permeate and retentate streams. The TR and MR were each packed with 3.1 g of Ni/ Al2O3 catalyst (type Ni-5256 E 3/64′′) furnished by Engelhard. Before reaction, the catalyst was preheated using N2 at 480 °C under atmospheric pressure for 6 h and then reduced using H2 at the same temperature for 3 h. To verify the flat profile of the temperature along the reactor during the reaction, a four-point thermocouple was inserted into the lumen in both ends of each the traditional reactor and the Pd/Ag membrane reactor. To increase the methane conversion, different kinds of sweep gas were used. In particular, both unreactive (N2, H2O) and reactive (O2, CO) gases were fed as the inlet stream of the shell side of MR. Furthermore, hydrogen was also used as sweep gas to test the equilibrium displacement due to the membrane. The sweep gas was used to keep the hydrogen partial pressure in the shell side as low as possible, compatible with our experimental apparatus. This produced a high driving force for hydrogen permeation. Consequently, as demonstrated in the experimental results presented below, the use of a sweep gas led to an increase in the methane conversion compared to that observed when no sweep gas was used. From an industrial point of view, the use of a sweep gas could correspond to keeping the shell side under vacuum. Otherwise, it is possible to use a sweep gas that is easy to remove after hydrogen extraction. In fact, it is possible to use steam as the sweep gas, thereby achieving a mixture of hydrogen and steam from which the steam can be removed by condensation so that a pure hydrogen stream is obtained.
Figure 5. Methane conversion versus temperature for both the TR and the MR. Experimental results of Shu et al.5 for the TR are also reported.
3. Results and Discussions 3.1. Effect of the Temperature: MR versus TR. The influence of temperature on methane conversion is shown in Figure 5, which provides a comparison between the TR and MR in terms of methane conversion at different temperatures for H2O/CH4 ) 3, plumen ) 1.22 bar, and pshell ) 1.1 bar (with N2 as the sweep gas at a flow rate of 2.17 × 10-3 mol/min). In the same figure, the experimental results obtained by Shu et al.5 for their TR are reported. It is quite clear that, in the range of temperature 400-500 °C, the experimental results of this work for the TR practically coincide with the earlier results. At each temperature, the MR offers a consistently higher methane conversion than the TR. Moreover, the slight influence of the Pd/Ag membrane as the catalyst was also studied: the maximum influence was found to be 5% at 450 °C and less than 1% for T < 450 °C. 3.2. Effect of the Feed Ratio (H2O/CH4). The influence of different H2O/CH4 feed gas ratios for the MR is given in Figure 6. As expected, an increase in
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Figure 6. Methane conversion versus H2O/CH4 feed ratio for the MR.
Figure 7. Methane conversion versus temperature for the MR: influence of the kind of sweep gas (sweep gas flow rate ) 2.17 × 10-3 mol/min).
Table 3. Effect of the Nitrogen Sweep Gas Flow Rate on Methane Conversion (T ) 450 °C) nitrogen flow rate (mL/min)
methane conversion (%)
0 53 140.6
31 42 50
the feed gas ratio corresponds to an increase in the methane conversion. In particular, increasing H2O/CH4 from 3 to 9 results in an increase of the methane conversion from 31 to 48% under our experimental conditions. For a TR, the methane conversion is about 18% at H2O/CH4 ) 3. 3.3. Effects of the Temperature and Sweep Gas on Methane Conversion. It is well-known that hydrogen permeation through a Pd/Ag membrane involves essentially the following five different steps: (1) adsorption of hydrogen molecules (reaction side or lumen); (2) dissociation of hydrogen molecules into atoms, (3) diffusion of hydrogen atoms from the lumen-side Pd/Ag surface layer, through the membrane bulk, to the shellside Pd/Ag surface layer; (4) recombination of hydrogen atoms to form hydrogen molecules; and (5) desorption of hydrogen molecules from the shell-side Pd/Ag surface layer to the gas bulk. The choice of the sweep gas is a very important problem in MRs and needs to be considered carefully. In fact, depending on both the kind of sweep gas and the sweep gas flow rate, there is a corresponding influence on the last step (step 5) of the sequence. Finally, the membrane surface conditions correspond to some changes in methane conversion. To obtain a better understanding of the influence of the sweep gas on the methane conversion, two different situations were considered, depending on the particular experimental conditions on the shell side: (a) three different nitrogen sweep gas flow rates, namely, 0, 53, and 140.6 mL/min (see Table 3), or (b) different kinds of sweep gases (hydrogen, nitrogen, carbon monoxide, steam, air, and oxygen; see Figure 7), whose flow rates were fixed at 2.17 × 10-3 mol/min. 3.3.1. Effect of the Nitrogen Sweep Gas Flow Rate. The influence of the sweep gas flow rate, when nitrogen is considered as an inert gas, on the methane conversion is reported in Table 3. From these experimental results, it is clear that the highest level of methane conversion using nitrogen as the sweep gas is
achieved by increasing the sweep gas flow rate, which corresponds to a higher driving force for hydrogen permeation through the Pd/Ag membrane. In other words, high methane conversions are achieved only at high sweep gas flow rates. 3.3.2. Effect of Different Kinds of Sweep Gases. The situation is well illustrated considering Figure 7, where the influence of different kinds of sweep gases on the methane conversion at various temperatures at a fixed flow rate (2.17 × 10-3 mol/min) is shown. In this figure, the effect of the kind of sweep gas on hydrogen removal is clearly demonstrated. In fact, as expected, when oxygen is used as the carrier gas, the highest methane conversion is obtained because of the chemical reaction between oxygen and hydrogen occurring on the palladium/silver surface of the membrane in the shell side. The highest methane conversion of 70% was obtained at 450 °C, with H2O/CH4 ) 3, plumen ) 1.22 bar, and pshell ) 1.1 bar. This is due to the reduction of the hydrogen partial pressure in the permeate side, which increases the driving force for permeation and results in the highest rates of hydrogen removal from the reaction zone. However, it is important to observe that, in this case, the hydrogen permeated through the membrane that is converted into water cannot be properly used as hydrogen. Air, because of its lower oxygen concentration, is not able to extract the same quantity of hydrogen as pure oxygen from the Pd/Ag surface. In this case, compared to pure oxygen, the lower driving force produces a lower methane conversion. Also, in the case of air as the sweep gas, the hydrogen converted into water is essentially considered to be lost. From the point of view of obtaining a greater methane conversion, the use of air as the sweep gas is, of course, better than the use of nitrogen. In fact, as confirmed by Hughes and co-workers,9 no nitrogen adsorption onto the membrane surface occurs. As a consequence, no reaction occurs on the Pd/Ag surface, and hydrogen is not removed through a chemical reaction with the carrier from the Pd/Ag surface, but rather, is simply washed from the boundary layer of the membrane in the shell side. Carbon monoxide, a gas adsorbed on the membrane surface,9,10 reacts on the shell side with the permeated hydrogen: reducing the hydrogen concentration on the shell side in this way gives higher methane conversions than nitrogen does.
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Because of this sequence (for example, at T ) 400 °C)
(CH4 conversion)O2 > (CH4 conversion)CO > (CH4 conversion)air > (CH4 conversion)N2 one might conclude that the hydrogen concentration on the shell surface side is also a function of the kind of sweep gas used as carrier: oxygen gives the greatest increase in methane conversion, followed by carbon monoxide and then by air and by nitrogen. In other words, one can say that the different rates of hydrogen removal from the Pd/Ag membrane surface are responsible for the differing methane steam reforming performances observed in an MR. To emphasize this idea, in the same figure, the effect of the use of hydrogen as the sweep gas is also presented. As expected, the presence of 1.1 bar of hydrogen in the shell side inverts the driving force: the sequence of the five steps mentioned above is inverted. As a result, hydrogen permeates from the shell side to the lumen side, and by Le Chatelier’s principle, the methane steam reforming reaction is shifted in the reverse sense. A direct consequence is a reduction of the methane conversion, also with respect to a TR. This situation demonstrates that an MR is not always able to give better-than-equilibrium conversions, but only when some particular experimental conditions are satisfied. Particular attention must be paid when steam is used as the sweep gas. The advantage of steam as a sweep gas arises from two main characteristics: (a) When oxygen is used as the sweep gas, the highest methane conversion is obtained. (b) Hydrogen separation from steam is easily done by condensation of the water/ hydrogen mixture. It is also important to observe that oxygen as the sweep gas consumes all of the hydrogen permeated through the membrane, whereas steam permits the recovery of all of the extracted hydrogen from the reaction ambient. The problem was already considered by Hughes and co-workers11 from a simulation point of view, with regard to microporous membranes with H2/N2 selectivities in the range 3.74-1000. However, because of the porous membrane, the increase in the methane conversion is due to an increase in the H2O/CH4 ratio in the lumen, as steam permeates through the porous structure of the membrane from the shell side to the lumen. Hughes and co-workers9 demonstrated a strong adsorption ability of steam in hydrogen permeation tests carried out using a Pd/stainless steel dense membrane, confirming the important role of water as a sweep gas. 3.4. Effect of Different Sweep Gases for Hydrogen Permeation Tests. To verify the influence of different kinds of sweep gases on hydrogen permeation, permeation tests were performed using the membrane in the absence of a chemical reaction. Figure 8 shows a plot of the hydrogen flow rate through the Pd/Ag membrane at two different sweep gas flow rates (1.43 and 2.17 × 10-3 mol/min). In this figure, it is quite evident that increasing the flow rate results in an increase in the hydrogen flux through the membrane for each sweep gas. This figure confirms the trend of the influence of the different abilities of the sweep gases to remove hydrogen. In particular, the effect of water as a sweep gas is confirmed at both sweep gas flow rates. 3.5. Comparison with Literature Data. From recent literature data, some experimental results of this
Figure 8. Influence of the kind of sweep gas on hydrogen permeation through the dense Pd/Ag membrane at two different sweep flow rates (1.43 and 2.17 × 10-3 mol/min).
Figure 9. Methane conversion versus temperature for TRs and MRs: comparison with literature data.
work in terms of methane conversion versus temperature are included in the already-discussed Figure 1, where the methane conversion is plotted as a function of temperature. In Figure 9, experimental results regarding both the TR and the MR (with the sweep gas at 2.17 × 10-3 mol/min) are reported. For the TR, our results are close to the thermodynamic equilibrium curve, whereas for the MR, the methane conversion falls in a middle region of the literature data. The literature data also show that, with good reactor design and optimal catalyst deployment, it is now possible to achieve methane conversions approaching the equilibrium value in TRs. However, this limit can be overcome by using MRs. Still, methane conversion in an MR depends on several parameters, such as temperature, pressure, catalyst, feed flow rate, feed ratio, membrane length and thickness, membrane surface, sweep gas flow
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rate, and kind of sweep gas. Because the reported literature data refer to experimental conditions that are different from those used in this work, a direct comparison cannot be make. Nevertheless, it is clear that MRs perform better than TRs. Probably, because of the limitations of our devices, it was not possible to use higher amount sof catalyst or lower feed flow rates, so our methane conversions were lower than those reported in the literature. It should be noted that, considering the experimental results for an MR with steam as the sweep gas, a remarkable improvement is observed with respect to the case of N2 as the sweep gas. In fact, at 450 °C, the methane conversion is 43% for MR with N2 as sweep gas, whereas using steam as the sweep gas, the methane conversion (calculated) is about 61%. 4. Conclusions Methane steam reforming has been experimentally studied using a defect-free Pd/Ag membrane reactor. It was found that the temperature, H2O/CH4 feed ratio, kind of sweep gas, and sweep gas flow rate influenced the methane conversion. The experimental results obtained here are compatible with those found in the literature. Both the kind and the flow rate of the sweep gas used affect methane conversion. Although the maximum methane conversion of 69% at 450 °C was achieved using a reactive gas, the most interesting result concerns the use of steam as the sweep gas, which gives a methane conversion approaching 60% at the same temperature. In each case, the use of a sweep gas led to an increase in the methane conversion compared to the case when no sweep gas was used. From an industrial point of view, the use of a sweep gas could correspond to keep the shell side under vacuum. Otherwise, it is possible to use steam as the sweep gas, which is easy to remove after hydrogen extraction. In this way, it is possible to achieve a mixture of hydrogen and steam from which the steam can be removed by condensation so that a pure hydrogen stream is obtained. From an industrial point of view, the results presented in this paper focus on an alternative way to carry out the methane steam reforming reaction with a simultaneous increase of the conversion of methane and the production of pure hydrogen production. This also means that it is possible to convert the same amount of methane at lower temperature with respect to a conventional packed-bed reactor, with a resulting energy saving.
Literature Cited (1) Kikuchi, E.; Uemiya, S.; Matsuda, T. Hydrogen production from methane steam reforming assisted by use of membrane reactor. Nat. Gas Convers. 1991, 509. (2) Chai, M.; Machida, M.; Eguchi, K.; Arai, H. Promotion of methane steam reforming using tuthenium-dispersed microporous alumina membrane reactor. Chem. Lett. 1993, 41. (3) Madia, G.; Barbieri, G.; Drioli, E. Theoretical and experimental analysis of methane steam reforming in a membrane reactor. Can. J. Chem. Eng. 1999, 77, 698. (4) Barbieri, G.; Violante, V.; Di Maio, F. P.; Criscuoli, A.; Drioli, E. Methane steam reforming analysis in a palladium-based catalytic membrane reactor. Ind. Eng. Chem. Res. 1997, 36 (8), 3369. (5) Shu, J.; Grandjean, B. P. A.; Kaliaguine, S. Methane steam reforming in asymmetric Pd- and Pd-Ag/porous SS membrane reactors. Appl. Catal. A: Gen. 1994, 119, 305. (6) Nam, S. W.; Yoon, S. P.; Ha, H. Y.; Hong, S.-A.; Maganyuk, A. P. Methane steam reforming in a Pd-Ru membrane reactor. Korean J. Chem. Eng. 2000, 17 (3), 288. (7) Jarosch, K.; de Lasa, H. I. Novel riser simulator for methane reforming using high-temperature membranes. Chem. Eng. Sci. 1999, 54, 1455. (8) Tosti, S.; Bettinali, L.; Violante, V. Rolled thin Pd and PdAg membranes for hydrogen separation and production. Int. J. Hydrogen Energy 2000, 25, 319. (9) Li, A.; Liang, W.; Hughes, R. The effect of carbon monoxide and steam on the hydrogen permeability of a Pd/stainless steel membrane. J. Membr. Sci. 2000, 165, 135. (10) Hughes, R. Breaking the thermodynamic barrier in catalytic petrochemical reactors using composite membranes. In Proceedings of the 15th World Petroleum Congress; John Wiley & Sons, 1998; p 773. (11) Oklany, J. S.; Hou, K.; Hughes, R. A simulative comparison of dense and microporous membrane reactors for the steam reforming of methane. Appl. Catal. A: Gen. 1998, 170, 13. (12) Basile, A.; Paturzo, L.; Lagana`, F. The partial oxidation of methane to syngas in a palladium membrane reactor: simulation and experimental studies. Catal. Today 2001, 67 (1-3), 65. (13) Koffler, S. A.; Hudson, J. B.; Ansell, G. S. Trans. AIME 1969, 245, 1735. Cited in ref 12. (14) Balovnev, Yu. A. Diffusion of hydrogen in palladium. Russ. J. Phys. Chem. 1974, 48, 409. Cited in ref 12. (15) Itoh, N.; Xu, W.-C. Appl. Catal. A: Gen. 1993, 107, 83. Cited in ref 12. (16) Itoh, N.; Xu, W.-C.; Haraya, K. J. Membrane Sci. 1991, 56, 315. Cited in ref 12. (17) Tosti, S.; Bettinali, L.; Violante, V.; Basile, A.; Chiappetta, M.; Criscuoli, A.; Drioli, E.; Rizzello, C. In Proceedings of the 20th Symposium on Fusion Technology, Marseille, France, Sept. 7-11, 1998; Saint Paul Lez Durance: Dedex, France, p 1033. Cited in ref 12.
Received for review June 12, 2003 Revised manuscript received October 14, 2003 Accepted November 21, 2003 IE030485A
