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Hydrogen Production Using Sorption-Enhanced Reaction Alejandro Lopez Ortiz and Douglas P. Harrison* Gordon A. and Mary Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803
This study examined the sorption-enhanced production of H2 via the steam-methane reforming process using a mixture of Ni-based commercial reforming catalyst and Ca-based sorbent obtained from commercial dolomite. The rates of the reforming, water-gas shift, and CO2 removal reactions are sufficiently fast that combined reaction equilibrium was closely approached, allowing for >95 mol % H2 (dry basis) to be produced in a single step. A dolomite pretreatment procedure was developed to remove sulfur, which was necessary to avoid poisoning of the reforming catalyst. The multicycle durability of the catalyst-sorbent mixture was studied as a function of regeneration temperature and gas composition using a laboratory-scale fixed-bed reactor. Twenty-five-cycle tests showed only moderate activity loss under most of the regeneration conditions studied. The primary loss in activity was associated with the inexpensive sorbent instead of the more expensive catalyst. Introduction Sorption-enhanced reaction involves the addition of a sorbent to the reaction mixture for the selective removal of one of the reaction products, thereby shifting the equilibrium of reversible reactions. The concept, which combines reaction and separation, can result in process simplification, improved energy efficiency, and increased reactant conversion and product yield. The important reactions in sorption-enhanced hydrogen production are
Reforming:
CH4(g) + H2O(g) S CO(g) + 3H2(g) (1)
Shift:
CO(g) + H2O(g) S CO2(g) + H2(g)
CO2 Removal: CaO(s) + CO2(g) S CaCO3(s) Overall:
(2) (3)
CH4(g) + 2H2O(g) + CaO(s) S 4H2(g) + CaCO3(s) (4)
The suggestion of sorption-enhanced hydrogen production is not new. Rostrup-Nielsen1 reports that the first description of the addition of a CO2 sorbent to a hydrocarbon-steam-reforming reactor was published in 1868. Williams2 was issued a patent for a process in which steam and methane react in the presence of a mixture of lime and reforming catalyst to produce hydrogen. A fluidized-bed version of the process was patented by Gorin and Retallick.3 Brun-Tsekhovoi et al.4 published limited experimental results and reported potential energy savings of about 20% compared to the conventional process. More recently, Kumar et al.5 reported on a process known as unmixed combustion (UMC) in which the reforming, shift, and CO2 removal reactions are carried out simultaneously over a mixture of reforming catalyst and CaO-based CO2 sorbent. In related work, Anand et al.6 reported on sorptionenhanced H2 production using a K2CO3-treated hydrotalcite sorbent. Although regeneration of the hydrotal* Author to whom correspondence should be addressed. E-mail:
[email protected]. Fax: 225 578 1476.
cite sorbent is possible at a lower temperature (∼400 °C) than with a calcium-based sorbent, the reported working CO2 capacity of the hydrotalcite sorbent is only a small fraction of the capacity of calcined limestone or dolomite. In earlier work in this laboratory, Balasubramanian et al.7 showed that >95% H2 (dry basis) could be produced in a single reactor containing reforming catalyst and CaO formed by calcination of high-purity CaCO3. The reactions were sufficiently rapid that combined reaction equilibrium was closely approached over a range of temperatures, steam-to-methane ratios, and volumetric feed rates. No shift catalyst was required for the reaction conditions tested. This paper also included a brief discussion of the process aspects of the production of H2 through sorbent-enhanced reaction. The economics of a commercial sorption-enhanced H2 production process require that the sorbent and catalyst maintain activity through numerous reaction-regeneration cycles, and none of the earlier studies, including the work in this laboratory, seriously examined the question of multicycle durability. This paper reports experimental results of multicycle tests from a laboratory-scale fixed-bed reactor using inexpensive dolomite as the sorbent precursor. Dolomite was chosen over limestone on the basis of earlier results of Silaban et al.8 and Han and Harrison,9 which showed that larger fractional calcium conversions and better multicycle durabilities could be achieved with dolomite. Sorbent regeneration was studied as a function of temperature and gas composition, and the effect of regeneration conditions was evaluated on the basis of performance in the subsequent H2 production cycle. Experimental Section A schematic diagram of the laboratory-scale fixed-bed reactor is shown in Figure 1. Additional details may can found in the dissertation of Lopez Ortiz.10 During the H2 production phase, CH4 and N2 diluent were obtained from high-purity cylinders, and their flow rates were controlled using mass flow controllers. Water was fed as a liquid using a high-pressure syringe pump. Feed lines were heat-traced, and N2 diluent was used in most runs to ensure complete vaporization of the H2O. The combined feed gases entered near the bottom of the
10.1021/ie001009c CCC: $20.00 © 2001 American Chemical Society Published on Web 06/02/2001
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Figure 1. Schematic of the laboratory-scale fixed-bed reactor system.
reactor and were preheated as they flowed upward in the annular area between the pressure vessel and reactor insert. The preheated gas flowed downward through the mixture of reforming catalyst and CO2 sorbent and exited from the bottom of the reactor. Excess steam was removed in the condenser and the pressure was reduced using a back-pressure regulator. The product gas was analyzed using a Shimadzu GC14A gas chromatograph equipped with flame ionization and thermal conductivity detectors. Regeneration was conducted in pure N2, in N2/O2 mixtures, and in pure CO2. The progress of the regeneration reaction in N2 and N2/O2 was followed by monitoring the CO2 content in the product gas. A fixed quantity of H2 was injected into the product gas downstream of the reactor during CO2 regeneration, and regeneration was complete when the H2 content of the product gas became constant. Complete regeneration was achieved in all cycles of all tests. Approximately 40 g of mixed pretreated dolomite and catalyst in a mass ratio of 2.2-2.7 was supported inside the reactor insert by a layer of quartz wool on top of a porous stainless steel disk. Selected properties of the catalyst and dolomite precursor are reported in Table 1. Both solids were crushed into powders with two particle size ranges, namely, 75 e dp e 150 µm and 300 e dp e 425 µm, selected for testing. The test conditions used in the reaction and regeneration phases are summarized in Table 2. The reaction conditions were the same in all tests and were chosen on the basis of favorable results from the earlier study of Balasubramanian et al.7 Regeneration tests were conducted at a total pressure of 1 atm because of reactor safety limitations at high temperature. Temperature and gas composition were chosen as the regeneration reaction variables. The regeneration gas compositions in Table 2 represent different regeneration options, with pure N2 used
Table 1. Properties of the Reforming Catalyst and Dolomite reforming catalyst (United Catalysts, Inc., C11-9-02) NiO-Al2O3 ∼18 wt % NiO crushed and sieved to 75-150 µm or 300-425 µm diameter dolomite (Rockwell Lime Co.) composition (wt %) CaCO3 53.78 MgCO3 45.89 sulfur 0.01 loss on ignition 47.53 crushed and sieved to 75-100 µm or 300-425 µm diameter Table 2. Reaction Conditions during H2 Production and Sorbent Regeneration sorbent-to-catalyst mass ratio 2.2-2.7 H2 production phase temperature 650 °C pressure 15 atm feed composition (mol %) CH4 12 H2O 48 N2 40 feed rate 500 cm3(STP)/min regeneration phase temperature 800-950 °C pressure 1 atm feed composition 100% N2, 4% O2/N2, or 100% CO2 feed rate 500 cm3(STP)/min
as the reference. In a commercial process, the energy for regeneration might be supplied by direct combustion of supplemental fuel, in which case the regeneration atmosphere would consist primarily of N2 and excess O2. This composition is important because, if catalyst and sorbent are both exposed to an oxidizing regeneration atmosphere, the active Ni catalyst would be reoxidized to NiO during each regeneration cycle and would have to be rereduced at the beginning of each reaction
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Figure 2. Equilibrium CO2 pressure as a function of temperature.
cycle. A regeneration atmosphere of pure CO2 can be used to limit atmospheric emissions of CO2 for greenhouse gas control purposes. With this option, a portion of the CO2 would be recycled and the energy required for regeneration would be supplied by indirect heating of the recycled CO2 by combustion of supplemental fuel or a portion of the H2 product. The remaining pure CO2 would be suitable for sequestration. The equilibrium CO2 pressure over CaCO3 as a function of temperature, shown in Figure 2, was used to select the experimental regeneration temperatures. Although CaCO3 decomposition in a CO2-free atmosphere is thermodynamically favored at all temperatures shown in the figure, a temperature of at least 800 °C is needed to ensure adequate decomposition kinetics. Temperatures from 800 to 950 °C were studied. In pure CO2 at 1 atm, the minimum regeneration temperature is about 900 °C, and CO2 regeneration was studied at 950 °C. The response from a typical H2 production reaction test is shown in Figure 3 where the mole percentages (dry basis) of H2, CH4, CO, and CO2 in the product gas are plotted versus time. The figure is divided into four regions indicated by the vertical dotted lines. Two factors are responsible for the startup period. First, there is a delay between the time when the valve to feed the reaction gas is opened and the time when the product gas reaches the chromatograph. In addition, the reforming catalyst is not prereduced, so that time is required to reduce NiO to active Ni. During the prebreakthrough period, the reforming, shift, and CO2 separation reactions occur at maximum efficiency, and the mole percent of each component in the product gas is very near the respective equilibrium values calculated using HSC Chemistry11 (represented
by the horizontal lines). The beginning of breakthrough corresponds to the leading edge of the CO2 separation reaction reaching the exit of the packed bed. The CO2 removal efficiency begins to decrease, which, in turn, reduces the extents of the reforming and shift reactions. The H2 content decreases, whereas the CH4, CO, and CO2 contents increase. The second steady-state or postbreakthrough period begins when the CO2 separation reaction is no longer effective, as a result of either conversion of the sorbent nearing completion or the rate of the CO2 separation reaction approaching zero. Only the reforming and shift reactions are active during this period. The decrease in the H2 content between the prebreakthrough and postbreakthrough periods, in this case from about 52 to 33%, represents the improvement associated with sorption enhancement. In a commercial system without N2 diluent but at the same temperature, pressure, and steam-to-methane ratio, the equilibrium H2 content (dry basis) during prebreakthrough would be about 96% compared to about 69% during postbreakthrough. Calcium conversion, shown on the right ordinate in Figure 3, was calculated by material balance and found to be about 50% at the beginning of breakthrough and 83% at the end of the test. Pretreatment of the commercial dolomite to remove sulfur was necessary to avoid sulfur release during reaction with resultant catalyst poisoning. The hydrogen concentration as a function of time using commercial calcined dolomite with and without pretreatment is illustrated in Figure 4. The reactor feed in these two tests contained only 6 mol % CH4, but the steam-tomethane ratio and other reaction parameters were the same as the standard values shown in Table 2. Without pretreatment, the H2 content peaked at about 22%, significantly below the 24.5% prebreakthrough equilibrium level, and then decreased continually, with the final value well below the postbreakthrough equilibrium level. The dolomite was pretreated by exposure to a mixture of 40% H2/N2 for 6 h, followed by a mixture of 40% H2O/ N2 for another 6 h with both steps at 900 °C. The first step reduced sulfates originally present in the dolomite to sulfides, and in the second step, the sulfides reacted with H2O to liberate sulfur as H2S according to the reactions
CaSO4(s) [or MgSO4(s)] + 4H2(g) S CaS(s) [or MgS(s)] + 4H2O(g) (4) CaS(s) [or MgS(s)] + H2O(g) S CaO(s) [or MgO(s)] + H2S(g) (5) Following this pretreatment, the experimental results were as expected, as shown in Figure 4 and other parts of this paper. Experimental Results Five-Cycle Tests. Five-cycle tests were carried out using the reaction conditions shown in Table 2, with regeneration in pure N2 and 4% O2/N2 at temperatures between 800 and 950 °C. In some tests, the sorbent and catalyst were separated by sieving after each reaction cycle, with only the sorbent exposed to regeneration conditions. In other tests, both the sorbent and catalyst were exposed to regeneration conditions, referred to in the following as in situ regeneration.
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Figure 3. Typical reactor response curves.
Figure 5. H2 concentrations in five-cycle tests following in situ regeneration at 800 °C in N2. Figure 4. Comparison of H2 production with and without dolomite pretreatment.
The hydrogen concentrations during the reaction phases of the five cycles following in situ regeneration under “mild” conditions of 800 °C in N2 are shown in Figure 5. Except for scatter during the startup period, the results are effectively identical showing negligible sorbent and catalyst deterioration under these conditions. The H2 concentrations during both the prebreakthrough and postbreakthrough periods of each cycle
were equal and were also equal to the respective equilibrium concentrations. The slopes of the curves during active breakthrough, which provide a measure of the global rate of the combined reactions, were also effectively equal. Similar H2 concentrations during the reaction phases of the five cycles following in situ regeneration under “severe” conditions of 950 °C in 4% O2/N2 are shown in Figure 6. Once again, the H2 concentrations during the prebreakthrough and postbreakthrough periods were effectively equal in each cycle and were also equal to
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Figure 6. H2 concentrations in five-cycle tests following in situ regeneration at 950 °C in 4% O2/N2.
the equilibrium concentration. There is, however, evidence of a slight decrease in the slope of the breakthrough curves during cycles 04 and 05. The larger time corresponding to the beginning of breakthrough in Figure 6 compared to Figure 5 is due to an increase of about 30% in the amount of sorbent in the reactor. The results shown in Figures 5 and 6 are typical of all five-cycle tests using in situ regeneration. Data scatter was somewhat greater in tests where the catalyst and sorbent were separated prior to regeneration and then remixed before the next reaction cycle. However, this was attributed more to inevitable small losses of catalyst and sorbent associated with sieving and to different catalyst-sorbent packing arrangements following remixing. Longer-Duration Tests. Because catalyst-sorbent separation following each reaction cycle more than doubled the time required to complete a cycle and because five cycle results showed little, if any, effect of separation, all longer-duration tests used in situ regeneration. In addition, obvious damage to the reactor insert following extended high-temperature exposure in an oxidizing atmosphere meant that no longer-duration tests using regeneration in 4% O2/N2 could be carried out. Instead, the longer-duration tests were limited to 25 cycles in N2 at temperatures of 800 and 850 °C, 15 cycles in N2 at 950 °C, and 25 cycles in CO2 at 950 °C. Hydrogen concentrations as a function of time from selected cycles of a 25-cycle test using N2 regeneration at 850 °C are shown in Figure 7. The maximum H2 concentrations during the prebreakthrough periods were effectively equal to the combined equilibrium concentration for the three simultaneous reactions in each cycle, and the postbreakthrough concentrations were very near the combined reforming and shift reaction equilibrium value. The slope of the breakthrough curves, which is related to the global reaction
Figure 7. H2 concentrations in selected cycles of a 25-cycle test following in situ regeneration at 850 °C in N2.
rate, also remained reasonably constant. Performance deterioration was largely associated with earlier onset of breakthrough with increasing cycle number. As will be shown below, multicycle results associated with N2 regeneration at 800 and 850 °C and CO2 regeneration at 950 °C exhibited similar behaviors. However, performance deteriorated more rapidly following regeneration in N2 at 950 °C. A more detailed comparison of the results from the longer-duration tests is presented in Figures 8-10. In Figure 8, the normalized maximum prebreakthrough H2 concentration is shown as a function of cycle number for the four longer-duration tests. The normalized H2 concentration is defined as the ratio of the maximum H2 concentration in cycle i to the maximum concentration in cycle 01. Similarly, in Figure 9 the normalized breakthrough time is defined as the ratio of the breakthrough time in cycle i to the breakthrough time in cycle 01, and the normalized sorbent conversion at the beginning of breakthrough in Figure 10 is defined in the same manner. For this comparison, the breakthrough time was defined as the time required for the H2 concentration to decrease from 50 to 35% (dry basis), two percentage points below the prebreakthrough equilibrium value to two percentage points above the postbreakthrough equilibrium. In the few cases (limited to 950 °C regeneration in N2) where the maximum H2 concentration was more than two percentage points below prebreakthrough equilibrium value, the beginning of breakthrough was taken to be the time when the H2 concentration decreased by two percentage points below the maximum H2 concentration. It is also important to note that evaluation of the breakthrough time requires interpolation between discrete data points so that some scatter is inherent in the calculation. The normalized maximum H2 concentration shown in Figure 8 appears to vary randomly between about 0.99
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Figure 8. Normalized maximum H2 content as a function of cycle number.
Figure 9. Normalized breakthrough time as a function of cycle number.
and 1.01 for N2 regeneration at both 800 and 850 °C and for CO2 regeneration at 950 °C. For N2 regeneration at 950 °C, the normalized H2 content remained between 0.99 and 1.0 for the first 6 cycles but decreased
Figure 10. Normalized sorbent conversion at the beginning of breakthrough as a function of cycle number.
thereafter and was only about 0.95 when that test was terminated after 15 cycles. Figure 9 presents the normalized breakthrough time as a function of cycle number and shows similar behavior. The breakthrough time was reasonably consistent through 25 cycles for N2 regeneration at 800 and 850 °C and for CO2 regeneration at 950 °C. However, for regeneration in N2 at 950 °C, the breakthrough time increased dramatically beginning in cycle 10 and reached a value near 6 by cycle 15. The breakthrough time provides a measure of the global reaction rate, and the large increase in the breakthrough time corresponds to a large decrease in the global rate. The normalized sorbent conversion at the beginning of breakthrough decreased gradually in all tests, as shown in Figure 10. In N2 at both 800 and 850 °C, the final values were about 70% of the first-cycle values, whereas in CO2 at 950 °C, the final value was only about 30% of the first-cycle value. However, much of the decrease in CO2 occurred between cycles 01 and 02. Thereafter, the slopes of the lines corresponding to N2 regeneration at 800 and 850 °C and CO2 regeneration at 950 °C were approximately equal. Once again, multicycle performance deterioration occurred more rapidly using 950 °C regeneration in N2, with a final normalized value after 15 cycles of only about 0.1. Two longer-duration dolomite calcination-carbonation tests, the first lasting 33 cycles and the second 148 cycles, were carried out using an electrobalance reactor to provide a comparison with the fixed-bed reactor results using CO2 regeneration. The results are shown in Figure 11. The electrobalance monitors the solid weight change associated with the reaction but provides no information on the product gas composition. In these tests, dolomite (no reforming catalyst) was exposed to pure CO2 at 1 atm while the temperature was continually cycled between 800 °C for carbonation and 950 °C for calcination. The temperature was held
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Figure 11. Comparison of fractional carbonation achieved in electrobalance and fixed-bed reactor tests.
constant for 5 min at both the maximum and minimum temperatures and was changed at a rate of 10 °C/min between the temperature limits. Calcination was complete in each cycle at the end of the 950 °C period, but carbonation was still occurring slowly when the temperature began to increase from 800 °C. Thus, the electrobalance results in Figure 11 represent the degree of carbonation achieved under the specified conditions, not the maximum achievable carbonation. The final fractional carbonation results from the 25-cycle reaction-regeneration test from the fixed-bed reactor using CO2 regeneration at 950 °C are included in Figure 11 for comparison. In the 33-cycle test, the fractional carbonation gradually decreased from 0.82 in cycle 01 to 0.49 in cycle 33. In the 148-cycle test, the fractional carbonation decreased from 0.83 in cycle 01 to 0.26 in cycle 97. This decrease was followed by an unexpected and unexplained increase to 0.34 in cycle 104 and a second decrease to 0.27 in cycle 148. The results from the fixedbed reactor test were similar, with the fractional carbonation decreasing from 0.87 in cycle 01 to 0.52 in cycle 25. The slightly larger fractional carbonation in the early cycles in the fixed-bed reactor occurs because these tests were carried out to completion whereas the electrobalance tests operated on a fixed-time cycle. Solid Characterization Limited sorbent and catalyst characterization tests were performed to supplement the reaction results. X-ray diffraction spectra of the unused catalyst, both as-received and following reduction, clearly matched library spectra for NiO and Al2O3 and Ni and Al2O3, respectively. Similarly, the spectra of pretreated dolomite clearly showed that CaO and MgO were the dominant species. There was concern that mixed metal compounds having an adverse effect on the sorbent and/or catalyst activity could be formed during the high-temperature multicycle tests. For example, Alzamora et al.12 reported that NiAl2O4 formation can begin near 700 °C with the
formation rate increasing at higher temperature. Agnelli et al.13 reported that Ca12Al14O33 was formed when the reforming catalyst support also contained CaO. Similar compounds could be formed in the sorption-enhanced reaction system through the interaction of the Al2O3 catalyst support with CaO from the sorbent. However, X-ray diffraction showed no evidence of mixed metal compounds after multicycle tests, although small concentrations would not necessarily be detectable from the X-ray spectra. The activity of reforming catalysts can also decrease as a result of agglomeration of nickel crystallites at high temperature. Numaguchi et al.14 reported an increase in Ni crystallite size from 25 to 40 nm when the catalyst was exposed to an atmosphere of H2O/H2/N2 for 430 h at temperatures ranging from 590 to 750 °C. XRD line broadening analysis using the MudMaster software developed by the U.S. Geological Survey15 was used to evaluate nickel crystallite agglomeration in this study. Texeira and Giudici16 also used the same technique to analyze reforming catalyst sintering. Experimental Ni crystallite agglomeration results are illustrated in Figure 12, where the crystallite diameter is plotted against the cycle number for multicycle tests using N2 regeneration at 800 and 950 °C. The initial crystallite diameter of 19.8 nm increased to 32.2 nm after 15 cycles using the 950 °C regeneration temperature, but increased to only 27.2 nm after 25 cycles using the 800 °C regeneration temperature. It is interesting to note that crystallite growth was almost equal through the first five cycles, reaching 25.4 nm at 800 °C compared to 26.3 nm at 950 °C. After five cycles, however, relatively little additional crystallite growth occurred at 800 °C, whereas significantly more growth occurred at 950 °C. Numerous researchers have investigated the sintering of calcined limestone and/or calcined dolomite. Of particular relevance to this study, Silaban et al.8 reported that dolomite calcined at 750 °C in N2 had a surface area of 21.3 m2/g. After a single carbonation cycle at 550 °C in 15% CO2/N2, the surface area
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growth results, the catalyst always maintained sufficient activity that postbreakthrough equilibrium was closely approached. Attributing activity loss to dolomite is economically important, as the cost of dolomite (∼1¢ per pound) is much less than the cost of reforming catalyst. Spent dolomite can be separated from the catalyst and replaced with minimal economic impact on the process. Acknowledgment The authors are grateful to the U.S. Department of Energy for financial support under Grant DE-FG0297ER12208. Literature Cited
Figure 12. Nickel crystallite growth in N2 regeneration tests.
decreased to 7 m2/g, and a second calcination cycle at 750 °C in N2 resulted in a surface area increase to only 16.3 m2/g. The pretreatment, reaction, and regeneration temperatures used in the current study were higher, and a limited number of sorbent surface area measurements following the reaction step (CaCO3/MgO) showed that the surface area decreased with increasing regeneration temperature and number of cycles. However, difficulties in accurately measuring low surface areas (generally less than 1 m2/g) coupled with limited data made it impossible to reach specific conclusions. Conclusions Previously reported results7 showed that >95% H2 (dry basis) could be produced in a single processing step via the sorption-enhanced steam-methane reaction using high-purity CaCO3 as the sorbent precursor. The rates of the reforming, shift, and CO2 separation reactions were sufficiently fast that combined reaction equilibrium was closely approached over a range of temperatures, feed gas compositions, and flow rates. This study showed that commercial dolomite, when pretreated for sulfur removal, also served as an effective sorbent precursor. Regeneration of the spent sorbent was carried out in atmospheres of pure N2, 4% O2/N2, and pure CO2 at temperatures between 800 and 950 °C. Multicycle tests showed no significant decrease in the maximum H2 concentration or increase in the breakthrough time (a measure of global reaction rate) except when regeneration was carried out in N2 at 950 °C. However, decreases in the fractional sorbent conversion at the beginning of breakthrough were detected in all multicycle tests. Some loss of activity is inevitable because of the severe conditions required for regeneration. The experimental evidence suggests that most of the multicycle activity loss is associated with the sorbent. The conclusion is supported by the results depicted in Figure 11, which show that the carbonation capacity decreases are similar in the electrobalance calcinationcarbonation tests without catalyst and in the fixed-bed reaction tests. Even though some catalyst deactivation might have occurred, as suggested by the Ni crystallite
(1) Rostrup-Nielsen, J. R. Catalytic Steam Reforming. In Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1984. (2) Williams, R. Hydrogen Production. U.S. Patent 1,938,202, 1933. (3) Gorin, E.; Retallick, W. B. Method for the Production of Hydrogen. U.S. Patent 3,108,857, 1963. (4) Brun-Tsekhovoi, A. R.; Zadorin, A. N.; Katsobashvili, Y. R.; Kourdyumov, S. S. The Process of Catalytic Steam-Reforming of Hydrocarbons in the Presence of a Carbon Dioxide Acceptor. In Hydrogen Energy Progress VII, Proceedings of the 7th World Hydrogen Energy Conference, Moscow, Russia, Sept 25-29, 1988; Veziroglu, T. N., Protsenko, A. N., Eds.; Pergamon Press: New York, 1988; Vol. 2, p 885. (5) Kumar, R.; Cole, J.; Lyon, R. Unmixed Reforming: An Advanced Steam Reforming Process. Presented at the Fuel Cell Reformer Conference, South Coast Air Quality District, Diamond Bar, CA, 1999. (6) Anand, M.; Hufton, J.; Mayorga, S.; Nataraj, S.; Sircar, S.; Gaffney, T. Sorption Enhanced Reaction Process (SERP) for Production of Hydrogen. Proceedings of the 1996 U.S. DOE Hydrogen Program Review; Report DE97000053; U.S. Department of Energy, U.S. Government Printing Office: Washington, D.C., 1996; Vol. 1, p 537. (7) Balasubramanian, B.; Lopez Ortiz, A.; Kaytakoglu, S.; Harrison, D. P. Hydrogen from Methane in a Single-Step Process. Chem. Eng. Sci. 1999, 54, 3543. (8) Silaban, A.; Narcida, M.; Harrison, D. P. Characteristics of the Reversible Reaction Between CO2(g) and Calcined Dolomite. Chem. Eng. Commun. 1996, 147, 149. (9) Han, C.; Harrison, D. P. Multicycle Performance of the CO2 Acceptor in a Single-Step Process for H2 Production. Sep. Sci. Technol. 1997, 32, 681. (10) Lopez Ortiz, A. Sorption Enhanced Process for the Production of Hydrogen. Ph.D. Dissertation, Louisiana State University, Baton Rouge, LA, 2000. (11) Roine, A. HSC Chemistry for Windows, User’s Guide; Outokumpu Research Oy: Pori, Finland, 1999. (12) Alzamora, L. E.; Ross, J. R.; Cruissink, E. C.; Van Reijnene, L. L. Interactions of Nickel With the Alumina Support. J. Chem. Soc., Faraday Trans. 1 1981, 77, 665. (13) Agnelli, M. E.; Demicheli, M. C.; Ponzi, E. N. Catalytic Deactivation of Methane Steam Reforming Catalysts. 1. Activation. Ind. Eng. Chem. Res. 1987, 26, 1704. (14) Numaguchi, T.; Shoji, K.; Yoshida, S. Hydrogen Effect on R-Al2O3 Supported Ni Catalysts for the SMR Reaction. Appl. Catal. 1995, 133, 241. (15) Eberl, D. D.; Drits, V.; Srodon, J. MudMaster: A Program for Calculating Crystallite Size and Size Distributions and Strain from the Shapes of X-ray Diffraction; Open-File Report OF 960171; U.S. Geological Survey: Denver, CO, 1996 (available at ftp:// brrcrftp.cr.usgs.gov/pub/ddeberl/). (16) Texeira, A. C.; Giudici, R. Deactivation of Steam Reforming Catalysts by Sintering: Experiments and Simulation. Chem. Eng. Sci. 1999, 54, 3609.
Received for review November 30, 2000 Revised manuscript received March 26, 2001 Accepted April 26, 2001 IE001009C
