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Energy & Fuels 1991,5,60-63
Catalyzed Hydrogasification of Yallourn Char in the Presence of Supported Hydrogenation Nickel Catalyst Shiro Matsumoto Governmental Industrial Research Institute of Kyushu, Shuku-machi, Tosu, Saga, 841 Japan Received April 17, 1990. Revised Manuscript Received July 27, 1990 The hydrogasification of Yallourn char and its catalysis by iron group metals were studied at the hydrogen pressure of 100 kPa. Iron group metals iron, cobalt, and nickel showed catalytic activity at loading levels as low as 0.1 wt 5%. The rate of hydrogasification increased on mixing a supported hydrogenation nickel catalyst with the catalyst-loaded char. This appeared to be due to the hydrogen atoms split-over from the supported nickel catalyst to catalytic species on char. Introduction The reaction between carbon and hydrogen has attracted much attention, especially in the past two decades,l12though not as extensively as other carbon-gas reactions because of the low gasification rate3 and strong thermochemical suppression at higher temperatures. The study of catalytic hydrogasification of carbonaceous materials is of potential importance for substitute natural gas production, and also hydrogasification facilitates the interpretation of catalytic reaction mechanism because it involves only the production of methane by the introduction of hydrogen. Many studies have been carried out on catalytic hydrogasification in order to search for active catalysts and to clarify the reaction mechanism. The former is successfully carried out by using nickel and iron as catalyst.As for the latter, including studies using controlled atmosphere electron microscopy:JO two mechanisms were proposed (1)spill-over mechanism,"J2 and (2) C-C bond weakening mechanism.13 For the catalyzed hydrogasification by iron group metals, the spill-over mechanism is more generally accepteda2 We previously reported that Ni/A1203or CoMo/A1203 catalyst enhances the catalytic activity of iron loaded on Saran char for steam gasification in the presence of hydrogen." We proposed that the synergism is probably due to a reduction of the iron surface by atomic hydrogen generated on Ni/A1203 or CoMo/A1203 catalyst. It is well-known that iron group metals, iron, nickel, and cobalt, are active as gasification catalysts in their reduced ~ t a t e . ' ~ J ~ The present study focuses on the new observation that the supported hydrogenation nickel catalyst markedly (1) Mckee, D. W. Chemistry and Physics of Carbon; Marcel Dekker: New York, 1981; Vol. 16, pp 1-118. ( 2 ) Pullen, J. R. "Catalytic Coal Gasification" Report No. ICTIS/ TR26; IEA Coal Research, London, 1984. (3) Walker, P. L., Jr.; Rusinko, F., Jr.; Austin, L. G. Advances in Catalysis; Academic Press: New York, 1959; Vol. 11, pp 133-221. (4) Tomita, A.; Ohtauka, Y.; Tamai, Y. Fuel 1983, 62, 150. ( 5 ) Ohtauka, Y.; Tamai, Y.; Tomita A. Energy Fuels 1987, 1, 32. (6) Nishiyama, Y.;Tamai, Y. ACS, Division of Fuel Chemistry, Preprints 1979, 24(2), 219. (7) Huttinger, K. J.; Krauss, W. Fuel 1981, 60, 93. (8) Hattinger, K. J. Fuel 1983, 62, 166. (9) Baker, R. T. K.; Chludzinski,J. J., Jr.; Sherwood, R. D. Carbon 1985, 23, 245. (10) Goethel, P. J.; Yang, R. T. J. Catal. 1988, 111, 220. (11) Robell, A. J.; Ballou, E. V.; Boudart, M. J. Phys. Chem. 1964,68,
-.
77AA --.
(12) Olander, D. R.; Balooch, M. J. Catal. 1979, 60, 41. (13) Holstein, W. L.; Boudart, M. J. Catal. 1981, 72, 328. (14) Mataumoto, S.; Walker, P. L., Jr. Carbon 1989, 27, 395. (15) McKee, D. W. Carbon 1974,12, 453. (16) Mataumoto, S.; Walker, P. L., Jr. Carbon 1986,24, 277.
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Table I. Properties of Yallourn Coals ultimate analysis - (wt %, daf) C H N S 0 FC." % ash. % as received demineralized
66.1 5.3 0.6 0.3 27.7 66.1 5.3 0.6 0.3 27.7
48.7 48.7
0.8
0.0
Fe 0.20% 14 ppm
"Yield after heat treatment at 1223 K for 1 h.
increases the rate of hydrogasification of iron group metal loaded char and its implications for the reaction mechanism of catalytic hydrogasification. Experimental Section Most experiments in this study were performed at atmospheric pressure in a micro flow quartz tube reactor of 12 mm diameter. The sample weight of char was 0.5 g and the flow rate of hydrogen gas was 100 mL/min. The sample was heated in high-purity argon a t about 50 K/min and the gas was changed to hydrogen as soon as the reaction temperature was reached. The effluent gas was introduced to an automatic gas sampler and analyzed by gas chromatograph every 5 min. The char conversion was calculated from the methane content in the effluent gas and the hydrogen flow rate. The purity of the hydrogen was most important because trace amounts of oxygen or water reacted with char to produce carbon monoxide and affected char conversion. The oxygen in the hydrogen was first converted to water in a copper furnace a t 773 K and the water was removed from the hydrogen with a magnesium perchlorate tube. Some runswere performed using a TGA apparatus with the same gas purifier. Yallourn brown coal from Australia was used as the starting carbonaceous material because of its low ash and sulfur content. The coal was first crushed to 32/60 mesh and demineralized with 6 N of HC1 a t 353 K. The coal was then washed with distilled water until chlorine was not detected. The change of impurities after acid treatment is shown in Table I. Iron, nickel, and cobalt were loaded on demineralized coal by wet impregnation of aqueous metal nitrate solution and heat treated to form char in high-purity argon at various temperatures higher than the reaction temperature, mostly 1123 K for 30 min. The supported hydrogenation nickel catalyst was a commercial material in which about 45 w t % of nickel was supported on diatomite (Ni/diatomite) and the particle size was under 150 mesh. The same weight of supported nickel catalyst, 0.5 g, was mixed with char in the experiments described.
Results The catalytic effects of iron group metals were examined for simple hydrogasification. Figure 1 shows the hydrogasification profiles of about 0.8 w t % of metal-loaded and demineralized Yallourn chars. The catalytic effect of the iron group metals is apparent. The catalytic activity de@ 1991 American Chemical Society
Hydrogasification of Yallourn Char
40
Energy & Fuels, Vol. 5, No. 1, 1991 61
t
80
!-
I
20
0 0 Timelmin I
400
200
600
Timetmin)
Figure 1. Hydrogasification profile of Yallourn chars at 1093 K: (0) 0.80wt % Fe, (A)0.92 wt % Co, ( 0 )0.83wt 90 Ni, ( 0 ) demineralized.
Figure 3. Hydrogasification profile of Yallourn chars at 993 K when mixed with Ni/diatomite: (0) 0.80wt % Fe, (A)0.92 wt % Co, (0) 0.83% wt % Ni, ( 0 )demineralized.
t
/
Fo2oF
?
I
01
0.5
I
1 .o
I
I
1.5
2.0
OO
Metal loading (ut$)
Figure 2. Catalytic activity of iron group metals for hydrogasification at 1093 K (0) Fe, (A)Co, ( 0 )Ni.
creases in the order of Co > Ni > Fe at this reaction temperature of 1093 K, using this catalyst loading method. One of the characteristics of the profiles is that the initial low reactivity is observed for all catalyst-loaded chars. Subsequently, the gasification rate gradually increases with reaction time to a steady state. The time of this initial low reactivity varied with reaction temperature and was less than 30 min at 1173 K by TGA analysis of char heattreated at 1173 K for 1h and lengthened with decreasing temperature, exceeding 300 min at reaction temperatures below 1043 K. An XRD study showed that all the catalyst was reduced to the metallic phase during heat treatment of metal-loaded coals to char at temperatures above 1123 K. These observations suggest that the initial low reactivity of catalyst-loaded char is due to poisoning by very small amounts of sulfur which is contained in coal and chemisorbs on the catalyst surface. The amount of catalyst loading was varied and the data obtained by using the TGA apparatus are plotted in Figure 2 as gasification rate versus catalyst loading. In this figure, the gasification rate is the steady-state value attained after the initial low reactivity. The catalytic activities are evident with only about 0.1 w t ?% loading for all the catalysts. However, the increase in gasification rate becomes small above 0.2 w t ?% and a leveling-off in the rate is observed for iron catalyst. The order of catalytic activities is complicated; that is, iron shows the highest activity at the lowest loading level under 0.1 wt 90, and cobalt shows the highest activity over 0.5 wt ?% of loading following nickel and then iron. It has been reported in many studies that nickel has the highest activity among iron group metals for gasificati~n.''-'~ On the contrary, cobalt had a higher activity (17) Hottinger, K.J.; Schleicher, P. Fuel 1981, 60, 1005.
T i m e lminl
Figure 4. Cobalt-catalyzed hydrogasification profile at various temperature when mixed with Ni/diatomite: (0) 943 K, ( 0 )893 K, (A)843 K.
than nickel in this study. There are many possible reasons for these differences, for example, the catalyst loading method, the properties of the carbonaceous materials used, catalyst dispersion, and inhibition by sulfur. It is not clear why cobalt has the highest activity at most loading levels, but this observation is not the main purpose of this report. Subsequently, the effect of Nildiatomite was examined for hydrogasification of iron group metal-loaded chars. Figure 3 shows the hydrogasification profile of chars when mixed with the same weight of Nildiatomite at the reaction temperature of 993 K. No methane was evolved at this reaction temerature without the addition of Nildiatomite, but higher reaction rates were observed for all chars than shown in Figure 1. Although the addition of Nildiatomite increases the gasification rate of demineralized char, the increase is larger for metal-loaded chars than for deminerlized char. Also, the duration of the initial low reactivity becomes shorter for all the catalyst loaded chars and is almost negligible for cobalt loaded chars. Those results indicate that the Nildiatomite not only increases the hydrogasification rate as catalyzed by iron group metals but also decreases the period of initial low reactivity, that is, removes the chemisorbed sulfur from catalyst surface. The gasification rate decreases with time after it reaches a maximum value. The decrease in rate is probably due to deactivation of Nildiatomite at high temperature. In fact, Nildiatomite had no effect on iron-catalyzed hydrogasification after it was heat treated with char at gasification temperatures for over 400 min. The decreasing (18) Tomita, A.; Tamai, Y. J. Catal. 1972, 27, 293. (19) Tamai, Y.;Watanabe, I.; Tomita, A. Carbon 1977, 15, 103.
Matsumoto
62 Energy (B Fuels, Vol. 5, No. 1, 1991
k
%ZRTED CATALYST
CHAR
Metal loadinq ( w t % )
Figure 6. Schematic diagram of hydrogen atom spill-over in the supported hydrogenation nickel catalyst and iron group metal loaded char system.
Figure 5. Catalytic activity of iron group metals for hydrogasification at 993 K when mixed with Ni/diatomite: (0) Fe, (A)
Co, ( 0 )Ni.
order of catalytic activity was again Co > Ni > Fe. The reaction temperature was changed to examine more precisely the effect of Ni/diatomite on the iron group metal catalyzed hydrogasification. The results are shown in Figure 4 for 0.92 wt % cobalt loaded char. A comparison of the gasification profile without Ni/diatomite in Figure 1 shows that the addition of Ni/diatomite can decrease the reaction temperature by about 200 K and hydrogasification took place at 843 K. The gasification rate was affected by the charring conditions and methane was evolved even at 793 K for the char heat treated at 823 K. This reaction rate was faster than that observed for 5 wt 3'% platinum catalyzed hydrogasification.m Ni/diatomite decreased the reaction temperature by about 150 and 100 K for iron- and nickel-loaded char, respectively. The gasification rate was measured when catalystloading levels were changed and Ni/diatomite was mixed with catalyst-loaded char. The curves of gasification rates versus catalyst loading are given in Figure 5. Here, maximum values were adopted as the gasification rates. The profiles of the rate versus catalyst loading are essentially the same as those found in the absence of the Ni/ diatomite as shown in Figure 2. That is, catalytic activities were observed at a low loading level of 0.1 wt 90 and the slow rate increase was observed above 0.2 w t %. Furthermore, iron shows the highest activity at its low loading level under 0.1 wt 90and cobalt surpasses other catalysts above 0.4 wt %. These results indicate that Nildiatomite increases the activity of iron group metals without changing the reaction mechanism or catalyst dispersion.
Discussion The synergism shown by Ni/diatomite and the iron group metal catalysts was first revealed in this study for iron group metal catalyzed hydrogasification. Therefore, it is necessary to point out the reason for the synergism. One of the ideas is a bimetallic effect reported by many that is, nickel on Ni/diatomite moved to char and formed an alloy with the iron group metal during gasification. To examine this idea, the same amount of iron and nickel was loaded on Yallourn char and gasification was measured after charring. No methane was evolved at reaction temperatures under 993 K. This result indicates that the synergism is not due to a bimetallic (20) Rewick, R. T.; Wentreck, P. R.; Wise, H. Fuel 1974, 53, 274. (21) Haga, T.; Nishjyama, Y. J. Catal. 1983,81, 239. (22) Adler, J.; Hilttinger, K. J. Fuel 1984, 63, 1393. (23) Mishima, M.; Suzuki, T.;Watanabe, Y. Fuel Processing Technol. 1987, 16, 45.
0.8
1 .o
l/T(K)
1.2
X lo3
Figure 7. Arrhenius plot of hydrogasification of 0.92 w t 70 cobalt-loaded Yallourn char (0)without Ni/diatomite, (0) mixed with Nildiatomite. effect. Ni/diatomite enhanced the activity of even nickel and it removed the sulfur from the catalyst surface. These results and the similarity between Figures 2 and 5 suggest that some kind of catalytic activity of Ni/diatomite is the reason for the synergism. Butler and Snelson mentioned the work on steam and hydrogasification catalyzed by transition metal incorporating molecular iodine.24 They concluded that the catalytic role of iodine at 873 K is to provide a finite steady-state hydrogen atom concentration according to the reaction between iodine and hydrogen. The synergism found in this study suggests that atomic hydrogen has some kind of role, but it is unreasonable in this system to consider atomic hydrogen in the gas phase. It is well-known that supported nickel catalyst for hydrogenation adsorbs hydrogen, where molecular hydrogen dissociates to atomic hydrogen, and the hydrogen atoms can spill-over to adjacent materials.% The supported nickel catalyst was mixed with chars mechanically in this study. However, Ni/diatomite is a very fine particle material under 150 mesh and there is a possibility that it contacts the catalyst particles of iron group metals on the char. Hence, hydrogen atoms dissociated on supported hydrogenation nickel catalyst can spill-over to the catalyst species on the char following the increase of activity of these catalysts as is illustrated in Figure 6. Two mechanisms were proposed for catalyzed and uncatalyzed hydrogasification as the spill-over mechanism and the C-C bond weakening mechanism. The idea (24) Butler, R.; Snelson, A. Fuel Processing Technol. 1979, 2, 17. (25) Conner, W. C., Jr.; Pajonk, G. M.; Teichner, S. J. Adoances in Catalysis; Academic Press: New York, 1986; Vol. 34, pp 1-79.
Energy & Fuels 1991,5, 63-68
mentioned above supports the spill-over mechanism in which the rate-determining step is the dissociation of molecular hydrogen. I t is also possible that hydrogen atoms spill-over to carbon directly and promote the breaking of C-C bonds. In fact, Ni/diatomite enhances the reaction rate of demineralized Yallourn char as depicted in Figure 3. However, demineralized char contains small amounts of iron as low as 14 ppm and iron has catalytic activity for hydrogasification at its very low loading level as shown in Figures 2 and 5. Therefore, the effect of Ni/diatomite on demineralized char can be said to be due to a trace of iron in the char. Ni/diatomite enhances the catalytic activity of a trace of iron in demineralized char. The Arrhenius plot is shown in Figure 7 for cobaltcatalyzed hydrogasification with and without Ni/diatomite. The activation energies were 119 and 97.2 kJ/mol with and without Ni/diatomite, respectively. The difference in the value of activation energy was not so large, although
63
the reaction rates are so different. If spilt-over hydrogen generated on Ni/diatomite is the explanation for the enhancement of cobalt-catalyzed hydrogasification, the value of the activation energy when mixed with Ni/diatomite refers to the activation energy of the next rate-controlling reaction, possibly C-C bond weakening. This speculation means that the energy between hydrogen dissociation and C-C cleavage is not very different, although the rate-determining step is hydrogen dissociation for iron group metals. Also, C-C bond breaking can be the rate-determining step for other group 8 metals like platinum which should have more ability to promote hydrogen dissociation than iron group metals. Furthermore, this study shows it is possible that iron group metals, especially nickel, can have high catalytic activity without Ni/diatomite by changing the loading method and/or improving the degree of dispersion of the catalyst. Registry No. Fe, 1439-89-6; Co, 1440-48-4; Ni, 1440-02-0.
A Novel Liquid Fluidized Bed Microreactor for Coal Liquefaction Studies. 1. Cold Model Results Henry W. Haynes, Jr.,* Ronald R. Borgialli, and Tiejun Zhang Chemical Engineering Department, University of Wyoming, P.O. Box 3295, University Station, Laramie, Wyoming 82071 Received April 19, 1990. Revised Manuscript Received August 23, 1990 The design of a novel liquid fluidized bed microreactor, intended for use in catalytic coal liquefaction studies, is based largely upon results obtained in a Plexiglas cold model. The catalyst is contained in a cylindrical basket having an impermeable wall and wire screen ends which sits near the bottom of a 100-cm3autoclave. An impeller directly above the basket circulates fluid down the annular space between the basket and vessel wall and up through the catalyst bed where the catalyst is maintained in a fluidized or “ebullated” state. The cold model studies have identified two types of impellers which perform very well depending upon the medium employed, i.e., whether liquid/solid or gas/ liquid/solid. Correlations have been developed for predicting the critical stirrer speeds corresponding to incipient fluidization and particle carryover.
Introduction The laboratory scale testing of heterogeneous catalysts for coal liquefaction applications poses a difficult experimental problem. High-pressure batch autoclave reactors take from many minutes to hours to bring to reaction temperature and therefore it is difficult to establish the reaction time with certainty. Of even greater concern is the fact that results from a batch autoclave experiment reflect a catalyst’s initial activity. It is well-known from experience with petroleum-processingcatalysts that initial activity and selectivity may not be a good indicator of “lined-out” performance which is typically achieved after several days on stream. It is not possible to evaluate catalyst deactivation rates in a batch reactor because deactivation kinetics is confounded with the main reaction kinetics. These considerations dictate that the catalysts be tested in a continuous flow unit. Several types of continuous flow laboratory reactors have seen service in multiphase (gas/liquid/solid) applications such as coal liquefaction. Mahoney et al. describe a
* To whom
correspondence should be addressed.
spinning basket configuration in which the catalyst is contained in an annular basket attached to a rotating shaft.’ Baffles inside, above, and below the basket direct fluid through the basket. In another design described by Mahoney, the annular catalyst basket is fixed and the impeller is designed to circulate fluid radially through the basket.2 Autoclave Engineers, Inc., Erie, PA, markets reactor internals corresponding to both of these configurations and several others as well. These are described in their Bulletin 1200. A potential problem with any reactor design that employs fixed or stationary beds of catalyst in a coal liquefaction application is the buildup of ash, metals, and carbonaceous residue on the external surface of the particles. In severe cases the particles may become cemented to one another, but in any case this buildup is likely to lead to a premature deactivation of the catalyst. Such deposits (1) Mahoney, J. A.; Robinson, K. K.; Myers, E. C. CHEMTECH 1978, 8, 758-763.
(2) Mahoney, J. A. In NATO Advanced Study Institutes Series E Applied Sciences Rodrigues, A. E., Calo, J. M., Sweed, N. H., Ede.; Sijthoff Noordhoffi Rockville, MD, 1981; No. 52, pp 487-513.
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