Ind. Eng. Chem. Res. 1987,26, 383-386
dependent data are required to distinguish between the two models. The kinetic parameters (k3/and k,) were used in the numerical simulation, incorporating 10 cycles of temperature modulation. B. Time-Dependent Methanation Rate. Figure 2 shows the time-dependent methanation rates during the tenth cycle of MCTA-T for different common temperatures. When the temperature is lower than the transition temperature, the methanation rate wave form is symmetric and similar to the input sinusoidal temperature wave. For this region there is good agreement with the experimental data. This is indicated by Figure 2a-c. Asymmetric wave forms of the methanation rate occurred above the transition temperature. For this region, the model based on direct surface suppression by carbon predicts a lower methanation rate than that of the experimental results. The basic model predicts a rate higher than that found experimentally because no steps in the overall model account for a possible transition in the kinetics. The kinetic parameters determined from the steady-state methanation data support the proposal by Yang to explain the transition in kinetics. When this model is employed, the simulation results show good agreement with the experimental results.
Conclusions The results reported here demonstrate that unidirectional site-blocking models are not applicable in the experimental range studied. Only small differences between the initial steady-state rate and that measured after temperature modulation indicate that the rate of formation of site-blocking material, less active carbon, is small, and virtually all the surface sites are still active after MCTA-T experiments. The deposited carbon can be removed by TPR. This procedure results in a single asymmetric peak. In addition to steady-state data, MCTA-T provides the time-dependent rate of the methanation reaction when the reactor is subjected to temperature modulation. Figure 2 shows the time-dependent differences between model predictions and the experimental results. The effect of a change in the rate constant in the model predicts the experimental results well.
383
Acknowledgment This work was supported by the Division of Chemical Sciences, Office of Basic Energy Research, under the Department of Energy Contract DE-AC02-84ER 13158.
Literature Cited Dalmon, J. A.; Martin, G. A. J. Catal. 1983, 84, 229. Goodman, D. W.: Kelly, R. D.; Madey, T. E.; Yates, J. T., Jr. J. Catal. 1980, 63, 226.Goodman. D. W.: Yates. J. T.. Jr. J. Catal. 1983.82. 255. Ho, S. V.{Harriott, P. C a t h 1980, 64, 272. Huang, Y. J. Master Thesis, Syracuse University, Syracuse, NY, 1984. Huang, Y. J.; Schwarz, J. A.; Heydweiller, J. C. Ind. Eng. Chem. Fund. 1986a, 25(3), 402. Huang, Y. J.; Schwarz, J. A. Appl. Catal. 1986b, in press. Huang, Y. J.; Schwarz, J. A., submitted for publication in Appl. Catal. 1986~. Lee, P. I. Ph.D. Dissertation, Syracuse University, Syracuse, NY, 1983. Lee, P. I.; Schwarz, J. A. J. Catal. 1982, 73, 272. Lee, P. I.; Schwarz, J. A.; Heydweiller, 3. C. Chem. Eng. Sci. 1985, 40(3), 509. Lee, P. I.; Schwarz, J. A. Ind. Eng. Chem. Process Des. Dev. 1986, 25(1),76. Martin, G. A.; Primet, M.; Dalmon, J. A. J . Catal. 1978, 53, 321. McCarty, J. G.; Wise, H. J. Catal. 1979, 57, 406. Polizzotti, R. S.; Schwarz, J. A.; Kugler, E. L. Proceeding of the Symposium on Advances in Fischer-Tropsch Chemistry; American Chemical Society: Washington, DC, 1978. Polizzotti, R. S.; Schwarz, J. A. J. Catal. 1982, 77, 1. Wentrcek, P. R.; Wood, B. J.; Wise, H. J . Catal. 1976,43, 363. Wolf, E. E.; Alfani, F. Catal. Rev.-Sci. Eng. 1982, 24(3), 329. Yang,C. H.; Soong, Y.; Biloen, P. Presented at the 6th International Congress on Catalysis, Burlin, Germany, 1984.
i.
,
I
*Author to whom correspondence should be addressed.
Yao-Jyh R. Huang, James A. Schwarz* Department of Chemical Engineering and Materials Science Syracuse University Syracuse, New York 13244 Received for review January 2, 1986 Reuised manuscript received September 19, 1986 Accepted October 30, 1986
Catalytic Gasification of Rice Hull. 3. Measurement of Reaction Efficiency in the Steam Reforming of Carbonaceous Material Steam reforming of hydrocarbons provides an effective method of releasing hydrogen from a water molecule. The efficiency of the reaction depends on the structure of the reactant and catalysts and the reaction conditions. T h e traditional index, AT, for measuring reaction efficiency using the temperature difference between the actual reaction temperature and the thermodynamic equilibrium temperature of the product gases is inadequate. A new index, REST, is proposed t o indicate the reaction efficiency of steam participation in the steam reforming reaction; this index is calculated from the ratio of actual steam consumption vs. the theoretical requirement of steam. With both AT and REST indexes, the efficiencies of steam reforming reactions of biomass (rice hull), methanol, and hydrocarbons (methane and naphtha) are analyzed with respect t o the effects of temperature, H,O/C ratio, and Ni surface area. T h e results indicate t h a t REST approach provides greater sensitivity and more universal applicability in the general scope of this reaction. Steam reforming of methane to produce hydrogen is usually carried out in the presence of catalyst at a reaction temperature higher than 600 OC (Allen et al., 1975; Karim and Metwally, 1979; Singh and Saraf, 1979). At these conditions the reaction is virtually complete and the reaction efficiency is often measured by the temperature differential approach (AT) to compare with thermodynamic equilibrium (Akers et al., 1970; Anderson et al., 1984). The temperature differential approach is defined 0888-5885181/2626-0383$0l.50/0
as the difference between the actual reaction temperature (TRx)and the corresponding temperature (TEq)of the equilibrium constant calculated from the composition of the actual reaction products (Akers et al., 1970). The smaller the value of AT, the closer the reaction is to the thermodynamic equilibrium. When heavier hydrocarbons are used as feedstock, the reaction becomes more complicated. Unless prior formation of methane is involved (Yarze and Lockerbie, 1962) 0 1987 American Chemical Society
384 Ind. Eng. Chem. Res., Vol. 26, No. 2, 1987 Scheme I r i c e hull
-
primary intermediates ( V P I )
char
H20
tar
HZ -t CO f
Cop
011
and steam reforming of it is the rate-determining step, the A T measurement will not be a suitable index of the efficiency of the overall steam reforming reaction. Recent workers (Ross et a1.,’1978; Tottrup, 1982; Bhatta and Dixon, 1969) appear to favor oxygenolysis reaction of heavier hydrocarbons to produce both hydrogen and carbon monoxide. Methane is than a secondary reaction product via the methanation reaction. The steam reforming of rice hull generally proceeds in two major stages as shown in Scheme I (Rei et al., 1986a, 1986b). A t the first stage the macromolecular solid structure of rice hull is broken down into volatile primary intermediates (VPI)and char; the former (containing less than 3 % methane) then undergoes multistaged oxygenolysis reaction in the presence of steam and catalyst. If the catalyst is an effective one, most of the VPI will eventually be converted into gaseous products; otherwise, some may go to the formation of tar oil and additional char. In most cases, the pyrolytic reaction to form char and VPI is quantitative when the contact time is long enough and reaction temperature is higher than 600 “C (Lin et al., 1986). In these cases, production of hydrogen and carbon oxides directly from steam reforming of methane will represent only a minor fraction of the total gas production. Therefore, the A T approach will not provide a fair assessment. Steam reforming of methanol to produce hydrogen is another interesting reaction and is gaining greater importance owing to the abundant supply of methanol from oil-producing countries. Methanol reacts readily with steam to yield hydrogen and carbon oxides. With CuOZnO or CuO/Si02 (Takahashi et al., 1982) as the catalyst, hydrogen and carbon dioxide are produced almost exclusively. Under these reaction conditions, the amount of hydrogen is more than what can be accounted for from the thermodynamic calculations, and the nearly absence of methane and carbon monoxide are imcompatible with the existences of thermodynamic equilibria of methane steam reforming and of water gas shift reactions. Accordingly, use of A T will not serve the purpose of measuring the efficiency of catalyst in this reaction having no steam reforming of methane. All these examples indicate that the traditional A T approach is inadequate to provide an efficient measurement for the general steam reforming of carbonaceous reactants other than methane. Furthermore, it is highly desirable to assess how far away the actual yield is from the ultimate yield of the reaction and how efficient the steam is being “split” to provide hydrogen by the given catalyst under the set of reaction conditions. Afterward, steam reforming of a carbonaceous reactant to produce hydrogen is mainly a way of obtaining extra hydrogen from the water molecule in addition to those from the hydrogen atoms of the reactant itself. The higher the efficiency of steam participation, the more desirable the reaction is if the energy cost in the endothermic nature of this reaction can be taken care of by other means. Therefore, it will be useful to have a new index to measure the efficiency of reactant and catalyst under a given reaction condition in “extracting” hydrogen from steam. This type of index will provide a guide for a process synthesizer to select the most
efficient catalyst and economical reactant, i.e., coke, biomass, gas oil, naphtha oil, or methane, in the most favorable reaction conditions to produce hydrogen from the steam reforming reaction. Accordingly, we would like to propose an index, namely NOST or the number of oxidizing steam, to indicate the actual molar amount of steam which oxidizes one carbon unit of carbonaceous material to form carbon oxides and splits itself into hydrogen. In addition, for any substrate there is a stoichiometrically determined maximum number of steam molecules available for steam reforming, (NOST), (eq 1). Using this concept, we can then define C,H,O, + (271 - x)HzO nCOs + (4n + m - 2x)/2Hz (1) +
the reaction efficiency of steam, REST = (NOST)/ (NOST),, in oxidizing carbonaceous species to hydrogen and carbon dioxide in a steam reforming reaction. For steam reforming of methanol, (NOST), is 1, for methane or naphtha it is 2, and for rice hull it is 1.18 (eq 2-5). CHSOH + lHzO COZ + 3Hz (2) CH,
+ 2H20
CnH2,Zn+ 2nHZO
-
-
-+
COP + 4H2
+ 3.lnHz CO2 + 2.12H2
nCOs
CH1.8800.82 + 1.28HZO --*
(3)
(4) (5)
By the use of this index for reaction efficiency of steam, a catalyst providing a high value of REST can be judged as an effective catalyst in the hydrogen production from steam through the steam reforming reaction; moreover, a high value of REST also indicates a high gas yield with a high percentage of hydrogen content.
Results and Discussion Tables I and I1 present the results from the steam reforming reactions of rice hull, methanol, methane, and naphtha oil. Both AT and REST approaches are used to compare the effect of temperature, catalyst surface area, steam/carbon ratio, and nature of feed on the variations of these two measurements. Steam Reforming of Rice Hull. An electrically heated fluidized-bed reactor (i.d., 5 cm; length 72 cm; stainless steel 316) containing 600 g of commercial nickel catalyst (G56H2 or RKS1, 125-600 pmj was fludized with steam (450 “C) and maintained at the desired reaction temperature by adjusting the electric current. Rice hull (300-600 wm, 8% moisture content by weight) was introduced into the fluid catalyst above the porous stainless steel distributor via a variable-speed screw feeder. Gasification took place immediately; the exit stream after passing through a cyclone and condenser was measured with a wet test meter for the gas volume. As shown in Table I, the first four runs illustrate the effect of reaction temperature; they indicate that a “good run” (high hydrogen fraction and gas yield) always has a high value of REST. As the reaction temperature is raised higher, more VPI is produced from the rice hull; this leads to more production and a higher hydrogen percentage until 600 “C is reached. At 550 “C, the pyrolysis is incomplete and the VPI formation is insufficient; this leads to a lower gas yield and, hence, a lower REST as shown in R1 and R2. As the pyrolysis approaches complete conversion (when the reaction is over 600 “C), the amount of VPI also reaches a constant level; therefore, the REST increment becomes saturated. This enables one to conclude that an optimum reaction temperature will be between 600 and 650 “C for the steam reforming of rice hull with the catalyst. With the AT approach, increasing AT values provide
Ind. Eng. Chem. Res., Vol. 26, No. 2, 1987 385 Table I. Effect of TemDerature. Catalyst, and Steam/Carbon Ratio on the Efficiency of Steam Reforming of Rice R5 R6 R3 R4 R7 R8 R1 R2 no. 650 700 650 650 650 650 550 600 T , "C 5.4 5.4 2.6 3.6 1.6 0 5.4 5.4 Ni" surface, m3/g 3.7 3.7 3.7 3.7 3.7 3.7 3.7 3.7 H,O/C, mol 0.37 0.37 0.37 0.37 0.37 0.37 0.37 WHSV, h-l 0.37 62.2 62.7 65.2 41.0 65.0 60.0 6i.9 62.8 H2, mol % 26.1 8.8 9.8 8.5 6.0 7.1 9.0 6.0 CO, mol 70 28.4 25.9 26.5 27.6 27.6 25.4 31.3 28.8 CO,, mol % 0.5 1.5 1.2 0.3 2.8 2.3 0.6 5.6 CH,, mol 70 1.76 1.61 1.10 1.75 1.45 1.70 1.78 0.72 gas yields, L/g 89.3 89.9 82.5 51.8 41.8 73.2 85.8 89.8 conversionb 0.81 0.70 0.22 0.54 0.82 0.84 0.85 -0.35 NOST' 68.6 59.2 18.9 70.9 72.1 -29.5 46.0 69.6 REST, '7c 21 190 20 56 90 AT, "C 0 25 50
Hull R9 650 1.6 2.5 0.37 59.9 6.5 27.6 1.2 1.16 50.3 0.19 16.1 85
'Ni surface area is determined by the hydrogen chemisorption from 600 "C downward to 20 "C. bCarbon conversion is defined as the total carbon atoms of gas products divided by the carbon atoms of rice hull. Mole number of participation steam per mole of carbon unit of reactant; here, VPI is taken as the reactant. NOST = '/,[(hydrogen atoms of gas products) - (hydrogen atoms of reactants)]/(carbon atoms of reactants).
Table 11. Effect of Catalyst, Temperature, and Steam/Carbon Ratio on the Efficiency of Steam Reforming of Methanol, Naphtha, and Methane substrate methanola naphthaC methaneC run no.
M1
M2
M3
M4
M5
M6
M7b
T , "c
300 4.0 0.12 65.2 32.9 0.8 1.1 0.66 32.7 0 0 -253
350 4.0 0.12 64.0 26.1 5.4 4.5 1.46 75.0 0.02 2.2 -220
400 4.0 0.12 64.4 14.1 16.1 5.4 1.97 100 0.12 11.5 -170
450 4.0 0.12 68.0 7.4 19.3 5.3 2.19 100 0.46 46.0 -110
500 4.0 0.12 70.0 4.8 21.3 4.1 2.33 100 0.60 60.0 -70
550 4.0 0.12 70.8 4.6 20.8 3.7 2.4 100 0.69 68.8 23.0
280 4.0 36.5 75.0
H,O/C, mol WHSV, h-l H,, mol % CO, mol % CO,, mol % CH,, mol % gas yield: L/g conversion NOST REST, 70 AT, "C
+o.o 25.0 +O.O
1.62 58 0.58 58.0
N1 760 444 3.75 0.27 63.9 9.9 17.4 8.4
N2 800 372 5.04 0.29 74.0 11.2 13.2 1.5
Me1 766 455 3.85 0.27 68.3 9.9 11.9 9.9
Me2 796 (out) 447 (in) 4.46 0.32 67.7 10.2 11.4 10.7
100 1.07 53.4 3.2
100 1.43 71.7 6.0
68.7 0.56 28.2 0.8
67.0 0.62 31.2 15.0
OExcept M7, runs M1-M6 were carried out in a fluidized-bed reactor containing nickel catalyst, Ni/a-AlzO, with nickel surface area of 2.8 m2/g. bA CuO-ZnO (40/60 wt % ) catalyst was used; the catalyst has a BET surface area of 32 m3/g. cThe steam reforming reactions were carried out with a commercial nickel catalyst according t o literature (Singh and Saraf, 1979). dGas volume corrected to standard conditions.
a reversed conclusion from the REST approach. The TEq, using the actual reaction products, is apparently further lagging behind the actual reaction as the reaction temperature is hiked; it is speculated that the supply of methane from either methanation of carbon monoxide or further degradation of VPI surpasses the actual capability of catalyst to consume it. Thus, one can readjust the reaction conditions to meet the catalyst activity or search for a more active catalyst to consume the methane intermediate. Here, the REST approach points to the appearance of a saturation of yield with a further increase of the reaction temperature, and an optimum reaction can then be determined for a catalyst with a given efficiency. The AT approach, on the other hand, indicates an incapacitation of catalyst efficiency and alerts the researcher to trace the cause of this drop. One way to trace catalytic efficiency is to look into the variation of the active surface area of a nickel catalyst. Runs R5-R8 and R3 show the effect of increasing the Ni surface area from 0.8 to 5.4 m2/g on the gas yield and hydrogen content. As expected, a higher surface area provides better results as judged from both AT and REST approaches. However, when corundum is used as the fludized medium, the REST value becomes negative. This is attributed to the net increase of water formation and hence loss of hydrogen from the reactant OF VPI itself, possibly, due to the dehydration of hydroxyl groups. The AT approach, however, is unable to pick up such a cause
because material balance is not used in the calculation of AT. Moreover, this series of reactions indicates a further decline of catalyst capacity to consume methane as the Ni surface area is decreased. Comparison of runs R3 and R8 provides another parameter. An increase of Ni surface area from 3.6 to 5.4 m2/g has caused a small increase of catalytic efficiency, and hence, the observed minor increase of the REST value from 68.6% to 70.0% can perhaps be attributed to the appearance of excessive Ni surface area for the given VPI concentration and the thermodynamic limitation via water gas shift reaction. Consequently, one can conclude that for the given reaction conditions (temperature, H,O/C, and WHSV) a catalyst of 3.6 m2/g will be sufficient. The slight difference in AT values of these two reactions probably is the results of experimental uncertainty in the measurement of CO and CH, concentrations. Finally, runs R6 and R9 compare the effect of the H,O/C ratio; here again REST enables one to understand the effect of H 2 0 / C more clearly than that provided by the AT approach. Steam Reforming of Methanol. Steam reforming of methanol provides an another important method of producing hydrogen; the composition of gas products depends keenly on the nature of the catalyst. With the CuO-ZnO catalyst, only very small amounts of methane and carbon monoxide are detected in the product mixture. It was reported that carbon dioxide is directly produced from the decomposition of the formate intermediate rather than from the water gas shift reaction of carbon monoxide
386 Ind. Eng. Chem. Res., Vol. 26, No. 2, 1987
(Takahashi et al., 1982). With the nickel catalyst, methanol decomposes directly to hydrogen and carbon monoxide in the absence of steam (Gates et al., 1985). In the presence of steam, these two primary products may undergo methanation and water gas shift reaction to form methane and carbon dioxide (Hsu, 1985). These two reactions, however, are insignificant at temperatures lower than 350 "C. In the case of methanol, methane is a net product rather than a reactant; the AT approach will be inappropriate due to its negative value. This prediction is confirmed from the negative values of A T shown in Table 11. These data were obtained from a fluidized-bed reactor system (i.d., 5 cm; length, 25 cm; stainless steel 304, containing 100 g of catalyst), and methanol was premixed with water, vaporized, and preheated to a desired temperature. As mentioned above, methanol can decompose to hydrogen and carbon monoxide in the absence of steam when nickel catalyst is used; in this case, REST will be zero. However, at low reaction temperatures, REST is also zero even in the presence of steam as shown in run M1. This can be interpretated as the sluggishness of steam participation in comparison with the direct decompositions of methanol. As the temperature increases, the formation of carbon dioxide and methane becomes significant owing to water gas shift and methanation reactions of carbon monoxide. The former, however, is favored by the large excess of steam; consequently, an increase of the REST value will be expected as shown by runs M5 and M6. Run M7 illustrates the extremely high efficiency of the CuOZnO catalyst in promoting steam participation, and the absence of methane in the reaction product will make the AT approach impractical. Steam Reforming of Hydrocarbons. Steam reforming of hydrocarbons, such as methane or naphtha, has been the major source of industrial hydrogen. These reactants carry 2.2-4 hydrogen atoms per carbon atom; therefore, they provide a ready source of hydrogen, albeit expensive. In the steam reforming reaction of hydrocarbons, a maximum of 2 mol of water molecules per carbon atom can be reformed to produce hydrogen. In order to compare the effect of different reactants, steam reforming reactions of naphtha oil (N series) and of methane (M series) over a commercial nickel catalyst in fixed-bed reactors are chosen (Singh and Saraf, 1979). As shown by runs N1 and N2, the NOSTs range from 1.0 to 1.4, much higher than those in the steam reforming of rice hull but a t par with those of methanol. The high RESTS of runs N1 and N2 are obtained at the expense of high reaction temperatures and slightly higher H,O/C ratios. In the steam reforming of methane, both NOST and REST are much lower than any one of the above three. Apparently hydrocarbon is not an effective reactant for steam reforming reactions with respect to the additional hydrogen production from water. In view of the desirability of the lower reaction temperature to achieve a comparable level of the REST value, an alcoholic reactant, methanol or rice hull, provides a more effective reactant for the steam reforming reaction.
Conclusion Although the AT approach has long been used as a measurement of catalyst efficiency in the steam reforming of methane or naphtha, it is, however, inadequate as a measure of reaction efficiency in the steam reforming of general carbonaceous reactants, particularly for those reactions in which methane is neither a reactant nor a viable intermediate such as rice hull or methanol. The newly proposed REST approach, on the other hand, offers a wide range of applicability to account for the extent of steam participation and the amount of gas yield as well as the percentage of hydrogen contained in the product. On the basis of the REST index, one can then decide how good a reactant is and how far away the gas yield is from the ultimate yield in the steam reforming of a carbonaceous reactant. (On the basis of the REST index shown in Tables I and 11, one reaches a tentative conclusion that alcoholic compounds, such as waste carbohydrates, are an ideal feedstock for the hydrogen production from steam reforming reactions). Therefore, the REST approach enables one to differentiate clearly the feedstock efficiency in steam reforming reactions for hydrogen productions. Registry No. CuO, 1317-38-0; ZnO, 1314-13-2; CH,OH, 6756-1; CHI, 74-82-8; CO, 630-08-0; COZ, 124-38-9; H,,1333-74-0; Ni, 7440-02-0.
Literature Cited Akers, W. W., et al. Catalyst Handbook; Imperial Chemical Industries Limited: London, 1970; p 87. Allen, D. W.; Gerhard, E. R.; Likins, M. R. Ind. Eng. Chem. Process Des. Deu. 1975, 14(3), 256. Anderson, J. R.; Boudart, M.; Rostrup-Nielsen, J. R. Catalysis Science and Technology; Springer-Verlag: Berlin, Heidelberg, 1984; Vol. 5, Chapter 1. Bhatta, K. S. M.; Dixon, G. M. Ind. Eng. Chem. Prod. Res. Deu. 1969, 8(3), 324. Gates, S. M.; Russell, J. N., Jr.; Yates, J. T., Jr. J. Cata2. 1985, 92, 25. Hsu, U. Z. MS Thesis, National Taiwan University, 1985. Karim, G. A.; Metwally, M. M. Int. J. Hydrogen Energy 1979,5,293. Lin, F. S.; Chang, T. S.; Rei, M. H. Agric. Wastes 1986, in press. Rei, M. H.; Yang, S. J.; Hong, C. H. Agric. Wastes 1986a, in press. Rei, M. H.; Lin, F. S.; Su, T. B. Appl. Catal. 1986b, 26, 27. Ross, J. R. H.; Steel, M. C. F.; Zeini-Isfahani, A. J . Catal. 1978, 52, 280. Singh, P. P. C.; Saraf,N. D.; Ind. Eng. Chem. Process Des. Dev. 1979, 18(1), 1. Takahashi, K.; Takezawa, N.; Kobayashi, H. Appl. Catal. 1982,2, 363. Tottrup, P. B. Appl. Catal. 1982, 4, 377. Yarze, J. C.; Lockerbie, T. E. Prespnted at the 137th National Meeting of the American Chemical Society, Columbus, OH, 1960; Chem. Abstr. 1962,57, 6209.
Min-Hon Rei,* Tien-Bau Su, Fwu-Shing Lin Process Synthesis Laboratory Department of Chemical Engineering National Taiwan University Taipei, Taiwan 10764, Republic of China Receiued for review March 7, 1986 Accepted October 29, 1986
