Ind. Eng. Chem. Prod. Res. Dev. 1980, 79, 385-388
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Brouwer, D. M., Chem. Ind., 1459 (1970). Brouwer, D. M., Oelderik, J. M.. Red. Trav. Chim., 87, 721 (1968). Burwell, R. L., Gordon, G. S., J. Am. Chem. Soc., 70, 3128 (1948); 71, 2325 (1949). Canterford, J. H., O'Donnell, T. A., Inorg. Chem., 5, 1442 (1966). Condon, P. R., in "Catalysis", Vol. VI, Chapter 2, P. H. Emmett, Ed., Reinhoid, New York, 1958. Emeleus, H. J., Guttmann, V., J . Chem. SOC., 2115 (1950). Everlng, B. L., Adv. Catal., 6, 197 (1954). Everlng, B. L., D'Ouvilie, E. L., Lien, A. P., Waugh, R. C., Ind. Eng. Chem., 45, 582 (1953). Gillespie, R. J., McMaster University, personal communication, 1979. Hogeveen, H., Bickel, A. F., J . Chem. SOC., Chem. Commun., 635 (1967). Hogeveen, H., Gaasbeek, C. J., Bickel, A. F., Red. Trav. Chim., 88, 703 (1969). Hogeveen, H., Gaasbeek, C. J., Red. Trav. Chim., 86, 719 (1969). Hyman, H. H., Ph.D. Thesis, University of Illinois. Leighton, P. A., Heldman, J. D., J . Am. Chem. SOC.,33, 822 (1946). Lien, A. P., McCaulay, D. A., US. Patent 2683763 (June 13, 1954). McCaulay, D. A., J . Am. Chem. SOC., 81, 6437 (1959).
385
Muetterties, E. L.,Castle, J. E., J . Inorg. Nucl. Chem., 18, 148 (1961). Nenitzescu, C. D., Avranc, M.,Sliam, R., Bull. SOC. Chim. Fr., 1268 (1955). Oblad, A. G., Gorin. M. H., J . Am. Chem. SOC.,33, 822 (1946). O'Donnell, T. A., "The Chemistry of Fluorine", vol. 5, p 1078, Pergamon Press, Oxford, England, 1973. Oelderik, J. M., Mackor, E. L., Platteeuw, J. C., van der Wiel, A., US. Patent 3 201 494 (Aug 17, 1965). Olah, G. A., US. Patent 3766286, (Oct 16, 1973). Otvos, Y. W., Stevenson, D. P., Wagner, C. D., Beeck, 0.. J. Am. Chem. Soc.. 73, 5741 (1951); 74, 3269 (1952). Passino, H. J., Rearick, J. S,,U.S. Patent 2423045 (June 24, 1947). Pines, H., Adv. Catal., 1, 201 (1955). Pines, H., Wackher, R. C., J. Am. Chem. SOC.,65, 595, 1642, 2518 (1946). Powell, T. M., Reid, E. B., J . Am. Chem. Soc., 67, 1020 (1945). Schneider, A., U S . Patent 3091 649 (1963). Siskin, M., Tetrahedron Lett. 527, (1978) (Part 7 of this series).
Received for reuiew April 23, 1979 Accepted April 23, 1980
Simultaneous Methanation of CO and CO, on Supported Ni-Based Composite Catalysts Tornoyuki Inui, Masaki Funabikl, and Yoshinobu Takegaml Department of Hydrocarbon Chemistry, Faculty of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606 Japan
Simultaneous methanation of CO and COPwas investigated using highly active novel type composite catalysts to provide the foundation for a new efficient process of methane production. The catalyst constituents such as 5% Ni-2.7% La203-0.6% Ru were supported on a spherical (3.0 mm) silica carrier having meso (5 nm) and macro (600 nm) bimodal pore structures. The reaction-gas mixture, containing a considerably high concentration of CO and COPwith excessive hydrogen, was allowed to flow at high space velocity at atmospheric pressure. With the rise in reaction temperature, C02 methanation was enhanced in succession to almost total consumption of CO, and the complete conversion of both CO and C02 into methane with 100% selectivity and a high space-time yield (61 mol dm-3 h-') at a low temperature such as 270 O C was achieved. On the basis of kinetic data in consideration with heat and mass transfer, the cause of this enhancement is discussed.
Introduction The methanation reaction for CO (1)has been applied CO
+ 3Hz
-+
CH4
+ HzO
(1)
for a long time in the purification process of gaseous materials for ammonia synthesis, since CO acts as an inhibitor to the catalyst. In this process, it is sufficient that only a small concentration of CO is hydrogenated into inactive methane. However, methanation nowadays has come to be utilized more and more for a new process of synthesizing a clean and high calorific gaseous fuel: methane. For this purpose, it is necessary for a gas containing a relatively high concentration of CO to be made to flow through a catalytic bed at a high flow rate and react with hydrogen a t a high conversion rate, if possible, with complete conversion. In order to achieve this, a conventional nickel catalyst is required to improve thermal stability because of the great exothermicity combined with the high reaction rate (Mills and Steffgen, 1973). On the other hand, COPis formed as a byproduct in the production process of synthetic gas by the steam-reforming reaction of petroleum naphtha or liquefied petroleum gas followed by methanation, but the COP is wasted as a nonreactive material. With regard to this process, a case even appeared in which C 0 2 was used as a retardative agent to inhibit the shift reaction between CO and H 2 0 0196-4321 /80/1219-0385$01 .OO/O
(2) during the CO methanation (Grabaski and Diehl, 1973).
CO
+ HzO
--*
COZ
+ Hz
(2)
However, importance should be attached to the effective use of COPfor methanation from the viewpoint of a cyclic use of carbon resources whose supply will be more limited in the near future. Hence a catalyst having a sufficient methanation activity for COz (3) would be needed. C02
+ 4H2
-
CH4
+ 2H20
(3) The authors have recently developed a certain novel type composite supported catalyst for CO or C 0 2 methanation (Inui et al., 1979a), and for methane formation by direct hydrogenation of active carbon (Inui et al., 1979b). For the catalyst, iron-group metals were used as the substrate, and small amounts of lanthanide oxides and very small amounts of platinum-group metals were used 85 promoters. The combined catalyst components had a remarkable effect for methanation rates, especially for C02 methanation with a lower activation energy than that for CO methanation. Consequently, an isokinetic temperature for both CO and C 0 2 methanation was observed in a very much lower temperature range than that expected in a conventional nickel catalyst. This distinctive catalytic performance suggested that if a reaction gas mixture containing CO, C02,and Hz would be flowed through the catalyst bed 0 1980 American
Chemical Society
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386 Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
a t a temperature close to or above the isokinetic temperature, simultaneous methanation of CO and COz, which was never realized effectively, would occur. Thus, this study attempted to apply the composite catalyst for simultaneous methanation with a complete conversion of CO and C 0 2 and to supply the foundation for a new more efficient process of methane production. Experimental Section Catalysts. Three kinds of catalysts supported on a spherical silica support of 3.0-mm diameter, having meso (5 nm) and macro (600 nm) bimodal pore structures, were used. The catalyst constituents were 5.0% Ni, 4.6% Ni2.6% La2O3, and 5.0% Ni-2.7% La203-0.6% Ru. Research grade Ni(N03)z-6Hz0,La(N0J3.6H20, and RuCl3.3H20 were used as reagents. The sequence of the procedures was incipient impregnation of the aqueous solution, drying, ammonia-water vapor treatment, thermal decomposition,hydrogen reduction, and thermal treatment in hydrogen stream a t 400 "C for 30 min. Details of the preparation methods and physical properties of the catalysts have been presented in our previous paper (Inui et al., 1979a). Apparatus and Technique. An ordinary flow reaction apparatus was used a t atmospheric pressure; 1-16 pieces of each kind of catalyst sphere were packed in single file in a Pyrex glass tubular reactor with an inside diameter of 3.77 mm. A reaction-gas mixture was allowed to flow through the catalyst bed changing the temperature from 160 to 400 "C a t a space velocity ranging from 10000 to 220000 h-l (STP); 0.06 atm of inlet partial pressure of both CO and C 0 2 was mainly used. The remainder was hydrogen. When the effect of additives in the reaction-gas stream on the reaction rate was examined, the additives were replaced with hydrogen amounting to 0.01-0.04 atm. The reaction temperature was measured by connecting the top of a thermocouple shielded with stainless steel (0.6 mm outer diameter) to the outer surface of the last catalyst sphere in the bed. The temperature measured by this method has been confirmed to be representative of the reaction temperature throughout the catalyst bed with 16 catalysts below 28% CO conversion under the following conditions: 0.12 atm of CO, 0.88 atm of H2, and 10000 hourly space velocity (Inui et al., 1979a). However, above 28% CO conversion, generation of temperature profile in the reactor was assumed. A quartz tube with an inner diameter of 2.3 cm, heated electrically from outside, was used as the heating bath. Part of the effluent gas was analyzed after drying with a -20 "C trap with a Yanagimoto TCD-FID type gas chromatograph with columns of Porapak Q, X-28, or MS-5A, and with He used as the carrier gas. Results a n d Discussion Preformance of Each Catalyst. The methanation by the use of a mixed gas containing 6% CO, 6% C02, and 88% H2 was carried out, respectively, on the three kinds of catalysts using 16 pieces with a space velocity of 11400 h-l. The reaction temperature was raised until carbon oxides were completely converted. The temperature dependence of CO and C 0 2conversion for each catalyst is shown in Figure 1. The rate of solomethanation of C02 is greater than that of CO on the Ni-La203 or Ni-La203-Ru catalyst in the lower reaction temperature range (Inui et al., 1979a). Nevertheless, in coexistence of CO and C 0 2 the methanation of CO occurred preferentially on every catalyst, and the COz methanation first proceeded after CO was almost completely consumed. Especially, on the bi- and tri-component catalysts, the C 0 2 methanation followed at an accelerated
100
80
2
60
z
0 m
t
40
z 0 u
20
n 250
200
350
300
400
TEMPERATURE ( " C !
Figure 1. Temperature dependence of CO and C02 conversion in the co-methanation on various catalysts: circle, Ni-Laz03-Ru; triangle, Ni-Laz03; square, Ni; open symbol, CO conversion; filled symbol, C 0 2 conversion.
-d l -B r
r
m> r
-m
0
I
I
I
I
I
1.5
1.6
1.7
1.8
1.9
2.0
2.1
lO3/T ( K - l )
Figure 2. Temperature dependence of the overall space-time yield of methane in the CO-C02 co-methanation: 0,Ni-La203-Ru; A, Ni-Laz03, 0 , Ni; dot-dashed line, 100% conversion level of the sum of CO and COz.
rate, and a complete conversion of both CO and COz was achieved a t such low temperatures as 270 "C (Ni-La2O,-Ru) and 300 " C (Ni-La203). These indicate that the adsorption strength of CO during the reaction is far stronger than that of COz on every catalyst, and the existence of CO inhibits the C 0 2 methanation. Some hydrocarbons of C2-C6 were produced as byproducts in a conversion lower than 75% for overall carbon oxides; however, above that percentage the selectivity of methane formation reached 100%. In the co-methanation on the composite catalysts, the amount of C 0 2 in the outlet gas was almost the same as that in the inlet gas in the temperature range where CO was exclusively converted. Hence the disproportionation reaction (4) during the course of 2CO c02 + c (4) +
the co-methanation deserves little consideration. In order to compare methanation rate including the integral rate at higher conversion range for each catalyst, space-time yield of methane, YMST,plotted in Arrheniustype coordinates, is shown in Figure 2. YMsT expresses the moles of methane formed per unit time (h) per unit catalyst-volume (dm3). As stated elsewhere (Inui et al., 1979a), the best expression for the activity of the practical catalyst particles should be the rate per catalyst volume or the reactor space occupied by the catalyst, because a well-designed individual catalyst sphere or particle should be an absolute and indivisible unit irrespective of its weight, inner structure, and composition. The YMSTcurve of the nickel catalyst has a tendency to reach a plateau before 100% conversion level. Such a tendency a t a high reaction rate or high conversion range is normal in a catalytic reaction which depends on partial pressures of reactants in positive orders, because of de-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980 387 TEMPERATURE
300
(-C)
TEMPERATURE
230
200
160
I
I
I
350
('C; 250
30C
3 -.\-.-.-
I
I
1 1.6
1.7
1.8
1.9
2 0
1 0 3 1 ~( ~ - 1 :
Downloaded by UNIV OF SUSSEX on September 10, 2015 | http://pubs.acs.org Publication Date: September 1, 1980 | doi: 10.1021/i360075a018
103r (~-1)
Figure 3. Comparison of space-time yield of products in the soloand co-methanation on the Ni-Laz03-Ru catalyst: SV, 11400 h-'; 0, C o 2 methanation; A, CO methanation; 0, CO-COz comethanation; solid line, methane; dotted line, ethane; broken line, propane; dot-dashed line, 100% conversion leveI.
crease in reactants and the diffusional limitations. To the contrary, in both the Ni-La203 and Ni-La203-Ru catalysts, YMSTkdeviate upward from each Arrhenius straight line and reach the level of complete conversion of both CO and COP(YMST = 61 mol dm-3 h-l). Such deviations naturally suggest that there are certain enhancing factors in the co-methanation on the composite catalysts. Comparison of Solo- and Co-Methanation. To determine whether the cause of the enhancement is in CO or C 0 2 methanation, comparison of solo- and comethanation of CO and C 0 2 over the Ni-La203-Ru catalyst was made. The partial pressure of CO or C 0 2 in solo-methanation was adjusted to the sum of CO and C02 in the co-methanation (0.12 atm). As shown in Figure 3, the Y M of~COP ~ solo-methanation is greater than that of CO in the lower temperature range; however, at high temperatures or high conversion range the ordinary tendency (that the YMST deviates downward from the Arrhenius straight line) is shown. On the other hand, an increase of the apparent activation energy appears in the CO solo-methanation as well as in the co-methanation. Compared with both solo-methanations, it is evident that the cause of the enhancement in eo-methanation on the composite catalyst is that the COP methanation is enhanced by the CO methanation in its higher conversion range. As can be seen in Figure 3, the lower boundary of the temperature, a t which the enhancement begins, corresponds to a temperature above which the formation of hydrocarbons of >C2 decreases abruptly. This suggests that some correlation exists between the enhancement and product distribution or properties of surface intermediates in CO methanation. In C 0 2 solo-methanation, the selectivity of methane and ethane was >99% and C2 as mentioned above. Accordingly, to examine the effects of some hydrocarbons of Cz-C8, each of them (0.01 atm) was added to the reaction-gas stream for CO methanation. However, as shown in Figure 6, coexistence of these hydrocarbons had no effect on the rate of CO methanation. Then, the surface-carbon species, during some steady state of CO methanation, were measured with varying CO conversion by means of the change in the reaction temperature. The steady state of the reaction was discontinued by a sufficient substitution of reacting gas by N2, and the residual surface-carbon species, C,, was determined by hydrogenation and the following oxidation. The detailed procedure was described in our previous paper (Inui et al., 1979a). As shown in Figure 7, the amount of carbon species decreases markedly with an increase in the CO conversion. This indicates that the presence of carbon species, which may relate to the formation of hydrocarbons of >C2, has a strong retardative effect on CO methanation, and the retardative action is released at higher CO conversion due to the decrease in the surface concentration of the carbon species. On thq other hand, the amount of surface-carbon species in C 0 2 methanation was very small compared with that in CO (Inui et al., 1979a). It has been concluded in the results that, in the comethanation of CO and C 0 2 using the Ni-La203-Ru cat-
2.0
1 0 3 1 ~( K - 1 )
Figure 6. Results of C3-C8hydrocarbon addition to the reaction-gas stream for CO methanation: SV, 220000 h-l; PCO,0.06 atm; P H ~0.93 , atm; PHC, 0.01 atm; 0,without addition; added hydrocarbon: 0 , C3H8; A, C3H6; A, 1-CdH8; 0,n-CsH18. 100,
,
I
\
i-r' 0
0 0
25
50
75
103
C0 C O N V E R S I O N ( 1 )
Figure 7. Variation of concentration of surface-carbon species (C,) with CO converstion in methanation on the Ni-La203-Ru catalyst: P,o, 0.06 atm; P H ~ 0.94 , atm; SV, 10 000 h-l.
alyst, the C02 methanation is retarded unless CO exists, in spite of the fact that in solo-methanation, C02 has a higher activity than CO. However, with a decrease of CO concentration by an increase in the reaction rate a t high temperatures, the surface-carbon species decreases and retardation of the CO methanation itself is released. Then the catalyst temperature rises and the temperature difference between the catalyst and the gas phase increases. This state causes C02 methanation to reach a complete conversion in addition to the complete conversion of CO. Acknowledgment The authors are grateful to the referees of this paper for their most valuable comments. This work was supported by the Iwatani Naoji Foundation's Research Grant. Literature Cited Grabaski, M. S., Diehl, E. K., 5th Synthetic Pipeline Gas Symposium, Oct 1973. Inui, T., Funabiki, M., Suehiro, M., Sezume, T., J. Chem. Soc.f a r a h y Trans. I , 75, 787 (1979a). Inui, T., Ueno, K., Funabiki, M., Suehiro, M., Sezume, T., Takegami, Y., J . Chem. SOC. faraday Trans. 1 , 75, 1495 (1979b). Mears, D. E., Ind. Eng. Chem. process Des. Dev., 10. 541 (1971). Mills, G. A,, Steffgen, F. W., Catal. Rev., 8, 159 (1973). Yoshida. F., Ramaswami, D., Hougen, 0.A,, Am. Inst. Chem. Eng. J., 8, 5
(1962).
Received for review July 19, 1979 Accepted May 21, 1980
