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Effect of potassium and sodium carbonate catalysts on the rate of gasification of metallurgical coke
Carbon Printed
Vol. 22, No. 2, pp. in Great Britain.
173-176,
ooO8-6223/84 $3.00 + .oO Pergamon Press Ltd.
1984
EFFECT OF POTASSIUM AND SODIUM CARBONATE CATALYSTS ON THE RATE OF GASIFICATION OF METALLURGICAL COKE Department
A. A. ADJORLOLO and Y. K. RAot of Mining, Metallurgical and Ceramic Engineering, FE10, Seattle, WA 98195, U.S.A.
University of Washington,
(Received 28 March 1983)
Abstract-The catalytic effects of potassium and sodium carbonates on the rate of gasification of metallurgical coke with carbon dioxide have been investigated by the thermogravimetric method. The experiments were carried out using metallurgical coke fines mixed with 5 wt% catalyst, the mixture pressed into disc-shaped pellets from which a small specimen (23 mg) was cut and used. The reaction was carried out at temperatures ranging from 725 to 900°C using pure CO, at 1 atm. The rates were measured for uncatalyzed coke-CO2 reaction and with K&O,, Na&O, and mixed (K, Na),CO, catalysts. Potassium carbonate exhibited the greatest catalytic activity closely followed by the mixed catalyst and sodium carbonate. For the K,CO,catalyzed reaction, an activation energy of 41.74 ( f 3.13) kcal/mole was found; and for the Na,CO,-catalyzed reaction the figure was 40.27 ( + 5.05) kcal/mole. The catalysis is thought to occur by the “vapor cycle” mechanism. It consists of reduction (M,CO,-+2M) followed by oxidation (ZM+M,CO,). When CO, is present in the system the alkali vapor _ (Mj quickly gets reconverted to the &bona& 1. INTRODUCTION
2. EXPERIMENTAL
Reports on the catalytic properties of alkali carbonates vis-a-vis carbon-gas reactions have appeared in the literature from time to time. Some of the earliest studies include those by Taylor and Neville[l] and Fox and White[2]. More recently, Veraa and Be11[3] studied the effects of alkali salts on the steam gasification of char and found that K&O, exerted the greatest influence. McKee and Chatterji[S, 61 investigated the effects of alkali carbonates on the gasification of high purity micro-crystalline graphite and found that L&CO3 is the most potent catalyst. Harker[4] determined the effect of alkali carbonate additions on the ignition temperature of coconut charcoal; the largest effect was due to Cs,CO,. Using a tracer technique, Orning and Sterling[7] and Mentser and Ergun[8] investigated the mechanisms of catalyzed and uncatalyzed C-CO, reactions. Yokohama et al.[91 found that K,CO, catalyst favors accumulation of trapped oxygen on carbon surface. Important contributions pertaining to alkali catalysts have also appeared in the metallurgical literature[lO, 111. The purpose of the present study is to obtain quantitative kinetic data on the catalysis of C-CO2 reaction. For this purpose, metallurgical coke impregnated with K&O, or Na2C0, is employed and the temperature range is kept purposely at moderate levels, viz. 70&9OO”C to take advantage of the fact that mass transfer effects are less likely to be significant at lower temperatures.
icurrently Honorary Visiting Professor, Department of Metallurgical Engineering, University of British Columbia, Vancouver, Canada V6T 1W5.
2.1
Apparatus
A schematic diagram of the apparatus used in the present study is shown in Fig. 1. it consists of a vertical tube furnace, a Cahn Electrobalance, a strip chart recorder and a quartz reaction tube. The reaction tube consisted of two parts: the top part having a standard taper at one end fitted snugly into a Pyrex joint on the balance housing and was left permanently attached to it; the lower part fitted into the top part also through a Pyrex standard taper joint which can be removed as needed to allow the sample to be placed. The furnace temperature was controlled by means of a Leeds and Northrup power controller. The temperature was monitored by means of a Pt-Pt . 13%Rh thermocouple placed slightly beneath the sample.
Fig. 1. Schematic diagram of the Experimental set-up. 173
174
A. A.
ADJOR~LO
2.2 Gases The gases used included COr, CO and N,.t The minimum purities of the gases are as follows: co: > 99.5 vol%; co*: > 99.995 vol%; N,: > 99.5 ~01%. Each of the gases was freed of moisture it contains by passing it through a packed drierite column. The CO and N, gases were further purified by passing them through tubes containing heated copper turnings; any oxygen present in these gases was trapped by copper. 2.3 Coke The metallurgical coke$ used in the present investigation contained 86% flxed carbon, 11% ash, 2% volatiles and O.SS%S, all figures being percentages by weight. The average particle size of the crushed coke was found to be 3.02pm. 2.4 Catalysts The catalysts8 used were in a finely divided form. Anhydrous reagent grade K2C03 and NarCOr were employed in this study. 2.5 Procedure Two grams of coke fines were carefully weighed and mixed with about 0.1 gm of potassium or sodium carbonate in a ceramic mortar. Agglomerates of coke or of catalyst were crushed and mixing continued until an even distribution of the catalyst was achieved. Three drops of honey-distilled water solution, used as the binder, was added to the cokecatalyst mixture. From this mixture small pellets some 170 mg in weight and disc-shaped (1.27 cm dia. x 0.13 cm thickness) were pressed using a stainless steel die at 2OOOpsi pressure. For experiments using mixed K,CO,-Na,CO, catalyst, 50 mg of each catalyst was used so that the proportion K,COS: Na2C0,: C in the hnal mixture became 1: 1: 40. This proportion translates into a catalyst concentration of 4.76 wt%; this has been rounded off to 5 wt% in reporting the results. For experiments using metallurgical coke with no catalytic additives similar procedure was employed in preparing the disc-shaped pellets. In actual weight loss experiments, be they with catalyzed or uncatalyzed coke, smaller specimens about 23 mg in size cut from the larger discs were employed. In a typical experiment the smaller specimen was laid flat on a shallow Pt-pan suspended from the balance using a 0.1 mm dia. nichrome wire. The removable part of the reaction tube was inserted in place and purified nitrogen was introduced to flush out the air in the reaction chamber and the balance housing. The furnace was then raised on two vertical guide frames and its final level was adjusted such that TAirco Industrial Gases, Seattle, Washington. $U.S. Steel Corp., Salt Lake City, Utah. §Mallinckrodt Inc., St. Louis, Missouri.
and Y. K. RAO
the Pt-pan containing the specimen was positioned in the middle of the equitemperature zone. The measuring thermocouple was about 2mm below the Pt-pan. The furnace was turned on at this point and its temperature was increased gradually until the desired value is obtained. Once the temperature has been stabilized at the level where gasification is to be carried out, the nitrogen flow was replaced by the CO2 flow into the reaction chamber. The weight loss sustained by the coke sample was recorded as a function of time. Discrete weight readings were taken at 2 or 5 or 10 or 15 min intervals depending upon the speed of the reaction, the faster rates requiring closer spacing of time intervals. The temperature of the reacting specimen was monitored continuously. All the samples were found to react during the initial stages at a fairly constant rate (whose value varied with temperature); and the period of steady-state rate may range anywhere from 10min to 1 hr depending on the temperature of the experiment. All runs were discontinued after the sample had lost about 20 to 30% of its original weight. The COr gas flow was terminated and the nitrogen flow was resumed. The reacted specimen was furnace cooled under nitrogen. 3. RESULTS For the uncatalyzed coke-CO, reaction the rates became measurable only at about 750°C and above. The small specimen used and the relatively high flow rates of reactant gases were so chosen to minimize the pore-diffusion effects. The rates of both uncatalyzed and catalyzed coke-CO? reaction were measured at several temperatures. The rates were obtained by transforming the experimental weight loss data into fractional coke bum-off F which then is plotted against time. It will be noted that AW F=W=- 0
W,-Wi wo
where W, and Wi are the original and the instantaneous weights of the coke specimen. Zero time was taken as the instant when CO2 gas was introduced into the reaction chamber. Of the large number of F-t plots constructed from the experimental data, for brevity, only a few are shown in Fig. 2. This figure shows the F-t plots for both the catalyzed and uncatalyzed coke-CO2 reaction at 850°C. It is clear that K,COr is the best among the three catalysts studied. Similar behavior was observed at other temperatures as well. From the F-t plots, the rate of reaction was deduced, in each case, by performing a linear regression analysis of the data points in the initial linear portion of the plots (i.e. up to 7% gasification). The activation energy, for each series of experiments, was determined by correlating the rate data at different temperatures with an Arrhenius-type relationship. These plots are presented in Figs. 3 and 4. For the uncatalyzed coke-CO, reaction, we have the follow-
175
Effect of potassium and sodium carbonate catalysts on the rate of gasification of metallurgical coke I
0
KpCO3
17 fK, A
5%
(K.
Na12C03
No)&03
Na,CO,
+coke 0,
“0
a_
6-
C
t
(min)
Fig. 2. Relative catalytic activities of K&O,, N&O, and mixed (K, Na), CO, catalysts vis-l-vis the coke-CO, reaction.
.-I
8
coke
uncatalyzed
Y I
II
/Tx104
Fig. 4. Arrhenius plots for the uncatalyzed and catalyzed coke-CO, reaction. Dashed line represents K,CO,-catalyzed reaction; 0, 5% K&O,; 0, 5% (K, Na),CO, and A, 5% Na,COz.
Potassium and sodium carbonates proved to be effective catalysts for the coke-CO2 reaction; the former was slightly more effective. For instance, at 9OO”C, the addition of K,C03 resulted in a rate enhancement of six times while the presence of Na,CO, produced an enhancement of about four times as compared to the uncataiyzed reaction. The respective rates may be represented as follows:
+
\
+
- 8\
+ \
-
9 I/Txi04
8
K,CO,:
I
I
I
91
IO
II
(K-l)
log(Rate) = 6.3lO(IfrO.640) - 4 1740( * 3 130) caljmole 2.303 RT
Fig. 3. Arrhenius plot for the uncatalyzed coke-CO* reaction.
Na,CO,:
ing relation: log(Rate)
= 6.6Sq~O.366) -
log(Rate) = 5.892( + 1.017) -
48 170( + 1860) Cal/mole 2.303 RT
The value of 48.17 (+ 1.86) k.cal/mole
’ obtained
40270( &-5050) caI/moie 2.303 RT
(K, Na),CO,: here
for the activation energy of uncatalyzed reaction is somewhat smaller than the values 5@70 kcal/mole reported by Wicke[lS]. The low value of activation energy is probably due to the effect of the impurities present in the coke. It is generally accepted that purer forms of carbon are associated with higher activation energies. Also the influence of pore-diffusion on the measured rates remains to be quantifi~. Where pore-diffusion effects are non-negligible, there is a tendency for the activation energy values to diminish.
log(Rate) = 6.983( f 0.706) - 45690( + 3480) cal/mole 2.303 RT ’ activation The energy values of 41.74 ( + 3.13) kcal/mole and 40.27 ( + 5.05) kcal/mole for K,CO, and Na,CO, catalysts respectively are not unreasonabie as compared to 48.04 kcal/mole reported by Jalan and Rao[l7] for the LiCO,-catalyzed reaction; the latter value was obtained after cor-
176
A. A.
ADJORLLXO
recting the rates for pore-diffusion effects. Inspection of Fig. 4 shows that for the catalyzed gasification (either with &CO, or Na,CO,) at higher temperatures the Arrhenius plot begins to flatten out indicating that pore-diffusion effects are significant in that region. The mathematical relations given above depict the straight-line portions of the Arrhenius plots. 4. DISCUSSION
One of the objectives of this study is to draw reasonable conclusions with regard to the mechanism of catalysis on the basis of the present results. There are a number of theories proposed in the literature[l2, 13, 15, 171 to interpret the observed catalytic activities of alkali salts. In the present study the indisputable facts are that K2C03 is a better catalyst than Na,CO, and that both catalysts are effective well below their melting points. This last observation casts doubts on the applicability of the electrochemical mechanism [ 171 for this case because such a scheme requires that the catalyst be molten at the temperature under investigation. The “intercalation compound” mechanism [ 15, 161 is weakened by the fact that these compounds become unstable at higher temperatures and also in environments containing CO2 gas. The calculations presented by Rao et al.[19] illustrate the instability of these intermediates. The “vapor cycle” mechanism originally formulated by Fox and White[2] and endorsed by McKee and Chatterji [5,6] appears to be the most suitable of the different schemes considered in the interpretation of catalysis. Indirect support for this mechanism comes from the observations on the behavior of alkali carbonates in the iron blast furnace[lO, 11, 141 and from experiments on the evaporation of alkalimetals[46]. The reactions involved in the “vapor cycle” mechanism are as follows: M,CO,(s, 1) + 2C(s) = 2M(g) + 3CO(g) 2M(g) + 2CO,(g) = M$O&
1) + CO(g).
(Ml) (M2)
The alkali vapor (M) is produced at catalyst/carbon junctions; it then gets reconverted to carbonate upon contact with CO2 gas. Rao et af.[19] showed how thermodynamic analysis may be applied to predict the driving force, 6, i.e. the differential in the alkali vapor pressures between the point where M is produced and the point where M is reconverted. At 800 K, these authors[l9] found that 6, is 3.72 x 10P6 atm whereas a,, is only 2.35 x 10m6atm;
and Y. K. RAO it is well to note that ai is the driving force for the ith type of alkali metal. These calculations led Rao et al. [ 191 to suggest that K&O, is likely to be a better catalyst than Na,COI. The experimental observations made in the present study certainly confirm the predictions made by Rao et al. [19]. This in itself is not a sufficient proof of the applicability of the vapor cycle mechanism; but its likelihood is high. One needs to know the kinetics of reactions (Ml) and (M2) for different carbonate catalysts before a full confirmation of “vapor cycle” mechanism is obtained for the K&O, and Na,CO, catalysts.
5. CONCLUSIONS The influences of K,CO,, Na,CO, and mixed catalyst on the coke-CO, reaction were investigated in the temperature range 70&9OO”C. It was observed that K,COs was the superior catalyst. The observation is generally supportive of the “vapor cycle” mechanism of catalysis. Acknowledgements-The authors wish to acknowledge their gratitude to the National Science Foundation for their support of this work through grant No. CPE 82-03725. RJWERENCES
1. H. S. Taylor and H. A. Neville, J. Am. Gem. Sot. 43, 2055 (1921). 2. D. A. Fox and A. H. White, Ind. Engrg. Chem. 23,259 (1931). 3. M. J. Veraa and A. T. Bell, Fuel 57, 194 (1978). 4. H. Harker, Proc. 4th Conf. on Carbon, p. 125. Pergamon Press, New York (1960). 5. D. W. McKee and D. Chatterji, Carbon 13, 381 (1975). 6. D. W. McKee and D. Chatterji, Carbon 16, 53 (1978). 7. A. A. Oming and E. Sterling, J. Phys. Chem. 58, 1044 (1954). 8. M. Mentser and S. Ergun, Carbon 5, 351 (1967). 9. S. Yokohama, K. Miyahara, K. Tanaka, I. Takakuwa and J. Tashiro, Fuel 58, 510 (1979). 10. K. P. Abraham and L. I. Staffanson, Stand. J. Met. 4, 193 (1975). of the Art, (Edited by 11. Alkalis in Blast Furnaces-State W. K. Lu), Symposium held at the McMaster University, Hamilton, Ontario, Canada (1973). 12. F. J. Long and K. W. Sykes, Proc. Roy. Sot. (London), Series A, A215, 100 (1952). 13. C. Heuchamps, ThPse Ingdnieur-Docteur, Universite de Nancy, Nancy, France (1960). 14. N. Nakamura, Y. Togino and M. Tateoka, Ironmaking Steelmaking 5(l), 1 (1978). 15. W. Y. Wen, Catal. Reu.-Sci. Engng 22(l), 1 (1980). 16. M. Ichikawa and K. Tamaru, J. Am. Chem. Sot. 93, 2079 (1971). 17. B. P. Jalan and Y. K. Rao, Carbon 16, 175 (1978). 18. E. Wicke, 5th Symp. on Combustion, 245. Reinhold, New York (1955). 19, Y. K. Rao, A. A. Adjorlolo and J. H. Haberman, Carbon 20, 207 (1982).
Vol. 22, No. 2, pp. in Great Britain.
173-176,
ooO8-6223/84 $3.00 + .oO Pergamon Press Ltd.
1984
EFFECT OF POTASSIUM AND SODIUM CARBONATE CATALYSTS ON THE RATE OF GASIFICATION OF METALLURGICAL COKE Department
A. A. ADJORLOLO and Y. K. RAot of Mining, Metallurgical and Ceramic Engineering, FE10, Seattle, WA 98195, U.S.A.
University of Washington,
(Received 28 March 1983)
Abstract-The catalytic effects of potassium and sodium carbonates on the rate of gasification of metallurgical coke with carbon dioxide have been investigated by the thermogravimetric method. The experiments were carried out using metallurgical coke fines mixed with 5 wt% catalyst, the mixture pressed into disc-shaped pellets from which a small specimen (23 mg) was cut and used. The reaction was carried out at temperatures ranging from 725 to 900°C using pure CO, at 1 atm. The rates were measured for uncatalyzed coke-CO2 reaction and with K&O,, Na&O, and mixed (K, Na),CO, catalysts. Potassium carbonate exhibited the greatest catalytic activity closely followed by the mixed catalyst and sodium carbonate. For the K,CO,catalyzed reaction, an activation energy of 41.74 ( f 3.13) kcal/mole was found; and for the Na,CO,-catalyzed reaction the figure was 40.27 ( + 5.05) kcal/mole. The catalysis is thought to occur by the “vapor cycle” mechanism. It consists of reduction (M,CO,-+2M) followed by oxidation (ZM+M,CO,). When CO, is present in the system the alkali vapor _ (Mj quickly gets reconverted to the &bona& 1. INTRODUCTION
2. EXPERIMENTAL
Reports on the catalytic properties of alkali carbonates vis-a-vis carbon-gas reactions have appeared in the literature from time to time. Some of the earliest studies include those by Taylor and Neville[l] and Fox and White[2]. More recently, Veraa and Be11[3] studied the effects of alkali salts on the steam gasification of char and found that K&O, exerted the greatest influence. McKee and Chatterji[S, 61 investigated the effects of alkali carbonates on the gasification of high purity micro-crystalline graphite and found that L&CO3 is the most potent catalyst. Harker[4] determined the effect of alkali carbonate additions on the ignition temperature of coconut charcoal; the largest effect was due to Cs,CO,. Using a tracer technique, Orning and Sterling[7] and Mentser and Ergun[8] investigated the mechanisms of catalyzed and uncatalyzed C-CO, reactions. Yokohama et al.[91 found that K,CO, catalyst favors accumulation of trapped oxygen on carbon surface. Important contributions pertaining to alkali catalysts have also appeared in the metallurgical literature[lO, 111. The purpose of the present study is to obtain quantitative kinetic data on the catalysis of C-CO2 reaction. For this purpose, metallurgical coke impregnated with K&O, or Na2C0, is employed and the temperature range is kept purposely at moderate levels, viz. 70&9OO”C to take advantage of the fact that mass transfer effects are less likely to be significant at lower temperatures.
icurrently Honorary Visiting Professor, Department of Metallurgical Engineering, University of British Columbia, Vancouver, Canada V6T 1W5.
2.1
Apparatus
A schematic diagram of the apparatus used in the present study is shown in Fig. 1. it consists of a vertical tube furnace, a Cahn Electrobalance, a strip chart recorder and a quartz reaction tube. The reaction tube consisted of two parts: the top part having a standard taper at one end fitted snugly into a Pyrex joint on the balance housing and was left permanently attached to it; the lower part fitted into the top part also through a Pyrex standard taper joint which can be removed as needed to allow the sample to be placed. The furnace temperature was controlled by means of a Leeds and Northrup power controller. The temperature was monitored by means of a Pt-Pt . 13%Rh thermocouple placed slightly beneath the sample.
Fig. 1. Schematic diagram of the Experimental set-up. 173
174
A. A.
ADJOR~LO
2.2 Gases The gases used included COr, CO and N,.t The minimum purities of the gases are as follows: co: > 99.5 vol%; co*: > 99.995 vol%; N,: > 99.5 ~01%. Each of the gases was freed of moisture it contains by passing it through a packed drierite column. The CO and N, gases were further purified by passing them through tubes containing heated copper turnings; any oxygen present in these gases was trapped by copper. 2.3 Coke The metallurgical coke$ used in the present investigation contained 86% flxed carbon, 11% ash, 2% volatiles and O.SS%S, all figures being percentages by weight. The average particle size of the crushed coke was found to be 3.02pm. 2.4 Catalysts The catalysts8 used were in a finely divided form. Anhydrous reagent grade K2C03 and NarCOr were employed in this study. 2.5 Procedure Two grams of coke fines were carefully weighed and mixed with about 0.1 gm of potassium or sodium carbonate in a ceramic mortar. Agglomerates of coke or of catalyst were crushed and mixing continued until an even distribution of the catalyst was achieved. Three drops of honey-distilled water solution, used as the binder, was added to the cokecatalyst mixture. From this mixture small pellets some 170 mg in weight and disc-shaped (1.27 cm dia. x 0.13 cm thickness) were pressed using a stainless steel die at 2OOOpsi pressure. For experiments using mixed K,CO,-Na,CO, catalyst, 50 mg of each catalyst was used so that the proportion K,COS: Na2C0,: C in the hnal mixture became 1: 1: 40. This proportion translates into a catalyst concentration of 4.76 wt%; this has been rounded off to 5 wt% in reporting the results. For experiments using metallurgical coke with no catalytic additives similar procedure was employed in preparing the disc-shaped pellets. In actual weight loss experiments, be they with catalyzed or uncatalyzed coke, smaller specimens about 23 mg in size cut from the larger discs were employed. In a typical experiment the smaller specimen was laid flat on a shallow Pt-pan suspended from the balance using a 0.1 mm dia. nichrome wire. The removable part of the reaction tube was inserted in place and purified nitrogen was introduced to flush out the air in the reaction chamber and the balance housing. The furnace was then raised on two vertical guide frames and its final level was adjusted such that TAirco Industrial Gases, Seattle, Washington. $U.S. Steel Corp., Salt Lake City, Utah. §Mallinckrodt Inc., St. Louis, Missouri.
and Y. K. RAO
the Pt-pan containing the specimen was positioned in the middle of the equitemperature zone. The measuring thermocouple was about 2mm below the Pt-pan. The furnace was turned on at this point and its temperature was increased gradually until the desired value is obtained. Once the temperature has been stabilized at the level where gasification is to be carried out, the nitrogen flow was replaced by the CO2 flow into the reaction chamber. The weight loss sustained by the coke sample was recorded as a function of time. Discrete weight readings were taken at 2 or 5 or 10 or 15 min intervals depending upon the speed of the reaction, the faster rates requiring closer spacing of time intervals. The temperature of the reacting specimen was monitored continuously. All the samples were found to react during the initial stages at a fairly constant rate (whose value varied with temperature); and the period of steady-state rate may range anywhere from 10min to 1 hr depending on the temperature of the experiment. All runs were discontinued after the sample had lost about 20 to 30% of its original weight. The COr gas flow was terminated and the nitrogen flow was resumed. The reacted specimen was furnace cooled under nitrogen. 3. RESULTS For the uncatalyzed coke-CO, reaction the rates became measurable only at about 750°C and above. The small specimen used and the relatively high flow rates of reactant gases were so chosen to minimize the pore-diffusion effects. The rates of both uncatalyzed and catalyzed coke-CO? reaction were measured at several temperatures. The rates were obtained by transforming the experimental weight loss data into fractional coke bum-off F which then is plotted against time. It will be noted that AW F=W=- 0
W,-Wi wo
where W, and Wi are the original and the instantaneous weights of the coke specimen. Zero time was taken as the instant when CO2 gas was introduced into the reaction chamber. Of the large number of F-t plots constructed from the experimental data, for brevity, only a few are shown in Fig. 2. This figure shows the F-t plots for both the catalyzed and uncatalyzed coke-CO2 reaction at 850°C. It is clear that K,COr is the best among the three catalysts studied. Similar behavior was observed at other temperatures as well. From the F-t plots, the rate of reaction was deduced, in each case, by performing a linear regression analysis of the data points in the initial linear portion of the plots (i.e. up to 7% gasification). The activation energy, for each series of experiments, was determined by correlating the rate data at different temperatures with an Arrhenius-type relationship. These plots are presented in Figs. 3 and 4. For the uncatalyzed coke-CO, reaction, we have the follow-
175
Effect of potassium and sodium carbonate catalysts on the rate of gasification of metallurgical coke I
0
KpCO3
17 fK, A
5%
(K.
Na12C03
No)&03
Na,CO,
+coke 0,
“0
a_
6-
C
t
(min)
Fig. 2. Relative catalytic activities of K&O,, N&O, and mixed (K, Na), CO, catalysts vis-l-vis the coke-CO, reaction.
.-I
8
coke
uncatalyzed
Y I
II
/Tx104
Fig. 4. Arrhenius plots for the uncatalyzed and catalyzed coke-CO, reaction. Dashed line represents K,CO,-catalyzed reaction; 0, 5% K&O,; 0, 5% (K, Na),CO, and A, 5% Na,COz.
Potassium and sodium carbonates proved to be effective catalysts for the coke-CO2 reaction; the former was slightly more effective. For instance, at 9OO”C, the addition of K,C03 resulted in a rate enhancement of six times while the presence of Na,CO, produced an enhancement of about four times as compared to the uncataiyzed reaction. The respective rates may be represented as follows:
+
\
+
- 8\
+ \
-
9 I/Txi04
8
K,CO,:
I
I
I
91
IO
II
(K-l)
log(Rate) = 6.3lO(IfrO.640) - 4 1740( * 3 130) caljmole 2.303 RT
Fig. 3. Arrhenius plot for the uncatalyzed coke-CO* reaction.
Na,CO,:
ing relation: log(Rate)
= 6.6Sq~O.366) -
log(Rate) = 5.892( + 1.017) -
48 170( + 1860) Cal/mole 2.303 RT
The value of 48.17 (+ 1.86) k.cal/mole
’ obtained
40270( &-5050) caI/moie 2.303 RT
(K, Na),CO,: here
for the activation energy of uncatalyzed reaction is somewhat smaller than the values 5@70 kcal/mole reported by Wicke[lS]. The low value of activation energy is probably due to the effect of the impurities present in the coke. It is generally accepted that purer forms of carbon are associated with higher activation energies. Also the influence of pore-diffusion on the measured rates remains to be quantifi~. Where pore-diffusion effects are non-negligible, there is a tendency for the activation energy values to diminish.
log(Rate) = 6.983( f 0.706) - 45690( + 3480) cal/mole 2.303 RT ’ activation The energy values of 41.74 ( + 3.13) kcal/mole and 40.27 ( + 5.05) kcal/mole for K,CO, and Na,CO, catalysts respectively are not unreasonabie as compared to 48.04 kcal/mole reported by Jalan and Rao[l7] for the LiCO,-catalyzed reaction; the latter value was obtained after cor-
176
A. A.
ADJORLLXO
recting the rates for pore-diffusion effects. Inspection of Fig. 4 shows that for the catalyzed gasification (either with &CO, or Na,CO,) at higher temperatures the Arrhenius plot begins to flatten out indicating that pore-diffusion effects are significant in that region. The mathematical relations given above depict the straight-line portions of the Arrhenius plots. 4. DISCUSSION
One of the objectives of this study is to draw reasonable conclusions with regard to the mechanism of catalysis on the basis of the present results. There are a number of theories proposed in the literature[l2, 13, 15, 171 to interpret the observed catalytic activities of alkali salts. In the present study the indisputable facts are that K2C03 is a better catalyst than Na,CO, and that both catalysts are effective well below their melting points. This last observation casts doubts on the applicability of the electrochemical mechanism [ 171 for this case because such a scheme requires that the catalyst be molten at the temperature under investigation. The “intercalation compound” mechanism [ 15, 161 is weakened by the fact that these compounds become unstable at higher temperatures and also in environments containing CO2 gas. The calculations presented by Rao et al.[19] illustrate the instability of these intermediates. The “vapor cycle” mechanism originally formulated by Fox and White[2] and endorsed by McKee and Chatterji [5,6] appears to be the most suitable of the different schemes considered in the interpretation of catalysis. Indirect support for this mechanism comes from the observations on the behavior of alkali carbonates in the iron blast furnace[lO, 11, 141 and from experiments on the evaporation of alkalimetals[46]. The reactions involved in the “vapor cycle” mechanism are as follows: M,CO,(s, 1) + 2C(s) = 2M(g) + 3CO(g) 2M(g) + 2CO,(g) = M$O&
1) + CO(g).
(Ml) (M2)
The alkali vapor (M) is produced at catalyst/carbon junctions; it then gets reconverted to carbonate upon contact with CO2 gas. Rao et af.[19] showed how thermodynamic analysis may be applied to predict the driving force, 6, i.e. the differential in the alkali vapor pressures between the point where M is produced and the point where M is reconverted. At 800 K, these authors[l9] found that 6, is 3.72 x 10P6 atm whereas a,, is only 2.35 x 10m6atm;
and Y. K. RAO it is well to note that ai is the driving force for the ith type of alkali metal. These calculations led Rao et al. [ 191 to suggest that K&O, is likely to be a better catalyst than Na,COI. The experimental observations made in the present study certainly confirm the predictions made by Rao et al. [19]. This in itself is not a sufficient proof of the applicability of the vapor cycle mechanism; but its likelihood is high. One needs to know the kinetics of reactions (Ml) and (M2) for different carbonate catalysts before a full confirmation of “vapor cycle” mechanism is obtained for the K&O, and Na,CO, catalysts.
5. CONCLUSIONS The influences of K,CO,, Na,CO, and mixed catalyst on the coke-CO, reaction were investigated in the temperature range 70&9OO”C. It was observed that K,COs was the superior catalyst. The observation is generally supportive of the “vapor cycle” mechanism of catalysis. Acknowledgements-The authors wish to acknowledge their gratitude to the National Science Foundation for their support of this work through grant No. CPE 82-03725. RJWERENCES
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