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Al2O3 Catalyst
Studies in Surface Science and Catalysis 153 S.-E. Park, J.-S. Chang and K.-W. Lee (Editors) © 2004 Elsevier B.V. All rights reserved.
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CCh Reforming of n-Heptane on a Ni/AkCb Catalyst K. Johanna Puolakka and A. Outi I. Krause Department of Chemical Technology, Helsinki University of Technology, P.O. Box 6100, FIN-02015 HUT, Finland; e-mail [email protected] CO2 reforming of »-heptane was studied on a 15 wt-% Ni/AkCb catalyst. In addition, thermal cracking of «-heptane was examined. Experimental results were compared with the thermodynamics of «-heptane CO2 reforming and of possible side reactions. The composition of the product at 900 °C under atmospheric pressure was close to that calculated from the thermodynamics.
1. INTRODUCTION Carbon dioxide is widely considered as a greenhouse gas causing global warming. Thus, carbon dioxide storage and catalytic activation for chemical reactions are of great interest. Carbon sequestration in saline aquifers, coalmines, oil and gas wells, and the ocean could be done during the changeover from fossil energy to renewable energy [1]. A potential reaction of carbon dioxide is CO2 reforming, in which the important raw material for chemical industry, synthesis gas (i.e. hydrogen and carbon monoxide) is formed. The CO2 reforming of methane has been already extensively studied, because also methane is regarded as a greenhouse gas [2]. Other potential feedstocks for the CO2 reforming are light, sulphur-free GTL (FischerTropsch) fractions, which are not suitable for gasoline due to low octane numbers. There are only few published studies on CO2 reforming of higher hydrocarbons thus requiring basic experiments with commercial catalyst to study if the reaction is feasible. In this work the reaction was studied with «-heptane (Equation 1) as the model compound for gasoline. C7H16 + 7 CO2 - 8 H2 + 14 CO
AH°298K = +1395kJ/mol
(1)
The typical side reactions of the CO2 reforming are the Boudouard reaction (Equation 2) and the water-gas shift reaction (Equation 3): 2 CO - C + CO2 H2O + CO - H2 + CO2
AH°298K = -172 kJ/mol AH°298K = -41 kJ/mol
(2) (3)
Major challenges in CO2 reforming are the highly endothermic reaction requiring high temperature and the catalyst deactivation by carbon deposition [3].
138
2. EXPERIMENTAL Experiments were carried out in a fixed bed flow reactor with an inner diameter of 6 mm. The reactor was heated by a three-zone tube furnace. Reaction products were analyzed with two gas chromatographs. Columns were DB-1 (J&W Scientific) for hydrocarbons and a packed column with activated carbon with 2% squalane for hydrogen, argon, carbon monoxide, carbon dioxide and small hydrocarbons. First, thermal cracking of w-heptane was studied with silicon carbide in a quartz glass reactor between 550 and 800 °C. Catalytic experiments were carried out at furnace temperatures of 700 and 900 °C under atmospheric pressure with a commercial 15 wt-% Ni/AkCb catalyst. The amount of the catalyst was 0.1 g and the particle size was 0.2-0.3 mm. The total feed rate in the experiments was 100 cm« min , and the composition 63 mol-% CO2 and 3 mol-% «-heptane balanced with argon. Threefold stoichiometric excess of carbon dioxide was used to reduce coke formation. The catalyst was reduced at 900 °C with a mixture of H2 and Ar (50% H2) for one hour. In addition, the performance of the catalyst was studied at a higher pressure of 4 bar using a stainless steel reactor (AISI 316), which limited the furnace temperature to 700 °C. The total feed rate was higher, 200 cm« min , and the catalyst reduction was done at 700 °C. Thermodynamic calculations of the CO2 reforming of w-heptane were performed with the HSC Chemistry 3.02 program.
3. RESULTS AND DISCUSSION 3.1. Thermodynamic calculations The composition of the product gas at the thermodynamic equilibrium was calculated as a function of temperature including the side reaction in addition to the »-heptane reforming. E.g. at 700°C the following net stoichiometric equation resulted: C7H16 + 5.98 CO2 ^ 11.72 CO + 5.16 H2 + 0.27 H2O + 1.29 CH4(4) The equilibrium conversions of rc-heptane and CO2 were 100% and 85% respectively. Compared to the basic CO2 reforming (Equation 1) the molar ratio of H2 to CO decreased from 0.57 to 0.44. However, the calculated enthalpy for reaction 4, +1084 kJ/mol, is somewhat smaller than the value for the mere CO2 reforming due to the lower enthalpies of the side reactions. At high temperatures the thermodynamic equilibria of side reactions are on the side of carbon dioxide consumption and carbon monoxide formation. For the reverse water-gas shift and the reverse Boudouard reactions AG < 0 kJ/mol, when temperatures are higher than 830 and 703 °C respectively. Thus, these temperatures define a suitable range for the reaction. Reaction temperatures higher than 703 °C are suitable in coke removal due to the reverse Boudouard reaction. In addition, the higher the temperature the larger the amount of hydrogen which is converted to water in the reverse water-gas shift reaction. This is not desirable. However, as the temperature dependency of AG of the water-gas shift reaction is relatively small, the upper limit of 830 °C is only suggestive.
139
3.2. Thermal experiments The conversion of «-heptane without catalyst was studied at the temperatures relevant for the reforming reactions. In these experiments «-heptane was cracked to smaller hydrocarbons, ethene being the predominant product. This was expected because rc-heptane is a good feedstock for steam cracking. No oxygen containing products were detected indicating reforming reaction not taking place. The conversion of «-heptane increased with temperature being 0% at 550°C and close to 100% at 800°C. The molar ratio of methane to ethene was 0.43 at 800°C. This ratio increased also with temperature, being 0.32 at 700°C. 3.2. Catalytic experiments In the catalytic experiments at 700 °C the main products were carbon monoxide (32 cm« min ) and hydrogen. In addition, smaller hydrocarbons (less than 1 cm« min each, methane to ethene ratio 1.09) were formed indicating that only some thermal cracking occurred besides the reforming reactions. The coking of the catalyst was also significant increasing the pressure in the reactor. In addition, the catalyst bed temperature increased indicating decrease of the endothermic reforming reaction. Initially the total conversion of «-heptane was 97%, of which 8% was cracked to lighter hydrocarbons. The initial conversion of carbon dioxide was 28% compared to the stoichiometric 33% (due to the threefold excess of carbon dioxide). At 900 °C the conversion of «-heptane was 100% and that of carbon dioxide 42%. The carbon dioxide conversion being higher than the stoichiometric one indicates that carbon dioxide was consumed in other reactions. Furthermore, the H2 to CO molar ratio of 0.32 was less than the stoichiometric ratio of 0.57 according to Equation 1. These facts point out to the occurrence of the reverse water-gas shift reaction and the reverse Boudouard reaction. The product composition agreed very well with thermodynamics, the calculated ratio of H2 to CO being 0.28 at 900 °C. The catalyst bed was coking only slightly and the pressure increase was much slower compared to the increase at 700 °C. In addition, no smaller hydrocarbons were formed. This all means that higher temperatures are more favourable for the reforming reactions. However, if high H2 to CO ratio is required, higher temperatures are not suitable due to the water-gas shift reaction, when larger than stoichiometric amount of CO2 is used.
Fig. 1. CO2 conversion and the outgoing CO flow at 4 bar and 700 °C.
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According to the thermodynamics coke formation is reduced at higher pressures. In addition, the pressure remains constant, when controlled. In larger scale industrial application the pressure will also be higher than atmospheric. In the experiments performed at 4 bar and 700°C the initial conversions for «-heptane and carbon dioxide were 83% and 13%, respectively. 31% of «-heptane was cracked and the initial molar ratio of methane to ethene was 0.61. The activity of the catalyst decreased clearly during the experiment of 6.5 hours. The conversion of carbon dioxide and the outgoing carbon monoxide flow are shown as a function of time in Figure 1. It seems that increasing the pressure worsened the situation, as was to be expected on the basis of the thermodynamic calculations considering conversion. This means that at higher pressures higher temperatures are needed. 4. CONCLUSIONS The CO2 reforming of «-heptane is a promising way of utilising carbon dioxide. High temperatures are required for the optimal production of synthesis gas. However, the overall reaction remains very endothermic even though the side reactions lower the reaction enthalpy slightly. Combining the CO2 reforming with partial oxidation could provide a solution as it decreases the need for outside heating. This could have a positive effect on the catalyst deactivation, too. ACKNOWLEDGMENTS The financial support for this work from the Technology Development Centre of Finland (TEKES) and Fortum Oil and Gas Oy is gratefully acknowledged.
REFERENCES 1. Hileman, B., How to Reduce Greenhouse Gases, Chem. Eng. News 80 (21) (2002) 37-41. 2. Bradford, M.C.J., Vannice, M.A., CO2 Reforming of CH4, Catal. Rev.-Sci. Eng. 41(1) (1999) 1-42. 3. Rostrup-Nielsen, J.R., Bak Hansen, J.-H., CO2-Reforming of Methane over Transition Metals, J. Catal. 144 (1993) 38-49.
137
CCh Reforming of n-Heptane on a Ni/AkCb Catalyst K. Johanna Puolakka and A. Outi I. Krause Department of Chemical Technology, Helsinki University of Technology, P.O. Box 6100, FIN-02015 HUT, Finland; e-mail [email protected] CO2 reforming of »-heptane was studied on a 15 wt-% Ni/AkCb catalyst. In addition, thermal cracking of «-heptane was examined. Experimental results were compared with the thermodynamics of «-heptane CO2 reforming and of possible side reactions. The composition of the product at 900 °C under atmospheric pressure was close to that calculated from the thermodynamics.
1. INTRODUCTION Carbon dioxide is widely considered as a greenhouse gas causing global warming. Thus, carbon dioxide storage and catalytic activation for chemical reactions are of great interest. Carbon sequestration in saline aquifers, coalmines, oil and gas wells, and the ocean could be done during the changeover from fossil energy to renewable energy [1]. A potential reaction of carbon dioxide is CO2 reforming, in which the important raw material for chemical industry, synthesis gas (i.e. hydrogen and carbon monoxide) is formed. The CO2 reforming of methane has been already extensively studied, because also methane is regarded as a greenhouse gas [2]. Other potential feedstocks for the CO2 reforming are light, sulphur-free GTL (FischerTropsch) fractions, which are not suitable for gasoline due to low octane numbers. There are only few published studies on CO2 reforming of higher hydrocarbons thus requiring basic experiments with commercial catalyst to study if the reaction is feasible. In this work the reaction was studied with «-heptane (Equation 1) as the model compound for gasoline. C7H16 + 7 CO2 - 8 H2 + 14 CO
AH°298K = +1395kJ/mol
(1)
The typical side reactions of the CO2 reforming are the Boudouard reaction (Equation 2) and the water-gas shift reaction (Equation 3): 2 CO - C + CO2 H2O + CO - H2 + CO2
AH°298K = -172 kJ/mol AH°298K = -41 kJ/mol
(2) (3)
Major challenges in CO2 reforming are the highly endothermic reaction requiring high temperature and the catalyst deactivation by carbon deposition [3].
138
2. EXPERIMENTAL Experiments were carried out in a fixed bed flow reactor with an inner diameter of 6 mm. The reactor was heated by a three-zone tube furnace. Reaction products were analyzed with two gas chromatographs. Columns were DB-1 (J&W Scientific) for hydrocarbons and a packed column with activated carbon with 2% squalane for hydrogen, argon, carbon monoxide, carbon dioxide and small hydrocarbons. First, thermal cracking of w-heptane was studied with silicon carbide in a quartz glass reactor between 550 and 800 °C. Catalytic experiments were carried out at furnace temperatures of 700 and 900 °C under atmospheric pressure with a commercial 15 wt-% Ni/AkCb catalyst. The amount of the catalyst was 0.1 g and the particle size was 0.2-0.3 mm. The total feed rate in the experiments was 100 cm« min , and the composition 63 mol-% CO2 and 3 mol-% «-heptane balanced with argon. Threefold stoichiometric excess of carbon dioxide was used to reduce coke formation. The catalyst was reduced at 900 °C with a mixture of H2 and Ar (50% H2) for one hour. In addition, the performance of the catalyst was studied at a higher pressure of 4 bar using a stainless steel reactor (AISI 316), which limited the furnace temperature to 700 °C. The total feed rate was higher, 200 cm« min , and the catalyst reduction was done at 700 °C. Thermodynamic calculations of the CO2 reforming of w-heptane were performed with the HSC Chemistry 3.02 program.
3. RESULTS AND DISCUSSION 3.1. Thermodynamic calculations The composition of the product gas at the thermodynamic equilibrium was calculated as a function of temperature including the side reaction in addition to the »-heptane reforming. E.g. at 700°C the following net stoichiometric equation resulted: C7H16 + 5.98 CO2 ^ 11.72 CO + 5.16 H2 + 0.27 H2O + 1.29 CH4(4) The equilibrium conversions of rc-heptane and CO2 were 100% and 85% respectively. Compared to the basic CO2 reforming (Equation 1) the molar ratio of H2 to CO decreased from 0.57 to 0.44. However, the calculated enthalpy for reaction 4, +1084 kJ/mol, is somewhat smaller than the value for the mere CO2 reforming due to the lower enthalpies of the side reactions. At high temperatures the thermodynamic equilibria of side reactions are on the side of carbon dioxide consumption and carbon monoxide formation. For the reverse water-gas shift and the reverse Boudouard reactions AG < 0 kJ/mol, when temperatures are higher than 830 and 703 °C respectively. Thus, these temperatures define a suitable range for the reaction. Reaction temperatures higher than 703 °C are suitable in coke removal due to the reverse Boudouard reaction. In addition, the higher the temperature the larger the amount of hydrogen which is converted to water in the reverse water-gas shift reaction. This is not desirable. However, as the temperature dependency of AG of the water-gas shift reaction is relatively small, the upper limit of 830 °C is only suggestive.
139
3.2. Thermal experiments The conversion of «-heptane without catalyst was studied at the temperatures relevant for the reforming reactions. In these experiments «-heptane was cracked to smaller hydrocarbons, ethene being the predominant product. This was expected because rc-heptane is a good feedstock for steam cracking. No oxygen containing products were detected indicating reforming reaction not taking place. The conversion of «-heptane increased with temperature being 0% at 550°C and close to 100% at 800°C. The molar ratio of methane to ethene was 0.43 at 800°C. This ratio increased also with temperature, being 0.32 at 700°C. 3.2. Catalytic experiments In the catalytic experiments at 700 °C the main products were carbon monoxide (32 cm« min ) and hydrogen. In addition, smaller hydrocarbons (less than 1 cm« min each, methane to ethene ratio 1.09) were formed indicating that only some thermal cracking occurred besides the reforming reactions. The coking of the catalyst was also significant increasing the pressure in the reactor. In addition, the catalyst bed temperature increased indicating decrease of the endothermic reforming reaction. Initially the total conversion of «-heptane was 97%, of which 8% was cracked to lighter hydrocarbons. The initial conversion of carbon dioxide was 28% compared to the stoichiometric 33% (due to the threefold excess of carbon dioxide). At 900 °C the conversion of «-heptane was 100% and that of carbon dioxide 42%. The carbon dioxide conversion being higher than the stoichiometric one indicates that carbon dioxide was consumed in other reactions. Furthermore, the H2 to CO molar ratio of 0.32 was less than the stoichiometric ratio of 0.57 according to Equation 1. These facts point out to the occurrence of the reverse water-gas shift reaction and the reverse Boudouard reaction. The product composition agreed very well with thermodynamics, the calculated ratio of H2 to CO being 0.28 at 900 °C. The catalyst bed was coking only slightly and the pressure increase was much slower compared to the increase at 700 °C. In addition, no smaller hydrocarbons were formed. This all means that higher temperatures are more favourable for the reforming reactions. However, if high H2 to CO ratio is required, higher temperatures are not suitable due to the water-gas shift reaction, when larger than stoichiometric amount of CO2 is used.
Fig. 1. CO2 conversion and the outgoing CO flow at 4 bar and 700 °C.
140
According to the thermodynamics coke formation is reduced at higher pressures. In addition, the pressure remains constant, when controlled. In larger scale industrial application the pressure will also be higher than atmospheric. In the experiments performed at 4 bar and 700°C the initial conversions for «-heptane and carbon dioxide were 83% and 13%, respectively. 31% of «-heptane was cracked and the initial molar ratio of methane to ethene was 0.61. The activity of the catalyst decreased clearly during the experiment of 6.5 hours. The conversion of carbon dioxide and the outgoing carbon monoxide flow are shown as a function of time in Figure 1. It seems that increasing the pressure worsened the situation, as was to be expected on the basis of the thermodynamic calculations considering conversion. This means that at higher pressures higher temperatures are needed. 4. CONCLUSIONS The CO2 reforming of «-heptane is a promising way of utilising carbon dioxide. High temperatures are required for the optimal production of synthesis gas. However, the overall reaction remains very endothermic even though the side reactions lower the reaction enthalpy slightly. Combining the CO2 reforming with partial oxidation could provide a solution as it decreases the need for outside heating. This could have a positive effect on the catalyst deactivation, too. ACKNOWLEDGMENTS The financial support for this work from the Technology Development Centre of Finland (TEKES) and Fortum Oil and Gas Oy is gratefully acknowledged.
REFERENCES 1. Hileman, B., How to Reduce Greenhouse Gases, Chem. Eng. News 80 (21) (2002) 37-41. 2. Bradford, M.C.J., Vannice, M.A., CO2 Reforming of CH4, Catal. Rev.-Sci. Eng. 41(1) (1999) 1-42. 3. Rostrup-Nielsen, J.R., Bak Hansen, J.-H., CO2-Reforming of Methane over Transition Metals, J. Catal. 144 (1993) 38-49.