CO2 Conversion and Utilization - American Chemical Society

Chapter 21

Computational Analysis of Energy Aspects of CO Reforming and Oxy-CO Reforming of Methane at Different Pressures

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Downloaded by UNIV OF NEW SOUTH WALES on September 19, 2017 | http://pubs.acs.org Publication Date: January 24, 2002 | doi: 10.1021/bk-2002-0809.ch021

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Wei

Pan

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and Chunshan Song

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Clean Fuels & Catalysis Program, The Energy Institute, and Department of Energy and Geo-Environmental Engineering, Pennsylvania State University, 209 Academic Projects Building, University Park, PA 16802 From State Key Laboratory of Cl Chemical Technology, Department of Chemistry, Tsinghua University, Beijing 100084, China Corresponding author: Fax: 814-865-3248; email: [email protected]

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Calculation on energy requirement with different feedstock compositions, reaction temperatures, and reaction pressures in CO reforming and oxy-CO reforming was conducted in this paper. The thermo-neutral state in oxy-reforming system at different temperatures and pressures was also estimated by this calculation. 2

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CO2 reforming of methane is attracting attention worldwide in the past decade due to its specific features. First, this reaction could produce CO-rich synthesis gas from methane, which is an advantage for certain applications when compared to steam reforming of methane that has been widely commercialized for producing hydrogen or hydrogen-rich syngas (1).

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© 2002 American Chemical Society Song et al.; CO2 Conversion and Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

317 Secondly, this reaction could be used in some remote natural gas fields to convert C0 -rieh natural gas on site into CO-rich syngas and further to more valuable and/or easily transportable liquid products (2). Finally, this reaction could be potentially integrated into coal- or natural gas-fired power plants for C 0 conversion by utilizing flue gas from these plants for tri-reforming without C 0 separation (3). Compared to steam-reforming of methane which usually gives H -rich syngas with H / C O ratio of 3 or higher, more CO-rich synthesis gas with H2/CO ratio of around 2 is required in methanol synthesis and FischerTropsch synthesis, for which C 0 reforming can also be used to adjust the H2/CO ratio (4). Most previous studies reported in literature on this reaction were conducted at atmospheric pressure. However, it is important to consider high-pressure reactions, and our on-going experimental study focuses on this reaction under high pressure (5-6, 11). High-pressure operation is preferred for syngas and H production in industrial operations due to higher process efficiency. A n d since synthesis gas from C 0 reforming of methane w i l l be used for methanol synthesis or Fischer-Tropsch synthesis, high-pressure reforming would eliminate the need to decompress the feed gas streams and the need to re-compress the produced synthesis gas in order to meet the pressure requirement in the preceding processes. In our recent computational study, we have examined the thermodynamic equilibrium conversions and carbon formation for the reaction systems of CO2 and o x y - C 0 reforming of methane under high pressures as well as atmospheric pressure (7). The effect of reaction pressures, reaction temperatures, and feedstock compositions on global carbon formation, methane conversion, CO2 conversion, and product distribution was extensively discussed (7). This paper presents the results of our computational study on another important aspect of these reaction systems - energy balance under various conditions. It is well known that C 0 or steam reforming of methane is a highly endothermic reaction. How much energy is required for this reaction at different reaction conditions and how to provide that energy would be critical information needed for the design of a reactor or even the whole reaction configuration. Unfortunately this aspect is somehow ignored in literature, especially the energy balance under high pressures. Only few research groups paid some attention to this aspect for reactions under atmospheric pressure (2). 2

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Computational Method The software system, H S C Chemistry, which has already been used to carry out all the thermodynamic calculation in our recent study (7) was employed in this work. It was obtained from Outokumpu Research Oy in Finland. The

Song et al.; CO2 Conversion and Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


318 thermodynamic properties (e.g. enthalpy or entropy) of each component in the reaction system were derived directlyfromthe database of this software. To calculate the energy balance, the first step is to calculate the equilibrium composition at different reaction conditions. Since all the results of these calculations have been presented in our previous paper, we have no intention to repeat those data here. We will directly use those data for the following energy balance calculation. By calculating and comparing the energy in reactants before reactions and products after reactions at a specific reaction condition, the reaction heat required or released in those reactions could be obtained. Subsequently, if all the reaction heat at different conditions is calculated, the effect of reaction conditions on reaction heat could be evaluated. The advantage of this computational analysis is that it enables us to calculate the energy or mass balance of not only one single reaction, but also the global reaction incorporating all the reactions taking place in the system. Therefore, the calculation results from this software could give us a global and comprehensive picture of this reaction system.

Results and Discussions

Effect of reaction temperature and pressure on reaction heat of

C0 +CH =2CO+2H 2

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It is well known that CO2 reforming of methane is a highly endothermic reaction. The enthalpy change of this reaction at STP condition is 247 kJ/mol. The data is obtained assuming that reactants are completely converted to products. And reactants and products are all at STP condition. However, this assumption does not apply in most practical reactions. Fig. 1 shows the trend of changes in reaction heat with temperatures and pressures. The data in this figure was obtained on the basis that reactants and products are at the same reaction temperature. Of course, calculation could also be done assuming reactants are at room temperature while products are at reaction temperature. The only difference between these two calculations is the energy required for heating up the reactants from ambient temperature to reaction temperatures. The more of this temperature difference, the more energy is required for pre-heating. Curves in Fig. 1 indicate that the reaction heat of this reaction is very low, almost close to zero at temperatures less than 400°C. When reaction temperature goes above 400°C, the reaction heat starts to gradually increase. The explanation for these phenomena is that thermodynamically C 0 reforming of methane have a very low conversion at temperatures lower than 400°C. Only when temperature goes above 40Q°C can its conversion become higher and higher. As 2



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Temperature(°C) Fig. 1 Temperature and pressure effects on reaction heat in the reaction of C 0 reforming of methane 2


320 discussed above, C 0 reforming is an endothermic reaction. The higher the conversion, the more the reaction heat required. For example, at 1 atm, the maximum reaction heat is required around 850°C or higher due to complete conversion of methane and C 0 into synthesis gas. Almost in all the temperature range, the reaction heat decreases when reaction pressure changes from 1 atm to 40 atm. Our calculation results on equilibrium composition (7) show that the conversion of methane conversion decreases with the increase of reaction pressure. At the same time, the reaction heat in Fig. 1 is expressed on the base of one mole of reactant (e.g. CH ). Low conversion of methane means less methane is converted which subsequently leads to lower reaction heat required for this reaction. 2

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Effect of O2/CH4 and CO2/CH4 ratios on reaction heat For C 0 reforming of methane, carbon deposition is one of the major problems. Besides that, energy supply for this reaction is another important issue due to its highly endothermic nature. Addition of 0 into this reaction system was examined in several laboratory studies to make the system more energy efficient or to suppress carbon formation (8-10). We have reported the calculation results on the effect of 0 addition on carbon formation and methane conversion (7). For 0 addition, it can not only suppress carbon formation, but also relieve the energy requirement due to the exothermic properties of oxidation reaction. Here we will present our study on the effect of 0 addition on the reaction heat and thus the energy supply and demand situations. In the reaction system containing O2 as one of reactants, the principle endothermic or exothermic reactions include: 2

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Endothermic reactions: C 0 + C H = 2CO + 2H C H + H 0 = CO + 3H C H = C+ 2H C + H 0 = CO + H 2

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Δ Η =247kJ/mol ΔΗ° = 206 kJ/mol ΔΗ° = 75 kJ/mol ΔΗ° = 131 kJ/mol

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Exothermic reactions: C 0 + H = CO + H 0 2

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2CO = C+CO2

CH4 + O.5 0 = C O + 2H 2CO + 0 = 2C0 C H + 20 = C 0 + 2H 0 2

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ΔΗ° = -41kJ/mol ΔΗ° =- 172kJ/mol ΔΗ° = - 35.5 kJ/mol ΔΗ° = - 440 kJ/mol AH° = -880kJ/mol

(1) (2) (3) (4)

(5) (6) (7) (8) (9)


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In our calculation, we have considered specific reactions individually and incorporated all those reactions as a whole globally. Therefore, the calculated reaction heat would represent the global reaction heat in the reaction system, not the reaction heat of a single reaction. Fig. 2 and Fig. 3 show the effect of CO2/CH4 and 0 / C H ratio in feedstock on reaction heat under 1 atm at 801°C. Fig. 2 was obtained assuming reactants are at room temperature and products at 801°C. And Fig. 3 was obtained assuming both reactants and products are all at the same reaction temperature (801°C). In general, Fig.2 shows that more reaction heat is required comparing with Fig. 3 at same conditions due to the required energy for heating reactants up to reaction temperature. In Fig. 2 and Fig. 3, it is apparent that the reaction heat could change from positive (requiring heat input from outside) to negative (releasing heat to outside) with the change of 0 / C H ratio in the feedstock. The more 0 / C H ratio, the more heat is released. This is due to the exothermic property of oxidation reaction related to 0 . At certain 0 / C H ratio, the reaction heat could be zero, which means the system does not require or release any energy to the outside. The system can maintain its temperature by itself through the balance between the exothermic and endothermic reactions in the system. This is the socalled thermo-neutral state. Comparing high CCVCFLi in the feedstock with low C 0 / C H , the reaction heat required is high or the reaction released is less at high C 0 / C H . Especially in Fig. 2, the difference is even larger. This is due to two reasons. One is that addition of C 0 will change the equilibrium composition in products. Another is that more heat is required to heat the additional C 0 up to reaction temperature.


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Comparison of effects of CO2/CH4 and 0 / C H ratios on reaction heat at different temperatures 2

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Fig. 4 compares the effects of C 0 / C H and 0 / C H ratio on reaction heat at the same pressure (1 atm) while at different temperatures. When reaction temperature goes from 642 °C to 702°C and 801 °C. The sensitivity of reaction heat to CO^CFLt ratio becomes less and less. At 642 °C, the 0 / C H value for thermo-neutral operation could range from 0.24 to 0.5 when C 0 / C H changes from 1 to 5. However, at 801°C, 0 / C H value only ranges from 0.47 to 0.53. This phenomenon could be attributed to the less impact of C 0 / C H on equilibrium composition at higher temperature (e.g. 801°C). 2

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500 ι

-300 -I 0 / C H ratio 2

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Fig. 2 Effect of C0 /CH and 0 /CH ratio in feedstock on global reaction heat in the reaction systems of C 0 or oxy-C0 reforming of methane (reaction pressure: 1 atm, only products are at reaction temperatures of 801°C) 2

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0 /CH ratio 2

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Fig. 3 Effect of C0 /CH and 0 /CH ratio in feedstock on global reaction heat in the reaction systems of C0 or oxy-C0 reforming of methane (reaction pressure: 1 atm, both reactants and products are at reaction temperatures of 801°C) 2

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O2/CH4 r a t i o

Fig. 4 Effect of C0 /CH and 0 /CH ratio in feedstock on global reaction heat in the reaction systems of C0 or oxy-C0 reforming of methane at (a) 642°C; (b) 702°C; and (c) 801°C (reaction pressure: 1 atm, both reactants and products are at reaction temperatures) 2

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0 /CH 2

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F i g . 4. Continued.


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Comparison of the effects of CO2/CH4 and different pressures.

on reaction heat at

Fig. 5 shows the effect of C 0 / C H and 0 / C H ratio on reaction heat at 702°C under 1 atm and 40 atm, respectively. In this case, both reactants and products are all at 702 °C. Apparently, reaction at 1 atm needs more energy to reach equilibrium while reaction at 40 atm needs less on the base of 1 mol of C H reactant. The Q>/CH range of thermo-neutral state at 40 atm is 0.06 - 0.18 when C 0 / C H changes from 1 to 5. However, at 1 atm, the 0 / C H range of thermo-neutral state is 0.4 - 0.5 depending the C 0 / C H ratio in feedstock. This difference is obviously related to the effect of high pressure on equilibrium composition. 2

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O2/CH4 ratio

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Summary By computational analysis of the reaction system of C 0 and oxy-C0 reforming of methane, we have investigated the effects of reaction temperature, pressure, and feedstock composition on the energy balance, namely the reaction heat needed as energy input for the reaction or released as energy output. Effects of pressure and temperature on the overall energy balance of the reaction system are closely related to their effects on equilibrium composition. Addition of oxygen in the feedstock could turn the global reaction from an endothermic system to an exothermic one. The 0 concentration in feedstock which could lead to the thermo-neutral state in the reaction system strongly depends on the pressure, and it could be calculated and compared under various conditions although only some examples are illustrated in this paper. These data could be very useful for future design of processing scheme and reactor design. 2

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Acknowledgement We are pleased, to acknowledge the partial financial support of this work by the Pennsylvania State University (PSU) and the partial support on C 0 reforming by the Air Products and Chemicals Inc. (APCI). We are grateful to Dr. John N. Armor of APCI, to Prof. Alan W. Scaroni and Prof. Harold H. Schobert of PSU for their encouragement and helpful discussions, to the members of the APCI/PSU SAMCOM for their support, and to Dr. Srinivas T. Srimat for technical assistance and helpful discussions. 2



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0 /CH ratio 2

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Fig. 5 Effect of C 0 / C H and 0 / C H ratio in feedstock on global reaction heat in the reaction systems of C 0 or oxy-C0 reforming of methane at different pressures (upper group: 1 atm, lower group: 40 atm, both reactants and products are at 702°C) 2

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328 Literature Cited


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Armor J.N., Appl. Catal. A: General 1999, 176, 159. Choudhary V.R.; Rajput A.M.; and Prabhakar B. Angew. Chem. Int. Ed. Engl. 1994, 33, 2104. 3. (a) Song, C. Chemical Innovation (formerly Chemtech, ACS), 2001, 31, 21; (b) Song C. Proc. 16th International Pittsburgh Coal Conference: Pittsburgh, USA, October 11-15, 1999, Paper No. 16-5, CD-ROM ISBN 1890977-16-0. 4. Gunardson, H.H.; Abrardo, J.M. Hydrocarbon Processing April 1999, 87. 5. Song, C.; Srinivas, S.T.; Sun, L.; Armor, J.N. Am. Chem. Soc. Div. Petrol. Chem. Prepr., 2000, 45, 143. 6. Srinivas, S.T.; Song, C. Am. Chem. Soc. Div. Petrol. Chem. Prepr., 2000, 45, 153. 7. Pan, W.; Srinivas, T.S.; Song, C. 16th International Pittsburgh Coal Conference: Pittsburgh, USA, October 11-15, 1999; Paper No. 26-2. CD-ROMISBN 1-890977-16-0. 8. Choudhary, V.R.; Rajput, A.M.; Prabhakar, B., Catal. Lett. 1995, 32, 391. 9. O'Connor, A.M.; Ross, J.R.H. Catal. Today, 1998, 46, 203. 10. Ruckenstein, E. and Hu, Y.H. Ind. Eng. Chem. Res., 1998, 37, 1744. 11. Song, C.; Murata, S.; Srinivas, S.T.; Sun, L.; Scaroni, A.W. Am. Chem. Soc. Div. Petrol. Chem. Prepr., 1999, 44, 160.


CO2 Conversion and Utilization - American Chemical Society

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