催化部分氧化甲烷制合成气的平衡热力学研究

Chinesc J. Chem. Eng, 10(1)56-62(2002)Thermodynamic Study on the Catalytic Partial Oxidation ofMethane to syngas*TQ03 Aⅹ U Jian(徐篾), WEI Weisheng(魏伟胜) and BAo Xiaojun(鮑晓军)The Key Laboratory of Catalysis, China National Petroleum Co., University of Petroleum, Beijing 102200, ChinaAbstract The catalytic partial oxidation of mcthane to syngas(CO+H2)has beenith the advanced process simulator PRO/I. The influences of temperature, pressure, CH4 O2 ratio and steamaddition in feed gas on the conversion of CHa selectively to syngas and heat dutywere investigated, andtheir effects on carbon formation were also discussed. The simulation results were in good agreement with theliterature data taken from a spouted bed reactorKeywords methane, partial oxidation, syngas, thermodynamic simulation1 INTRODUCTIONdownstream processes. Economic studies show thatEfficient utilization of natural gas resource, in- reduction of 25% of syngas production cost would notcreasing importance of environmental protection and only result in a strong improvement of the existinghealth concerns have stimulated numerous global technologies, but also allow the natural gas converR&d efforts for converting natural gas into more valu- sion routes to become much more competitive whenable chemical feedstocks and liquid fuels. In addition, coupled with oil refining/ 31it desirable to convert the gas on-site to liquid prod- tion have been proposed in the past decades G.remote locations of many discovered gas reserves make Several alternative routes for syngas products that can be transported more economicallcluding autothermal reforming, combined reforming,Methane can be converted to chemical feedstocks non-catalytic partial oxidation and catalytic partialand liquid fuels in either direct or indirect routes. In oxidation). Among these routes, catalytic partialthe direct route, methane can be converted to ethylene oxidation of methane with oxygen or air is a potenand ethane by oxidative coupling and or to methanol tial alternative to steam reforming, because it givesby partial oxidization. However, all these processes the desired H2: CO ratio(nHa 7co =2: 1)requiredhave the disadvantages of relatively low conversion for methanol or Fischer-Tropsch synthesis. Since itsand selectivity, and the indirect conversion remains mild exothermicity, this process would be much moreto be the main path for the utilization of natural gas energy-efficient than the energy-intensive steam re-in the future[llforming process. Economic analyses suggest that thisBy indirect route, methane is converted to syngas new route to methanol requires 10%--15% less energy(mixture of CO and H2), which is then synthesized and approximately 25%-30% less capital investmentto methanol, or converted to liquid hydrocarbons by in comparison to the typical steam reforming 5Fischer-Tropsch synthesis. Meanwhile, the hydrogenExtensive effort is presently being devoted to theproduced can be further utilized for ammonia synthe- catalytic partial oxidation of methane to syngas, withthe focus on both catalyst development and reactionThe dominating commercial method for producing engineering aspectsl6l. Generally, most of the reportedsyngas is by steam reforming of natural gas( 21. Be. experiments on catalytic partial oxidation were undercause of high endothermicity of the reaction proceed- the conditions close to the equilibrium state. Theing at nH, nco=3: 1, this process is characterized knowledge on the thermodynamic equilibrium of theby high investment costs and energy consumption. It reaction at different operating conditions is necessaryis estimated that in most applications of syngas such so that the catalyst and reactor performance can bemethanol synthesis, Fischer-Tropsch synthesis and evaluated. Some of thermodynamic analyses appearedammonia synthesis, about 60%--70% of the the over- in the literature discussing the effects of temperature,all process cost is associated with syngas generation 2). pressure and methane-to-oxygen ratio in the fecd onCost reduction in syngas generation would have a large the methane conversion and syngas selectivity 6-81and direct influence on the overall economics of these Thermodynamic data were also reported by many中国煤化工Received 2001-09-20, accepted 2001-11-27Supported by the China National Petroleum Co.(CNPC)CNMHGTowhomcorrespondenceshouldbeaddressedFax:+86(10)86971019;E-mail:baoxj@www.bjpeu.cdu.cnThermodynamiedy on the Catalytic Partial Oxidation of Methane to Syngasthorsl9-1d for comparing with the catalyst and or (D)water gas shift reactionreactor performance. But none of them gave an inte-grated discussion, especially on the carbon formationCO+H2O→H2+CO2and the effect of steam addition and heat duty at var-△H2sK=-41.2 k. mol-1(9)ious operating temperature and pressure. The aim ofthe present investigation is to give a thorough inves-(E)carbon formation reactionstigation on the infuence of operating conditions, suchas temperaand feed composition on theCH4→C+2H2equilibrium composition and heat duty, and therefore△H2y8K=74.9kJ·mol-provide some fundamental information for the processevaluation, design and integration2CO→CO2+C△H9sK=-1724 kJ. mol-1(l1)2 PRO/II SIMULATIONThe catalytic partial oxidation of methane to syn-Because the GIBBS REACTOR model can onlyas was simulated as an isothermal reaction using the solve single phase reactions, only gas components suchGIBBS REACTOR model in PRO/IL2. By minimiz- as CH4, 02, co, CO2, H2 and H20 are included asing Gibbs free energy, it calculates product composi- product components in the calculations of methanetion and thermal conditions subject to overall heat conversion and syngas selectivity. To overcome thiand material balances. The main reactions occurring shortcoming, in the present investigation, the formain the partial oxidation are listed below(reactions 1 tion of solid component carbon in the system was de-termined by comparing the values of equilibrium conA)main reactionstants,P. /pCH, and pCo/pCo of the carbon forma-tion Reactions(10) and(11), respectively. The conCH4+0502→CO+2Hversion of methane and the selectivities of H2 and cO△H29k=-3559 kj. mol-1(1) in the present work were definedFCHa, in- FCHaoutCO2+ 2H2O△H29K=-802 kJ. molH, out +FH2O,outCH4+1502→CO+2H2OH519kJ. mol-Iwhere Fi is the molar flow rate of species iCH4+1502→CO2+H2+H2O△H2k=-56lkJ.mol-1(4)3 RESULTS AND DISCUSSION3.1 Effects of temperature and pressureCH4+O2→CO2+2H2The effects of temperature and pressure on theequilibrium composition and heat duty were studied△H29K=-319kJ·mol(5)with CH /02 ratio at stoichiometric ratio(2/1), tem-perature ranging from 700 K to 1600 K and pressureCH4+O2→CO+H2+H2Ofrom atmospheric pressure to 4 MPa. CHA conversion△H2sK=-278 kJ. mol-(6)and Ha and Co selectivity vs. temperature and pressure are shown in Figs. 1, 2 and 3, respectively(C)reforming reactionsIt can be seen that an increase of temperatureresults in a considerable rise of CHa conversion andCH4+H2O→CO+3H2product selectivities. For example, at atmosphericEAI,1 he selectivities to△H298K=206kJ·molH2中国煤化工92.30% at temperatiCN MH Geased to 97.91%CH4+CO2→2CO+2H299.21% and 99.45% at temperature of 1200 K respec△H2sK=217kJ·mol-l8) tively. Conversion and selectivity of over 95 can behinese J.Ch.E.10(1)56(2002achieved at approximately 1100 K to 1200K at atmo- plant(l3l. Figs. 1-3 show that as the pressure is in-spheric pressure. However, both decrease sharply if creased the conversion of met hane falls, and so doesthe temperature falls below 1100Kthe selectivity to syngas. For example, at 1200 K theseatmospheric pressure to 66. 27%, 81.36% and 86.37%at 4 MPa respectivelyIn the downstream Fischer-Tropsch synthesis andmethanol synthesis, low methane content in syngas isnecessary for increasing the partial pressure of CO and3 MPaH2 to prevent kinetic and thermodynamic factors fromdecreasing the methane conversion rates. table l illus-trates the temperatures needed at different pressuresFigure I Effect of temperature and pressure on CHto achieve a 90% CHa conversion. These data indicateconversionthat increasing the reaction temperature can compensate for the effect of pressure. But high temperatureoperations at elevated pressure will increase considerably the capital investment because special alloys and101. 3 kParefractories have to be used in the construction of thehigh temperature can cause disintegration of the cataor agglomeration of catalyst particles. Thereforbe accepted between reaction temperature and pres-Figure 2 Effect of temperature and pressure on H2 sure. Calculation results suggest that changing thesteam, which will be discussed later, can lower thetemperature required for a certain methane conversion-- I MPTable I Temperature required to achieve 90% CHconversion at different→4MPaessure kPaEffect of temperature andpressure on Co20004000From the stoichiometry of Reaction (1), it is evlent that increasing pressure will make the equilibFor methanol synthesis and Fischer-Trupsch syn-rium shift to the left side. It is ideal to conduct the thesis, a H2/co ratio of 2 in syngas is desirable. Fcprocess at lower pressure for higher methane conver- syngas having a H2/CO ratio much higher than 2(e. gsion and syngas quality. However, most commercial syngas from steam reforming processes), the excess H2downstream processes are operated at high pressure needs to be reduced in a shift reactor that utilizes the(e.g, methanol synthesis, Fischer-Tropsch synthesis), reverse water-gas shift reaction [Reaction(9)). Fig 4and the natural gas is typically transported by pipeline gives the H2/ Co ratios of the equilibriun product atat a pressure of about 4MPa. Thus high pressure op- different temperature and pressure. It indicates thateration should be employed in commercial syngas pro the H2/Co ratio is kept at 2 when the temuperatureduction processes from the point of view of heat recov- is above 1050 K and that the pressure has no distinctery and compression efficiency. An energy assessment effect, on H2/CO ratio, especially when temperature isof a catalytic partial oxidation reactor on a commer- above 1050Kcial scale methanol plant shows that the syngas com中国煤化工essure onpression load in the methanol syuthesis loop decreases duty isCNMHG the heat dutyrapidly with the increase in pressure, resulting in sig- is decrenigher tenperature and leprcsificant reduction in energy consumption for the entire sure. This can be explained by the fact that the mildlyFebruary, 200Thermodynamic Study on the Catalytic Partial Oxidation of Methane to Syngasexothermic partial oxidation occurs more easily than 3.2 Effect of feed compositionthe highly exothermic combustion of CH4 [ Reaction 3.2.1 Effect of CH4/O2 ratio(2)) at higher temperature and lower pressureFigures 7 and 8 show the effects CH4/O2 ratio onthe CHa conversion, product selectivities, heat duty一101,3kPaand H2/Co ratio when varying nCH,/no, from 1.4/1o 4/1 with the temperature and pressure fixed. Since-P-3 MEsimilar tendency was found in all the calculations at4 MPaeach temperature and pressure only the results at1400 K and 3 MPa are given here8001000I20014001600p=101.3kPaFigure 4 Effect of temperature and pressure on101.3kP百0,164 MPa(a) Methane decomposition reaction, Reaction(10)p=101.3kPa120014001600-P-P=3 MPaP=4 MPaFigure 5 Effect of temperature and pressure on heatduring the catalytic partial oxidation of methaneL4001600syngas, carbon deposition on catalysts, especiallyon Ni-based ones, can take place due to methane de-(b) Boudouard reaction, Reaction(11)composition [Reaction(10))and or the boudouard re-Figure 6 Effect of temperature and pressure onaction(Reaction(1l)in the forms of both encapsu-carbon formationlation and whisker 8 14, 15). The encapsulating carboncauses a direct deactivation of the catalyst, becausethe active sites are covered. The whisker carbon givesrise to the loss of active sites after regeneration of thecatalyst. Therefore, one of the most important considerations for the industrial application of the process isto make these reactions thermodynamically unfavorable. i.e10-1.52.02.3:7PH/pcH. > KionU11/No/pCo>KilEffect of CH4/Oconversion and syngas selectivitiesere Kio and KilI are the equihbriumconstants(T=1400K,P=3when carbon formation occurs. Fig. 6 shows the natu-I logarithm of P /pcH, and pCoa/pCo at differentthe O2 feed composition increases, the con-temperature and pressure, in which the natural loga-n of CHa increases and the selectivity to syn-hm of the equilibrium constants of these two car-nd nH2/nco decreases, more H20 and CO2 arebon formation reactions are represented by the dotted formethe heat dutv increases. At 1400 K andlines. It can be inferred that carbon formation can be 3 MP中国煤化工 he selectivities toavoided if the temperature is carefully controlledcOCNMHGnge from 90.19%above about 1200 K to 1300 K, while pressure has no 98.00701.3] willl nCh/no, =2/1 trominent effect on the carbon formation98.38%,9114%,81.82%andl.0 with ncH,/no2Chinese J. ch.E.10(1)56(2002)Chinese J. Ch. E(Vol 10, No. 1)1. 4/1, respectively. The increase of O2 content in formation occurs when CH4/O2 ratio is slightly abovethe feed implies a decrease of temperfor achiving a certain conversion. For example, at are prone to be deactivated by carbon, care must bethe pressure of 3 MPa, the temperature required for taken to keep the CH4/O2 ratio not exceeding 2/1a 90 percent methane conversion is about 1400 K with 3.2.2 Effect of stean additionTCH. n0,=2/1, whereas it decreases to 1250K withThe function of steam addition in the catalyticncH/7o2=14/1partial oxidation of methane has been discussed inthe literature6, 16, including enhancing the methaneconversion, achieving safer operation by lowering thepartial pressures of potentially explosive fced gas, in-hibiting the formation of carbon on the catalyst surface, minimizing gas phase reaction before the catalyst08bed, and controlling exothermic heat of the partial oxidation processThe results of steam addition on equilibrium com3.03.54,0position and heat duty of the reaction system at1400K and 3 MPa with the nH o/ncH, ranging fromFigure 8 Effect of cha/Oz ratio on the heat duty 0.0/1.0 to 1.0/1.0 were shown in Figs. 10 and 11. Ob-and H2/co(T=1400K,P=3MPa)viously, steam addition can enhance the CHa conversion, thus lower the temperature required for a givenAs CHa in feed is increased, the conversion of CHA conversion. Table 2 lists the temperature required todecreases and the selectivity to syngas increases, less achieve 90% CHa conversion at different pressure withH2O and CO2 are formed, so that the heat duty de- steam addition (ncH. /no, /nH2o =1/0.5/0.5)creases. It also can be found from Fig 9 that the car. comparing with Table 1, it can be seen that thebon formation occurs when the CH,/Oz ratio is above perature is decreased considerably (over 100 K on thein average)lI cases studied herein(a)Methane decomposition reaction(Reaction 10)Figure 10 Effect of steam addition on CHA(T=1400K,]01.3kPaIn(Kii)2.0Figure 9 Effect of CH4/O2 ratio on carbon中国煤化工The effects of the CHaO2 ratio on carbon forma- Figure 11CNMHtion are presented in Fig 9, suggesting that the carbonT=1400K,P=3MPa)Thermodynamic Study on the Catalytic Partial Oxidation of Methane to SyngasTable 2 Temperature required to achieve 90% CH, 4 CONCLUSIONSonversion at differentBased on the above simulation and discussion the(ncH4/no2/ma12o=1/0.5/0.5)following conclusions can be madePressure, kPaTemperature(1)To achieve high CHA conversion and syngas970selectivity, it is desirable for the partial oxidation ofmethane to syngas to be operated at high temperature and low pressure. Conversions and selectivitiesof over 95% can be achieved at approximately 1100 Kto 1200 K at atmospheric pressure. Increasing the re-However, the steam addition also causes the de- action temperature can compensate for the effect ofrease of CO and the increase of H2/CO ratio, perhapspressurebecause of the promptitude of water gas shift reaction(2)At higher temperature and lower pressure, theAt 1400 K and 3 MPa, the CH4 conversion, CO selec- mildly exothermic partial reaction occurs more eas-tivity and nHa /nco change from 90.19%, 98.01% and ily than the highly exothermic combustion of CHa,so1.95/1 with no steam addition to 99.19%, 84.94% and that the heat duty decreases2.5/1 with feed ratio of nCH./no/nH20=1/0.5/0.53)Carbon formation is thermodynamically unfarespectively. As shown in Fig. 11, the heat duty of the vorable with temperature above approximately 1200Kreaction system decreases as the steam addition in- to 1300K Pressure has no significant cffect on the car-creases because of the occurrence of steam reforming bon formationreaction.The H2/ Co ratio is kept at 2 when the temper3.3 Validation of thermodynamic analysisature is above 1050 K; pressure has no distinct effectWhile there are several new processes for syngas on the H2/CO ratiomaking via partial oxidation of methane in bench-and (5)As the O2 feed composition increases,pilot-scale development, only limited information is version of CHA increases, the selectivity to syngas de-available in literature, especially in the case dealing creases, and the heat duty increases. As the CH4with conditions of high pressure and high tempera- feed composition increases, the conversion of CHa de-ture. In the present investigation, the recent results creases, the selectivity to syngas increases, and thegiven by Marnasidou et al 10I from a pilot-plant-scale heat duty increases. The carbon forms when thespouted bed were used to validate the theoretical anal- CH,/O2 ratio isyses. Based on the feeding gas composition,reactor(6 )Steam addition can enhance the CH4 converpressure and assumed equilibrium temperature pro- sion, thus lower the temperature required for a certainvided by those authors, a comparison of the Pro/iI conversion. It also causes the decrease of co and thesimulation results obtained in the present investiga- increase of H2/CO ratio. The heat duty of the reactiontion with experimental results by Marnasidou et al. system decreases as the steam addition increases;thewas made in Table 3. It is observed from Table 3 that reaction system can then be operated under adiabaticthe calculated CH, conversions, CH4, CO and Hno condition by the addition of steam.but the calculated CO2 composition deviates from the paring calculation results with the experimental dataexperimental data seriously, either due to the inaccu- obtained in a spouted bed reactor and good agree-rate estimation of the equilibrium temperature or badTable 3 Comparison of the thermodynamic analysis with the experimental results obtained in apilot-plant-scale spouted bed by Marnasidou et al. 0Production gas(dry, molar fractionQ, tf taCHcO24622012.912627425820073793783.097071418.916.7126.1270.948.148.03097074310.29中国煤化工086786677152138075911.1CNMHGChinese J. Ch. E. 10(1)56(2002)Chinese J. Ch. E.(Vol 10, No. 1)ACKNOWLEDGEMENTThe authors would like to acknowledge Prof. ChenHydrocarbon Processing, 77(3), 77-79Jiayong of Inst. Process Engineering, CAs for his6 Foulds, G. A, Lapszewicz, J. 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