An Innovative Approach for Ethylene Production from Natural Gas

Studies in Surface Science and Catalysis J.J. Spivey, E. Iglesia and T.H. Fleisch (Editors) 9 2001 Elsevier Science B.V. All rights reserved.

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An Innovative Approach for Ethylene Production from Natural Gas ZHU Aimin**, TIAN Zhijian*, XU Zhusheng, XU Yunpeng, XU Longya, LIN Liwu Dalian Institute of Chemical Physics, CAS, Dalian 116023, China A natural gas conversion process (denoted here as the EPNG Process) in which a fixedbed reactor is used to combine the exothermic oxidative coupling of methane (OCM) and the endothermic oxidative dehydrogenation of ethane with CO2 (ODE), is described in this paper. The results at various operating conditions demonstrate that this process is an efficient way to product ethylene from natural gas. In a 130-hour-running of a 100mL-scale reactor (CH4 1.71/min, C2H6 0.51/min, and 02 0.631/min), a conversion of 27% and 66% for methane and ethane was maintained, and a total carbon yield of 27% for ethylene was obtained. 1. I N T R O D U C T I O N Among the direct routes that have been explored for methane conversion using the abundant worldwide natural gas resources, the oxidative coupling of methane (OCM) to ethane and ethylene appears to be most promising. However, the ethylene yield of less than 10% with an ethylene content of only about 5% in the product gas presents a challenge [ 1]. In order to improve the technical and economic feasibility of the OCM process, considerable research effort has been devoted in recent years to seeking new approaches to increase ethylene production. One approach is to enhance the ethylene yield during the OCM reaction by using a simulated countercurrent moving-bed chromatographic reactor (SCMCR)[2], a gas recycle reactor-separator [3], or a membrane contactor [4]. Another approach is the combination of exothermic OCM and endothermic conversion of ethane and naphtha to ethylene in the same reactor [5-9]. The IFP oxypyrolysis process [6] combines the OCM (a heterogeneously catalyzed reaction producing ethane as a primary product) with the steam cracking of ethane (a homogeneous gas-phase reaction producing ethylene and hydrogen). Methane, separated from natural gas, is mixed with oxygen or air, then added to the OCM reactor. After molecular oxygen has been nearly quantitatively consumed, the ethane separated from natural gas is added to the OCM effluent. The heat of the exothermic OCM reaction is then partially utilized in the endothermic conversion of ethane to ethylene and hydrogen. In the OXCO process [5,7], a single fluidized bed reactor is used to combine the OCM step with the pyrolysis of ethane and higher alkane components present in natural gas. Such a one-stage process could be attractive for its higher heat utilization efficiency. Moreover, an additional methanation process was proposed to hydrogenate the COx resulting from non-selective oxidation, for the recovery of valuable carbon [5-9].

"*Present address: Laboratoryof Plasma Physical chemistry, Box 288, Dalian University of Technology, Dalian 116024, P. R. China. E-mail:[email protected] "Corresponding author. E-mail:[email protected]

70 The oxidative dehydrogenation of ethane (ODE) with CO2 over a catalyst, as an alternative route for producing ethylene, has been proposed [ 10-13 ]. It has been found that the addition of CO2 promotes the ethylene yield and prevents carbon deposition [14-16]. Actually, the OCM reaction is always accompanied by the production of COx [ 17]. With most OCM catalysts, carbon dioxide is the major COx, and its selectivity is 4-6 times that of CO [5]. Therefore, the inevitable by-product CO2 from the OCM step can act as one of the reactants required by the second step of ODE with CO2. Moreover, the heat of the exothermic OCM reaction could be also utilized by the strongly endothermic reaction of ODE with CO2: C2H6+CO2--~CEH4+CO+H20(g) (AHE98K=+ 178.4kJ/mol) (1) Thereby, an innovative approach to ethylene production from natural gas (denoted the EPNG Process), which combines the OCM process and the process of ODE with CO2 in one reactor packed with the OCM catalyst and the ODE with CO2 catalyst, has been proposed and explored by the present authors. In the EPNG Process, the inevitable by-product CO2 from the OCM process is required by the process of ODE with CO2 over a catalyst, and the heat generated by the exothermic OCM process is partially or even completely utilized by the endothermic reaction of ODE with CO2. Also, ethane produced by the OCM process, together with added ethane, dehydrogenates to ethylene over the catalyst used in the ODE-CO2 process, producing a high content of ethylene in the product gas. 2 EXPERIMENTAL The La0.2Ba01/CaO was selected as the catalyst for the OCM process for its high selectivity to CO2 rather than CO [1]. The preparation of the La0.2Bao.l/CaO catalyst is described elsewhere [1]. The MnO/SiO2 catalyst for the ODE with CO2 was prepared by impregnating silica gel with an aqueous solution of Mn(NO3)2. The flow diagram of the EPNG process and a tubular quartz reactor modeled on a jacket heat exchanger are shown schematically in Fig.1. The La0.2Ba0.1/CaO catalyst was placed in the jacket of the reactor and the MnO/SiO2 was placed in the inner tube. A 10ml-scale reactor packed 1.Sml La0.2Bao.l/CaO and 5 ml MnO/SiO2 and a 100ml-scale reactor packed 30ml La0.2Ba0.~/CaO diluted with 60ml quartz sand and 90 ml MnO/SiO2 were electrically heated in flowing nitrogen. The catalyst zone of the 10ml-scale reactor was centred inside a tubular furnace (40mm I.D.). The catalyst zone of the 100ml-scale reactor was inserted into the bottom part of a tubular furnace (90mm I.D.), as shown in Fig.1. The feedstock mixtures in the jacket were heated indirectly by the effluent gas in the inner tube outside the furnace, before it contacted the heated catalyst. It is evident that this reactor cannot function as an adiabatic reactor. When the catalysts were at the required Product temperature the nitrogen flow was stopped and a Fig. 1 Schematics of the reactor methane and oxygen mixture was admitted to the 1 Lao2BaoI/CaO 2 quartz sand reactor. By adjusting the furnace temperature or the 3 MnO/SiO2 4 furnace tube composition of CH4/O2 mixture, a desired reaction temperature could be achieved at which the oxygen was totally consumed. The metered stream

71 of ethane was then directly injected into the oxygen-flee zone between the OCM catalyst and the ODE catalyst and mixed with the OCM product. Thus the OCM process and the reaction of ODE with CO2 were combined in one fixed-bed reactor. In the comparison experiments, the Lao.2Bao.i/CaO catalyst or the MnO/SiO2 catalyst were replaced by quartz sand, and the OCM and ODE with CO2 were run separately in the 10ml-scale reactor without combining the reactions. For the ODE with CO2 reaction, the nitrogen was introduced as a diluent with the CO2 and ethane mixture to the MnO/SiO2 catalyst in order to simulate the flow condition of the ODE stage of the EPNG process. The composition of the reactant mixture was controlled by a set of mass flow controllers. The effluent gas from the reactor was analyzed by an on-line gas chromatograph equipped with a thermal conductivity detector after removing water by condensation at 273K. The overall results of the conversions and the ethylene yield were defined as follows: CH4 conversion (%)=100(moles of methane consumed/moles of methane introduced) C2H6 conversion (%)=100(moles of ethane consumed/moles of ethane introduced) CO2 conversion (%)=100(moles of CO2 consumed by ODE reaction/moles of CO2 produced by OCM reaction) C2H4yield (%)=200(moles of ethylene produced/total moles of carbon atom introduced) 3. RESULTS AND DISCUSSION 3.1 Coupled effect on the yield of ethylene To examine the effect of ethylene on the yield, three comparative experiments shown in Fig. 1were carried out in the 10-ml scale reactor: OCM process alone, ODE with CO2 process alone and EPNG Process. The operating conditions are presented in Table 1. Table 1. Operating.conditions for the comparative experiments in Fig. 1 Parameters OCM process ODE With CO2 'EP?qG Process c . .alonea'-- . process alone b " Temperature of the ()CM part (K) 1023 1023 Temperature of the ODE part (K) 1073 1073 Feed flow rate (ml/min): CH4 80 0 80 C2H6 0 20 20 a CH4/O2=2, b CO2)C2H'6= 1, c CH4/O2/CzH"6=4/2/'i' ,,,

At 1023K, oxygen was fully consumed in the OCM experiment. The yield of C2 hydrocarbons was 13.2% at 34.7% methane conversion and 38:0% C2 hydrocarbons selectivity, giving an ethylene yield of 3.49 ml/min (Fig.2; the ethylene yield is presented at different flow rates for comparison). During the ODE-CO2 process alone, nitrogen was added to the OCM catalyst bed and a stoichiometric gas mixture of CO2 and C2H6(CO2/C2H6=1) was introduced to the catalyst bed. To make a corresponding comparison of these runs, the sum of the flow rate of CO2 and N2 was made equal to the flow rate of effluent gases in the OCM process alone. As a result, 3.87 ml/min of ethylene yield was formed in the ODE-CO2 process (Fig.2).

72 In the EPNG process, a mixture of methane and oxygen (CH4/O2=2) was passed through the OCM catalyst bed first, then together with ethane for ODE with CO2. As can be seen from Fig.2, the EPNG process gave l l.41mL/min of ethylene yield, which is much higher than the sum of the yields of ethylene produced in the OCM process alone (3.49 mL/min) and the ODE with CO2 process alone (3.87 mL/min). As expected, the EPNG Process, composed of OCM and ODE with C02, greatly increases the yield of ethylene. This can be referred to as a coupled effect.

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Fig.2 The yields of ethylene in the comparative experiment

3.2 The effect of C2HdCH4 molar ratio Fig.3 indicates the conversion of methane, ethane, and carbon dioxide and 60 the yield of ethylene as a function of C2H6/CH4 molar ratio in the feed gases. The results in Fig.3 were obtained at the 40 same conditions as the EPNG run in Table :__~_~ 1 except for the flow rate of ethane fed to the reactor. As the C2H6/CH4 molar ratio in 20 feed increased, the methane conversion gradually decreased, the ethane conversion did not change significantly, while the l n , . . l , l , | l . . | . - 0 conversion of carbon dioxide as well as the CoJ 0 0.1 0.2 0.3 0.4 yield of ethylene increased. According to the product distribution in C2H6/CH4 the EPNG process at various C2H6]CH4 Fig. 3 The i nfl uence of molar ratios in feed, it was inferred that in c2Wo~ the ODE with CO2 zone, ethane is 9 O-14Conv. --Ii-121-15 Conv. converted to ethylene via two parallel 9-A.- C~ Qmv. ~ C 2 H 4 Yiel d pathways: ethane dehydrogenation with CO2 (reaction 1) and thermal pyrolysis of ethane. It is estimated that 40-50% of ethane converted ethylene is via reaction 1 and 50~60% is via pyrolysis. Moreover, the side reaction, reforming of ethane and CO2 to syn-gas (reaction 2), is negligible when the C2H6/CH4 molar ratio in feed less than 0.325. C2H6+2CO2--->4CO+3H2 (2) Another side reaction of hydrogenolysis of ethane (reaction 3) also occurred in the ODE with CO2 zone. C2H6+Hz---~2CH4 (3)

73 Table 2 Variation of the relative C2 increment with C2H6/CH4 in the feedstocks* relative C2 increment C2H6/CH4 C2 in feedstocks C2 in products (C2 in products/C2 in (C mol%) (C mol%) feedstocks) 0 0 13.2 0.163 24.6 29.7 1.20 0.250 33.3 36.2 1.09 0.325 39.4 41.2 1.05 i.

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The amount of the C2 in the products was always greater than that in the feed for C2H6/CH4 feed molar ratios of 0 to 0.325 (Table 2). However, the higher the CzH6/CH4 molar ratio, the greater the rate of reaction 3. The relative C2 increment decreased from 1.20 to 1.05 (Table 2) and the overall methane conversion decreased from 34.7% to 27.6% (Fig.3), as the C2H6/CH4 molar ratio in feed increased from 0 to 0.325. 3.3 A bench scale test for the EPNG Process

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Time (h) Fig.4 Product distribution of the EPNG Process over the La0.2Ba0.1/CaO and MnO/SiO2 catalysts at 1073 K ( D, CH4, &, C2H4; x, CO2; O, C2H6; 0, CO ). A(0-2.3h): T=1023K, FcH4-1700ml/min, Fc2H6=0ml/min, Fo2=630ml/min; B(2.3-~23.0h): T increased to 1073K, FcH4=1700ml/min, Fc2H6=500ml/min, Fo2=630ml/min; C(23.0-77.0h): T=1073K, FCH4= 1700ml/min, Fc2H6=500ml/min, Fo2=630ml/min; D(77.0-80.0h): T=993K, FCH4=1700ml/min, Fc2H6=0ml/min, FoE=630ml/min; E(80.0-~128.8h): T=1073K, FCH4--1700ml/min, Fc2H6=500ml/min, Fo2=630ml/min.

74 A bench scale test for the EPNG was carried out with 30mL of the La0.2Ba0.1/CaO catalyst (10-20 mesh) for OCM and 90ml of the MnO/SiO2 catalyst (10-20 mesh) for ODE with CO2. The distribution of the EPNG products is given in Fig.4. It should be noted that with an C2H6/CH4 feed ratio of 0.29/1, the EPNG carbon-containing product contained -17 tool% ethylene. This level of ethylene in the EPNG product gas is much higher than that in the product gas from the OCM alone (-5 tool%). When the reaction temperature was kept at 1073K, and the flow rate of CH4, C2H6and 02 were controlled at 1700 ml/min, 500ml/min and 630ml/min (STP), a constant methane conversion of 26%, ethane conversion of 67%, and a ethylene yield of 27% were achieved in a 130 hour test. ACKNOWLEDGEMENTS This work was supported by the Ministry of Science and Technology of China. REFERENCES

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