Energy & Fuels 2005, 19, 877-881
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Plasma Methane Conversion in the Presence of Dimethyl Ether Using Dielectric-Barrier Discharge Yu Wang and Chang-jun Liu Key Laboratory of Green Chemical Technology, School of Chemical Engineering, Tianjin University, Tianjin 300072, People’s Republic of China
Yue-Ping Zhang* Department of Chemistry, Tianjin University, Tianjin 300072, People’s Republic of China Received July 22, 2004. Revised Manuscript Received January 15, 2005
An experimental investigation was conducted to convert methane in the presence of dimethyl ether (DME), using dielectric barrier discharge (DBD) at atmospheric pressure and 100 °C. The co-feed of DME induces an increase in methane conversion, from 18.22% to 38.46%. The major products include C2 and C3 hydrocarbons, methoxy-containing oxygenates (methyl ethyl ether (MEE), dimethoxymethane (DMM), and dimethoxyethane (DMET)) and syngas (CO and H2). The selectivity of products changes with the CH4/DME feed ratio. The larger the CH4/DME feed ratio, the higher the selectivity of light hydrocarbons. The highest selectivity of all the oxygenates is observed for a CH4/DME feed ratio of 1:1.
Introduction Methane (CH4) is a principal component of natural gas, coalbed methane, associated gas of oil fields, and some byproduct gases of chemical plants. The utilization of methane as a raw material is important to maintain a safe and reliable energy and chemical supplies in the future. However, the synthesis of liquid fuels and oxygenates from methane still requires a multistep process via syngas (CO + H2). Steam reforming of methane to syngas with further conversion to methanol was one of the successful industrialized technologies. This process was operated at high temperature (a high endothermic reaction) and high pressure (to satisfy a reasonable conversion to methanol). The production and compression of syngas constituted >60% of the total cost in methanol synthesis. The high-energy consumption and high investment has limited the application of methane conversion via syngas. Recently, a worldwide investigation has been conducted to attempt to convert methane directly to chemicals and fuels.1-6 However, because of the difficulty in the activation of stable C-H * Author to whom correspondence should be addressed. Fax: +86 22 27890078. E-mail:
[email protected]. (1) Xu, X.; Fu, G.; Goddard, W. A., III; Periana, P. A. Selective Oxidation of CH4 to CH3OH Using the Catalytica (bpym)PtCl2 Catalyst: A Theoretical Study. In Natural Gas Conversion VII; Bao, X., Xu, Y., Eds.; Studies in Surface Science and Catalysis, Vol. 147; Elsevier: Amsterdam, 2004; pp 499-504. (2) Wilcox, E. M.; Roberts, G. W.; Spivey, J. J. Catal. Today 2003, 88 (1-2), 83-90. (3) Li, L.; Borry, R. W.; Iglesia, E. J. Catal. 2002, 57 (21), 45954604. (4) Huang, W.; Xie, K. C.; Wang, J. P.; Gao, Z. H.; Yin, L. H.; Zhu, Q. M. J. Catal. 2001, 201 (1), 100-104. (5) Zhang, Q. J.; He, D. H.; Zhang, X.; Zhu, Q. M.; Yao, S. L. Direct Partial Oxidation of Methane to Methanol in a Specially Designed Reactor. In Utilization of Greenhouse Gases; Liu, C.-J., Mallinson, R. G., Aresta, M., Eds.; ACS Symposium Series, No. 852; American Chemical Society: Washington, DC, 2003; pp 280-290.
bonds in the CH4 molecule, methane conversion directly to more useful chemicals and fuels remains a challenge.7 Further improvements in the catalysts are being performed worldwide. Many studies have also been conducted simultaneously to exploit other nonconventional conversion technologies. Among the nonconventional technologies under development, the nonequilibrium plasma is very promising.8-23 (6) Okumoto, M.; Rajanikanth, B. S.; Katsura, S.; Mizuno, A. IEEE Trans. Ind. Appl. 1998, 34 (5), 940-944. (7) Lunsford, J. H. Catal. Today 2000, 63 (2-4), 165-174. (8) Li, Y.; Liu, C.-J.; Eliasson, B.; Wang, Y. Energy Fuels 2002, 16, 864-870. (9) Wang, J.-G.; Liu, C.-J.; Zhang, Y.-P.; Eliasson, B. Chem. Phys. Lett. 2003, 368, 313-318. (10) Okumoto, M.; Su, Z.; Katsura, S.; Mizuno, A. IEEE Trans. Ind. Appl. 1999, 35 (5), 1205-1210. (11) Okazaki, K.; Kishida, T.; Ogawa, K.; Nozaki, T. Energy Convers. Manage. 2002, 43 (9-12), 1459-1468. (12) Matsumoto, H.; Tanabe, S.; Okitsu, K.; Hayashi, Y.; Suib, S. L. J. Phys. Chem. A 2001, 105 (21), 5304-5308. (13) Hwang, B. B.; Yeo, Y. K.; Na, B. K. Korean J. Chem. Eng. 2003, 20 (4), 631-634. (14) Larkin, D. W.; Lobban, L. L.; Mallinson, R. G. Ind. Eng. Chem. Res. 2001, 40 (7), 1594-1601. (15) He, J. X.; Han, Y. Y.; Gao, A. H.; Zhou, Y. S.; Lu, Z. G. Chin. J. Chem. Eng. 2004, 12 (1), 149-151. (16) Kado, S.; Urasaki, K.; Sekine, Y.; Fujimoto, K. Fuel 2003, 82 (11), 1377-1385. (17) Zhu, A. M.; Zhang, X. L.; Li, X. S.; Gong, W. M. Sci. China Ser. B. 2002, 45 (4), 426-434. (18) Liu, C. J.; Xue, B. Z.; Eliasson, B.; He, F.; Li, Y.; Xu, G. H. Plasma Chem. Plasma Process. 2001, 21 (3), 301-310. (19) Wang, J.-G.; Liu, C.-J.; Eliassion, B. Energy Fuels 2004, 18 (1), 148-153. (20) Schmidt-Szalowski, K.; Opalinska, T.; Sentek, J.; Krawczyk, K.; Ruszniak, J.; Zielinski, T.; Radomyska, K. J. Adv. Oxid. Technol. 2004, 7 (1), 39-50. (21) Zhang, J. Q.; Zhang, J. S.; Yang, Y. J.; Liu, Q. Energy Fuels 2003, 17 (1), 54-59. (22) Malik, M. A.; Jiang, X. Z. Plasma Chem. Plasma Process. 1999, 19 (4), 505-512. (23) Eliasson, B.; Liu, C.-J.; Kogelschatz, U. Ind. Eng. Chem. Res. 2000, 39 (5), 1221-1227.
10.1021/ef049823q CCC: $30.25 © 2005 American Chemical Society Published on Web 02/17/2005
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Energy & Fuels, Vol. 19, No. 3, 2005
Figure 1. Schematic representation of dielectric-barrier discharge (DBD) reactor system. Legend is as follows: 1, AC power source; 2, high-voltage electrode; 3, dielectric barrier material (quartz tube); 4, grounded electrode; 5, discharge gap; and 6, discharge region.
The advantage of nonequilibrium plasma lies in its nonequilibrium property: it has a high electron temperature (104-105 K), whereas the gas temperature remains low (as low as room temperature), which leads to some unusual chemical performance. Direct methane conversion using discharge plasmas to syngas,21 acetylene or other C2 hydrocarbon,12,14-16,19,20 liquid fuels (methanol or higher hydrocarbons),8,23 and oxygenates (such as acids and alcohols)8-11 have been extensively investigated. Especially, direct synthesis of oxygenates and liquid fuels from methane using nonequilibrium plasmas has recently attracted much attention. A plasma “gas-to-liquid” race is being conducted.24 Methane conversion using nonequilibrium plasmas normally involves a co-reactant or dilution gas, to improve the yield or selectivity and avoid any possible coke formation. For example, carbon dioxide (CO2),8,9,18,19,23 nitric oxide (NOx),12 and helium10 have been used as the co-reactant or dilution gas. During our previous investigations on synthesis of oxygenates from methane using dielectric barrier discharge (DBD),8,9 we observed that the newly produced water has an important role in the formation of oxygenates.25 In this work, we attempt to use dimethyl ether (DME) as a co-feed for methane conversion using DBD. Here, DME is principally applied as the oxygen atom supplier. The oxygenates produced here are totally different from those formed with a CO2 co-feed and are methoxycontaining hydrocarbons, such as dimethoxymethane (DMM) and dimethoxyethane (DMET). We have previously reported a production of diesel fuel additives (DMM, DMET, and others) directly from DME plasmas.26,27 Compared to the feed of pure DME,26,27 a very different product distribution has been achieved with the co-feed of methane and DME, as discussed below. Experimental Section The DBD reactor (as shown in Figure 1) used in this investigation was similar to that reported previously.8,25,26 The gap for the discharge was 1.1 mm, and the length of the (24) Remote Gas Strategies Newsletter, February 1998. (25) Zhang, Y.-P.; Li, Y.; Liu, C.-J.; Eliasson, B. Influence of Electrode Configuration on Direct Methane Conversion with CO2 as a Co-reactant Using Dielectric-Barrier Discharges. In Utilization of Greenhouse Gases; Liu, C.-J.; Mallinson, R. G.; Aresta, M., Eds.; ACS Symposium Series, Vol. 852; American Chemical Society: Washington, DC, 2003; pp 100-115. (26) Jiang, T.; Liu, C.-J.; Fan, G.-L. Chem. Lett. 2001, (4), 322323. (27) Jiang, T.; Liu, C.-J.; Rao, M.-F.; Yao, C.-D.; Fan, G.-L. Fuel Process. Technol. 2001, 73 (2), 143-152.
Wang et al.
Figure 2. Effect of the methane/dimethyl ether (CH4/DME) ratio on the conversion of CH4 and DME. discharge zone was 300 mm. The width of discharge gap was equal to the distance between the inner surface of the steel tube and the outer surface of the quartz tube. The inner diameter of the steel tube and the outer diameter of quartz tube were 11.8 mm and 9.6 mm, respectively. The total flow rate was ∼40 mL/min and set via mass-flow controllers. The residence time in the reactor was ∼16.6 s. The high-voltage electrode was an aluminum foil that was attached to the inner surface of the quartz tube. A stainless-steel tube around the quartz tube served as the grounded electrode. All the reactions were conducted at 373 K, which was maintained using circulating oil. A high-voltage generator (made in Tianjin University) supplied up to 10 kV of sinusoidal signal at a frequency of 25 kHz to the high-voltage electrode. The voltage and current were measured with a high-voltage probe (Tektronix model P6015A) and a pulse current transformer (Pearson Electronics model 411) via a digital oscilloscope (Tektronix model 2440). The discharge power was measured via a digital multimeter (Keithley model 2000). The discharge power was fixed at 50 W in this work. The products were analyzed using an online gas chromatography/mass spectroscopy (GC/MS) system (Hewlett-Packard model HP5890, equipped with thermal conductivity detection (TCD) and flame ionization detection (FID) devices) with a mass-selective detector (Hewlett-Packard, model HP5971). The liquid products were collected in a trap that was cooled by a mixture of ice and water. The gaseous products were analyzed online with a HP-Plot Q capillary column (30 m × 0.53 mm), and the liquid oxygenates were analyzed with a SimplicityWax capillary column (50 m × 0.25 mm). The conversions of methane and DME, and the selectivity of gaseous and liquid products (based on carbon balance), were defined as follows:
methane conversion (%) ) DME conversion (%) )
moles of CH4 converted × 100 moles of CH4 in the feed
moles of DME converted × 100 moles of DME in the feed
selectivity (%) ) moles of a certain product produced × number of C atoms moles of DME converted × 2 + moles of CH4 converted × 100
Result and Discussion The Effect of Methane Feed Content on the Conversions and Selectivities. Figure 2 shows the effect of methane feed content on conversions of DME and methane. From this figure, the co-feed of DME enhances methane conversion, because methane con-
Plasma Methane Conversion in the Presence of DME
Figure 3. Production distribution in plasma pure methane or DME conversion.
Figure 4. Effect of the CH4/DME ratio on the selectivity of gaseous hydrocarbons.
version is normally
