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J Description of Technologies for Direct Conversion of Natural Gas Numerous direct methane conversion routes are being studied at the bench scale by various companies, government agencies, and universities. These are briefly discussed in the following sections. COLD FLAME OXIDATION Cold flame oxidation involves the conversion of a pressurized mixture of methane and oxygen at moderate temperatures of 660° to 930°F (350° to 500°C). The reaction mixture is very fuel rich, with methane-to-oxygen ratios of about 20:1. The main chemical reaction is the oxidation of meth- ane to methanol (CH4 + i/: O2 = CH3OH). However, the further oxidation of methanol to formaldehyde often takes place simultaneously (CH3OH + i/z O2 = CH2O + H2O). The best results, achieved at the University of Manitoba, report 90 percent methanol selectivity at 7.5 percent single-pass conversion in an isothermal reactor (Kuo, 1987~. DIRECT OXIDATION Direct oxidation involves the catalytic coupling (oxidative coupling) of methane and an oxidant in the presence of a catalyst at moderate (700°C) temperatures and at about atmospheric pressure to produce C2+ hydrocar- bons. The oxidants include air, oxygen, and nitrous oxide. Phillips Petro- leum has described the use of catalysts, such as CaO/Li2CO3, that lead to relatively high conversions (15 to 20 percent) and good selectivities (60 to 70 percent) in the formation of ethylene and ethane. Work has been done by J. H. Lunsford and T. Ito at Texas A&M University on the chemistry of direct oxidation. Also, Akzo Chemie, Amoco, University of California at 197
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198 APPENDIX J Berkeley, the universities of Pittsburg and Tokyo, Idemitsu Kosan, and Union Carbide have also pursued development of direct-oxidation catalysts. British Petroleum has described experiments at higher temperatures (1100°C) and at short residence times at which conditions methane reacts with oxygen to produce syngas as well as C2+ hydrocarbons. Several cata- lysts, including zirconia, gave C2+ selectivities of over 50 percent. OXYCHLORINATION Oxychlorination involves the catalytic reaction of methane with a mix- ture of hydrogen chloride and oxygen to produce methyl chloride. The methyl chloride is then reacted over a shape-selective zeolite catalyst to produce a mixture of aliphatic and aromatic hydrocarbons. Methane can also be oxidized to methyl halides using chlorine, bromine, or iodine. British Petroleum has developed catalysts that are selective in the con- version of methane and also the conversion of methyl chloride to hydrocar- bons. The Pittsburgh Energy Technology Center has announced a similar process. The chloromethanes are reacted over ZSM-5 to produce gasoline and HC1, which is recycled. Imperial Chemical and Mobil have described catalysts that support the oxychorination process. INDIRECT OXIDATION (OXIDATIVE COUPLING TO ETHYLENE) Indirect oxidation of methane takes place at high temperature (about 760°C) using various reducible metal oxides as oxygen carriers as well as catalysts. A typical reaction can be represented by 2 CH4 + MOx + 2 = C2H4 + 2 HALO + MOx, where MOx + 2 and MOx represent the metal oxide and reduced metal oxide, respectively. The reducible oxides are reoxidized with oxygen ac- cording to the reaction MOx + O2 = MOx + 2. Combination of the above reactions gives the following "coupling" reactions: CH4 + I/2 O2 = I/2 C2H4 + H2O. The indirect oxidation route can use air rather than purified oxygen and operates at a temperature that allows efficient recovery of the reaction heat. This is done by circulation of the metal oxide between a zone where it reacts with methane and a reoxidation chamber where it is regenerated with air. This configuration is similar to a fluid bed catalytic cracking unit, in which the catalyst circulates between a cracking section and a regeneration section (where coke is burned off the cracking catalyst). To make a liquid
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APPENDIX J 199 fuel, the oxidative coupling product would be cascaded over a reactor used in the methanol to gasoline process to oligomerize the ethylene to gasoline. ARCO Oil and Gas Company currently appears to be the leader in indi- rect oxidation, with 45 patents. ARCO recently reported success in devel- oping a two-step process that first converts methane to olefins (called REDOX process), followed by a catalytic reaction of the olefins to high-octane gaso- line. In the first step, ARCO has reported conversions of 25 percent with C2+ selectivities greater than 75 percent. According to ARCO, this technol- ogy has been proven in pilot-scale studies and is ready for larger-scale testing in a demonstration plant. CATALYTIC PYROLYSIS Direct methane conversion through catalytic pyrolysis involves contact of methane with a catalyst at a relatively high temperature (1100° to 1200°C), pressures near about 1 atm, and at a short contact time. Under these condi- tions methane undergoes catalytic dehydrogenation (2 CH4 = C2H4 + 2 Ho. Chevron has described several catalysts, such as Al2O3/Th/Cs, that lead to relatively high conversions (20 percent) with high selectivity (90 to 100 percent) to C2+ hydrocarbons, including both light olefins and aromatic hydrocarbons. The University of Houston, Phillips Petroleum, Sohio, and the University of Taiwan have also pursued catalytic pyrolysis. STRONG ACID CONVERSION Strong acid catalysts can promote polycondensation of methane. The University of California, Exxon Corporation, and Firestone have investi- gated this chemistry. BIOLOGICAL CONVERSION Biological conversion utilizes organisms that consume methane and oxy- gen for growth, thereby producing methanol. Exxon, several Japanese insti- tutions, and the University of Michigan have reported work on the biologi- cal conversion route.
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