15, pp. 1748-1754, 1996 Copyright 0 1996Elsevier Science Ltd Printed in Great Britain. All rights reserved 0016-2361/96$15.00+0.00
Fuel Vol. 15, NO.
methanol yield from methane in a non-isothermal reactor
Qihai Liu, Jan Rogut,
Baoshu Chen, John L. Falconer
Department of Chemical Engineering,University of Colorado, Boulder, CO 80309-0424, USA (Received 7 September 1995; revised 12 April 1996)
Partial oxidation of methane to methanol was carried out homogeneously in a non-isothermal reactor that contained a non-permselective membrane. A tubular reactor was used with a smaller-diameter tubular membrane of 5 nm pore diameter alumina or 0.5 pm pore diameter metal. The membrane provided auniform flow distribution and separated the hot reactor wall from a cooling tube located in the centre of the reactor. The cold region in the reactor rapidly quenched further reaction. The selectivity for CH30H formation at 4.6% conversion increased from 34 to 52% when quenching was used. The highest yield (selectivity times conversion) obtained was 3.8% at 55 MPa and 800 K. Methanol selectivityincreased with increasing pressure and decreased with increasing temperature, residence time and O2 concentration. The combined selectivity to partial oxidation products (CO, CH30H, CH20) was almost constant at 86%. Copyright 0 1996 Elsevier Science Ltd. (Keywords:methane; oxidation; methanol)
Methanol is produced commercially from methane via high-temperature catalytic steam reforming to formsynthesis gas. This energy-intensive, endothermic reaction is followed by catalytic conversion of the synthesis gas to CH30H. A less expensive process is desired, particularly to utilize the natural gas that is located in remote wells. Direct partial oxidation of CH4 to CH30H has been studied because it is a one-step, exothermic process. However, complete oxidation is favoured thermodynamically,and thus high selectivities to CH30H have usually been observed only at relatively low conversions’ . Both homogeneous and heterogeneous processes have been reported for CH4 oxidation, and high selectivities for CH30H have been obtained only at high pressures. At lower pressure, CH20 (formaldehyde) is the favoured partial oxidation product. Selectivities for CH30H have ranged from 10 to 80% at CH4conversions of 5 to 12%2-4. These high selectivities have been difficult to reproduce in other laboratories5>6. Most studies report CH30H selectivities between 30 and 40%, at CH4 conversions of 3 to 60h7-12. Methane conversion is limited by the O2 feed concentration, which is kept low to avoid an explosive mixture. Moreover, most studies report that as the OZ concentration increases, the CH30Hselectivity decreases6”-12. For example, Foulds et aZ.8>9 observed that selectivity decreased from 47 to 23% as the O2 concentration increased from 2.5 to 9Svol.%. At a fixed 02 concentration, Helton6 reported that CH30H selectivity decreased as O2 conversion increased. In contrast, Burch et al.’ reported that product selectivities are relatively insensitive to O2 concentration, reactor type,residence time and diluent gases.
Because CH30H can be subsequently oxidized to CO, C02, and HzO, it is desirable to remove CH30H continuously from the reactor zone to increase selectivity. Armor13 suggested that a permselective membrane could be added to a reactor to accomplish this. Others have quenched the product stream leaving the reactor14’Is. In the present study, a non-permselective membranewas combined with a non-isothermal homogeneous reactor. The membrane was used to separate high- and lowtemperature regions in a non-isothermal reactor. By maintaining a large thermal temperature gradient and distributing the gas flow evenly, selectivity was increased significantly above that obtained in the absence of the temperature gradient. The highest yield (selectivity times conversion)...