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Ind. Eng. Chem. Res. 2001, 40, 5908-5916
Comparative Study on Oxygen Permeation and Oxidative Coupling of Methane on Disk-Shaped and Tubular Dense Ceramic Membrane Reactors F. T. Akin, Y. S. Lin,* and Y. Zeng† Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0171
Oxygen permeation and oxidative coupling of methane (OCM) on fluorite-structured Bi1.5Y0.3Sm0.2O3-δ (BYS) membrane reactors of disk-shaped and tubular geometries were studied at high temperatures (800-900 °C). Their oxygen fluxes under an oxygen partial pressure gradient of air/helium are similar and in the range of (1-3) × 10-8 mol/cm2‚s at 900 °C. The highest C2 yields obtained in the tubular and disk-shaped BYS membranes are 22% and 10.4%, respectively. The oxygen permeation fluxes through both the disk-shaped and tubular BYS membrane reactors under OCM reaction conditions are 1-2 orders of magnitude higher than those under oxygen permeation conditions with He as the purge. Oxygen permeation flux through the tubular BYS membrane is an order of magnitude smaller than that through the disk-shaped membrane under the OCM reaction conditions. The results show that the membrane geometry and downstream flow conditions have a significant effect on the reaction results. The tubular geometry gives more favorable results in terms of C2 yield and selectivity. Introduction In most membrane reactors, reaction and separation steps are integrated into a single step.1 This feature enables the selective removal of the products so that yields greater than the nominal equilibrium values are attained. A membrane reactor can be also used to avoid the direct contact of the reactants. The reactants are introduced into the separate chambers; they then diffuse into the membrane, where they meet and react. An advantage of this reactor design is self-regulation of the process, which is suitable for reactions that require a strict stoichiometric flow of reactants. Another advantage of these types of reactors is that thermal runaways do not occur, making the reactor attractive for processing of fast, heterogeneous, exothermic reactions such as partial oxidation reactions.2 Partial oxidation of light alkanes is one route for their direct conversion to more useful products. These reactions usually proceed through bimodal pathways where the alkyl radical forms first and coupling and oxidation of these radicals take place second.2 However, alkane reaction with oxygen over all known catalysts results in the thermodynamically much more favored formation of carbon oxides, thereby decreasing the selectivity for higher alkanes. Oxidative coupling of methane (OCM) to ethane and ethylene (C2 products) is a partial oxidation reaction where selectivity, not conversion, is the problem. In a novel membrane reactor for OCM where a dense semipermeable ionic conductor is used, the contact mode of oxygen and methane is controlled. Two approaches have been reported using these types of membrane reactors to improve the selectivity and yield of OCM. In the first approach, the membrane is used as the * Corresponding author. E-mail:
[email protected]. Fax: 513-556-3473. † Current address: Gases Technology, The BOC Group, 100 Mountain Avenue, Murray Hill, NJ 07974.
oxygen separator (if the air is used as the reactant) and oxygen distributor.3,4 A regular OCM catalyst is packed in the tube or shell side of the membrane reactor. The OCM reaction in this membrane reactor follows the same mechanism as that in the conventional co-feed reactor. The other approach requires that the surface of the oxygen semipermeable membrane is catalytically active and selective for OCM. In this case oxygen permeates through the membrane and reacts, in the form of nonmolecular oxygen, with methane on the other surface of the membrane. This approach minimizes the amount of the gas-phase oxygen in the methane stream, therefore reducing the total oxidation reactions in the gas phase. The second approach has a different reaction mechanism for OCM. Table 1 summarizes work published on the second approach for OCM. The membranes used in these studies were made of both perovskite- and fluorite-type materials. In most cases, the membrane surface is not modified with other catalysts except for the work reported by Nozaki and Fujimoto5 and Guo et al.6 Most reactors employed disk-shaped perovksite-type ceramic membranes and offered very low C2 yields (
