(128d) Oxidative Coupling of Methane Enhanced by Thermally Optimized Reactors and Intermediate Separation

Few direct methane conversion schemes (i.e. no syn-gas intermediates) have been designed that offer simple and economical production of transportable liquid fuels from the abundance of stranded natural gas fields in the world. For example, the production of C2 hydrocarbons (i.e. ethylene + ethane) through the oxidative coupling of methane (OCM) has been prohibited from commercial practice due to low overall yield (conversion x selectivity). In particular, today's catalysts exhibit either high selectivity (>70%) coupled with low conversion (<5%) or high conversion (>75%) with low selectivity (<15%) (Hutchings, 1992). To overcome these problems, Mesoscopic Devices has designed a novel multi-bed system with optimized thermal integration that may enable commercial practice of oxidative coupling with low cost and high yield. In addition to increasing conversion and selectivity for the oxidative coupling of methane, the system design can be applied to other oxidation processes resulting in similar benefits.

Most of the research conducted on the oxidative coupling of methane deals with increasing activity and yield of the OCM catalyst. However, many studies have come to the conclusion that economical practice of OCM is not possible without both high activity catalysts and a novel reactor or process design to achieve high yield. The need for a novel reactor design is due to the limitations of present catalysts to achieve both high selectivity and high conversion through a single pass (Kalenik, 1992). Su and co-workers have shown through theoretical modeling that the limit to ethylene yield by OCM using conventional, packed bed, continuous feed operation is approximately 28% (Su, 2003). For oxidative coupling to be economically competitive with other processes, researchers have shown the processes must achieve C2 selectivities and CH4 conversions of approximately 85 and 35%, respectively (Matherne, 1992 and Kuo, 1992).

Two process designs have been published in Science and patented, one by Tonkovich and one by Jiang, that report overall C2 yields of approximately 50 and 85%, respectively (Tonkovich, 1993 and Jiang, 1994). Both process designs use sorbents to collect the C2s to minimize further oxidation of the products to CO2 and H2O. Though both process designs achieve the yields deemed necessary for economical practice of OCM, the complexity of the process designs, which consists of numerous switching valves and beds or gas recycle, have prohibited the technologies from reaching commercialization. Mesoscopic Devices' design builds on the above processes by increasing system single-pass product selectivity and methane conversion through a multi-bed arrangement of thermally optimized reactors and separation units. Furthermore, system complexity of the multi-bed arrangement is minimized with the use of a simple rotating valve.

Mesoscopic Devices' OCM process is based on 4 key elements: 1) multiple thermally optimized reactors, 2) multiple product separation units, 3) a single rotating valve and 4) a unique multi-bed arrangement that incorporates all of the above elements. The reactors are based on a thermally optimized design capable of achieving higher overall yield and production rates compared to traditional reactors in a single pass. After the catalytic reaction, the desired ethylene product is separated from the by-products and unreacted feed in an adsorption unit. Our regenerable sorbents have high ethylene breakthrough and saturation capacity and are capable of removing ethylene from a mixture of OCM products (C2H4, C2H6, CO2, CO, H2, N2, CH4). The combination of the thermally optimized reactors and separation units increases product selectivity by reducing over-oxidation of the desired ethylene product.

To increase methane conversion and overall product yield, the aforementioned series of unit operations is implemented in a multi-bed system where all of the necessary fluid flow switching is controlled by a single rotating valve. The rotating valve operates with a single moving part eliminating numerous mechanical switching valves, which are expensive and potentially unreliable. The rotating bed switches the adsorption units between two phases, ethylene adsorption and desorption, to maximize methane conversion, product selectivity and sorbent efficiency.

This paper will present the overall design concept as well as individual component designs (reactor, separation unit, rotating valve) enabling the conversion of methane gas to ethylene through the oxidative coupling of methane with the multi-bed arrangement. We will present results from our feasibility studies in our dual bed assembly as well as our 8 bed prototype system at the kilogram per day scale. Preliminary models of a scaled-up version of the OCM system to 100 kg/day ethylene will also be presented.

References:

Hutchings, Graham J. and Michael S. Scurrel. ?Studies of the mechanism of the oxidative coupling of methane using oxide catalysts?. Methane Conversion by Oxidative Processes: Fundamentals and Engineering Aspects, edited by E.E. Wolf, Van Norstrand Reinhold Catalysis Series, New York 1992.

Jiang, Y., I.V. Yentekakis and C.G. Vayenas. ?Methane to ethylene with 85 percent yield in a gas recycle electrocatalytic reactor-separator?. Science, 264 (1994) 1563-1566.

Kalenik, Zbigniew and Eduardo E. Wolf. ?The role of gas-phase reactions during methane oxidative coupling?. Methane Conversion by Oxidative Processes: Fundamentals and Engineering Aspects, edited by E.E. Wolf, Van Norstrand Reinhold Catalysis Series, New York 1992.

Kuo, James C.W. ?Engineering evaluation of direct methane conversion processes?. Methane Conversion by Oxidative Processes: Fundamentals and Engineering Aspects, edited by E.E. Wolf, Van Norstrand Reinhold Catalysis Series, New York 1992.

Matherne, J.L. and G.L. Culp. ?Direct conversion of methane to C2's and liquid fuels: process economics?. Methane Conversion by Oxidative Processes: Fundamentals and Engineering Aspects, edited by E.E. Wolf, Van Norstrand Reinhold Catalysis Series, New York 1992.

Su, Yee San, Jackie Y. Ying, and William H. Green, Jr. ?Upper bound on the yield for oxidative coupling of methane?. Journal of Catalysis, 218 (2003) 321-333.

Tonkovich, Anna Lee, Robert W. Carr and Rutherford Aris. ?Enhanced C2 yields from methane oxidative coupling by means of a separative chemical reactor?. Science, 262 (1993) 221-223.