Simulation studies of a membrane water-gas shift reactor under non-isothermal conditions

M.ESPERANZA ADROVER; EDUARDO LÓPEZ; DANIEL O. BORIO; MARISA N. PEDERNERA

Malta

Conferencia; XVIII International Conference on Chemical Reactors (CHEMREACTOR-18),; 2008

Introduction: Hydrogen production processes and their subsequent purification steps have been extensively studied in the last two decades for fuel-cell feeding. Reforming or partial oxidation of hydrocarbons or alcohols have been reported as the main routes to obtain the required hydrogen. In most cases, the CO content also evolving from the production reactor has to be reduced to avoid the poisoning of the fuel-cell anode catalyst. The water-gas shift reaction (WGSR) is selected to accomplish this task, with the additional effect of increasing the H2 production. This moderately exothermic reaction is strongly controlled by chemical equilibrium. An attractive alternative to increase the CO conversion is the use of membrane reactors (MR) to provoke an equilibrium displacement by the selective permeation of hydrogen from the reaction medium [1-2]. Many studies have been reported concerning water-gas shift membrane reactors (WGS-MR) [3]. In the vast majority of them isothermal operation has been considered. This assumption is reasonable in low-scale (laboratory) equipment due to the high relations transfer area / reaction volume under use. Nevertheless, when several membrane tubes are installed in parallel inside a shell through where the sweep gas flows, the hypothesis of isothermal-operation could be no more realistic. In a previous publication, a theoretical study of the influence of thermal effects on a WGS-MR performance is carried out for different configurations of the sweep-gas flow on the reactor shell (co and counter-current) [4]. The present contribution presents results regarding the effect of the operating pressure and the sweep-gas flowrate on the performance of the WGS-MR. A 1-D, pseudohomogeneous mathematical model has been selected to represent the reactor operation under adiabatic conditions (energy transfer between process and permeate streams only due to the permeation flow). A comparison with a conventional fixed-bed reactor (CR, without H2 permeation) is also reported. Results and discussion: The operation of a WGS-MR to process the outlet of a 1kWth methanol reformer was analyzed. 30 membrane tubes of 8mm I.D. and 150 mm length,  located in parallel, have been selected. Simulations show a strong influence of the operating pressure (P) for a constant sweep gas pressure of 1 atm, flowing co-currently with the process gas. In fact, higher pressures lead to an appreciable increase of the CO conversion (XCO) due to higher H2 permeation flows and enhanced equilibrium displacement. This increase in XCO is also associated to higher thermal effects. The increment of temperature in the membrane reactor when comparing with the conventional reactor is due to both the increase in CO conversion and the diminution of the molar flowrate of the process gas [5]. The reduction of the catalyst volume (reactor length) to reach a required XCO level was also analyzed for increasing operating pressures (results not shown). Medium pressures of around 5 atm appear favorable to achieve a noticeable diminution of the reactor length with moderate increase on compression and manufacture costs. The performance of the non-isothermal WGS-MR for operation without sweep-gas has been also studied. This alternative arises as highly attractive since no separation / purification steps are required for the pure H2 permeated gas. The increase in operating pressure results also beneficial for the operation without sweep gas but in a lower extent when compared with the operation with sweep gas. This behavior obeys to the elimination of the dilution effect provided by the sweep gas. The operation without sweep gas leads to lightly lower CO conversions but a pure H2 stream is attained in the reactor shell. Besides, an improved utilization of the catalyst in the reactor length is achieved. References: [1] J.S. Oklany, K. Hou, R. Hughes; App. Catal. A, 170 (1998) 13. [2] S. Tosti, L. Bettinali, V. Violante; Int. J. Hyd. Energy, 25 (2000) 319. [3] A. Criscuoli, A. Basile, E. Drioli; Catal. Today. 56 (2000) 53. [4] M. E. Adrover, E. López, D. Borio, M Pedernera; Studies on Surface Science and Catalysis, Elsevier, Amsterdam, Vol. 167, 2007, 183-188. [5] M. E. Adrover, E. López, D. Borio, M Pedernera; “Análisis de un reactor de membrana no isotérmico para la reacción de water-gas shift”, XV Argentinian Congress of Catalysis, La Plata, Argentina, 12-16/11/2007. Acknowledgments: Support of this work through Universidad Nacional del Sur (UNS), Consejo Nacional de Investigaciones Científicas y Tecnológicas (CONICET) and Agencia Nacional de Promoción de Ciencia y Tecnología (ANPCyT) is gratefully acknowledged