Analysis of heat effects in a membrane reactor for the WGSR

M. E. ADROVER; E. LÓPEZ; M. N. PEDERNERA; D O. BORIO

IXTAPA-ZIHUATANEJO

Congreso; Mexican Congress on Chemical Reaction Engineering (MCCRE 2008); 2008

The Water Gas Shift Reaction (WGSR) is a reversible and moderately exothermic reaction widely used in industry. In ammonia plants, two adiabatic reactors at different temperature levels are disposed in series (HTS and LTS) to reach high CO conversions. In smaller scale applications, such as fuel processing for PEM fuel cells, the WGSR reactor is instaled downstream the reformer to increase the H2 production rate and purify the process stream. In these designs, the WGSR is carried out in a single adiabatic reactor at intermediate temperature [1]. However, the chemical equilibrium limitations make neccesary to introduce another purification stage (e.g., CO PrOx) to reach the specified CO content for the fuel cell (<10 ppm). An attractive alternative to the Fixed Bed Reactor (FBR) is the Membrane Reactor (MR). A dense membrane (e.g., Pd supported on a porous substrate) is used to shift the equilibrium by means of selective and continuous permeation of one of the reaction products (e.g., H2) [2]. The advantages of the MR to carry out equilibrium limited reactions have been demonstrated by different authors [3-5]. In most of the literature contributions the heat effects are neglected and the MR operation is supposed to be isothermal. This common modeling assumption agrees with the temperature measurements inside the reactor at laboratory scale, due to the high ratios between heat-transfer area and reactor volume. However, this condition may not be true for higher process scales, e.g.: several membrane tubes installed in parallel within a shell where a sweep gas is circulated. In this article, the operation of a non-isothermal MR for the WGSR is simulated and compared with that of a FBR. A simple seudohomogeneous 1-D model is selected to simulate both the MR and the FBR. Mass and heat balances are considered for the reaction side (catalyst tubes) and the permeation side (shell) and two different flow configurations of the sweep gas: co- and countercurrent [6]. The kinetic model [7] corresponds to a high temperature WGS catalyst. It is assumed that the permeation flow (JH2) follows the Siervert´s law.  [1] J.A. Francesconi, M.C. Mussati, P. Aguirre.XX SICAT-Simpósio Ibero-Americano de Catálise (2006) [2] E. Kikuchi. Catalysis Today, 25 (1995) 333. [3] A. Basile, V. Violante, F. Santella, E. Drioli. Catalysis Today, 25 (1995) 321. [4] W. Yu, T. Ohmori, T. Yamamoto, A. Endo, M. Nakaiwa, T. Hoyakawa, N. Itoh. Int. J. Hyd. En. 30(2005) 1071. [5] F.C. Gielens, et al. J. Memb. Sci., 279 (2006) 176. [6] M.E. Adrover, E. López, D. Borio, M. Pedernera. Stud. Surf. Sci. Catal. 167, pp. 183-188, Elsevier (2007). [7] W.F. Podolski, Y.G. Kim. Ind. Eng. Chem.,13(4) (1974) 415.