BIMOLECULAR EXOTHERMIC REACTION WITH VAPORIZATION IN THE HALF-WETTED SLAB CATALYST

The steady-state behavior of a partially wetted, porous slab which catalyzes an exothermic, bimolecular reaction (A + D --> B) is simulated. One face of the catalyst is wetted by a flowing liquid film and the other is exposed directly to the gas. The interplay between partial external wetting, exothermic reaction, vaporization, imbibition and intraparticle and external transport processes is assessed by examining the dependence of catalyst effectiveness, degree of pore filling, and other state variables on the catalytic activity (Thiele modulus). The simulations reveal an interesting competition between the gas and liquid phases for the void space. While liquid imbibition, capillary condensation, and the endothermic demands of the vaporization process promote the filling of the pores with liquid, the exothermic reaction, rapid transport in gas-filled pores, and ineffective heat removal from the nonwetted face combine to help fill the pores with gas. The outcome of the competition has a significant impact on the catalyst effectiveness. The effectiveness dependence on Thiele modulus can consist of hysteresis and multiple local minima and maxima. The analysis demonstrates a smooth transition between the case of the completely filled, essentially isothermal pellet within which only diffusion and reaction occur, and the mostly gas-filled pellet within which dramatic nonisothermal and nonisobaric transport and reaction occurs. Performance comparisons between the half-wetted slab and a completely nonwetted slab elucidate the differences between the single-phase and multiphase situations