Answer to Question #300559 in Chemical Engineering for SAVARA RAVITEJA

Question

A solid catalyzed gaseous reaction has the form A + B ↔ C (6) assuming the operating condition of the catalyst is such that the external and internal diffusion steps have negligible pressure gradients, propose rate equations for the following cases: (a) The adsorption of A on the catalyst is controlling (b) The surface interaction between adsorbed A and adsorbed B is controlling (c) The desorption of C from the surface is controlling

Answer 1

When a reaction occurs inside a catalytic particle, the reagents are consumed for giving products and a certain amount of heat is consumed or released according to the thermal characteristic of the reaction (exothermic or endothermic). The concentration of the reagents decreases from the external geometric surface of the particles toward the center. The concentration of the products, on the contrary, increases. The temperature changes as a consequence of the heat consumed or released by the reaction, increasing or decreasing from the external surface to the center of the catalytic particle. In other words, the reaction is responsible of the concentration and temperature gradients originating inside the particle that act as driving forces for both the mass and heat transfer inside the catalyst particle. The faster the reaction, the steeper the gradients. In the case of high reaction rate, this effect is propagated toward the external part of the catalyst particle, generating other gradients of concentration and temperature between the catalyst surface and the bulk fluid. When the fluid flow regime is turbulent, as normally occurs in industrial reactors, the external gradients are confined to very thin layer, named the boundary layer, that surrounds the solid surface. The boundary layer is quiescent, and consequently mass and heat transfer occur through it, with a relatively slow process characterized by the molecular diffusion mechanism

External diffusion and chemical reaction are consecutive steps, and their contributions to the overall reaction rates can be considered separately. A similar approach cannot be adopted for the internal diffusion as it occurs simultaneously with the simultaneously with the chemical reaction. To describe the influence of internal diffusion on reaction rate requires solving the mass and heat balance equations related to any single particle for evaluating the concentration and temperature profiles inside the pellet.