Adv. Chemical Reaction Engineering Lectures. Topic 3 Diffusion & reaction in porous catalysts Part 1

In this lecture we start Topic 3, where we look at what really happens inside a porous catalyst pellet when diffusion and reaction occur at the same time. So far we have focused on surface kinetics (adsorption, surface reaction, desorption). Now we add the next layer: internal diffusion through pores, which can limit how quickly reactants reach the active sites (especially the internal surface area). You’ll learn: The difference between external diffusion (bulk to pellet surface) and internal diffusion (through pores) The main pore diffusion mechanisms: molecular diffusion, Knudsen diffusion, and surface diffusion How we estimate an effective diffusivity inside real, tortuous pores (including porosity, tortuosity, and constriction effects) How to set up a steady-state material balance on a spherical catalyst pellet, leading to a diffusion–reaction differential equation How different reaction rate definitions relate to each other (per surface area, per mass of catalyst, and per volume) We then introduce a key design tool: the Thiele modulus, which tells you whether a catalyst pellet is reaction-limited or diffusion-limited, and what to change (for example, particle size) to reduce internal diffusion limitation. Part 2 will solve the dimensionless equation, give the concentration profile inside the pellet, and introduce the effectiveness factor, plus a brief look at non-isothermal effects and how to apply all of this in the problem sheets. #Catalysis #PorousCatalysts #Diffusion #ThieleModulus #EffectivenessFactor #ChemicalEngineering