Adv Chemical Reaction Engineering. Topic 4 External Mass Transfer Effects on Heterogeneous Catalysis скачать в хорошем качестве

Adv Chemical Reaction Engineering. Topic 4 External Mass Transfer Effects on Heterogeneous Catalysis

In this lecture we move one step outward in the overall catalytic process and focus on external mass transfer. So far we have looked at surface reaction kinetics (adsorption, surface reaction, desorption) and internal diffusion inside porous pellets. Now we consider the stagnant film around each pellet, and how transport from the bulk fluid to the pellet surface can limit the observed rate. You’ll learn: • What external mass transfer is, and how it links bulk concentration to surface concentration • How to test whether external mass transfer can be neglected using the Mears criterion • How stagnant film theory leads to a mass transfer coefficient and a concentration driving force • How to estimate the mass transfer coefficient using correlations (via Sherwood, Reynolds, and Schmidt numbers) • How internal diffusion plus reaction is captured by the internal effectiveness factor • How external and internal effects combine into an overall effectiveness factor, so you can write rates in terms of bulk concentration • The Biot number as a quick comparison of internal vs external diffusion resistance • How to build packed-bed reactor models: o A homogeneous model (single concentration field, pellet effects lumped together) o A heterogeneous model (separate bulk and surface balances, linked by mass transfer) • How the packed-bed model simplifies when dispersion is negligible, giving an exponential concentration decay and a simple conversion–length design equation By the end, you should be able to connect mass transfer, pellet limitations, and reactor modelling, and understand when a packed bed is limited by transport rather than intrinsic kinetics. In the next lecture we continue developing the packed-bed framework and look more closely at additional resistances and design implications. #Catalysis #HeterogeneousCatalysis #ExternalMassTransfer #MearsCriterion #EffectivenessFactor #OverallEffectivenessFactor #BiotNumber #PackedBedReactor #MassTransfer #ChemicalEngineering