Chapter 16 - Mass Transfer Effects: Immobilized and Heterogeneous Reaction Systems

Abstract

How does mass transfer affect biotransformations? Chemical kinetics or bioprocess kinetics in general is first derived based on homogeneous systems. For large particles and immobilized cell or enzyme systems, mass transfer can be a limiting factor. A general Monod (or Michaelis-Menten) rate expression is employed as the intrinsic reaction rate to elucidate the effects of mass transfer in the fluid phase (external mass transfer) and inside the solid matrix (internal mass transfer). The external mass transfer effects are weaker if the diffusion coefficient is high, flow is strong, or the particle is small. The internal mass transfer effects are stronger if the Thiele modulus is large. The Thiele modulus is a function of particle size, diffusivity, and intrinsic reaction kinetic parameters. Porous catalyst, encapsulated biocatalyst, biofilm, and intraparticle thermal effects can affect reaction rates. For a biocatalyst immobilized system via encapsulation, capsule size, biocatalyst loading (inside the capsule), capsule shell material or permeability, and the capsule shell thickness all play important roles in the reaction system as a whole. In general, iterative solutions are necessary when both internal and external mass transfer effects are important. Reactor analysis including the mass transfer effects can be incorporated by including the effectiveness factor with the reaction rate. The shrinking core model is applicable in areas ranging from pharmacokinetics (eg, dissolution of pills in the stomach), to biomass gasification (solid to gas phase), to biomass extraction (solid to liquid phase), to porous catalyst regeneration (solid to gas phase), to coal particle combustion (solid to gas phase), to pulping and bleaching of fibers (solid to liquid phase), to pyrolysis. Mass transfer, especially diffusion in the solid matrix, is often the rate-limiting step in biomass conversion processes.