Diffusion of hydrolytic enzymes into biomass particles is a potential limiting step, which has yet to be studied in detail separate from intrinsic hydrolysis kinetics. We developed and applied a pore-enzyme diffusion model for both adsorbing and non-adsorbing enzymes and coupled them to reactor-level mass balance equations. With this multi-scale model, the effects of biomass particle porosity, size, and adsorption capacity on the characteristic time of enzyme diffusion and adsorption were predicted over an expected range of these parameters. Using a hydrolysis limiting threshold characteristic time for enzyme diffusion of 6 h, this model mapped the transport parameter space between two distinct zones: diffusion limiting and non-diffusion limiting. The model also predicted a decrease in characteristic time with an increase in the biomass-to-enzyme loading ratio. At the particle level, characteristic time was most strongly affected by firstly adsorption capacity, then particle radius, adsorption affinity, and porosity.
Merugu et al. (Tue,) studied this question.