We have studied the mass transfer restrictions imposed by the structure of biomass on the hydrolysis of xylan during dilute acid pretreatment of biomass. We have shown that this restriction significantly reduces the observed rates of hydrolysis from the intrinsic hydrolysis rates that are possible in the absence of mass transfer. We also believe that these restrictions are responsible for the biphasic kinetics that has been observed for xylan hydrolysis. The effects of mass transfer on xylan hydrolysis were studied by dilute acid pretreatment of xylobiose, beechwood xylan, and poplar wood at particle sizes ranging from 50 µm to 10 mm. The hydrolysis results show the rate of xylose formation from beechwood xylan is about an order of magnitude larger than from the smallest particle size poplar (50 µm) we have studied. Moreover, we observed no significant difference in the rates of xylan hydrolysis in poplar over two orders of magnitude difference in particle size. Therefore, we believe that that the xylan hydrolysis rate in this particle size range are limited primarily by the diffusion of the products across the plant cell walls.
A mathematical model has been developed to describe the kinetics of xylan hydrolysis that includes diffusion of the products through biomass. The model has been used to fit experimentally measured xylose yields at different temperatures. The modeling results suggest that diffusion rates are only modestly affected by temperature. This suggests that the mass transport restrictions cannot be significantly overcome by increasing temperature.