(519e) Mass Transfer in High Solids Biomass Slurries

Enzymatic saccharification rates and yields are hindered in high solids slurries, which is generally attributed to mass transfer limitations resulting from the high viscosity of the slurries. However, other possible mechanisms exist and no evidence has yet been presented directly linking the lower rates and yields to the mass transfer limitations.  The objective of this work was to compare saccharification results for identical high solids slurries in a conventional Erlenmeyer flask with another system capable of overcoming mass transfer limitations in high viscosity slurries.

In order to achieve high mass transfer rates in the high viscosity slurries, a resonating acoustic mixer was employed for the saccharification reaction. Dispersion coefficients in the mixing vessel were determined as a function of viscosity in order to quantify the mass transfer improvements and determine if the system is a valid choice for studying reactions in the absence of mass transfer limitations.  Since conventional means of measuring dispersion could not be performed due to the slurry characteristics, a computational fluid dynamics model was developed that simulated motion of the slurries in the mixer.  The model was validated by comparing experimentally determined mixing times using an electrolytic tracer with mixing times from the simulations.  Computational and experimental mixing times differed by 2% or less in all cases.  Dispersion values were achieved on the order of values for water flow in a pipe, indicating the system was a valid choice for studying reactions in the absence of mass transfer limitations.

At low initial solids where good mixing could be achieved in the shake flask, the glucose release in the mixer was just 4% higher than in the shake flask, while for 30% initial total solids, the glucose release in the mixer was approximately 40% higher than in the shake flask.  These results provide a direct link between low rates and yields of saccharification of viscous biomass slurries and mass transfer limitations.