Characterization of stirrers for screening studies of enzymatic biomass hydrolyses on a milliliter scale

The evaluation of mixing quality is an important factor for improving the geometry of stirred-tank reactors and impellers used in bioprocess engineering applications, such as the enzymatic hydrolysis of plant materials. Homogeneity depends on different factors, including the stirrer type and the reactor type (e.g., ratio of diameter/height, ratio of impeller tip diameter/reactor diameter) with or without baffles. This study compares two impellers for enzymatic hydrolysis of suspensions of biomass particles on a milliliter scale. Both impellers were derived from industrially relevant geometries, such as blade and grid stirrers, although the geometry of the second stirrer was slightly modified to an asymmetric shape. The stirrers were investigated with different stirrer-reactor configurations. This was done experimentally and with the aid of computational fluid dynamics. The flow field, mixing numbers, power characteristics and initial conversion rates of sugars were considered to compare the two stirrers. The simulated mixing numbers and power characteristics in baffled and unbaffled milliliter-scale reactors were found to be in good agreement with the measured mixing times and power consumption. The mixing numbers required to reach homogeneity were much higher for the symmetric impeller and remained at least twice as high as the mixing numbers required when using the asymmetric impeller. The highest initial sugar releases from milled corn stover suspensions were achieved with the asymmetric impeller shape. Regardless of the differences in the flow fields or mixing times, diverging enzymatic sugar releases could be confirmed for Newtonian media only.