13-45: Mixing of lignocellulosics in stirred tank reactors

Designing reactors for viscous slurries and suspensions is complicated due to the non-Newtonian flow behavior and the high power requirements.  For biomass slurries, this is further complicated by changes in the slurry characteristics as the reaction proceeds due to changing insoluble solids concentration (ISS).  Pretreated corn stover slurries were used in trials designed to understand how ISS affects mixing and power draw at large-scale, with the objective of maintaining good mixing and suspension of solids while minimizing power draw.  A computational fluid dynamics (CFD) model representing a conventional mixing tank with a pitched-blade impeller was created and used to predict solids distribution throughout the tank and torque requirements for low and high solids concentrations.  The low solids were represented by a 5% slurry where the system exists multiphase.  The high solids were represented by a 12.5% slurry, where the system behaves as a single phase.  Model validity was established by comparing CFD results to experimentally measured solids concentration strata and torque in a lab-scale reactor.  ISS stratification was determined by pipette sampling at varying depths. Power was measured with a torque sensor aligned with the shaft.  The rheology of the slurries was determined using a cup-and-vane rheometer and the data were fit to a Herschel-Bulkley model.  Interestingly, power requirements were actually higher for the 5% slurry above a certain rpm due to the shear thinning nature and low flow away from the impeller of the thicker slurry.  The validated model was applied to predict power and solid suspensions at large-scale.