4-29: 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 significant changes in the slurry characteristics as the reaction proceeds due to changing insoluble solids concentration (ISS).  Pretreated corn stover (PCS) slurries were used in trials designed to understand how ISS affects mixing and power draw at large-scale, with the objective of designing a system capable of maintaining good mixing and suspension of solids while minimizing power draw.  A computational fluid dynamics (CFD) model representing a conventional baffled mixing tank with a pitched-blade impeller was created and used to predict solids distribution throughout the tank and torque requirements for varying solids concentrations. Model validity was established by comparing CFD results to experimentally measured solids concentration strata and torque on the impeller shaft in a lab-scale reactor.  ISS stratification was established by both pipette sampling and conductivity measurements at varying depths. Power requirements were measured with a torque sensor aligned with the shaft.  The rheology of the PCS slurries, an important parameter input for the CFD model, was determined using a cup-and-vane rheometer and the data was fit to a Herschel-Bulkley model.  Upon validation, the CFD model was used to determine how changing ISS affects mixing and power requirements.  The CFD model can be applied for scale-up considerations.