Although exceptionally difficult to transport due to their high viscosities, processing of high-solids slurries will be necessary for the economic operation of lignocellulosic biorefineries. In this work, we examine the changing rheology of high-solids pretreated corn stover (PCS) slurries prior to and during enzymatic saccharification. Prior to saccharification, PCS slurries with fractions of insoluble solids (FIS) ranging from 5-20% (w/w) were found to exhibit rheological properties characteristic of soft solids, including the presence of an apparent yield stress and shear-thinning behavior. Notably, yield stress and viscosity were found to be strong functions of PCS concentration, decreasing roughly ten-fold for each 5% decrease in FIS. During saccharification, FIS drops as glucose and other sugars are liberated by the hydrolysis reaction and move into the liquid phase. As a result, the viscosity and yield stress of PCS slurries within a saccharification reactor diminish rapidly with conversion, by more than an order of magnitude within a few hours. The rheological properties of unsaccharified and saccharified stover agree well when compared at equivalent volume fractions of insoluble particles.

Because the rapid drop in yield stress with conversion is an important design parameter, indicating when a suspension becomes “pourable” or “pumpable”, this work will help the process designer identify an optimal residence time during saccharification. In this light, we also present preliminary results suggesting that viscosity-modification / flow-assurance additives may significantly reduce the yield stress and viscosity of PCS slurries, ensuring their downstream processability.