The determination of the rheological behavior of biomass slurries is important for designing reactors and processes for conversion to cellulosic ethanol, especially at large-scale. This will become even more important as cellulosic ethanol moves closer to commercialization. The settling of biomass particles during viscosity measurements has always been a serious problem causing erratic results. Maintaining homogeneity by uniformly suspending all the solid particles is necessary for obtaining accurate measurements of the viscosity of particulate slurries.

Settling of solids is observed for solids concentrations less than ~12% for pretreated corn stover (PCS) solids, above which the instantaneous separation of solids and liquid phases does not occur. A method is introduced here that incorporates both the determination of the uniform suspended speed (USS) in a vane/cup rheometer and the time needed for premixing to reach a steady-state viscosity prior to viscosity measurements. The USS for various solids concentrations of PCS slurries was determined both experimentally and with computational fluid dynamics (CFD). CFD simulations also provide visual verification of the distribution of solids in the viscometer cup. The density of wet PCS solids, which is difficult to determine experimentally, is determined to be 1100 kg/m3 based on the volume fraction distribution of solids at the USS.