Uniform distribution of heat, chemical catalysts, and enzymes as well as absence of product gradients within conversion reactors are all dependent on the mixing properties of biomass slurries being processed, which in turn are determined by rheological characteristics. Biomass rheology poses several challenges because of the fibrous nature of the particles, their ability to absorb water and become unsatu­rated at relatively low solid concentrations of 25-35% (w/w), and the continually changing particle chemical/physical properties during flow through the process. Free water content appears to be the largest factor contributing to slurry rheology. This is especially true at the high solid concentrations that are desired to make the overall process economical by lowering equipment volume and thereby cost [27]. At solid concentrations beyond the point of unsaturation, the slurries become wet granular material that agglomerate and can compact under their own weight if not adequately mixed. At lower concentrations, adequate mixing is still required to pre­vent settling. To further complicate matters, as biomass gets broken down into its constitutive sugars, changes occur in particle size as well as chemical properties. Water retaining polymers, such as hemicellulose and pectin, are broken down and the previously hygroscopic biomass has lower capacity for water absorption result­ing in an increased amount of free water, and thereby altered slurry rheology. These dynamic changes in solid properties necessitate studies to understand rheological behavior of slurries through various process treatments.

In simplest terms, biomass slurries can be described as non-Newtonian pseudo­plastic (shear-thinning) fluids [27, 61, 62]. Whereas the exact mechanism leading to pseudoplasticity in biomass slurries is unknown, a possible explanation of the behavior can be ascribed to formation of three dimensional network structure of the fibrous particles and subsequent breakdown of this structure under shear [63]. Previous studies show that while free water is present, apparent viscosity values under continuous shear increase with increasing solid concentrations. These mea­sured apparent viscosities can be modeled with simple Casson, Bingham or Power Law models [27, 61, 62]. Thick slurries with little or no free water do not exhibit a further increase in apparent viscosity with increasing solid concentrations under continuous shear [27]. Other viscoelastic properties, such as storage and loss mod — ulii could continue to change; however, these measurements have not yet been reported for biomass slurries.

The relatively sparse data and lack of fundamental understanding of rheologi­cal properties of biomass slurries makes calculations on mixing requirements for biomass conversion processes uncertain. Also, transport properties within biomass slurries, such as convective/conductive heat transport and convective/diffusive mass transport, and their effects on conversion are hard to discern or estimate. For exam­ple, Fig. 6 shows enzyme digestibility data obtained during digestion of pretreated corn stover at high solids pretreatments (>15% solids). Each data point was gen­erated as a single measurement from triplicate reactors after 5 days of digestion. As can be seen from Fig. 6a, conversion of cellulose to glucose decreases steadily as solids concentrations increase suggesting inhibition of enzymes, possibly due to poor mass transfer resulting in localized accumulation of sugars as suggested by Hodge and coworkers [22]. Clearly, slurry properties will play a major role in determining these transport parameters that are crucial to determine optimal process performance across multiple scales. As another example, Fig. 7 shows experimental data from tests performed to evaluate heating time in a closed reactor containing biomass slurries of varying concentrations. These data show significant retarda­tion of heat transfer, even with the moderate density slurries containing 10% solids (w/w). Simple heat transfer simulation models have been developed for biomass slurries assuming conductive heat transfer and a one-dimensional system; however, their validity has not been verified with experimental data [64, 65]. In unsatu­rated biomass slurries containing discrete aggregates, the accurate determination and prediction of transport properties might be a challenging exercise.