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High solids processing of biomass slurries provides the following benefits: maximized product concentration in the fermentable sugar stream, reduced water usage, and reduced reactor size. However, high solids processing poses mixing and heat-transfer problems above about 15% for pretreated corn stover solids. High solids slurries exhibit high viscosities and require high power consumption in conventional stirred tanks because they must be run at high rotational speeds to maintain proper mixing. An 8 L scraped surface bioreactor (SSBR) is employed here for enzymatic saccharification experiments to handle high solids loading and as a means for scale-up from laboratory-scale shake flasks. The scraping action of the blades keeps the reactor surface clear, which improves the heat-transfer characteristics. The horizontal rotation of the shaft and blades provides mixing and prevents particle settling much more effectively than in conventional stirred tanks, even at very low rotational speeds. The reactor is designed to be easily scaleable to pilot plant or production scale and operates at a specific power consumption of 0.56 kW/m 3 or less, which is well below the typical power requirement range of industrial-scale reactors. The role of the viscosity of biomass slurries in power consumption of the reactor is presented. Insoluble solids are seen to be the dominant factor affecting viscosity when the remaining insoluble solids level is higher than about 12%, regardless of the initial solids concentration. Below this level, viscosity characteristics are attributed to multiple factors in addition to insoluble solids, such as solids composition, solids morphology, and liquid-phase composition. The efficiency of the saccharification reaction is defined as sugar released per unit energy input. Tests were performed for initial solids loadings between 10 and 25%, with results showing that the efficiency factor is the highest for 20% initial solids concentrations. This efficiency factor can be used to optimize future design and processing strategies.

The effect of varying initial particle sizes on enzymatic hydrolysis rates and rheological properties of sawdust slurries is investigated. Slurries with four particle size ranges (33 microm < x < or = 75 microm, 150 microm < x < or = 180 microm, 295 microm < x < or = 425 microm, and 590 microm < x < or = 850 microm) were subjected to enzymatic hydrolysis using an enzyme dosage of 15 filter paper units per gram of cellulose at 50 degrees C and 250 rpm in shaker flasks. At lower initial particle sizes, higher enzymatic reaction rates and conversions of cellulose to glucose were observed. After 72 h 50 and 55% more glucose was produced from the smallest size particles than the largest size ones, for initial solids concentration of 10 and 13% (w/w), respectively. The effect of initial particle size on viscosity over a range of shear was also investigated. For equivalent initial solids concentration, smaller particle sizes result in lower viscosities such that at a concentration of 10% (w/w), the viscosity decreased from 3000 cP for 150 microm < x < or = 180 microm particle size slurries to 61.4 cP for 33 microm < x < or = 75 microm particle size slurries. Results indicate particle size reduction may provide a means for reducing the long residence time required for the enzymatic hydrolysis step in the conversion of biomass to ethanol. Furthermore, the corresponding reduction in viscosity may allow for higher solids loading and reduced reactor sizes during large-scale processing.

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Computational fluid dynamic simulations are employed to predict flow characteristics in a continuous auger driven reactor designed for the dilute acid pretreatment of biomass. Slurry containing a high concentration of biomass solids exhibits a high viscosity, which poses unique mixing issues within the reactor. The viscosity increases significantly with a small increase in solids concentration and also varies with temperature. A well-mixed slurry is desirable to evenly distribute acid on biomass, prevent buildup on the walls of the reactor, and provides an uniform final product. Simulations provide flow patterns obtained over a wide range of viscosities and pressure distributions, which may affect reaction rates. Results provide a tool for analyzing sources of inconsistencies in product quality and insight into future design and operating parameters.