Rheology Modification and Enzyme Kinetics of High-Solids Cellulosic Slurries: An Economic Analysis

An analysis was completed to assess the economic benefits of rheology modifiers because of reduced viscosity and yield stress of high-solids pretreated corn stover (PCS) suspensions in a biomass-to-ethanol conversion facility. A process design developed by the National Renewable Energy Laboratory was used as a base scenario. Rheological modifiers can result in economic benefits because of (1) decreased utility costs associated with pumping and mixing of slurries, (2) decreased capital costs associated with increased pump and reactor lifetimes, and (3) decreased capital costs associated with smaller reaction vessels. Standard mixing and transport analyses for power-law and Herschel-Bulkley fluids were used to develop correlations for power consumption as a function of process variables, such as pipe diameter, volumetric flow rate, impeller geometry, and mixing speed. A 3-fold reduction in the yield stress can result in 50-60% reductions in the pressure gradient of PCS suspensions flowing through a length of pipe, while a 25% reduction in the viscosity can result in a 25% reduction in the mixing power. Because of enhanced reaction kinetics associated with the modifiers, reactor sizes and capital costs can be reduced by 20%. These combined savings can reduce the minimum ethanol selling price by roughly 0.5% if the price of the rheology modifiers remains low.