Improved Mixing of High Biomass Concentrations by Double Helical Impeller

High biomass substrate concentrations are very desirable for reducing bioprocessing costs by minimizing bioreactor size, minimizing fluid handling and maximizing product concentrations. However, biomass slurries containing more than 5%-10% solids form non-Newtonian mixtures that cannot be mixed using conventional devices. Small turbine or axial flow impellers are not effective in mixinghigh-viscosity pseudoplastic fluids distant from the impeller and larger diameter impellers are used to extend flow to the vessel wall. However, these large impellers, including the conventional anchor agitator, provide poor vertical mixing. Helical ribbon impellers are more efficient than conventional large-diameter impellers. But biomass will adhere to conventional helical ribbons at highsolids concentrations without mixing, resulting in wide temperature fluctuations for microbial processes. Such process instability can destroy microorganisms when operating near the maximum temperature tolerated by the microbes. This is true during simultaneous saccharification and fermentation where the enzyme optimum temperature is far above the maximum temperature tolerated by the cultures ofchoice. This poster reports on the investigation of several improved mixing devices for a high solids bench-scale bioreactor. Mixing is quantitated by measuring first-order temperature time constants. Improved mixing resulted in excellent temperature control with a double helix countercurrent device providing the best performance. This device can handle fed batch or continuous feeding of dry,high absorbent pretreated fibrous biomass at initial or cumulative solids levels well in excess of 20% (w/w). An improved drive system is presented along with calculations which project improved mixing at lower power per volume requirements.