Granular mixing processes are important to many industries including the pharmaceutical, agricultural, and biotechnology industries. These processes often require both a high degree of homogeneity and a high degree of customizability. As granular mixing processes are so widely employed, a thorough understanding of the mixing dynamics is necessary to understand and control the resulting products. Research into granular mixing processes has been, thus far, largely focused on laboratory scale mixers with simple geometries, while actual industrial processes often require large mixers with complex geometries. Moreover, granular mixing processes are often very sensitive to changes in operating conditions and any solutions provided to deal with specific mixing problems are highly system-sensitive and do not readily carry over to other mixer types. These sensitivities mean it is necessary to study more complicated mixer geometries, more complicated operating condition, and industry scale mixers in order to apply experimental and theoretical knowledge of granular mixing to industrial processes.

One specific example of a complicated industrial mixer is a double screw pyrolyzer used in the bioenergy industry to produce bio-oil via fast pyrolysis. Bio-oil can be converted into synthetic gasoline, diesel, and other transportation fuels, or can be converted into biobased chemicals for a wide range of purposes. Double screw pyrolyzers utilize a granular mixing process by mechanically conveying and mixing a biomass and heat carrier media together using two intermeshing screws. Fast pyrolysis is still a relatively new technology and much of the research that has been done with double screw pyrolyzers has focused on the products and not on the mixing dynamics within the mixer. However, understanding the xxv mixing dynamics is important because bio-oil yields are dependent on the heat transfer between the heat carrier and the biomass. Improving the mixing effectiveness within the pyrolyzer will improve the heat transfer rates, and thus, will improve the bio-oil yields.

The purpose of this project is to expand upon previous work done with double screw mixers, designed to geometrically replicate double screw pyrolyzers, by investigating the effects of various operating conditions and by developing, characterizing, and optimizing larger double screw mixers. Select operating conditions featuring changes in screw rotation speed, dimensionless screw pitch, and screw rotation orientation, previously investigated by Kingston and Heindel (2014b, c), were repeated while varying additional operating conditions such as material injection configuration, mass flow rate ratio, particle size and density, and mixer scale. Insights gained though this study hope to improve the viability of double screw pyrolyzers by increasing the mixing effectiveness through adjustments to operating conditions, and by bridging the gap between laboratory scale mixers and industry scale mixers.