Scale effects on double-screw granular mixing

Abstract

Double screw pyrolyzers, featuring twin rotating screws, can be used to convert biomass into bio-oil via fast pyrolysis. Understanding the granular mixing dynamics within a double screw pyrolyzer is crucial to maximizing bio-oil yields. This study aims to investigate the effect of scale on mixing effectiveness in a cold-flow double screw pyrolyzer. Select operating conditions featuring changes in screw rotation speed, dimensionless screw pitch, screw rotation orientation, and particle size are compared for three double screw mixers of differing scales. Conclusions from this study provide additional insights into pyrolyzer scale-up and the mixing dynamics in larger scale granular mixers and the role that particle size plays in mixer scale-up. Advanced 360° optical visualization techniques, paired with non-invasive composition analysis were used to determine the mixing effectiveness of the screw mixer. In terms of mixer scale, counter-rotating down pumping screw rotation conditions saw decreases in the mixing effectiveness of the system with increasing mixer scales. The up-pumping screw rotation orientation remains a poor mixing condition at all mixer scales, while for a co-rotating screw rotation condition, increasing the mixer scale from 1x to 1.5x greatly increased the mixing effectiveness, but further increasing the mixer scale to 2x decreased the mixing effectiveness. In terms of the effects of particle size on mixer scale-up, at a mixer scale of 1x, decreasing the biomass particle size yields dramatic increases in mixing effectiveness, but at a 2x mixer scale, the same decrease in biomass particle size yields significantly less improvement in mixing effectiveness. Additionally, results indicate that scaling up with smaller biomass particle sizes results in significantly greater losses in mixing effectiveness than when scaling up with larger biomass particles.