Floating and motion characteristics of single and multiple wheat straw particles in a stirred tank

Wheat straw (WS), produced abundantly in China, is an attractive substrate for anaerobic digestion (AD). However, its low density and poor wettability cause it to float in the digester, reducing AD efficiency. Considering the opaque nature of slurry, the floating and motion characteristics of WS are still uncovered. In this study, a visualization system was constructed to capture the motion of WS particle in the stirred tank with simulated fluid. Besides, the effects of particle size, blade angle, blade height, liquid phase viscosity, solid loading, rotational speed, and pretreatment conditions on the motion of single particle and dispersion of multiple particles were investigated. The results showed the motion of single particle contained four stages based on the growth rate of dimensionless velocity. Small particle size, large blade angle, low blade height, and low liquid phase viscosity enabled particles to be drawn down in short period. Pretreatment could enhance the drawdown efficiency and reduce the rotational speed. This study not only provides novel insights on the motion of WS particle in stirred tank but also guides the design and optimization of mixer in the anaerobic digesters, enabling an efficient valorization of WS and similar feedstocks.