Understanding the hydrodynamics of anaerobic digesters for bioenergy production

Anaerobic digestion is an efficient way of treating animal wastes and biomass byproducts to reduce its pollution threat and obtain renewable bioenergy in the form of methane (biogas). The high failure rate of anaerobic digesters coupled with the lack of fundamental research prohibits the widespread use of anaerobic digestion in USA. Assessing the mixing and hydrodynamics of gaslift anaerobic digesters and their influence on digester design, scale and operation via experimental studies is the focus of this work. A new and unique Multiple Particle Tracking (MP-CARPT) technique to track up to eight particles simultaneously was successfully developed and validated by tracking single and dual particles of same and different densities. For the first time, Computer automated radioactive particle tracking (CARPT) and Computational Fluid Dynamics (CFD) are used to study the effects of operating and design variables, internals, and scales on the mixing pattern and the detailed hydrodynamics of the anaerobic digesters. The CARPT experimental results show that gaslift digester with draft tube diameter half of the reactor diameter and multiple point sparger provides better mixing than other digester configurations. CARPT data concluded that the geometric similarity and equal power input per unit volume is not sufficient to obtain the same digester performance at two different scales. Further, successful development and implementation of the multiple-particle tracking technique (MP-CARPT) in this work will overcome the limitations of the single-particle CARPT in future research on dense multiphase systems including anaerobic digesters. Performance studies in laboratory and pilot-scale digesters treating cow manure show that large-scale experimentation is required to obtain reliable information for design and scale-up of digesters. The knowledge gained from this dissertation will be useful for further investigations that can lead to better understanding and design of anaerobic digesters.