(477d) Mechanical Mixing of Non-Newtonian Fluid Flow in Anaerobic Digestion | AIChE

(477d) Mechanical Mixing of Non-Newtonian Fluid Flow in Anaerobic Digestion

Authors 

Yu, L. - Presenter, Bioprocessing and Bioproduct Engineering Laboratory
Chen, S. - Presenter, Washington State University


Anaerobic Digestion (AD) has been becoming an important practical technology to convert organic waste into clean energy since the traditional energy was in short supply. How to achieve process intensification is a key point to reduce the digester size. High Solid Anaerobic Digestion (HSAD) is a new application of the proven and conventional low-solids anaerobic digestion technology used throughout the world. It will typically have a lower land requirement due to the lower volumes associated with the moisture. However, high solid concentration also devastates mass and heat transfer in digester. Mixing is required to enhance this HSAD process. Several essential roles can be played in anaerobic digestion, such as enhancing substrate contact with the microbial community, improving pH and temperature uniformity, preventing stratification and scum accumulation, facilitating the removal of biogas from the digesting, and aiding in particle size reduction. Mechanical mixing is considered as the most effective mode in the three mixing of anaerobic digestion. It is also typically used in anaerobic digestion of high solid slurries. The mechanical mixing is usually assumed as Continuously Stirred Tank Reactors (CSTR) in AD research and production. Few people took time to study the flow field in anaerobic digesters. Especially in HSAD, the fluid properties have been changed from Newtonian to non-Newtonian. Therefore, it is necessary to understand how the mechanical mixing impacts the flow field in non-Newtonian fluid.

This paper wants to study mechanical mixing of non-Newtonian fluid flow in anaerobic digestion by Computational Fluid Dynamics (CFD) technology. In this case, a general mathematical model was developed to integrate theories of single-phase turbulence and non-Newtonian fluid in mixing-flow of anaerobic digesters. The flow pattern was qualitatively identified between Newtonian and non-Newtonian liquid manure. It shows non-Newtonian fluid induces rheological complexities such that zones of significant motion (called a cavern) around impellers are formed with essentially stagnant regions elsewhere. The numerical simulations of the flow fields was conducted to qualitatively and quantitatively characterize the mixing and dead (stagnant) zones. The theoretical model was validated against the experimental data from literature.