CFD simulation of gas mixing in anaerobic digesters

•A CFD model is developed to simulate gas mixing in anaerobic digesters.•Confined gas mixing generally performs better than unconfined gas mixing.•Simulations of velocity gradient and the breakup number of a floc are performed.•Several key issues on gas mixing in anaerobic digesters are discussed.

A computational fluid dynamics (CFD) model that characterizes gas mixing in anaerobic digesters was developed. The four gas mixing designs studied are: (1) unconfined mixing by two bottom diffusers, (2) confined mixing by one draft tube, (3) unconfined mixing by two cover mounted lances, and (4) confined mixing by two bubble guns. The flow fields for each design were obtained by solving an Eulerian multiphase flow model, assuming that the liquid phase is a non-Newtonian power-law fluid. The commercial CFD software, Fluent 14.5, was applied to simulate gas–liquid two-phase flow in the digester. A qualitative description of the fluid motion due to the generation of gas bubbles and quantitative identification of the flow fields of the liquid phase were made to compare the four mixing designs, in which the average velocity and a uniformity index for velocity were used to evaluate the mixing performance for each design. In addition, the velocity gradient for gas mixing along with its application to calculate the breakup number of a floc was investigated. Further, the mixing intensity that impacts the biological process and the justification of the simulation results were discussed.