Computational fluid dynamic for improving design and performance of an external loop airlift reactor used in electrochemical wastewater treatment

Abstract

An innovative semi pilot reactor was designed for waste water treatment. It consists of an external loop airlift reactor used as electrochemical reactor in which electrocoagulation/electroflottation was performed to treat waste water. The computational fluid dynamic consisting of multiphase Eulerian-Eulerian formulation and a structured mesh was used in order to investigate the flow in two configurations of external-loop airlift reactors: a semi pilot airlift reactor (first configuration) and an improved external loop airlift reactor (second configuration). The simulation produced the experimental results for the first configuration and highlighted the weak efficiency in the case of continuous mode of the first configuration. The improved design was proposed to realize a total flotation of sludge in both modes: batch and continuous. The goal of this study is to illustrate the influence of different parameters in the flow pattern and the average liquid velocity. Interesting results were obtained by simulation showing that the electrodes position, hydrogen production related to the current intensity and the bubble diameter are the predominant parameters influencing the flow pattern in this reactor. The simulation concerning the improved second configuration exhibits that: (1) Average liquid velocity increases when the electrodes are positioned in depth (lower position in the riser). (2) The total flotation is obtained independently of the position of the electrodes while erosion of the flocks takes place for lower position of electrodes in the case of the first configuration. This constitutes an improvement in reactor performance in the case of batch mode. (3) Average liquid velocity decreases when the bubble diameter increases. The numerical procedure and simulation can be used as tools for the design and scale-up of external loop airlift reactors. An application is then presented to show the efficiency of electrocoagulation/electroflotation in removing color for textile effluent.