Hydrodynamic behavior of an airlift reactor with net draft tube with different configurations: Numerical evaluation using CFD technique

Abstract

In terms of gas holdup, liquid velocity, and volumetric mass transfer coefficient for oxygen (KLa), the hydrodynamic behavior of four configurations of an airlift reactor (ALR) with a net draft tube (NDT) of different net mesh sizes (ALR-NDT-3, 6, 12, and ALR) have been numerically simulated for a range of inlet air flow rates. The effect of various levels of ratio of height (H) to inner tube diameter (D) of the net draft tube (H/D: 9.3, 10.7, 17.5, and 20) and ratio of inner cross-sectional area of the riser (Ar) to the inner cross sectional area of the downcomer (Ad) (Ad/Ar: 1.3 and 7) for different air flow rates is also evaluated for each reactor configuration operating with an air–water system. The two-fluid formulation coupled with the k–ε turbulence model is used for computational fluid dynamics (CFD) analysis of flow with Eulerian descriptions for the gas and liquid phases. Interactions between air bubbles and liquid are taken into account using momentum exchange and drag coefficient based on two different correlations. Trends in the predicted dynamical behavior are similar to those found experimentally. A good agreement was achieved suggesting that geometric effects are properly accounted for by the CFD model. After a comparison with experimental data, numerical simulations show significant enhanced gas holdup, liquid velocity, and KLa for the ALR-NDTs compared with the conventional ALR. Higher gas holdup values are achieved for ALR-NDT-3 than that for the other ALRs because it acts like a bubble column reactor as the holes present in the NDT are large. Maximum liquid velocities are seen in ALR-NDT-12, which operates like a conventional ALR. Moreover, the interaction between the NDT and upward gas flow leads to cross flow through the net, small bubbles, and high interfacial area as well as good mass transfer. This was significant in ALR-NDT-6 with maximum KLa value of 0.031 s−1. The applied methodology provides an insightful understanding of the complex dynamic behavior of ALR-NDTs and may be helpful in optimizing the design and scale-up of reactors.