CFD Modeling of Hydrocarbon-Air System Hydrodynamics in Three Types of Column Reactors

In this study, hydrodynamic characteristics such as the gas holdup (ε), the liquid phase velocity, and the mass transfer coefficient (kLa) for an air-diesel system were modeled for the bubble column (BCR), the airlift (ALR), and the airlift with the net draft tube (ALR-NDT) reactors at different superficial gas velocities ranging from 0.008 to 0.085 m s-1. A 3D two-fluid Eulerian-Eulerian model was developed using the computational fluid dynamic (CFD) technique to model the three configurations of column reactors and predict the hydrodynamic parameters. The results of the 3D-CFD modeling showed good agreement with the experimental data where the average error was less than 14 and 9 % for ε and kLa respectively. Although the vortex occurred in BCR and ALR at high gas velocities, the optimum liquid and the gas circulation and distribution were observed in ALR-NDT. Furthermore, the formation of the dead zone (kLa = 0) in the reactors was studied, and the results revealed that ALR-NDT has a lower volume of dead zones (about 8 %) in comparison with BCR and ALR. In order to reduce the dead zone in BCR and ALR systems, the location of the gas diffuser and the draft tube was investigated. The dead zone was decreased by 12 % with shifting the gas diffuser to the bottom of the BCR. Also, by increasing the distance of the gas diffuser from the draft tube, the dead zone was decreased by 40 % specifically near the walls of ALR. Meanwhile, the simultaneous shifting of the gas diffuser and the draft tube to a lower position in ALR had no effect on the dead zone formation and its distribution.