CFD analysis of multiphase flow in an airlift reactor: superficial velocity and gas holdup influence on the loop recirculation

Abstract

Several products of great importance in the food and pharmaceutical industries are produced by biotechnological processes, using high conversion and high specificity chemical reactions carried out by microorganisms. Such processes can be conducted in airlift reactors (ALRs), which are capable of operating multiphase systems. The ALRs are pneumatically agitated, have a high surface area of contact between the phases and have satisfactory homogenization, favoring the transfer of mass and energy. However, thus yet, the comprehensive investigation of the relationships among flow patterns, dead zones, gas holdup, phase velocity profiles, and the loop recirculation has not been extensively investigated. The purpose is to analyze the dynamics of multiphase flow at a local level, which is scarcely explored in the literature. This will provide a complete understanding of the recirculation pattern found in the ARL reactor. In this work, Computational Fluid Dynamics (CFD) technique was used to study the effects of different air inlet velocities (0.27 m/s, 0.36 m/s, 0.45 m/s and 0.54 m/s) and the average Sauter diameter of the bubbles injected by the sparger (12 mm, 15 mm, 18 mm and 21 mm) on the type of flow (stagnant, recirculation and preferential path zones), gas holdup, alpha parameter (α) and phase velocity profiles. The model was validated through experimental data. The simulation results indicated that the higher the air velocity injected by the sparger, the greater the amount of gas in the riser relative to the downcomer, which results in a lower alpha parameter (useful information for reactor sizing). It was also observed that the velocity profile of the liquid phase is closely related to that of the gas phase. The operating conditions that provided the most satisfactory results (better mixing time, more effective homogenization, and better establishment of the recirculation circuit) were obtained for an inlet air velocity of 0.54 m/s and an average Sauter bubble diameter of 21 mm.