Hydrodynamics of an external-loop airlift reactor with inserted membrane

3D CFD Simulation of Gas Hold-up and Mass Transfer in a Modified Airlift Reactor with Net Draft Tube

This paper documents CFD simulations of the gas hold-up (ɛg) and volumetric mass transfer coefficient (KLa) for three kinds of airlift reactors (ALRs) namely, conventional ALR, ALR with net draft tube (ALR-ndt), and packed-bed ALR with net draft tube (PBALR-ndt). The 3D two-fluid Eulerian-Eulerian model was adopted to predict the influence of superficial gas velocity on ɛg and KLa. The simulation results were consistent with the trends described previously in the experimental work regarding ɛg and KLa values and a good agreement was obtained (absolute error less than 20 %). Based on the simulation results, axial flow is the dominant flow in the conventional ALR while in ALR-ndt and PBALR-ndt radial flow streamlines are appeared in the reactors due to the presence of concentric net draft tube which improves their performance. The effective role of the net draft tube is proven since consequence of generation of small bubbles by passing through net draft tube is the entrainment of a larger percentage of gas bubbles from the riser into the downcomer which results in improvement of gas holdup and the KLa values. An exponential correlation is used for relating gas hold-up and mass transfer coefficient. Higher power obtained for ALR-ndt and PBALR-ndt (n ≈ 1.22) compared to ALR (n = 0.95) was indicative of high sensitivity of KLa value to gas hold-up in these reactors due to presence of the concentric net draft tube. The CFD modeling is considered to be an invaluable tool allowing us to analyze and visualize the impact of fluidic forces on hydrodynamic properties and consequently, reactor performance.