Modeling of oxygen mass transfer in a gas–liquid airlift reactor

Abstract

This article focuses on the physical modeling and numerical simulation of mass transfer in two-phase bubbly flow in an airlift internal loop reactor. The objective of this article is to show the ability of computational fluid dynamics (CFD) to correctly simulate mass transfer in such a bubbly reactor. The modeling of two-phase bubbly flow is based on the socalled two-fluid model derived from Reynolds-averaged Navier-Stokes equations in twophase flow. From the hydrodynamic perspective, the flow is steady state. Given the steadystate distributions of phases, interfacial area, and velocity field in the whole volume of the airlift, mass transfer is computed and the evolution of oxygen concentration in the two phases is predicted. Numerical simulations are discussed after comparison to experimental data. The simulations are validated in terms of oxygen concentration in the liquid vs. time. Then, different points are discussed, in particular, the perfectly mixed reactor assumption in the liquid phase and the spatial and temporal heterogeneity of oxygen concentration in the gas arising from oxygen impoverishment in bubbles in the downcomer. This leads to heterogeneity of the transfer driving force between the gas and the liquid. Bubble age or residence time in the airlift loop reactor has been calculated to show the weak renewal of oxygen in bubbles in the downcomer. This discussion generates questions on the estimation of a global mass transfer coefficient from experiments in such a heterogeneous airlift reactor.

Towards a robust CFD model for aeration tanks for sewage treatment – a lab-scale study

“…The simplified simulation strategy used assumed that the multiphase flow in the reactor was hydrodynamically in steady state, and thus only the transport equation for a source term representing oxygen recovery from air bubbles was solved. While numerical results obtained by Cockx et al (2001) were relatively close to the experimental data, the discrepancies in values of k L a observed in the work by Talvy et al (2007) were explained by the variable concentration of oxygen in bubbles assumed to be of constant size in simulations, and nonuniform mass transfer in the reactor.…”

“…Similarly, the need for bubble diameter calibration has been emphasized in more recent CFD work by Terashima, So, Goel, and Yasui (2016), involving use of the same modeling approach to determine k L a values in a number of full-scale activated sludge systems and clean water tanks that differed in their dimensions, diffuser types (coarse and fine-pore, ceramic, plastic and membrane diffusers), their configuration (single and dual spiral roll) and operating airflow rates. Others (Cockx et al, 2001;Talvy, Cockx, & Line, 2007) have explored the use of a Eulerian model to determine axial dispersion and oxygen mass transfer in a pilot-scale airlift reactor. The simplified simulation strategy used assumed that the multiphase flow in the reactor was hydrodynamically in steady state, and thus only the transport equation for a source term representing oxygen recovery from air bubbles was solved.…”

Numerical simulations of gas–liquid mass transfer in bubble columns with a CFD–PBM coupled model

“…Darmana et al (2005) used a three-dimensional discrete bubble model to investigate the complex behavior involving hydrodynamics, mass transfer and chemical reactions in a gas-liquid bubble column reactor. Talvy et al (2007) studied the physical modeling and numerical simulation of mass transfer in two-phase bubbly flow in an airlift internal loop reactor.…”

Modeling transport phenomena and reactions in a pilot slurry airlift loop reactor for direct coal liquefaction

“…Although there have been significant research efforts concerning the fundamental characteristics of IALRs under ambient conditions (Heijnen et al, 1997;Huang et al, 2007Huang et al, , 2008Lin et al, 1997aLin et al, , 1997bMudde and Van den Akker, 2001;Talvy et al, 2007aTalvy et al, , 2007b, the scale effect on the flow structure is still not fully understood. Sound understanding of the hydrodynamic behavior in IALRs (e.g., flow regime, phase holdup, bubble characteristics, interfacial phenomenon, backmixing, and interphase transport) is therefore essential in designing and scaling up these slurry reactors, especially under the condition of elevated pressure and high temperature with relevance to industrial reactive processes.…”