Hydrodynamics and mass transfer in airlift bioreactors: experimental and numerical simulation analysis

Abstract

Airlift bioreactors are widely used in industrial applications such as wastewater treatment, gas decontamination and other biochemical processes, and such systems have generated increasing academic and industrial interest. Many bioprocesses occurring in airlift bioreactors are limited by the gas-liquid mass transfer, which is determined by the hydrodynamics of the multiphase flow. The hydrodynamics of multiphase flow in airlift bioreactors is complex and involves a large number of interrelated hydrodynamic parameters that are influenced by factors such as aeration mode, liquid physical properties and reactor scale. In study of bioreactor performance, detailed numerical data on local hydrodynamics, which is crucial for design of multiphase reactors and optimization of operating conditions, is often limited by available measurement techniques. Furthermore, while many simulation studies of airlift reactors have been reported for Newtonian fluids, there is a lack of numerical simulation study of airlift bioreactors incorporating the non- Newtonian fluid rheology commonly encountered in bioprocesses.

The aim of this thesis is to improve understanding of the hydrodynamic and mass transfer performance of airlift bioreactors and thereby contribute to their improved application, design and scale-up, as well as optimization of aeration. Both experimental and numerical simulation approaches are used in the research work.

In the thesis work, novel and advanced measurement techniques – three-dimensional electrical impedance tomography and particle image velocimetry – were applied in the experimental studies, together with conventional methods. The experimental results bring better understanding of the hydrodynamics of industrial airlift bioreactors.

Detailed quantitative analysis of local hydrodynamics and mass transfer characteristics of center- and annulus-rising airlift bioreactors was performed, which provided important insights into the effect of the aeration mode on airlift bioreactor performance. Experimental results presented in the thesis facilitate the design and effective utilization of airlift reactors, particularly for biochemical applications, by providing valuable information about key phenomena such as local flow structure, shear rate field and foam layer thickness.

Investigation of gas distribution and local gas holdup was carried out using tomographic imaging techniques with non-Newtonian fluids commonly encountered in industry. Novel experimental results of visualized gas distribution are presented, bringing new insights into potential approaches for improvement of aeration and effective gas-liquid contact. Better understanding of the effect of the hydrodynamics on mass transfer was achieved by separation of the liquid-side mass transfer coefficient and the gas-liquid interfacial area.

Numerical simulation results of the three-phase model comprising the liquid phase and two gas bubble phases developed in this work are presented and compared with experimental results from both this study and published literature. Comparison of the experimental and simulation results showed that the developed three-phase model is capable of predicting well the hydrodynamics and mass transfer for low viscous fluids over the three circulation flow regimes studied. The effects of the reactor scale are predicted reasonably well. To give more confident prediction for airlift bioreactors with viscous non-Newtonian fluids, a computational fluid dynamic model with wide bubble size distribution due to bubble breakage and coalescence was developed and used in simulations with ANSYS Fluent Solver in homogeneous and heterogeneous flow regimes. Good agreement between the experimental and simulated results indicated that the developed computational fluid dynamic model is able to describe and predict the hydrodynamics in airlift bioreactors with non-Newtonian fluids with reasonable accuracy.