Multiscale study of hydrodynamics, mixing and gas-liquid mass transfer in a stirred-tank bioreactor

The growth of industrial biotechnology has created a pull for advancing bioreactor design. The requirements of the culture system have led to a variety of technical issues that generally involve transfer of mass and energy. Predicting bioreactor performance has proved to be complex as it requires not only a deep knowledge of all the biological aspects, but also a proper characterization of transport and transfer phenomena within the bioreactor which are equipment design and scale dependent. In stirred-tank bioreactors, hydrodynamics governs bulk fluid mixing and gas-liquid mass transfer. The understanding and quantification of these three physical key aspects and of their interactions are required within the framework of scale-up or scale-down. Due to their simplicity, traditional scaling criteria based on global quantities are obviously not able to account for the intricacy of the local hydrodynamics, mixing and mass transfer properties.

This dissertation is part of a project aiming at a better mastering of phenomena linked to gas-liquid transfer that govern the performance of biochemical processes. It studies the influence of mixing and circulation imposed by hydrodynamics, within a baffled stirred-tank reactor, on the gas-liquid transfer through the liquid free surface and on the spatiotemporal distribution of the dissolved gas concentration. The major thrust of this work is to improve the description of fluid dynamics, mixing and gas-liquid mass transfer in stirred-tank bioreactors. The main input is the development and validation of a characterization experimental and computational approach that allows understanding, quantifying and modeling these multiscale transport and transfer phenomena during bioreactor implementation, in particular the selection of agitation configuration and operating conditions.