Marie-Laure Collignon

Experimental Characterisation and Modelling of Homogeneous Solid Suspension in an Industrial Stirred Tank

In this work, we study the conditions needed to reach homogeneous distribution of aluminium salts particles in water inside a torispherical bottom shaped stirred tank of 70 L equipped with a Pfaudler RCI type impeller and three equispaced vertical baffles. The aim of the present study is to develop a CFD model describing the quality of particle distribution in industrial scale tanks. This model, validated with experimental data, is used afterwards to develop scale-up and scale-down correlations to predict the minimum impeller speed needed to reach homogeneous solid distribution Nhs. The commercial CFD software Fluent 14 is used to model the fluid flow and the solid particle distribution in the tank. Sliding Mesh approach is used to take the impeller motion into account. Assuming that the discrete solid phase has no influence on the continuous liquid phase behaviour, the fluid flow dynamics is simulated independently using the well-known k-∊ turbulence model. The liquid-solid mixture behaviour is then described by implementing the Eulerian Mixture model. Computed liquid velocity fields are validated by comparison with PIV measurements. Computed Nhs were found to be in good agreement with experimental measurements. Results from different scales allowed correlating Nhs values to the volumetric power consumption.

Revisiting the determination of hydromechanical stresses encountered by microcarriers in stem cell culture bioreactors

Background Expansion of mesenchymal stem cells (MSC) is one of the key steps for their use in tissue engineering or cell therapies. Today, expansion processes are mainly based on the use of microcarriers to allow large interfacial adherence areas [1]. However, this culture technology is known to be practically limited to low agitation intensity and microcarrier concentrations due to possible cell damage arising from particle hydromechanical stress or collisions between microcarriers [2]. Unfortunately, the description of the relationship between bioreactor hydrodynamics, microcarrier suspension and occurrence of collisions was neither clearly established in the case of stem cell cultures, nor based on a local description of the bioreactor hydrodynamics heterogeneity. Thus, in the present study, it is proposed to use numerical simulations to describe not only the liquid phase but also the microcarrier dispersion and the occurrence of hydromechanical stress encountered by the microcarriers. Two kinds of hydromechanical stress can be distinguished: (i) fluid-solid interactions (fluid shear stress) arising from turbulent eddies and (ii) solid-solid interactions arising from collisions between microcarriers or between microcarriers and bioreactor walls [2].