Christophe Wylock

Nonmonotonic Rayleigh-Taylor instabilities driven by gas-liquid CO2 chemisorption

Density variations induced by gas absorption in reactive aqueous solutions often trigger buoyancy-induced motions, generally in the form of plumes monotonically sinking into the bulk liquid and enhancing the absorption rate. Here, we contrast two types of CO2-absorbing alkaline solutions, studying their dynamics inside a vertical Hele-Shaw cell by interferometry. While the first one indeed behaves as expected, the second one leads to a quite unusual oscillatory (phase-slipping) dynamics of convective plumes, which moreover does not lead to a significant transfer enhancement. Thanks to a simplified model of momentum and species transport, we show that this particular dynamics is related to a nonmonotonic density stratification, resulting in a stagnant layer close to the interface. Conditions for this to occur are highlighted in terms of the ratios of species diffusivities and their contribution to density, a classification deemed to be useful for optimizing chemisorption (e.g., for CO2 capture or sequestration) processes.

Structured model of VERO cells metabolism in a fixed-bed bioreactor

In this work, a structured mathematical model of the VERvet Origin (VERO) cell metabolism in a fixed-bed bioreactor is developed. Mass balance equations for the extra- and intracellular species are written considering the main pathways of the animal cell metabolism. From the model equations, an undetermined set of equations relating the consumption or production velocities of the extracellular species and the velocities of the metabolic reactions can be developed. The monitoring of a reference cell culture enables to transform this undetermined set of equations in a determined one. The resolution of this determined set of equations enables to analyze and characterize the cell metabolism. From the model and the cell metabolism characterization, a monitoring tool is developed to simulate the time evolution of the species concentrations when the time evolution of the cell concentration is known. This simulation tool is used to simulate five experimental cell cultures. The comparison of the experimentally determined and computed time evolutions of the species concentrations enables to validate the developed monitoring tool. The validated monitoring tool can be used to optimize the operating conditions of the culture, as the medium feeding rate, the range of the species concentrations.

Computational Fluid Dynamics Characterization of a Bioreactor Mixing Device for the Animal Cell Culture

During a cell culture in a bioreactor, the cells are exposed to the shear stresses mainly generated in the culture medium by the mixing device. Beyond a critical shear stress, this exposition induces cell damages. Therefore, the limitation of the shear stress is an important criterion for the design of bioreactors. An accurate modeling of the flow and the induced shear stresses in the medium is a tool to achieve an effective design of a bioreactor. In this work, a new design of a mixing device is considered. The aims of this work are to develop a methodology to study the flow and the induced shear stresses in the device, to study and to model the relation between the flow, the induced shear stresses and the cell viability, to use the developed model as an optimization tool, and to study the design of the bioreactor mixing device and its scale-up. In a first step, the flow and the induced shear stresses in the device are simulated by Computational Fluid Dynamics. In a second step, the model of the influence of the flow and the induced shear stresses on the cell viability is established by a comparison between the computed flow and the induced shear stresses and experimental measurements of cellular viabilities for different impeller rotation speeds. Finally, the influence of another design of the mixing device on the cell viability is studied.

Segregated Model of Adherent Cell Culture in a Fixed-Bed Bioreactor

In this work, a mathematical model of a fixed-bed bioreactor for the animal cell culture is developed to study the optimization and the scale-up of this bioreactor. Several cell populations are considered: the cells in suspension in the medium at the beginning of the process and the adhering cells to the fixed-bed. The model includes a capture rate kinetic of the cells in suspension by the fixed-bed and a spatial distribution of the nutrient and by-product concentrations in the fixed-bed. Therefore, the model reports the potential gradients of the cell concentrations in the fixed-bed. Some model parameters are experimentally identified and the model is validated using experimental data obtained with two pilot bioreactors. The model is used as a simulation tool to study the influence of the bioreactor design or the velocity field of the culture medium on the cell concentration gradients in the fixed-bed bioreactor and to optimize the operating conditions, the design, and the scale-up of this bioreactor.