F. Delvigne,* T. El Mejdoub, Soler E. Scuder and P. Thonart

Faculté Universitaire des Sciences Agronomiques de Gembloux

Unité de bio-industries

CWBI, passage des Déportés 2

5030 Gembloux, Belgium

Phone: 0032 81 62 23 05

Fax: 0032 81 61 42 22

e-mail: delvigne.f@fasgx.ac.be

When performing a bioreaction in a stirred vessel, care should be taken with the operating parameters of the process. Microorganisms are very sensitive to nutrient concentration fluctuation (carbon, oxygen); therefore, mixing systems must ensure good homogenisation of the broth as well as efficient oxygen transfer.

In this study, fed-batch cultivation of a Pichia pastoris strain was performed in a bioreactor equipped with different mixing systems. We used a radial system (composed of two rushton disk turbines with 6 blades), and an axial system (composed of a high efficiency propeller pumping downward coupled with a high efficiency propeller pumping upward). For these systems, mixing efficiency (evaluated by mixing time measurement) and oxygen transfer efficiency (evaluated by gassing in/gassing out method) were quantified.

These results can be used to form mathematical correlations to calculate the mixing time and the k_La (oxygen transfer coefficient) for different operating conditions and impeller types, but this approach is global and does not represent the internal dynamic of the system. An alternative is to exploit the mixing time and oxygen transfer data to model the gas-liquid hydrodynamic of the vessel in a structured view. This structured modelling approach allows us to break the black box model and see what happens locally in the stirred vessel.

One of the most-used structured models is the compartment model in which the vessel is a cascade of perfectly mixed cells in series. The adjustable parameters are the flow between adjacent cells. General oxygen transfer equation (dC_L/dt = k_La. (C^* - C_L), with C_L: oxygen concentration in the liquid phase; C^*: oxygen concentration in the liquid phase at saturation; t: time) can be added to the compartment model to include oxygenation effect in the structured approach.

After the validation step, our models were used to quantify fed-batch process efficiency for the mixing systems. This structured approach has shown very different hydrodynamic behaviour in the function of the stirring system used (radial or axial). A purely radial stirring system led to a drop in the homogenisation efficiency, which can be attributed to the radial flow produced by turbines which have a barrier effect and lead to axial gradient in the vessel. The use of propellers increases homogenisation efficiency, but the aeration efficiency is modified. Compartment models were used to calculate sugar and oxygen concentration at different points of the bioreactor and a classical Monod-type growth equation was used to quantify the impact of the hydrodynamic on P. pastoris cell multiplication.