Modelling of a series of continuously stirred tank reactors for thermal processing of liquid foods

A series of stirred tanks for the thermal treatment of liquid foods is easy to model and operate. Good modelling leads to accurate predictions and small uncertainties in the lethality, giving a safe food product because of a small chance of underprocessing. Mathematical proof is presented that the minimum overall reactor volume for a given degree of thermal destruction of cells or spores is obtained when the stirred tanks are of equal size. Analytical techniques based on theoretical residence time distributions are employed to find the optimum number of tanks in the series using capital and operation costs in the definition of the objective function. If typical scaling factors are used, these optima depend only on the degree of reduction of viable cells/spores sought and a dimensionless economic parameter relating operating and capital costs if a single stirred tank were employed

AXIAL IMPELLER SELECTION FOR ANCHORAGE DEPENDENT ANIMAL CELL CULTURE IN STIRRED BIOREACTORS: METHODOLOGY BASED ON THE IMPELLER COMPARISON AT JUST-SUSPENDED SPEED OF ROTATION

Animal cells, which are nowadays essential for the industrial production of proteinic compounds, are commonly cultivated inside stirred tank bioreactors. In case of anchorage dependent cells, they are usually fixed on microcarriers. The choice of agitation conditions (impeller type, rotational speed…) in this type of process is not an easy task as it has to fulfil three potentially conflicting goals: (1) maintaining microcarriers in complete suspension, (2) homogenizing the culture medium, and (3) limiting mechanical constraints generated by the hydrodynamics on the cells. The aim of this study is to present an original methodology to select the most appropriate axial impeller for this specific application. Seven propellers are preselected on basis of their characteristics available in the literature. Instead of comparing impellers at a given rotational speed or a given power input, they are compared at their respective minimum impeller rotational speed that leads to a complete microcarrier suspension, i.e. at their respective just-suspended speed Njs. They are then compared at higher rotational speeds N, expressed as multiples of Njs. The impeller classification is based on the intensity of mechanical constraints they produced, evaluated from: (1) the macro-shear rate quantified by the spatial derivative of time average velocity fields measured by P.I.V, (2) the micro-shear rate characterized by the ratio between the microcarrier diameter to the average Kolmogorov scale computed from power input measurements, and (3) the impact of microcarrier collisions on cells described via the Turbulent Collision Severity index also computed from power input measurements. Results show that the 125 mm diameter TTP impeller (Mixel) and the 150 mm diameter Elephant Ear impeller (Applikon) produce the smallest mechanical constraints at their just-suspended speed (50 rpm and 20 rpm, respectively). Moreover, the mechanical constraints they produce increase more slowly with the N/Njs ratio than the mechanical constraints produced by other impellers. These propellers are thus even more advantageous if rotational speeds higher than the just-suspended speed have to be used.Aspirant FNRS dossier n° 1.1.202.10 .