FLOW AND MIXING TIME STUDIES IN BIOREACTORS

Abstract

Rheological behaviour of three commercially relevant biological cultures: E.coli BL21, Botryococcus braunii and Pichia stiptis is investigated in the present work. These cultures were rheologically characterized and correlation between viscosity and biomass concentration was studied. For all the three cultures studied, all samples taken were proven to be Newtonian fluids and effective viscosity was quite close to that of the nutrient medium. Dynamic viscous properties such as storage modulus, loss modulus and complex viscosity were measured to determine if the culture exhibit viscous or elastic dominant behavior. It was found that all the cell cultures investigated exhibit viscoelastic behavior. Response Surface Methodology (RSM) study was conducted to determine the optimum operational parameters of a bioreactor for Newtonian cultures. It was done for two different combinations of impellers: Rushton-Rushton impeller and Rushton impeller- Marine propeller. Further, the mixing behaviour of different cultures in a laboratory scale 7L bioreactor was investigated numerically assuming single phase flow behaviour, i.e. in the absence of air. For the numerical modelling of flow and mixing behaviour in the bioreactor k-ɛ model for turbulence and moving reference frame (MRF) approach for rotating flow was employed. Mixing time study was conducted for all the cultures at varying agitation rates (i.e. 100, 200, 350 and 600 rpm). The species transport model was used to determine mixing characteristics in the bioreactor. The mixing behaviour was reported at different locations in the reactor, where the dead points are observed with flow behaviour predictions. The results obtained can be useful for designing of bioreactor for the processing for these cultures for commercial purposes.