SUBSTRATE GRADIENTS IN BIOREACTORS - ORIGIN AND CONSEQUENCES

Gradients of glucose in time and space are shown in a 30 m(3) cultivat ion of Saccharomyces cerevisiae grown in minimal medium to a cell dens ity of 20 gl(-1). The fed-batch concept was used with glucose as the l imiting component which was fed continuously to the process. As the me an glucose concentration declined throughout the process, the level of glucose was at all times different in three sampling ports (bottom/mi ddle/top) of the reactor. These gradients were furthermore shown to de pend on the feed position. This means that if the feed was supplied in the relatively stagnant mixing zone above the top impeller, the gradi ents were more pronounced than by feed in the well mixed bottom impell er zone. A rapid sampling system was constructed, and continuous gluco se samples of every 0.15 s were analysed from a point of the reactor. Fifty samples were collected with this system, but the amount and freq uency is possible to change. The results of these series show a varian ce of the glucose concentration where at one stage, a peak appeared of a relative difference in concentration of 40 mgl(-1). The pattern of these rapid glucose fluctuations was shown to depend on the turbulence level at the location of the feed. It was shown, that the fluctuation s were more pronounced when the feed was localised in a relatively sta gnant area than in the well-mixed impeller area, where the deviation f rom the mean was negligible. The fluid flow, in the impeller (gassed a nd ungassed) and bulk area (ungassed) of the reactor, was characterise d by turbulence measurements using thermal anemometry. These types of areas resembles well the different areas of sampling as mentioned abov e. The turbulent frequencies in these areas were in the range of 10(-1 ) to 10(4) Hz with the highest amplitudes at low frequencies. The spec tra depicts a uniform time scale for all zones, especially at the low frequencies. The dominance of low frequency, high amplitude flow varia tions and the observed short-time oscillations in substrate concentrat ion support the hypothesis of substrate transport over fairly long dis tances without substantial mixing both in the impeller, but especially , in the bulk zone of the reactor. Simulations with an integrated CFD and biokinetic model were performed. The predictions of the glucose gr adients of this model were compared to measurements.