5.3 Bioreactor Oxygen Balance

Let us now consider the case of oxygen balance within a bioreactor in which cells are growing and in the process consuming oxygen. There is a continuous inflow of air at a constant volumetric flow rate. The liquid broth is agitated by a Rushton agitator (flat blade stirrer ). Le the metabolic oxygen uptake rate be qO2 and cell concentration is X. Let us examine the reactor system over a sufficiently short period that we can treat X as a constant. Consider oxygen balance over the liquid phase of the bioreactor.

O2 transfered from Gas Phase - O2 consumed by Cells = Accumulation

For constant liquid phase volume, the above can be simplified to

The concentration, CDO is readily measured using an dissolved oxygen electrode. A later segment of the course on Biosensors, will deal with principle of measurement and construction of DO electrodes.

If oxygen being supplied is in exact balance with the oxygen consumed by the cells, we expect the dissolved oxygen concentration to remain constant; that is, the derivative in Eq(5-7) will vanish. That is,

One useful application of the above is in estimating the maximum cell concentration a particular bioreactor is capable of supporting in terms of oxygen supply. See the example below.

Example 5-1.

A bioreactor has an oxygen mass transfer coefficient capability of 400 h-1. What is the maximum concentration of E. coli that can be grown aerobically in this reactor. Respiration rate of E. coli is 0.35 g O2 (g Cell)-1 h-1. Critical oxygen concentration is 0.2 mg/L. Assume oxygen saturation with air to be 6.7 mg/L.

Solution

From Eq(5-8), we have

The maximum oxygen concentration driving force that can be expected is

= ( 6.7 - 0.2) = 6.5 mg/L.

Therefore, maximum cell concentration that can be grown at maximum growth rate is