Oxygen gradients in animal-cell bioreactors.

An estimation is made of oxygen gradients in animal-cell bioreactors, using straightforward engineering calculations. Three types of bioreactor are considered: stirred vessel, bubble column and air lift, of sizes between 0.01 and 10 m(3). First, the gradient is estimated in the stagnant layer surrounding a cell (15 μm), a microcarrier (185 μm) with 300 cells attached to it, a macroporous support (1.25 mm) containing 185,00 cells and one (6 mm) containing 4.25 million cells. It is assumed that oxygen consumption is 10(-16) mole O(2)·cell(-1)·s(-1), while mass transfer coefficients are obtained from Sherwood relations. Circulation and liquid-retention times of the bioreactors are compared with the oxygen-exhaust times of suspensions with 10(12), 10(13) and 10(14) cells/m(3) to estimate if oxygen gradients are likely to exist in the bulk-liquid phase. Finally, the gradient in the liquid film surrounding air bubbles is estimated using k( l )A-values obtained from empirical correlations. It is clear from all these estimations that in many situations severe gradients can be expected. The question remains, however, whether gradients should be avoided as much as possible, or may be tolerated to a certain extent or even created on purpose because of possible beneficial effects.