Question

Air is blown into the bottom of a bioreactor just below the stirrer which breaks the air bubbles into smaller air bubbles, resulting in the concentration of oxygen in the liquid solution being 20% of the saturation concentration. It is desired to increase the mass transport of oxygen to the reactor, i.e. moles of oxygen transferred from air to water per second. Should you increase or decrease the speed of the stirrer? Explain based on the two-film theory.

Answer 1

According to the film theory; the mass transfer resistance is given by

Rm = δ / D

where;

δ = thickness of the film

D = Diffusion coefficient

The resistance to mass transfer will decrease if δ decreases of D increases.

D is a function of temperature and pressure and hence will not change unless we change T and P.

δ is a function of stirring speed. Higher the stirring speed; lower will be the value of film thickness δ. Hence increasing the speed of stirrer will reduce the film thickness and hence the Mass Tansfer resistance.

As the mass transfer resistance decreases; the rate of mass transfer will increase.

Therefore; to increase the mass transport of oxygen to the reactor ; one should increase or the speed of the stirrer.