Question

Which size of bubbles is relevant for mass transfer?

(a) Small

(b) Very small

(c) Large

(d) Very large

I got this question in a job interview.

My question is taken from Oxygen Transfer in Fermenters in section Mass Transfer of Bioprocess Engineering

Answer 1

Correct answer is (a) Small

To explain I would say: The most important property of air bubbles in fermenters is their size. Small bubbles have correspondingly slow bubble-rise velocities; consequently they stay in the liquid longer, allowing more time for the oxygen to dissolve. While it is desirable to have small bubbles, there are practical limits. Bubbles << 1 mm diameter can become a nuisance in bioreactors. Oxygen concentration in these bubbles equilibrates with that in the medium within seconds, so that the gas hold-up no longer reflects the capacity of the system for mass transfer.