Question

The rising tube and the down coming tubes are the features of which type of fermenters?

(a) Bubble column

(b) Air-lift

(c) Packed bed

(d) Fluidized bed

I have been asked this question during an internship interview.

This intriguing question comes from Oxygen Transfer in Fermenters in section Mass Transfer of Bioprocess Engineering

Answer 1

The correct choice is (b) Air-lift

To explain: There are two types of air-lift fermenters which are the internal loop and the external loop. The internal loop has a draft tube in its inner tube, in which the up-flowing gasses liquid and the down-flowing liquid is separated by the draft tube. Draft tubes are used in some processes to promote better mass transfer, mixing and inducing circulatory motion to reduce bubble coalescence. The external loop has two streams flow in two separate pipes connected at top and bottom. In this way, the air-lift fermenters improve the circulation and oxygen transfer and equalize shear forces in the reactor.