Fermentation - Food and Industrial Bioproducts and Bioprocessing - page 188

(a) (b) (c)

Gas out Gas out

Gas out

Draft tube

Riser

Downcomer

Sparger Sparger

Gas in Gas in Gas in

Figure 7.6 Types of bubble column reactors. (a) Bubble column; (b) gas-lift with internal loop; (c) gas-lift
with external loop.

Bubble columns have been used in the industrial production of single cell protein, in
which low back-mixing is desired to achieve high substrate conversion. The main advantages
of bubble column reactors compared to STRs include (Shah et al ., 1982 ):

(1) less maintenance due to simple construction;

(2) high heat transfer rates per unit volume of the reactor;

(3) high liquid residence time for slow reactions to proceed.

On the other hand, bubble coalescence, back mixing and high pressure drop are disadvantages
of bubble column reactors. In addition, it is difficult to maintain consistent concentration of
substrate, nutrients and oxygen along the height of the column, which could starve the
microorganism at points where these levels are low. Multiple feed points to the column can
reduce this risk.

7.4.1.3 Hollow fiber membrane reactor

A hollow fiber membrane module is the most commonly used membrane bioreactor
(Figure 7.7). The hollow fiber membrane module contains hundreds to thousands of hollow
fibers in an assembly similar to a shell and tube heat exchanger. The feed can be applied to
either the fiber (lumen) side or to the shell side. However, the feed is usually applied to the
shell side because the flow path from the lumen side to shell side is too narrow, which
increases the risk for membrane clogging. The permeate stream from the membrane module
usually contains the product and the retentate contains the concentrated feed stream.
The driving force in membrane bioreactors with conventional biomass separation is the
transmembrane pressure, which also affects the flux rate through the membrane.
Microfiltration membranes with pore sizes between 0.1 and 5 μm can be used to confine
cells in the reactor (Salmon and Robertson, 1995). The selection of the pore size depends on