Civil Engineering Reference
In-Depth Information
Agitated culture is considered as the most suitable cultivation technique for the com-
mercial production of BC once, in comparison with static culture, requires less space
and work force and higher production rates may be possibly achieved [45, 48].
Several attempts have been made to produce BC using the stirred-tank reactor.
However, cellulose production under intensive agitation encounters many problems,
including the spontaneous appearance of mutations in the bacterial strains, which
causes a decline in the biopolymer synthesis; accumulation of BC in i brous form
during cultivation which increases the viscosity of the broth enabling proper oxygen
supply; and an easy attachment of BC to the shat of reactors, making it hard to collect
and also to clean up the reactors [45, 49].
In order to overcome such drawbacks, several ef ective agitated culture bioreactors
have recently been designed and tested. h e airlit reactor was i rstly described by Chao
et al.
[50] for the production of BC (Figure 2.8). Accordingly, oxygen-enriched air is sup-
plied to increase the amount of dissolved oxygen in the broth and also promote the mixing
of the culture medium. h is way, no mechanical powered agitation is required, granting
it with low power requirements. Furthermore, the homogenous shear stress throughout
the bioreactor and mild agitation resulted in unique elliptical pellets of BC, instead of
the i brous materials attained when using the stirred-tank reactor. h e results reported
a BC production of 3.8 g.L
-1
using normal air supply. However, the concentration of BC
doubled (8 g.L
-1
) when oxygen-enriched gas was used in the system [51]. Furthermore,
Chao
et al.
also discovered that the addition of 0.1% (w/w) agar to the culture medium
increased BC production from 6.3 g.L
-1
, in the control sample, to 8.7 g.L
-1
[52] .
Several modii cations of the air-lit reactor have been proposed. For instance, Cheng
et al.
[53] produced a modii ed air-lit reactor containing a rectangular wire-mesh drat
tube that was easier to scale up and construct. h ese new drat tubes are capable of sub-
dividing the bubbles into smaller ones which resulted in a higher volumetric oxygen
transfer coei cient and mixing capability than conventional reactors. In fact, without
using oxygen-enriched air, the dissolved oxygen in the modii ed airlit reactor could be
maintained above 35% throughout the cultivation process. As a result of the use of such
Air out
Gas out
ID = 460
Downcomer
Riser
Draft Tube
120
Gas In
Sparger
136
Air in
Figure 2.8
Dif erent types of bioreactors used to produce BC: airlit bioreactor (let ) (reproduced with
permission from [37]); and spherical type bubble column bioreactor (right) (reproduced with permission
from [54] ).
