Biomedical Engineering Reference
In-Depth Information
stripping. Membrane-based systems show a high selectivity for solvents but might
suffer from clogging and fouling and seem to be more suitable for use with
immobilized cells. Liquid-to-liquid extraction also has high selectivity, but emul-
sions might form to render the process less suitable. All these procedures can be
designed to allow online product recovery so that butanol toxicity and inhibition
to the fermentation is reduced. High concentrations of substrates and cells result
in high productivity. However, a sugar concentration of more than 60 g
L
1
in
a nonintegrated process without product removal is not used, which leads to a
low concentration of cells because of butanol inhibition. The usual maximum
concentration of total solvents in the fermentation broth is 20 g
L
1
, of which
L
1
. Ezeji et al. [
29
] devised the integration of a
fed-batch and gas-stripping system to recover solvent in acetone-butanol-ethanol
fermentation; a total of 500 g glucose was used to produce 232.8 g
butanol is only about 13 g
L
1
solvents.
Qureshietal.[
30
] discussed the separation of butanol from aqueous solutions
and fermentation broth by adsorption of silicalite, resins, bone charcoal, activated
charcoal, bonopore, and polyvinylpyridine. Silicalite appeared to be more attractive
to concentrate butanol from dilute solutions (5 to 790-810 g
L
1
) and required less
energy for butanol recovery (1,948 kcal kg
-1
butanol). To improve efficiency and
reduce adsorbent costs, the method of biomass absorption to produce anhydrous
ethanol appeared domestically and abroad [
31
]. Hassaballah and Hills [
32
] reported
the use of cornmeal as adsorbent to condense 85 % (w/w) alcohol vapor, and the
product yield was up to 99.8 % (w/w). Westgate et al. [
33
] indicated that starch was
a well-established adsorbent for drying ethanol, and the purity of ethanol and the
energy consumption were both promising.
7.2.5
Present Situation and Problems of Cellulosic Butanol
Fermentation Technology
The traditional acetone-butanol fermentation substrates used are corn and molasses;
the production of butanol is generally 9-13 g
L
1
, and the raw material cost
accounts for 60-70 % of the total cost, which is an important reason to limit the
development of the butanol fermentation industry. Another factor that cannot be
ignored is the toxicity of solvent on the microbial cells; especially, when the concen-
tration of butanol production reaches 13 g
L
1
, the fermentation will stop. It is also
a limiting factor affecting solvent production. In addition, in the acetone-butanol
fermentation process, energy consumption for traditional distillation recovery of
butanol is high. Steam consumption accounts for 70 % of the cost of the entire
power, and too much wastewater is produced by distillation recovery of butanol,
which increases environmental protection costs.
Lignocellulose has the most potential as a fermentation substrate, but there are
also many problems in the process. Now,
Clostridium
as a major industrial strain
cannot effectively hydrolyze lignocellulose. Therefore, it is necessary to carry out
