Environmental Engineering Reference
In-Depth Information
Fre,S,P,Xc
Vapor
Fγ,PγYγ=Sγ=0
Fp,S,P,
X=0
Feed
F0,S0
Fpu, X,S,P
Flash
Membrane
Fr,SrPr
Purge
Fpu, X,S,P
Fermentor
Pump
F, S,X,P
FIGure 21.3
Schematic diagram of extractive process under vacuum for bioethanol production.
mentioning that the conventional mode of operation produces 13 L of vinasse/L of ethanol (Navarro
et al. 2000). Vinasse is one of the main polluting byproducts of alcoholic fermentation because of
its low pH, high solids content, etc. Further details of the technical features of extractive processes
can be found in Silva et al. (1999) and Atala and Maugeri Filho (2006).
The extractive process was developed by Atala (2004), as cited by Mariano et al. (2008) in an
application for the butanol purification. This configuration consists of three interconnected units:
fermentor, filter (tangential microfiltration for cell recycling) and vacuum flash vessel (for the con-
tinuous removal of ethanol from the broth).
The concentration of sugarcane molasses in the feed stream is from 180 to 330 kg/m
3
of total
reducing sugar, and the
Saccharomyces cerevisiae
in the steady state reaches 30 kg/m
3
. The low
ethanol concentration is maintained at 40-60 kg/m
3
. These characteristics of operation guarantee
higher yield (10 kg/m
3
per hour) than in fed-batch and continuous modes of operation. Figure 21.3
depicts the extractive process formed by a fermentor, pumps, flash vessel, and a membrane system.
The kinetic model takes into account the substrate and product inhibitions, the volume occupied
by the cells, intracellular ethanol and terms of cell death (considering that continuous processes
are operated for long periods). The term ρ is a relation between dry cell mass and the volume of
wet cells, and γ, the relation between the inter- and extracellular ethanol concentrations. Having
a higher ethanol concentration in the fermentation stage is an interesting alternative, and has a
significant impact on the whole process costs because of the reduced effort required in the ethanol
purification.
21.10 cellulosIc ethanol ProductIon
Plants are structurally sustained by their walls, which are composites analogous to liquid crystals.
Cellulose microfibrils are deposited in large quantities in the vascular tissues and fibers. These
microfibrils are covered by hemicellulosic polysaccharides, which in the case of sugarcane are
arabinoxylan and β-glucan. The deposition of cellulose is one of the most efficient packing pro-
cesses in nature. Glucose chains linked by β-1,4-glycosidic linkages are packed together so that very
little water is left among the polymers. These interactions and the lack of water prevent the access
of enzymes to the glycosidic linkages, avoiding the attack of microorganisms and consequently
defending plant tissues against pathogen attack.
As a result of this efficient packing, the plant cell wall is, by far, the most adequate form in
which to store carbon and energy in high quantities but at the same time protect against the attack
of microorganisms. The potential for production of biomass will therefore increase as a plant makes
proportionally more wall. On the other hand, this highly packed form of storage imposes a tremen-
dous barrier to access the energy stored. Enzyme hydrolysis is probably the most efficient way to
gain access to the energy stored in glycosidic linkages. What is lacking, though, is the necessary
