Environmental Engineering Reference
In-Depth Information
Cellulase
Steam
To fermentation
Bagasse
Steam
explosion
Hydrolysis
Acid solution
Lime
Precipitation
Overliming
To cogeneration
FIGURE 13.11
Simplified block scheme of bagasse pretreatment.
The main features of this pretreatment scheme are:
A size reduction step is not necessary in this case.
For the breakage of bonds and hydrolysis of hemicellulose, acid-catalyzed
steamexplosion is used. The fibers are first heated up to 100
C in a steaming vessel
and thenmixedwith hot (100
C) acid solution (1%H
2
SO
4
(w/w) dry bagasse) and
pumped into the steam explosion vessel using a high-pressure (14 bar) pump.
For detoxification, flash cooling and overliming are applied. Since the steam
explosion is carried out at high temperature, no extra heat input is required for
steam stripping, and just by flashing the effluent of the steam explosion, the vol-
atile toxic materials (acetic acid and sugar degradation products) can be stripped
of the medium. The liquid portion is removed from the fibers and cooled down to
the overliming temperature (50
C). Overliming is performed at a pH of 10; after-
ward, the pH is brought down to 4.5 by using a bypass from the liquid fraction
after steam explosion. This pH adjustment results in precipitates that are removed
by clarifiers. The remaining toxic materials, which are phenolic lignin degrada-
tion products, can be absorbed by activated charcoal if the concentrations are
above the threshold inhibition concentrations (not shown in Figure 13.11).
The detoxified liquid fraction contains the pentose (C
5
) sugars. This stream is
used to dilute the fibers after steam explosion to about 20 wt% solids. These
fibers contain the cellulose and the lignin. Commercial cellulase enzymes are
added to break the cellulose in hexose (C
6
) sugars. Cellulose hydrolysis takes
place at 65
C with a large residence time (36 h), after which 90% of the cellulose
is hydrolyzed. The enzymes are not recovered.
The lignin residue is separated from the mixed sugar solution and sent to
cogeneration to be combusted for the generation of energy.
The liquid fraction (or mixed sugar solution) is sent to fermentation for ethanol
production.
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