Environmental Engineering Reference
In-Depth Information
Thorough comparison of ScCO
2
extraction under varying temperatures and
pressures with hexane extraction (a traditional solvent of similar polarity) demon-
strates that, although the chemical composition of hexane and supercritical
extracts do not vary greatly, the proportions of unwanted co-extracted components
such as pigments, polar lipids, and free sugars is well known to be far greater in
hexane extracts than from ScCO
2
[18].
3.2.2.2
Ammonia Fiber Explosion
The major processing impediment to the production of economically viable
commercial ethanol or other chemicals from biomass through biological processing
is the inherent resistance of lignocellulosic materials to conversion to fermentable
sugars. In order to improve the efficiency of enzymatic hydrolysis, a pretreatment
step is necessary to make the structural carbohydrate fraction accessible to cellulases.
Probably one of the most effective biomass pretreatments is ammonia fiber explo-
sion (AFEX), which is a combination of chemical and physical digestion [19]. In
AFEX pretreatment, the biomass is treated with liquid anhydrous ammonia at mod-
erate temperatures (60-100°C) and high pressure (250-300 psi) for 5 min followed
by a rapid pressure release. In this process, the combined chemical and physical
effects of lignin solubilization, hemicellulose hydrolysis, cellulose decrystallization,
and increased surface area enables significant enzymatic conversion of cellulose and
hemicellulose to fermentable sugars. The AFEX treatment has some unique features
that distinguish it from other biomass treatments, as described below:
• Nearly all of the ammonia can be recovered and reused, while the remaining
serves as nitrogen source for microbes in downstream processes.
• There is no wash stream in the process. Dry matter recovery following the
AFEX treatment is essentially 100%. Treated biomass is stable for long periods
and can be fed at very high solids loadings in enzymatic hydrolysis or fermenta-
tion processes.
• Cellulose and hemicellulose are well preserved in the AFEX process, with little
or no degradation.
• There is no need for neutralization prior to the enzymatic hydrolysis of AFEX-
treated biomass.
• Enzymatic hydrolysis of AFEX-treated biomass produces clean sugar streams
for subsequent fermentation processes.
The effectiveness of AFEX treatment has been clearly demonstrated with corn
stover. The rate and the extent of both glucose and xylose produced were substan-
tially greater than the untreated sample. Clearly all of the pretreatment variables
(temperature, moisture content, ammonia loading, and treatment time) influence
the reactivity of the treated biomass. AFEX treatment is a dry process, which per-
mits much higher solid loadings in the fermentation process than liquid phase
treatments. This implies that much higher product concentrations such as ethanol
