Page 115 - Lignocellulosic Biomass to Liquid Biofuels

Bioconversion of lignocellulosic biomass to bioethanol and biobutanol 89

[182]. The recovery of other cellulases by membrane processes was already
reviewed elsewhere [194].

3.3.4 Final comments
1. Hydrolysis is affected by structural characteristics of cellulose and the
presence of lignin and hemicellulose. The removal of lignin, solubiliza-
tion of hemicelluloses, and decrystallization of cellulose by pretreat-
ment strongly induce cellulose accessibility to enzyme.
2. The pretreatment of biomass by ionic liquid, addition of surfactant,
applying microwave irradiation or ultrasound enhances cellulose
conversion.
3. Glucose yield increases with increasing hydrolysis period but decreases
with substrate loading above a certain limit. This is caused by end
product inhibition and mass transfer limitation.
4. Most of the industries use distillation in downstream purification to
dehydrate ethanol. But this technique is highly energy demanding and
hence results in high power consumption. Energy consumption should
be reduced in order to attain energy-efficient dehydration method.
5. Azeotropic and extractive distillations are industrially acceptable etha-
nol dehydration processes.
6. The use of highly available, low-cost glycerol as entrainer in extractive
distillation dehydrates ethanol as well as minimizes energy
consumption.
7. The use of ethylene glycol, n-pentane, or ionic liquid as mass separat-
ing agent minimizes energy consumption.

3.4 Simultaneous saccharification and fermentation

The fermentation of the hexoses (C6 sugars) and pentoses (C5 sugars)
derived from the cellulose and hemicellulose portion of lignocellulosic
biomass is performed by microorganisms, such as bacteria, yeasts, and
fungi. Currently, it is possible to develop fermenting microorganisms
genetically engineered by metabolic engineering approaches to convert
both C5 and C6 sugars from the hydrolyzate into ethanol [201,202]. The
following chemical equation summarizes the fermentation reaction of the
six-carbon sugar, glucose:
C 6 H 12 O 6 -2C 2 H 5 OH 1 6CO 2 1 heat
Glucose-ethanol 1 carbon dioxide gas 1 heat