Page 107 - Lignocellulosic Biomass to Liquid Biofuels

Bioconversion of lignocellulosic biomass to bioethanol and biobutanol 83

3.3.2 Traditional downstream purification
Aqueous ethanol that is produced after conventional fermentation process
is yet to be accepted as fuel ethanol. Ethanol and water form azeotropic
mixture and make it difficult to get fuel-grade ethanol. Thus new research
directs to initiate reducing high-energy demanding and industrially
acceptable distillation process to break the azeotrope nature for achieving
almost pure ethanol. Since many years azeotropic and extractive distilla-
tion processes are used widely for this purpose [183].
Distillation is separating liquid mixtures based on their boiling point
by evaporation followed by condensation. Gill et al. designed the use of
glycerol in extractive distillation processes to dehydrate ethanol. The rea-
son is that glycerol has high availability, relatively low cost, and enhanced
relative volatility, which impart a positive effect on azeotropic mixture.
The study of the impact of this entrainer on feed stage, feed temperature,
reflux ratio, feed molar ratio has been done to get the most appropriate
design for minimizing the requirement of energy [184].
A novel approach of extractive dividing-wall columns (DWC), which
combines with solvent recovery system, and preconcentration column in
a single distillation unit, is efficient to separate 99.8 wt.% bioethanol. This
ethanol dehydration configuration can be applied in large-scale bioethanol
production anywhere in this world [185]. While using ethylene glycol
and n-pentane as mass separating agent, DWC in extractive and azeotro-
pic distillation can separate highly pure, that is, 99.8% ethanol, and also
saves 10% 20% of overall energy [186]. Using ionic liquid as mass sepa-
rating agent during extractive distillation lowers energy consumption for
ethanol purification. This process can achieve 0.995 (in mass) ethanol
purity and 99.9 wt.% ethanol recovery [187].

3.3.3 Possibilities of application of membrane
in bioethanol production
Membrane, a permeable barrier when placed between two mediums and
applying required driving force, allows the selectively transfer of one or
more constituent from one phase to another. Membranes that are mostly
used in bioseparation processes are generally porous in nature. The consti-
tuents that pass through the pores of the membrane and collected in other
side are called permeate, whereas the components that retain are called
retentate. Most useful pressure-driven membrane-based separation com-
monly used methods include microfiltration (MF), NF, ultrafiltration