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The non-food based feedstocks mostly used for
production of second generation bioethanol which consist of cellulosic biomass
such as complete energy crops (e.g., switchgrass, miscanthus) and agricultural
and wood residues (e.g., woodchips, corn stover, sugarcane bagasse, and
sawdust). (Kang et
al., 2014) Cellulosic biomass mainly
consists of three polymers such as cellulose, hemicellulose, and lignin that
are interconnect in a heterogeneous matrix. Cellulose is a linear
polysaccharide chain contain several ? (1-4) linked with D-glucose units.
Hemicellulose is a heteropolymer structure of xylose, mannose, galactose,
rhamnose and arabinose (Rocha-Meneses
et al., 2017).Lignin is a complex
polymer of cross-linkage aromatic compounds. Lignin which acts as a protective
barrier and hamper the depolymerization of cellulose and hemicellulose to
fermentable sugars. Pretreatment is a first processing step to make
lignocellulosic biomass more manageable to biological conversion at high yields
that otherwise agonize from low yields and high processing costs.(I &
H, 2015)

In pretreatment step separate hemicellulose and lignin
from cellulose, that are organize into a complex structure, which is
incompliant to decomposition. Pretreatment methods are divided into: physical,
chemical, physio–chemical and biological pretreatments.

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 Physical pretreatments:
Physical pretreatment method include some physical techniques like chipping,
grinding, and milling particles.

 Chemical pretreatments:
Chemical pretreatment include treat with chemical like dilute acids,
concentrated acids, alkali, ozone and ionic liquids.

 Physio–chemical pretreatments:
Physio–chemical pretreatments include uncatalyzed steam, acid catalyzed steam,
liquid hot water/hydrothermal, ammonia fiber explosion, ammonia recycling
percolation, soaking aqueous ammonia, wet oxidation, CO2 explosion. Biological
pretreatments include pretreatments with microorganisms.(R. Kumar
et al., 2016)

It is also possible to further improve the efficiency
of pretreatment by combine different pretreatment methods. Most common use
pretreatment methods are dilute acid pretreatment, hydrothermal pretreatment and
alkaline pretreatment. Pretreatments that are reported as efficient for the
pretreatment of lignocellulosic biomass are Nitrogen explosion, compressed air
and steam explosions are known as effective methods of pretreatment that
increase glucose and ethanol yields.

A few recently
developed pretreatments techniques including co-solvent enhanced
lignocellulosic fractionation (CELF), co-solvent based lignocellulosic
fractionation (COSLIF), extractive ammonia (EA) pretreatment, ?-valerolactone (GVL)
pretreatment, pretreatment applying ionic liquid(s), sulfite pretreatment to
overcome recalcitrance of lignocellulose (SPORL), and switchable butadiene
sulfone pretreatment.Hydrolysis

After pretreatment next step is hydrolysis, also known
as saccharification. In this hydrolysis step, cellulose is converted into
glucose. This conversion is take place by using some specific acids or enzymes.
Table5: presents the main
characteristics of enzymatic and acid hydrolysis. Acid hydrolysis is the most
commonly use method then enzymatic method and  because in this method, utilizes dilute or
concentrated acids, such as sulfuric acid (H2SO4), hydrochloric acid (HCl),
nitric acid (HNO3), trifluoracetic acid (TFA) or phosphoric acid (H3PO4).(Onuki,
2008)

 Fermentation
process is used to convert glucose into ethanol. Simultaneous saccharification
and fermentation (SSF) and separated hydrolysis and fermentation (SHF) (Mohr & Raman, 2013)
are the two most dominant fermentation processes used in the fermentation of
lignocellulosic hydrolysate.

In Simultaneous saccharification and fermentation
(SSF), use enzymes to hydrolyze cellulose into sugars and then ferment the
hexoses into ethanol, at the same time. This process has several well-known
advantages. It has low costs process, low enzyme requirements, increase
hydrolysis efficiency and give high ethanol yields, reduced process time
consumption, reduced risk of inhibition and contamination, and it does not
require reactors with large volumes. However, this method SSF has some
limitations, both cellulose hydrolysis and hexose fermentation occur in the
same reactor. Result of this method gives the product inhibition on the cellulases as the sugars that are
immediately consumed by the microorganism which are used for fermentation.
There is a barter between the cost of enzymes production and hydrolysis
fermentation process. In SSF, the rate of enzyme production effected the rate
of alcohol production.

 In Separate
Hydrolysis and Fermentation (SHF) configuration, the enzyme production,
hydrolysis of biomass, hexose and pentose fermentation, all are carry out in
separate reactors. The disadvantages of SHF lead to the development of
Simultaneous Saccharification and Fermentation (SSF) (Axelsson, 2011)
process.

In Simultaneous Saccharification and Co-fermentation
(SSCF) process, take co-fermented in the same reactor of glucose and xylose.
Strains of S. cerevisiae and Zymomonas mobilis are genetically modified to
co-ferment both glucose and xylose.

Another technique used, integration is the
Consolidated BioProcessing (CBP), in which only one or single microorganism is
used for hydrolysis and fermentation steps. This prospect reduces the capital
costs and increases process efficiency. However, microorganisms which can done
both produce enzymes for hydrolysis of biomass and then ferment released sugars
are still under development stage. 

Fermentation by-products are mostly
removed by distillation process. However, immense volatile by-products of
fermentation are obtained in ethanol. Also, mostly high concentration of
ethanol is not required for drinking or pharmaceutical purpose  (Axelsson,
2011). 

 Further
distillation is just waste of energy and money. There are many techniques for
purification processes of ethanol which comprise distillation. Distillation is the
mostly using process and well known for industrial purification technique of
ethanol. It consumes various volatilities of different components in a mixture. (Dufey, 2006)

 The
basic principle of this techniques is heating a mixture, low boiling point
components are concentrated in the vapor phase. By condensing this vapor, more
concentrated less volatile compounds are obtaining in liquid phase.
Distillation is one of the mostly using and efficient separation techniques.
However, this technique have several problems during process. In ethanol
production, a distillation tower is designed to separate water and ethanol effectively
rather than separation of volatile compounds . (Onuki,
2015)Water
is obtained from the bottom of the distillation tower and ethanol is obtained
from the top of the tower. It is assuming that impurities with similar boiling
points to ethanol remain in ethanol even after distillation. Second is its prohibitive
cost. Distillation is a repeating process of vaporization and condensation.
Therefore, it has prohibited costs process.

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