cellulose-hemicelluloses hydrolysis and fermentation were performed in single
reactor. SSF minimized microbial contamination as well (Srichuwong et al. 2009;
Kaur et al. 2018). As two processes are combined, therefore, there is a low need of
enzyme and low overall cost. The main limitation of the process is the difference in
temperature requirement for saccharification (50
C) and fermentation (usually
32–37
C) (Danquah et al. 2011; Sarkar et al. 2012). However, several recombinant
strains capable of producing hydrolyzing enzymes and mediating fermentation are
developed for mediating the SSF and overcoming the limitations of temperature
variance (McBride et al. 2018).
7.3.4 Distillation Process for Recovery of Bioethanol
The fermentation broth is subjected to distillation where water or any other
impurities are removed for generation of anhydrous high-quality ethanol that can
be used as transportation fuel. The principle of removing impurities through distillation uses the difference in boiling point of different constituents of the mixture. The
boiling point (BP) of ethanol is 78.2
C, which get vaporized and get separated from
other component (water BP: 100
C). The different distillation processes used for
obtaining anhydrous ethanol are adsorption distillation, azeotropic distillation,
chemical dehydration, diffusion distillation, extractive distillation, membrane distillation, and vacuum distillation.
The variation in molecular size of ethanol and water is used during adsorption
distillation. The mixture is passed through the molecular sieve that allow ethanol to
pass while entrap the water. Azeotropic distillation utilizes the application of binary
azeotropic mixtures which disobeys Raoult’s law (Kumar et al. 2010). An additional
chemical called entrainer is used to amend the relative volatilities of azeotropes and
help in the recovery of azeotropes that can be reused. In dehydration distillation
process, a hygroscopic chemical substance (calcium oxide, quicklime) is introduced
into the ethanol-water mixture causing rapid dehydration of the mixture (in liquid or
vapor phase) by forming soluble calcium hydroxide leaving ethanol on the surface.
The method of diffusion distillation involves ethanol recovery by diffusing the
mixture through the voids of inert gas followed by condensation. Extractive distillation is based on variation in volatility of different components of the mixture, and
thus an additional solvent mixture is added. The highly volatile components (lighter
component) are present at the top of the column, while other components with low
volatility (heavier component) along with additional solvent are at the bottom. The
process involves two distillation runs where during the first run, light component is
extracted and the left out heavy component is recovered during the second distillation run (Ravagnani et al. 2010; Gryta 2012). In membrane distillation, the principle
of permeability is used where semi-permeable membrane is used for mass transfer
through a semi-permeable membrane. The volatile component is crossed through the
membrane as the surface tension associated with the membrane blocks the feed.
Usually, membranes are made up of polymers; however in some cases, metals and
ceramic material are also used (Wang and Chung 2015). Efficiency of the distillation
174
B. Kumar et al.
reactor. SSF minimized microbial contamination as well (Srichuwong et al. 2009;
Kaur et al. 2018). As two processes are combined, therefore, there is a low need of
enzyme and low overall cost. The main limitation of the process is the difference in
temperature requirement for saccharification (50
C) and fermentation (usually
32–37
C) (Danquah et al. 2011; Sarkar et al. 2012). However, several recombinant
strains capable of producing hydrolyzing enzymes and mediating fermentation are
developed for mediating the SSF and overcoming the limitations of temperature
variance (McBride et al. 2018).
7.3.4 Distillation Process for Recovery of Bioethanol
The fermentation broth is subjected to distillation where water or any other
impurities are removed for generation of anhydrous high-quality ethanol that can
be used as transportation fuel. The principle of removing impurities through distillation uses the difference in boiling point of different constituents of the mixture. The
boiling point (BP) of ethanol is 78.2
C, which get vaporized and get separated from
other component (water BP: 100
C). The different distillation processes used for
obtaining anhydrous ethanol are adsorption distillation, azeotropic distillation,
chemical dehydration, diffusion distillation, extractive distillation, membrane distillation, and vacuum distillation.
The variation in molecular size of ethanol and water is used during adsorption
distillation. The mixture is passed through the molecular sieve that allow ethanol to
pass while entrap the water. Azeotropic distillation utilizes the application of binary
azeotropic mixtures which disobeys Raoult’s law (Kumar et al. 2010). An additional
chemical called entrainer is used to amend the relative volatilities of azeotropes and
help in the recovery of azeotropes that can be reused. In dehydration distillation
process, a hygroscopic chemical substance (calcium oxide, quicklime) is introduced
into the ethanol-water mixture causing rapid dehydration of the mixture (in liquid or
vapor phase) by forming soluble calcium hydroxide leaving ethanol on the surface.
The method of diffusion distillation involves ethanol recovery by diffusing the
mixture through the voids of inert gas followed by condensation. Extractive distillation is based on variation in volatility of different components of the mixture, and
thus an additional solvent mixture is added. The highly volatile components (lighter
component) are present at the top of the column, while other components with low
volatility (heavier component) along with additional solvent are at the bottom. The
process involves two distillation runs where during the first run, light component is
extracted and the left out heavy component is recovered during the second distillation run (Ravagnani et al. 2010; Gryta 2012). In membrane distillation, the principle
of permeability is used where semi-permeable membrane is used for mass transfer
through a semi-permeable membrane. The volatile component is crossed through the
membrane as the surface tension associated with the membrane blocks the feed.
Usually, membranes are made up of polymers; however in some cases, metals and
ceramic material are also used (Wang and Chung 2015). Efficiency of the distillation
174
B. Kumar et al.