Figure 4 presents the major barriers in the commercialization of bioalcohols production (Hegde et al. 2018).
10 Ethanol Recovery
Ethanol is formed as a dilute component (10–15% (w/w)) in
the fermentation broth due to the presence of other
by-products in the mixture. It is, therefore, subjected to
various degrees of purification to obtain ethanol with a
purity of about *99% or more.
The steps involved in the most widely used recovery
technique are fractional distillation followed by dehydration
(Cheng and Timilsina 2010).
Fractional distillation: This unit operation is based on the
principle of vapor–liquid equilibria (VLE) of the components present in a mixture to be distilled, with a difference in
their boiling points. Ethanol and water form a non-ideal
solution with a positive deviation from Raoult’s law. Ethanol
recovery takes place in a fractionating column with a choice
of packings or plates. Depending on the azeotropic characteristics of the mixture, the ethanol concentration range can
be divided into two categories for maximum theoretically
achievable ethanol concentration during fractional distillation, the first concentration range of 0–93% (w/w) and the
other range of 93–100% (w/w). The higher the number of
theoretical plates in the column, the better will be the mass
transfer between the components, and thus it signifies that
with a count of enough number N of theoretical plates, the
vapor-rich mixture at the top of the fractionator can have
93% (w/w) ethanol concentration and the liquid-rich mixture
at the bottom of the column can be approximately 0%
(w/w) in ethanol. Therefore, the theoretical maximum for
ethanol concentration in fractional distillation is 93% (w/w).
Dehydration: Further enrichment of the fractionally distilled ethanol to a concentration of *99% calls for the
dehydration mechanism using molecular-sieve adsorption
with the size of pores of around 0.3–0.35 nm. The difference
in the sizes of the ethanol (0.40 nm) and water (0.28 nm)
molecules allows for the water molecules adsorption, thus
resulting in concentrated ethanol of *99% (w/w).
11 Concluding Remarks
• Bioethanol produced using food competitive and
energy-intensive crops like starch and sugarcane using
conventional technology (1G) is at its early stages of
commercialization. The scope of the present work was
limited to the study of food wastes for bioethanol production (2G). The maturity status of this 2G technology is
contained still at the pilot-plant setup and demonstration
stage, due to the hassles in the incorporation of many
desirable yet incompatible characteristics in the technology. The development of the 2G technology from pilot to
commercial scale is in progress still and is expected to be
achieved in the next few years (Stephen and Periyasamy
2018).
• This chapter is suggestive of the fact that pectin-rich
biomasses have significant benefits when utilized for
bioethanol production. The notable ones include the high
degree of hydrolysis and the number of fermentable
sugars achieved (John et al. 2017; Parmar and Rupasinghe 2013; Mamma and Christakopoulos 2014). It has
also been mentioned that the most commonly adopted
pretreatment techniques are acid hydrolysis, alkaline
hydrolysis, and enzymatic hydrolysis. However,
depending upon the physical complexity of the biomass
used, many hybrid pretreatment techniques have been
designed based on the level of interlinkages of different
fundamental principles of conventional pretreatment
techniques (Kumar et al. 2019).
• Commercializable bioethanol (1G) finds its way through
the regional storage centers to the retail fueling stations. It
is then blended with gasoline as per the government
norms of the respective geographical areas of fuel distribution (Singh et al. 2016). However, the evolution of the
conventional processes (1G) into advanced processes (2G
and 3G) is still ongoing and their commercialization is
expected in a few years to come.
• The production of 3G biofuels is based on the usability of
algae or cyanobacteria, an undervalued substrate source.
The algal biomass is mainly composed of lipids, proteins,
and sugars, and its cultivation does not demand huge
water-or farmable-land requirements (Stephen and Periyasamy 2018; Ibrahim et al. 2018). It, therefore, has the
potential to produce sustainable competitive bioethanol
using valorization of low-valued algal biomass. The
advancement toward the green-design approaches, with a
need for further intensified research toward bioethanol
and other biofuels production because of their advantages, marks the beginning of an energy-secure habitat.
Acknowledgements The authors thank Indian Institute of Technology, Delhi, for enabling the writing of this chapter.
References
Akbas, M. Y., & Stark, B. C. (2016). Recent trends in bioethanol
production from food processing byproducts. Journal of Industrial
Microbiology and Biotechnology, 43(11), 1593–1609.
Ángel Siles López, J., Li, Q., & Thompson, I. P. (2010). Biorefinery of
waste orange peel. Critical reviews in biotechnology, 30(1), pp. 63–69.
Bailey, R. B., Benitez, T., & Woodward, A. (1982). Saccharomyces
cerevisiae mutants resistant to catabolite repression: use in cheese
Bioconversion of Food Waste into Ethanol: A Review
55
10 Ethanol Recovery
Ethanol is formed as a dilute component (10–15% (w/w)) in
the fermentation broth due to the presence of other
by-products in the mixture. It is, therefore, subjected to
various degrees of purification to obtain ethanol with a
purity of about *99% or more.
The steps involved in the most widely used recovery
technique are fractional distillation followed by dehydration
(Cheng and Timilsina 2010).
Fractional distillation: This unit operation is based on the
principle of vapor–liquid equilibria (VLE) of the components present in a mixture to be distilled, with a difference in
their boiling points. Ethanol and water form a non-ideal
solution with a positive deviation from Raoult’s law. Ethanol
recovery takes place in a fractionating column with a choice
of packings or plates. Depending on the azeotropic characteristics of the mixture, the ethanol concentration range can
be divided into two categories for maximum theoretically
achievable ethanol concentration during fractional distillation, the first concentration range of 0–93% (w/w) and the
other range of 93–100% (w/w). The higher the number of
theoretical plates in the column, the better will be the mass
transfer between the components, and thus it signifies that
with a count of enough number N of theoretical plates, the
vapor-rich mixture at the top of the fractionator can have
93% (w/w) ethanol concentration and the liquid-rich mixture
at the bottom of the column can be approximately 0%
(w/w) in ethanol. Therefore, the theoretical maximum for
ethanol concentration in fractional distillation is 93% (w/w).
Dehydration: Further enrichment of the fractionally distilled ethanol to a concentration of *99% calls for the
dehydration mechanism using molecular-sieve adsorption
with the size of pores of around 0.3–0.35 nm. The difference
in the sizes of the ethanol (0.40 nm) and water (0.28 nm)
molecules allows for the water molecules adsorption, thus
resulting in concentrated ethanol of *99% (w/w).
11 Concluding Remarks
• Bioethanol produced using food competitive and
energy-intensive crops like starch and sugarcane using
conventional technology (1G) is at its early stages of
commercialization. The scope of the present work was
limited to the study of food wastes for bioethanol production (2G). The maturity status of this 2G technology is
contained still at the pilot-plant setup and demonstration
stage, due to the hassles in the incorporation of many
desirable yet incompatible characteristics in the technology. The development of the 2G technology from pilot to
commercial scale is in progress still and is expected to be
achieved in the next few years (Stephen and Periyasamy
2018).
• This chapter is suggestive of the fact that pectin-rich
biomasses have significant benefits when utilized for
bioethanol production. The notable ones include the high
degree of hydrolysis and the number of fermentable
sugars achieved (John et al. 2017; Parmar and Rupasinghe 2013; Mamma and Christakopoulos 2014). It has
also been mentioned that the most commonly adopted
pretreatment techniques are acid hydrolysis, alkaline
hydrolysis, and enzymatic hydrolysis. However,
depending upon the physical complexity of the biomass
used, many hybrid pretreatment techniques have been
designed based on the level of interlinkages of different
fundamental principles of conventional pretreatment
techniques (Kumar et al. 2019).
• Commercializable bioethanol (1G) finds its way through
the regional storage centers to the retail fueling stations. It
is then blended with gasoline as per the government
norms of the respective geographical areas of fuel distribution (Singh et al. 2016). However, the evolution of the
conventional processes (1G) into advanced processes (2G
and 3G) is still ongoing and their commercialization is
expected in a few years to come.
• The production of 3G biofuels is based on the usability of
algae or cyanobacteria, an undervalued substrate source.
The algal biomass is mainly composed of lipids, proteins,
and sugars, and its cultivation does not demand huge
water-or farmable-land requirements (Stephen and Periyasamy 2018; Ibrahim et al. 2018). It, therefore, has the
potential to produce sustainable competitive bioethanol
using valorization of low-valued algal biomass. The
advancement toward the green-design approaches, with a
need for further intensified research toward bioethanol
and other biofuels production because of their advantages, marks the beginning of an energy-secure habitat.
Acknowledgements The authors thank Indian Institute of Technology, Delhi, for enabling the writing of this chapter.
References
Akbas, M. Y., & Stark, B. C. (2016). Recent trends in bioethanol
production from food processing byproducts. Journal of Industrial
Microbiology and Biotechnology, 43(11), 1593–1609.
Ángel Siles López, J., Li, Q., & Thompson, I. P. (2010). Biorefinery of
waste orange peel. Critical reviews in biotechnology, 30(1), pp. 63–69.
Bailey, R. B., Benitez, T., & Woodward, A. (1982). Saccharomyces
cerevisiae mutants resistant to catabolite repression: use in cheese
Bioconversion of Food Waste into Ethanol: A Review
55