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fossil fuels, and simultaneously depleting fossil fuel sources alarm us to look for
their alternatives. Today, all the energy sources are in great demand due to the industrial advancement through which countries are prospering economically. According
to the Energy Information Administration (United States), a total energy of 406
quadrillion British thermal units (BTU) was consumed around the world in the year
2000, and the energy demand is anticipated to increase to 769.8 quadrillions BTU
by 2035 [3]. There is an approximately 47.25% increase in anticipated energy
demand between the years 2000 and 2035. A large portion of energy demand is met
using the combustion of the main source of energy like fossil fuel. However, the
rapid exhaustion of fossil fuels and environmental concerns due to their usage
evoked the search for many promising alternative green energy sources like biofuels
[4]. Exploration and exploitation of various alternative energy sources have gained
huge attention over the past few decades. The contribution of biofuels in meeting
inevitably increasing energy demand is very important because of the aforementioned concerns with mainstream fossil fuels.
Among the portfolio of biofuels, biodiesel is considered to be the best alternate
for diesel fuel as a result of its high net energy returns (~90%). Moreover, biodiesel
has been recognized around the world as a green fuel and considered as the best
option in contrast to diesel fuel. Citing the eco-friendly nature of the biodiesel, central and state governments of various countries are framing policies to enlarge its
utilization [5]. Biodiesel is considered a renewable fuel, which produces lesser
greenhouse gas emissions and has a par excellence lubricating property. Also, biodiesel is inherently free of sulfur unlike petroleum-based diesel [6]. Biodiesel is an
alkyl ester of long-chain fatty acids derived from the reaction between oil/fat and
alcohol. Industrially, biodiesel is produced by transesterification of oil and alcohol
using a base catalyst like KOH or NaOH [7]. This process is limited by low mutual
solubility between the oil and alcohol phases. These phases are nonhomogeneous or
immiscible in each other, limiting the mass transfer through interphase. This raises
the need for a phase transfer catalytic process involving the formation of an intermediate complex that can solubilize the organic (oil) and inorganic (alcohol) phases.
The commercialization of biodiesel on a large scale is limited by the high cost of
production. The high cost of production using current technology is due to its
requirement of high-quality feedstock and also its intolerance to impurities (free
fatty acid (FFA) and water) present in the low-quality feedstock. The common feedstock for biodiesel production involves edible and non-edible oils. Pure fatty acids
are costlier compared to edible oils which in turn are costlier than non-edible oils.
Moreover, the widespread use of edible oils for fuel may elevate the crisis of food
against the comfort of fuel. Therefore, non-edible and waste vegetable oils are the
most preferred feedstock to lower biodiesel production costs. However, non-edible
oils may have higher amount of FFA content than edible oils. The FFA present in
oils reacts with the alkali to form soap which results in the difficult product purification and low biodiesel yield [8]. Subsequently, it is essential to restrict the FFA
content present in feed oil preceding the base-catalyzed transesterification reaction.
Therefore, need arises to adopt certain reaction criterion which governs the adaptability of the single-step (only transesterification) or two-step (esterification to
Z. Hussain et al.
fossil fuels, and simultaneously depleting fossil fuel sources alarm us to look for
their alternatives. Today, all the energy sources are in great demand due to the industrial advancement through which countries are prospering economically. According
to the Energy Information Administration (United States), a total energy of 406
quadrillion British thermal units (BTU) was consumed around the world in the year
2000, and the energy demand is anticipated to increase to 769.8 quadrillions BTU
by 2035 [3]. There is an approximately 47.25% increase in anticipated energy
demand between the years 2000 and 2035. A large portion of energy demand is met
using the combustion of the main source of energy like fossil fuel. However, the
rapid exhaustion of fossil fuels and environmental concerns due to their usage
evoked the search for many promising alternative green energy sources like biofuels
[4]. Exploration and exploitation of various alternative energy sources have gained
huge attention over the past few decades. The contribution of biofuels in meeting
inevitably increasing energy demand is very important because of the aforementioned concerns with mainstream fossil fuels.
Among the portfolio of biofuels, biodiesel is considered to be the best alternate
for diesel fuel as a result of its high net energy returns (~90%). Moreover, biodiesel
has been recognized around the world as a green fuel and considered as the best
option in contrast to diesel fuel. Citing the eco-friendly nature of the biodiesel, central and state governments of various countries are framing policies to enlarge its
utilization [5]. Biodiesel is considered a renewable fuel, which produces lesser
greenhouse gas emissions and has a par excellence lubricating property. Also, biodiesel is inherently free of sulfur unlike petroleum-based diesel [6]. Biodiesel is an
alkyl ester of long-chain fatty acids derived from the reaction between oil/fat and
alcohol. Industrially, biodiesel is produced by transesterification of oil and alcohol
using a base catalyst like KOH or NaOH [7]. This process is limited by low mutual
solubility between the oil and alcohol phases. These phases are nonhomogeneous or
immiscible in each other, limiting the mass transfer through interphase. This raises
the need for a phase transfer catalytic process involving the formation of an intermediate complex that can solubilize the organic (oil) and inorganic (alcohol) phases.
The commercialization of biodiesel on a large scale is limited by the high cost of
production. The high cost of production using current technology is due to its
requirement of high-quality feedstock and also its intolerance to impurities (free
fatty acid (FFA) and water) present in the low-quality feedstock. The common feedstock for biodiesel production involves edible and non-edible oils. Pure fatty acids
are costlier compared to edible oils which in turn are costlier than non-edible oils.
Moreover, the widespread use of edible oils for fuel may elevate the crisis of food
against the comfort of fuel. Therefore, non-edible and waste vegetable oils are the
most preferred feedstock to lower biodiesel production costs. However, non-edible
oils may have higher amount of FFA content than edible oils. The FFA present in
oils reacts with the alkali to form soap which results in the difficult product purification and low biodiesel yield [8]. Subsequently, it is essential to restrict the FFA
content present in feed oil preceding the base-catalyzed transesterification reaction.
Therefore, need arises to adopt certain reaction criterion which governs the adaptability of the single-step (only transesterification) or two-step (esterification to
Z. Hussain et al.