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Advances in bio-oil extraction from nonedible oil seeds and algal biomass 201

7.8 Chemical extraction

7.8.1 Solvent extraction
Solvent extraction, also known as leaching, is a technique for removing concentrated
materials from a solid by liquid solvent. Particle size, type of solvent, temperature, and
agitation of solvent are the most important criteria during the extraction process. The
viscosity of liquid solvent plays a crucial role such that the selected solvent should be
able to circulate freely. The increase in solubility increases temperature with a simul-
taneous increase in transfer of material from the surface of the particles. The solvent
extraction is economically feasible in large-scale industries as it finds its application
widely in chemical industries. The three type of extraction in this section are (1) hot
water extraction, (2) solvent extraction, and (3) ultrasonication with solvent. The
extraction with n-hexane was used for the extraction of oil from Jatropha and
Pongamia pinnata with the yield of 41% and 95%–99%, respectively [83].The
soxhlet extractor with n-hexane as the solvent was used for the extraction of oil from
Cerbera odollam (Seamango), Coriander (Coriandrum sativum L.), Moringa oleifera,
and Guizotia abyssinica [84–87].
Selection of solvent during the extraction of the lipid from algal biomass is impor-
tant as the chemical solvent has high selectivity and solubility toward the lipid. The
organic solvent should be nonpolar for the effective extraction because the lipid con-
tains triacylglycerides [66, 67, 88]. The lipids are extracted through diffusion across
the algal cell wall and the extraction is carried out by physical contact with the bio-
mass and the seed of the extract [89]. The main disadvantage of using chemical sol-
vents is their toxicity toward humans and the surrounding environment. The solvents
such as n-hexane, methanol, chloroform-methanol, and ethanol are widely used for the
extraction of lipids. The percentage of lipids extracted mainly depends on the micro-
algal strains and the combined use of cosolvents. The rate of diffusion is always a lim-
iting factor during the extraction of a lipid as it restricts the diffusion into the cell wall
[90]. Hexane solvent extraction in combination with impellers is used for the extrac-
tion of oil. The extracted oil in the impellers is separated by mixing it with cylo-
hexane. The extraction of 95% of oil from algae is done using the cold press and
hexane as the solvent. Cyclo-hexane dissolves oil into it and the pulp is filtered out. Then
with the help of distillation, oil and cyclohexane are separated. The use of n-hexane is
widely carried out during lipid extraction but it is highly toxic. Moreover, it reduces the
selectivity of the lipid due to the nonpolar nature of the solvent. Ethanol, a greener
solvent, has drawn wide attention due to its being less toxic. But the extraction efficiency
is low when compared to other solvent sdue to the azeotrop mixture containing water
[54, 91]. It was reported that the total lipid recovery and FAME recovery can be
increased through utilizing a solvent mixture of polar and nonpolar solvents [91a].

7.8.2 Supercritical fluid extraction

Supercritical carbon dioxide extraction is one of the available green technologies that
replaces the traditional organic solvent. The extraction and reaction field have new
dimensions with the introduction of supercritical fluid technology. The basic principle