Page 105 - Advances in Eco-Fuels for a Sustainable Environment

78 Advances in Eco-Fuels for a Sustainable Environment

common practice. Pretreatment of the seeds, such as cooking, can increase the oil
yield of screw pressing up to 89% after a single pass and 91% after a dual pass
[56]. Ofori-Boateng et al. [57] in their study revealed that the mechanical extraction
method can be used in extracting oil from the Jatropha curcas seed. Yang et al. [58]
indicated that the mechanical expeller can be used to extract oil from Idesia
polycarpa with high yield.
There are some challenges associated with this method of extraction. First, the oil
extracted needs further purification such as filtering and degumming. Second, the
design of the mechanical extractor is limited to certain seeds [7]. The problem of a
large number of seeds, its slow processes [20], and low yield [22] are also inclusive.
However, this method is advantageous in that it is easy to use, no solvent is required
[20], and it is less expensive [22].

3.3.2 Chemical extraction method
Chemical extraction, also known as solvent extraction or leaching, is the technique of
removing one constituent from a solid by means of a liquid solvent. The rate of extrac-
tion is affected by some factors such as particle size, type of liquid chosen, temper-
ature, and agitation of the solvent [22]. In allowing for greater interfacial area between
the solid and liquid, it is essential to increase the surface area of the seed. The liquid
chosen should be a good selective solvent and its viscosity should be sufficiently low
to circulate freely. The solubility of the material will increase with the increasing tem-
perature. Agitation of the solvent also increases the eddy diffusion and therefore
increases the transfer of material from the surface of the particles. Organic solvents,
such as benzene, cyclohexane, hexane, acetone, and chloroform, have been shown to
be effective when used on microalgae paste; they degrade microalgal cell walls and
extract the oil because oil has a high solubility in organic solvents [20]. This method
was used in extracting grape seed oil with a good yield [59]; likewise from Gomez
et al. [60] worked on the same seed oil. A valuable oil yield was obtained from rose
hip seed, which is generally a waste material, via solvent extraction method [61].
According to Shah et al. [62], Jatropha seed kernels gave a yield of 44g oil/100g
through this method.
Chemical extraction was found to be a more effective technique due to its higher oil
yield and consistent performance [63]. Solvent extraction is only economical at a
large-scale production of >50 tonnes of biodiesel per day [7, 64]. However, Adriaans
[64] and Rosenthal et al. [65] did not recommend the conventional n-hexane solvent
extraction because of environmental impacts (generation of waste water, higher spe-
cific energy consumption, and higher emissions of volatile organic compounds) and
human health impacts (working with hazardous and inflammable chemicals). The
production of VOCs in the conventional process is particularly bothersome because
these can react in the atmosphere with other pollutants to produce ozone and other
photochemical oxidants, which can be hazardous to human health and cause damage
to crops. Besides this, the volatile organic compounds (VOCs) are themselves green-
house gases that are carcinogenic and have toxic properties [65].