Page 133 - Advances in Eco-Fuels for a Sustainable Environment

Prospective ecofuel feedstocks for sustainable production 105

Thus, WCO must be treated before the transesterification process. Techniques such as
acid esterification with methanol and sulfuric acid, esterification with ion-exchange
resins, neutralization with alkalis followed by soap separation by a decanter, and
extraction of polar liquids along with acid esterification and distillation of free fatty
acids have been proposed to reduce it [57]. Wang et al. [58] carried out the two-step
catalyzed process of biodiesel production from WCO because it contains more free
fatty acids. The conversion rate of free fatty acids reached 97.22% for a reaction sys-
tem containing methanol to triglyceride ratio of 10:1 with 2wt% ferric sulfate at 95°C
for 4h followed by transesterification of remaining triglyceride at 65°C in a reaction
time of 60min for methanol to triglyceride ratio of 6:1 and 1% (wt of oil) KOH cat-
alyst. A final biodiesel conversion around 97% has been obtained. Some of the prop-
2
erties of WCO biodiesel include a kinematic viscosity of 5.3mm /s, a cetane number
of 54, flash point of 196°C, and a pour point of 11°C [58]. The fatty acid profile of
WCO and used cooking oil is presented in Table 4.3.

4.2.3 Soapstock as feedstock for biodiesel

The addition of water and alkali during the refining process causes the precipitation of
a semisolid material known as soapstock. The soapstock contains fatty acids, triglyc-
erides, mono and diglycerides, phosphoglycerides, pigments, some of the lesser com-
ponents of crude oil, and water [59]. A substantial amount of water is present in the
soapstock, which can be emulsified with the lipid constituent and is difficult to
remove. The transesterification becomes more complicated due to the presence of free
fatty acids and acyl glycerol. Because soapstock has high free fatty acids, alkali catal-
ysis cannot be utilized. The biodiesel production from soapstock involves a high-
efficiency method that involves two steps. Alkaline hydrolysis of all lipid-linked fatty
acid ester bonds takes place in the first step and in the second step, the acid-catalyzed
transesterification of the resulting fatty acid sodium salt is carried out [11]. Park et al.
[60] carried out work on production of biodiesel from soybean soapstock in which the
soapstock was converted to high-acid acid oil by KOH hydrolysis. This was followed
by transesterification, resulting in fatty acid methyl esters with conversion of 91.7%
for soap stock and 81.7% for acid oil using amberlyst-15 catalyst for a methanol to oil
ratio of 6:1 and also 9:1. In biodiesel production using a lipase catalyst carried out by
Su and Wei [59], the maximum conversion of 95.2% was obtained for a methanol to
oil molar ratio of 5:1 in a reaction time of 10h and at a reaction temperature of 45°C.
The fatty acid profile of soybean soapstock is presented in Table 4.3.

4.2.4 Grease as feedstock for biodiesel

Grease is another low-cost feedstock used for the production of biodiesel and it con-
tains triglycerides, diglycerides, monoglycerides, and free fatty acids [11]. If the
grease contains free fatty acid levels <15%, it is known as yellow grease. Grease con-
taining free fatty acid levels >15% is known as brown grease. Because of the presence
of more free fatty acids, the transesterification of grease is complicated, just like
soapstock and WCO [19]. As per Issariyakul et al. [61], the maximum conversion