Page 302 - Advances in Eco-Fuels for a Sustainable Environment

Ecofuel conversion technology of inedible lipid feedstocks to renewable fuel 259

biodiesel based on dried spent coffee grounds under optimal conditions of immersion
in 20wt% H 2 SO 4 solution at 70°C and transesterification for 12h. The achieved yield
was quite close to the conventional process of oil extraction and two-step acid-base
(trans)esterification that was found to be able to produce 17.32wt% biodiesel based
on dried spent coffee grounds.
Biodiesel production without a catalyst has been proven as a promising process
requiring fewer unit operations. To serve this purpose, the supercritical state of meth-
anol/ethanol is used. The main features of this method are the higher rates of ester
formation by better solubility of oils in the methanol/ethanol, a higher reaction rate
at high temperature, and its ability to process feedstock with high FFA and water con-
tent. Those features come with the fact that the high thermal and potential energy of
pressure supplied into the system has disrupted interaction between polar molecules,
particularly alcohols as well as the possible formation of acidified methanol and
methoxide ion from the methanol-water interaction, which is similar to the acid catal-
ysis mechanism [106]. Better oil solubility and reaction rate offer occurrence of simul-
taneous extraction and (trans)esterification could have the potential to significantly
reduce process cost.
In 2011, Calixto et al. reported supercritical methanol in situ biodiesel production
from spent coffee grounds [107]. They found that the CO 2 presence in the reactor had
certain influence on lipid solubility in methanol, depending on the temperature and
pressure used. At 300°C and 10.0MPa, where oil and methanol were completely mis-
cible, the introduction of CO 2 to reach a pressure of 20.0MPa altered the lipid-rich
phase from the lipid-methanol phase into the lipid-CO 2 phase. It was demonstrated
that at 300°C, 10.0MPa, and a CO 2 /methanol molar ratio of 0.11, a biodiesel yield
of 93.4% can be obtained, which was considerably higher than the 84.9% yield
achieved by that of a pure methanol system at 30.0MPa and 330°C.
Kwon et al. demonstrated that water contained in oil interacted more with methanol
than oil [55]. Such interaction provides an advantage by giving high tolerance to crude
lipids with high water content. In this study, supercritical methanol showed a constant
biodiesel yield around 97.5wt% even at 5wt% water content, in contrast to the yield of
the acid-catalyzed process that dropped to about 76wt% due to the presence of just
1wt% of water in oil. Biodiesel conversion was performed continuously in a
charcoal-packed reactor with the volumetric flow rate of the spent coffee oil, alcohol,
and CO 2 at 10, 3, and 100mL/min, respectively. The optimum reaction condition was
at 430°C and the oil to alcohol (methanol or ethanol) weight ratio of 2.5.
The studies discussed above demonstrated that reaction behavior (i.e., catalysis and
the reaction mechanism) of spent coffee oil (trans)esterification was pretty similar to
the other oil feedstocks used to produce biodiesel. Special remarks that appeared in the
biodiesel production from spent coffee oil are the presence of the unsaponifiable part
and the remaining caffeine in the extracted oil, as shown in Fig. 9.1, which may reduce
the attainable amount of biodiesel and the emission quality of the produced biodiesel.
The unsaponifiable part in spent coffee oil was identified as cyclic terpenes, namely
cafestol and kahweol [50]. Jenkins et al. found that those compounds appeared as
dark-blue precipitate in the acid-catalyzed process [48, 50]. It was possible that the
natural ester of those compounds in coffee lipid was broken down by heat and the acid