Biofuels in Perspective

Production of Biodiesel from Waste Lipids 163
in the presence of a large excess of methanol, which drove the reaction to completion
(99 % in 4 h) was observed using an oil:methanol:acid ratio of 1:245:38 at 70 ◦ C and a
ratio range of 1:75:1.9–1:245:3.8 at 80 ◦ C. In these conditions also the FFA were rapidly
converted into esters in only a few minutes. Also diglycerides present in the initial oil
were rapidly transformed into FAME and very little monoglycerides were detected. For
large-scale industrial productions this procedure seems to be less suitable due to the high
excess of methanol which must be recovered.
Also waste palm oil has been transesterified in acid conditions (Al-Widyan and
Al-Shyaukh, 2002).
Sulphuric acid and different concentrations of hydrogen chloride and ethanol at different
levels of excess were used. Higher concentrations of catalyst (1.5–2.5 M) produced
biodiesel in a much shorter time and of a lower specific gravity. Sulphuric acid was a much
better catalyst than hydrogen chloride at 2.25 M.
Moreover, a 100 % excess of alcohol reduced the reaction time. The best process combination
was the use of 2.25 M H2SO4 with 100 % excess of ethanol ina3hperiod.
Due to its low cost, H2SO4 seems to be the best catalyst for the acid transesterification of
triglycerides in acid medium. The advantage is that simultaneously the FFA are converted
into esters. However, when the amount of FFA is too high a quantitative ester formation
can be prevented due to the water formation. A two-step reaction with water removal after
the first step enables the formation of biodiesel with an acceptable level of FFA.
A calcium and barium acetate catalyst was developed for the production of biodiesel using
feedstocks with high amounts of free fatty acids. A calcium and barium acetate catalyst
was developed (Basu and Norris, 1996). However, the process is carried out at 200–220 ◦ C
and pressures of 2.76–4.14 MPa. In addition, the biodiesel produced contains too high levels
of soaps and monoglycerides and the use of barium compounds is not environmentally
friendly.
Another study compares the use of KOH and calcium and barium acetate (Rose and
Norris, 2002). Using a methanol:oil ratio of 0.38, a mixture of 0.12 % barium acetate and
0.34 % calcium acetate at high temperature and high pressure during 2–3 hours reaction
time resulted in similar esters yields (85–95 %) as 0.01 %–2 % KOH (methanol/oil ratio
0.2–0.28) for a reaction time of 1–2 h.
Other catalysts are acetates and stearates of calcium, barium, magnesium, mangane,
cadmium, lead, zinc, cobalt and nickel (Di Serio et al., 2005). A ratio of oil:alcohol of 1:12
and a temperature of 200 ◦ C for 200 min was used. Stearates gave better results due to
the higher solubility than acetates in the lipophilic phase. These catalysts seem to be very
promising due to their higher performance at lower catalyst concentration than Brönsted
acids using lower alcohol to oil ratios and are less sensitive to the water content of the
feedstock.
It can be concluded that homogeneous acid catalysts are suitable for the conversion of
waste oils containing high levels of free fatty acids. However, the long reaction time, the
high ratio of alcohol to oil, high concentrations of catalysts, separation of the ester layer
and the extraction (washing) of the catalyst can jeopardize the economical conversion of
waste oils into biodiesel.
A recent report (Lotero et al., 2005) reported the use of a solid acid catalyst for the
conversion of high acidic oils into biodiesel and showed that these catalysts are performing
a simultaneous esterification of FFA and transesterification of the tri-, di- and