Biofuels in Perspective

162 Biofuels
due to the presence of K2SO4 which can be used as fertilizer. In general, sodium salts are
cheaper than potassium salts.
Although they are more expensive, NaOMe and KOMe are also used, which have the
advantage of greater safety, easier handling, better layer separation and purer glycerol
(Markolwitz, 2004).
A typical transesterification reaction using waste and crude oil with less than 3 % FFA
involves a six times molar excess of alcohol (in most cases methanol), 1–1.5 % (weight %)
of catalyst at 20–65 ◦ C for 30–90 minutes under vigorous stirring. As transesterification is
a reversible reaction, the reaction is carried out in two steps, in many cases the first step
yields a 75 % conversion into methyl esters.
The glycerol layer which contains residual alcohol, catalyst and soap is separated and
fresh methanol and catalyst is added.
A two-step procedure gives normally a yield of 95–98 %. In a one-step procedure more
catalyst is used. The reaction can be carried out either in a batch or continuous system.
According to this procedure, biodiesel is obtained fulfilling the EU standards.
Several studies have been carried out by various research groups to determine the ratio
oil/methanol/catalyst in order to produce biodiesel (see Section 9.3.2.7).
9.3.2.7 Production of Biodiesel by Acid-Catalysed Interesterification
The efficiency of the alkali-catalysed transesterification of waste oils is decreased in the
presence of water and FFA mainly due to the soap – of and/or monoglycerides formation
which is giving rise to poor separation of the ester and glycerol layers. Especially when the
FFA content is higher than 1.5 %, acid-catalysed transesterification should be considered.
On the other hand acid catalysis is much slower.
Acid catalysis involves the use of strong acids such as sulphuric acid and hydrogen
chloride. The transesterification of vegetable oils using sulphuric acid has been studied
by Canacki and Van Gerpen (1999). The most economical conditions involve the use of a
molar ratio of 20:1 methanol: oil; 3 % weight of sulphuric acid at 60 ◦ for 48 hours.
The percentage conversion is increased with higher amounts of methanol and catalyst,
and decreased dramatically in the presence of water with a conversion of 90 % for 0.5 %
water to 32 % when 3 % water was present. The effect of the presence of FFA is far less
dramatic: a level of 5 % FFA gives a 90 % conversion in comparison to a 75 % conversion
at a 20 % FFA content.
A comparison has been made of transesterification with methanol in the presence of
KOH and H2SO4 (Nye et al., 1983). The process using 0.1 % H2SO4 (65 ◦ /40 h) and 0–4 %
KOH (50 ◦ /24 h) gives rise to an ester yield of 79.3 % and 91.9 % for a 3.6:1 methanol:oil
ratio. However, the use of higher alcohols (ethanol-butanol) provided higher yields for the
acid catalysed transesterification.
The reaction kinetics of the acid-catalysed transesterification of waste frying oil in excess
of methanol to form FAME has been studied by Zheng et al. (2006). Rate of mixing, feed
composition (molar ratio oil:methanol:acid) and temperature were independent variables.
There was no significant difference in the yield of FAME when the rate of mixing was in
the turbulent range 100–600 rpm. The oil:methanol:acid molar ratios and the temperature
were the most significant factors affecting ester production. A pseudo-first-order reaction