Refined edible oils with less than 1 wt% free fatty acids (FFA) are
mainly used as feedstocks for industrial biodiesel production and
the feedstock price is one of the most significant factors affecting
the economic viability of fuel production [1–4]. The byproducts,
such as soapstock, acid oils and deodorized distillates, obtained
during the refining of edible oils have been suggested as alternative
and cheaper feedstocks for biodiesel [5–7]. Especially, the crude oils
of palm and rice bran have a high content of FFA and the amount of
acid oils discharged during the refining of these edible oils, is large,
at ca. 10–20 wt% refined oil output, e.g. 5.6–11 million tons for
palm. The main components of acid oils are FFA, remainder comprising
triglycerides, and hence the oils are almost solid at room
temperature owing to the higher melting points of FFA. Such oils
with a high FFA content could not be used as feedstocks using conventional
processing methods, using homogeneous alkali catalysts.
Many researchers have studied biodiesel production from the
waste acid oils [8–11]. One of the methods proposed is a two-step
process, involving a homogeneously acid catalyzed esterification of
FFA followed by the homogeneous alkali catalyzed transesterification
of triglycerides [12–17]. This process (Fig. 1(a)) can smoothly
convert both FFA and triglycerides present in the acid oil to biodiesel.
However, drying/dewatering of the oil and the addition of
excess alcohol (10–20 mol eq FFA) are required to shift the equilibrium
toward the product because of the reversible nature of the
esterification reaction. Subsequently, additional operations are
required to remove the acid catalyst and the byproduct water
and to add excess alcohol for transesterification. The additional
steps result in increased production costs. There is no commercially
feasible process to convert these acid oils to biodiesel, which
fully satisfies international specifications.
We have proposed a simple continuous production process for
obtaining biodiesel from oils with high content of FFA [18]. In
the process (Fig. 1(b)), the expanded-bed reactor was packed with
a cation-exchange resin catalyst for esterification of FFA and a
second reactor loaded with an anion-exchange resin catalyst for