Biodiesel From Conventional Feedstocks - Biotechnology in China: Biofuels and Bioenergy - page 58

carbonate. The traditional production process is the use of alkali metal hydroxide
as the homogeneous catalyst, which has a higher solubility in methanol.
Generally speaking, with homogeneous alkali as the catalyst for biodiesel
production, control for water content and free fatty acids (FFA) contained in raw
materials is required. It has been demonstrated extensively that when the moisture
content of raw materials is more than 0.06% and acid content is more than 1 mg/g
oil, the saponification reaction occurs, resulting in emulsification, which weakens
catalyst activity and reduces biodiesel yield [ 9 - 12 ].
Many Chinese scholars have studied soybean oil, cottonseed oil, rapeseed oil,
rice bran oil, sunflower oil, fried oil, and a variety of waste oil and other fats and oils
as raw materials with alkali as the catalyst for biodiesel production. Their studies
have also shown that, when the raw material contains water and FFA, saponification
occurs, not only consuming the alkali catalyst, but also reducing the reaction rate
and leading to difficulties in separating the by-product glycerol [ 9 - 12 ]. Therefore,
the current homogeneous alkali-mediated biodiesel preparation is mainly limited
to academic research in China. In fact, since only low-quality oil feedstocks
are currently available for biodiesel production, the use of the homogeneous alkali-
mediated process for preparing biodiesel on a large scale is not realistic in China.
3.1.2 Homogeneous Acid-Catalyzed Reaction
The homogeneous acids used for biodiesel production are mainly concentrated
sulfuric acid, phosphoric acid, and other inorganic acids. Due to the fact that
concentrated sulfuric acid is cheap and abundant, it is the most commonly used
homogeneous acid catalyst at present in China. However, this inorganic acid leads
to serious issues such as severe corrosion of the equipment and environmental
concerns. Since low-quality oils, which usually contains large amount of water and
FFA, are the main oil feedstocks in China, alkali-based technology cannot be
adopted for biodiesel production [ 13 - 15 ]. Therefore, nowadays, vast numbers of
small and medium enterprises adopt the acid-catalyzed technology to produce
biodiesel, leading to more waste, a more complicated production process, and
other issues. These consequences cast uncertainty over the future of the acid-
catalyed process for biodiesel production.
3.1.3 Two-Step Catalytic Process With Acid and Alkali
Some enterprises in China use the acid-alkali two-step catalytic process for
biodiesel production, in which pre-esterification with an acid catalyst is conducted
prior to alkali catalyst-mediated transesterification [ 16 , 17 ]. Although this kind of
acid- and alkali-catalyzed two-step catalytic process can be used with different
oil feedstocks for biodiesel production, the complexity of the procedures and
operations diminishes its economic and environmental benefits.