Biomedical Engineering Reference
In-Depth Information
enzyme, solid metal catalyst, and solid super base. Each type has its advantages and
disadvantages which are discussed below.
Acid Catalysis
The acid catalyst acts by donating a proton to the carbonyl group which increases the
ester activity [21]. This protonation makes the oxygen in the carbonyl group positively
charged and open to receiving electrons from the alcohol. The advantages of using acid are
that it does not produce the saponification side reaction that base does. The soap produced
from saponification causes difficult separation downstream. Acid's downside is that the
reaction rate is much slower than using base [22, 23]. Conversion is also highly controlled by
water and fatty acid content; less of both lead to high conversions [18]. Therefore, acid
catalysts are generally only used for pretreatment in an esterification reaction [19] as shown
below:
Common acids used in this process are sulfuric acid and phosphoric acid [19], but acids
are not used commercially as of 2003 [7].
Base Catalysis
Conversely, base catalysts act on the alcohol by deprotonating it. This allows it to more
readily react with the feedstock [21]. Base-catalyzed reactions occur up to 4000 times faster
than acid-catalyzed and the base is less corrosive to materials [22, 16]. In base-catalyzed
reactions, its typical to use 6:1 mole ratio of alcohol to triglyceride in order to drive the
reaction forward [19]. Since bases will react with water and FFA in a saponification reaction
as shown below, base catalysts are frequently used when vegetable oil is the feedstock as
there is less FFA and water content [19]. If there is greater than 1% FFA content,
pretreatment is required [19]. Because of the saponification reaction, it is important to
carefully analyze how much base to use. Base catalysis has the same conversion dependence
as acid [18]. To use the best of both types of catalysis, acid-base catalysis is often used. This
takes advantage of the very fast reaction rate of base-catalysis while reducing saponification
with acid. This occurs in a two step process. First, the acid catalyst is used to convert the free
fatty acids to esters which decreases the free fatty acid content (preferably to 1%)[21]. Free
fatty acids are what react with the base to produce soaps, so reduction in its content will
likewise reduce the amount of soap produced. The feedstock is then base-catalyzed for
transesterification.
