Environmental Engineering Reference
In-Depth Information
CH 2 -OOC-R 1
R 1 -OOCCH 3
R 2 -OOCCH 3
CH 2 -OH
Catalyzer
CH-OOC-R 2
+ 3CH 3 -OH
+
CH-OH
CH 2 -OOC-R 3
R 3 -OOCCH 3
CH 2 -OH
Figure 14.4 Production of biodiesel by transesterification of triglycerides with methanol.
and hydrolysis, side reactions produce significant amounts of hydroxymethylfurfural (HMF),
from glucose, and furfural from xylose. Additionally, other undesirable compounds develop
including extractives, organic acids, and phenolic compounds. Therefore, hydrolyzates need a
detoxification step before fermentation to eliminate these compounds that are inhibitors of
microorganisms. Detoxification can be performed by physical, chemical, and biological
methods, and each one has a different degree of elimination of toxic compounds (Sánchez and
Cardona, 2008).
As can be seen, conversion of lignocellulosic materials into bioethanol is not a straight-
forward process. To make glucose contained in cellulose available to microorganisms, the
amount of energy needed makes this process not currently economically feasible. Even
when cellulosic materials are more abundant and inexpensive than sugary or starchy feed-
stocks, the conversion process makes ethanol via this route more expensive than traditional
counterpart.
Biodiesel
Biodiesel is a fuel made by transesterification of fatty acids with an alcohol, typically metha-
nol. Triglycerides contained in vegetable oils or animal fats are combined with methanol via a
transesterification reaction catalyzed by bases or enzymes. The end products of this reaction
are alkyl esters (biodiesel) and glycerol (Fig. 14.4). Commercial biodiesel is currently pro-
duced using the catalysis with an alkali route.
When feedstock fats contain high levels of free fatty acids (more than 4 percent), fats are
pretreated with diluted acid and methanol to convert the free fatty acids into methanol esters
(biodiesel) via acid esterification. The remaining triglycerides are then converted into bio-
diesel via normal transesterification.
Transesterification is conducted by dissolving first the catalyst, potassium hydroxide,
into methanol and adding it into the oil or pretreated fat. Extra base is added to neutralize
the acid if the fat went through pretreatment (Fig. 14.5). At the end of the reaction, the
products separate into two layers. The light layer contains the biodiesel and the heavier a
mixture of glycerol, unreacted methanol, and the catalyst. Methanol can be recovered and
reused. Before use, biodiesel needs a refining process to eliminate residual methanol, catalyst,
and soaps.
Theoretically, pure biodiesel (B100) can fuel diesel engines without modifications.
However, fuel lines and rubber seals sometimes need replacement. In addition, biodiesel
tends to increase its viscosity as ambient temperature decreases, so cold weathers requires the
addition of a fuel heating system. To avoid some of the problems with B100, different degrees
of blending of biodiesel with petroleum diesel are a common practice. The most common
blend is 20 percent biodiesel and 80 percent petroleum diesel or B20, but other blends are
possible.
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