Environmental Engineering Reference
In-Depth Information
The most popular biodiesel production process is
transesterification
(produc-
tion of the ester) of vegetable oils or animal fats, using alcohol in the presence of a
chemical catalyst. About 3.4 kg of oil/fat are required for each gallon of biodiesel
produced (Baize, 2006). The transesterification of degummed soybean oil produces
ester and glycerin. The reaction requires heat and a strong base catalyst such as
sodium hydroxide or potassium hydroxide. The simplified transesterification reac-
tion is shown below:
Triglycerides + Free fatty acids (<4%) + Alcohol → Alkyl esters + Glycerin
Some feedstocks must be pretreated before they can go through the transesteri-
fication process. Feedstocks with less than 4% free fatty acids, which include veg-
etable oils and some food-grade animal fats, do not require pretreatment. Feedstocks
with more than 4% free fatty acids, which include inedible animal fats and recycled
greases, must be pretreated in an acid esterification process. In this step, the feed-
stock is reacted with an alcohol (such as methanol) in the presence of a strong acid
catalyst (sulfuric acid), converting the free fatty acids into biodiesel. The remaining
triglycerides are converted to biodiesel in the transesterification reaction:
Triglycerides + Free fatty acids (>4%) + Alcohol → Alkyl esters + Triglycerides
Figure 5.7
i
illustrates the basic technology for processing vegetable oils (such as
soybeans) and recycled greases (used cooking oil and animal fat). When the feed-
stock is vegetable oil, the extracted oil is processed to remove all traces of water, dirt,
and other contaminants. Free fatty acids are also removed. A combination of methyl
alcohol and a catalyst, usually sodium hydroxide or potassium hydroxide, breaks the
oil molecules apart in the esterification process. The resulting esters are then refined
into usable biodiesel.
When the feedstock is used-up cooking oil and animal fats refined to produce
biodiesel, the process is similar to the way biodiesel is derived from vegetable oil,
except there is an additional step involved (
Figure 5.7
). Methyl alcohol and sulfur
are used in a process called
dilute acid esterification
to obtain a substance resem-
bling fresh vegetable oil, which is then processed in the same way as vegetable oil to
obtain the final product.
a
lgae
to
B
iodiesel
The focus of our discussion of biofuels to this point has been on terrestrial sources
of biomass fuels. This section is concerned with photosynthetic organisms that
grow in aquatic environments—namely, macroalgae, microalgae, and emergents.
Macroalgae, more commonly known as seaweed, are fast-growing marine and fresh-
water plants that can grow to considerable size (up to 60 m in length). Emergents are
plants that grow partially submerged in bogs and marshes. Microalgae are, as the
name suggests, microscopic photosynthetic organisms. With regard to the mass pro-
duction of oil for alternative renewable energy purposes, the focus of this discussion
is mainly on microalgae—organisms capable of photosynthesis that are less than
