Environmental Engineering Reference
In-Depth Information
TABLE 5.9
Biodiesel Physical Characteristics
Specific gravity
0.87 to 0.89
Kinematic viscosity at 40°C
3.7 to 5.8
Cetane number
46 to 70
Higher heating value (Btu/lb)
16,928 to 17,996
Sulfur (wt%)
0.0 to 0.0024
Cloud point (°C)
-11 to 16
Pour point (°C)
-15 to 13
Iodine number
60 to 135
Lower heating value (lb/lb)
15,700 to 16,735
•
Acid esterification
—Oil feedstocks containing more than 4% free fatty
acids go through an acid esterification process to increase the yield of bio-
diesel. These feedstocks are filtered and preprocessed to remove water and
contaminants and are then fed to the acid esterification process. The cata-
lyst, sulfuric acid, is dissolved in methanol and then mixed with the pre-
treated oil. The mixture is heated and stirred, and the free fatty acids are
converted to biodiesel. Once the reaction is complete, it is dewatered and
then fed to the transesterification process.
•
Transesteriication
—Oil feedstocks containing less than 4% free fatty acids
are filtered and preprocessed to remove water and contaminants and then
fed directly to the transesterification process along with any products of the
acid esterification process. The catalyst, potassium hydroxide, is dissolved
in methanol and then mixed with the pretreated oil. If an acid esterification
process is used, then extra base catalyst must be added to neutralize the acid
added in that step. Once the reaction is complete, the major co-products,
biodiesel and glycerin, are separated into two layers.
•
Methanol recovery
—The methanol is typically removed after the biodiesel
and glycerin have been separated to prevent the reaction from reversing
itself. The methanol is cleaned and recycled back to the beginning of the
process.
•
Biodiesel reining
—Once separated from the glycerin, the biodiesel goes
through a clean-up or purification process to remove excess alcohol, resid-
ual catalyst, and soaps. This consists of one or more washings with clear
water. It is then dried and sent to storage. Sometimes the biodiesel goes
through an additional distillation step to produce a colorless, odorless, zero-
sulfur biodiesel.
•
Glycerin reining
—The glycerin byproduct contains unreacted catalyst and
soaps that are neutralized with an acid. Water and alcohol are removed to
produce 50 to 80% crude glycerin. The remaining contaminants include
unreacted fats and oils. In large biodiesel plants, the glycerin can be further
purified, to 99% or higher purity, for sale to the pharmaceutical and cos-
metic industries.
