Environmental Engineering Reference
In-Depth Information
combustion leading to carbon deposition, clogging, and sticking of moving parts inherent to
use of unmodified vegetable oils persisted [1, 3, 7].
2.2.3. Transesterification
. The transesterification of triglyceride fatty acids with alcohols,
yielding esters (Figure 14), has emerged as the technology that yields products most similar to
conventional diesel fuel in chemical and combustion characteristics (Table 3). This is a
stepwise process, esterifying one fatty acid at a time, that ultimately yields fatty acid esters as
well as glycerol. Because each step is reversible, the product yield is enhanced greatly by
providing an excess of alcohols; in fact, the molar ratio of alcohols to triglycerides is one of
the most important parameters in the process. While greater proportions of alcohol favor more
rapid and extensive reaction, they also create a greater volume of alkali waste to be treated;
optimal ratios have therefore been reported ranging from 6:1 to 30:1, varying with the nature
of the fat or oil and alcohols in use. The optimization of the alcohol : triglyceride molar ratio
is also a key challenge for enzymatic catalysis [1, 7, 18].
Table 3. Physical and chemical properties of biodiesel in comparison to petroleum-
derived diesel fuel. Adapted from [1]
Vegetable oil
methyl ester
Kinematic
viscosity
(mm2/s)
Cetane
number
Lower
heating
value
(MJ/l)
Cloud
point
(°C)
Flash
point
(°C)
Density
(g/l)
Sulfur
(wt
percent)
Peanut
4.9 (37.8°C)
54
33.6
5
176
0.883
--
Soybean
4.5 (37.8°C)
45
33.5
1
178
0.885
--
Soybean
4.0 (40°C)
45.7-56
32.7
0.880
--
--
--
(15°C)
Babassu
3.6 (37.8°C)
63
31.8
4
127
0.879
--
Palm
5.7 (37.8°C)
62
33.5
13
164
0.880
--
Palm
4.3-4.5 (4O°C)
64.3-70
32.4
0.872-0.877
--
--
--
(15°C)
Sunflower
4.6 (37.8°C)
49
33.5
1
183
0.860
--
Tallow
12
96
--
--
--
--
--
Rapeseed
4.2 (40°C)
51-59.7
32.8
--
--
0.882
--
(15°C)
Used rapeseed
9.48 (30°C)
53
36.7
192
0.895
0.002
--
Used
corn oil
6.23 (30°C)
63.9
42.3
166
0.884
0.0013
--
Diesel fuel
12-3.5 (40°C)
51
35.5
--
--
0.830-0.840
--
(15°C)
JIS-2D (Gas
oil)
2.8 (30°C)
58
42.7
--59
0.833
0.05
Transesterification can be accomplished at a variety of temperatures, with temperature
optima depending on the oils involved; process temperatures of 25-100°C are common [7].
Catalysis of the process is essential (except in supercritical solvents) and may be
accomplished by acids, bases, or enzymes: alkali catalysis employing NaOH, KOH, or a
corresponding alkoxide is typically the most rapid and efficient in fats and oils with extremely
low-water contents and low concentrations of free fatty acids [1, 7]. While this process is the
