Biomedical Engineering Reference
In-Depth Information
cases, there is not a concrete optimal temperature, but instead, the highest temperature in the
investigated range was considered the best; 65 and 70
°
C respectively. Still, a more common
reaction temperature used is 60 °C. [11, 14] This is still reasonable since the conversion
differences between 60 and 70
°
C is very small. In general, using a range of 50 to 70
°
C is
recommended. While higher temperatures result in slightly better conversions, using a lower
temperature would save in operating costs. Newer processes propose utilizing supercritical
conditions where temperatures are above 280°C. These proceses will be discussed later.
2.3. Improving Miscibility
A constant issue with transesterification is that fat and oil do not mix with alcohol. The
reaction is thus in two-phases and is mass transfer limited. [9] There are three methods for
improving miscibility: adding a cosolvent, mixing during the reaction, and using a membrane
reactor. A common cosolvent used it tetrahydrofuran (THF).
2.4. Alcohol Reagent
Methanol, isopropanol, butanol, and ethanol are some of the alcohols that can be used for
the transesterification. Methanol is the most frequently used alcohol because of its lower cost
and smaller molecular mass, which means less material is used up in relation to the amount of
esters produced. Because three moles of alcohol is required for the reaction, regardless of
which alcohol, it ends up being cheaper to use methanol (as shown in Table 3)
Furthermore, FAMEs, produced using methanol, are considered to have the proper
characteristics as an ideal replacement of petrodiesel fuel, including viscosity, boiling point,
and cetane number. [15] The importance of these characteristics is explained later. Alcohol
choice can depend on the type of catalyst used. Tamalampudi et al. showed that methanol is
best for use in enzyme catalyzed production of biodiesel with the Jatropha oil feedstock [16].
It was hypothesized that low molecular weight and high polarity of methanol may allow it to
more easily diffuse and access the enzyme [16].
Table 3. Comparison of the cost of methanol and ethanol (based on price of 2004)
Alcohol
Cost/gallon
Gram moles/gallon
Cost/gram-mole
Methanol
US$ 0.61
93.56
US$ 0.0065
Ethanol
US$ 1.45
64.82
US$ 0.0224
However, a common downside of biodiesel from methanol is the high cloud point, which
makes it less desirable in colder climates. Work has shown that cloud point and pour point
can be lowered when longer chain alcohols are used instead of methanol [17], as shown in
Table 4. In conventional base-catalyzed methods, absolute ethanol (99.9%) must be used to
limit water content that would neutralize the catalyst. Absolute ethanol is also very
hygroscopic, or absorbs and retains water, so is not commonly used in industry [18]. Ethanol
forms an azeotrope with water, so it is difficult to remove and recycle [19]. Therefore, though
methanol is more toxic, it is considered better overall.
