Environmental Engineering Reference
In-Depth Information
2.3 Comparison of Technologies
A conventional base-catalyzed reaction is used in the majority of transesterifica-
tion processes to produce biodiesel. Sodium hydroxide is used as the catalyst when
methanol is the acyl acceptor, and potassium hydroxide is used when ethanol is the
acyl acceptor, due to solubility considerations [15]. The ethyl esters have a slightly
higher energy value than the methyl esters due to the presence of the additional
carbon atom, and ethanol can be more easily produced from renewable sources,
such as corn. Typical reactions take place with a high molar ratio of alcohol to oil
of about 6:1 with methanol, and 12:1 for ethanol [15]. The excess alcohol allows
for complete conversion of the triglycerides to the fatty acid esters. An advantage
of base-catalyzed transesterification is the relatively short reaction time to achieve
conversion levels of 98% or greater, compared to other processes. The reaction is a
direct process, needing no intermediate steps, and operates at a relatively low tem-
perature and pressure of about 66 C and 1.4 atm, respectively. However, a major
disadvantage of the base catalyzed process is the formation of soap when water or
free fatty acids are present in the feedstock. Thus the feedstock should be anhy-
drous but the process still requires a large amount of base to be added to neutralize
the fatty acids [15]. Soap formation results in additional downstream separation
problems combined with a reduction in the fatty acid ester yield. The process also
requires two steps and uses large amounts of chemicals as catalysts.
Acid-catalyzed transesterification is a viable alternative, in which sulfuric acid
is typically used. One advantage over the base-catalyzed method [26] is that it is
not as susceptible to soap formation. The resulting downstream product is easily
separated and produces a relatively high quality glycerol byproduct. The process
also requires only one step, compared to two steps in the base-catalyzed process.
However, acid-catalysis reactions are slower and result in lower yields than base-
catalysis, ranging from 56.8 to 96.4% depending on the feedstock [27]. A major
disadvantage to either base or acid transesterification process is the disposal of the
glycerol byproduct. Glycerin is already inexpensive, easily available, and is used
in a wide array of pharmaceutical formulations. The major issue is with the purity
of the glycerin; the byproduct glycerin from the production of biodiesel is 80-88%
while industrial grade is 98% or higher [15]. The low market value of glycerin does
not make purification economical. Many researchers are investigating innovative
chemical and biological processes for the conversion of glycerin into value-added
products including antifreeze agents, hydrogen, and ethanol [28].
A relatively new and promising development in the production of biodiesel is via
enzymatic transesterification with lipase as the catalyst. Several microbial strains
of lipases have been found to have transesterification activity; Pseudomonas cepa-
cia [29], Thermomyces lanuginosus [30], and Candida antarctica [31] are a few
that have been reported. The products of an enzyme-catalyzed reaction can easily
be collected and separated. Unlike alkali-based reactions, enzymes can be recycled
since they are not used up and require much less alcohol to perform the reaction.
However, enzyme reactions take much longer to complete and can have lower yields
due to inhibition of the enzyme caused by glycerol formation. Methanol, the acyl
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