(289j) Solid Acid Catalyzed Esterification of Free Fatty Acids in Oil Using Co2 Enhanced Media

The potential
applications of alkyl esters include intermediates for value added products and
a renewable, biodegradable, and nontoxic diesel fuel substitute,
biodiesel[1-4].  The production of alkyl esters typically occurs via the
transesterification of vegetable oils with alcohols using alkaline catalysts in
a batch process.  Large scale use of alkyl esters, and biodiesel, is
currently prohibitively expensive due to costs associated with purifying the
oil feedstock [1,5].  Cheaper feedstocks are available, but the high free
fatty acid and water content lead to increased separation costs [6] and
decreased catalyst efficiency[7].

This paper
presents the results of exploiting dense CO₂-enahnced media to
esterify the free fatty acids in waste oil.  Recently, solid acid
catalysts have been reported as environmentally beneficial alternatives to
sulfuric acid for the esterification of palmitic acid in soybean oil using
methanol [8].  However, acid sites accessibility can be an issue depending
on the solid acid used.  In addition, supercritical methanol has been used
for the simultaneous methyl esterification of free fatty acids and
transesterification of triglycerides [9].  However, the high temperatures
and pressures required for supercritical methanol are cost prohibitive. 
The use of CO₂ to expand the reaction mixture can result in
decreased mass transfer limitations at milder pressures and temperatures than
supercritical methanol conditions.  In addition to the phase behavior
results presented, the effect of a) the CO₂-expanded media, b) the
incorporation of co-solvents such as hexane, c) the feedstock composition, and
d) the type of solid acid catalyst employed on the methyl ester yield is
discussed. 

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