To study the effect of acid addition on the transesterification reaction
under SCW condition, 0.25% or 0.5% sulfuric acid (% of total reaction
mixture) was added to the reaction mixture containing 1.0 g
biomass, 0.50 mL water, and 30 mL methanol. The mixture in the reactor
was then heated to 175 °C for 2 h. As shown in Table 4, by
adding a small (0.25%) amount of sulfuric acid, a FAME content of
92.82% in the reaction product can be obtained in 2 h. This FAME content
is about the same as the FAME content obtained (92.18%) without
adding sulfuric acid; however, at a much longer reaction time
(8 h) as shown in Table 3.
Biodiesel synthesis from soybean oil was investigated under supercritical
condition and base-catalyzed subcritical condition of
methanol [21]. Under supercritical condition, a maximum methyl
ester yield of over 98% was obtained. At 160 °C and a methanol to
oil ratio of 24, with 0.1 wt.% potassium hydroxide as the catalyst a
98% methyl esters yield was observed in 20 min. Compared to these