Page 195 - Biofuels Refining and Performance

178 Chapter Six

mechanism. The soap forms emulsions and makes recovery of methyl
esters (ME) difficult. Saponification consumes the base catalyst and
reduces product yields. The use of alkaline catalysts requires that the
oil reagent be dry and contain less than about 0.3 wt% FFA [27, 28].
Acid catalysts can handle large amounts of FFA and are commonly
used to esterify FFA in fat or oil feedstock prior to base-catalyzed FFA
alcoholysis to ME [29]. Though it solves FFA problems, it adds additional
reaction and cleanup steps that increase batch times, catalyst cost, and
waste generation.
Generally, acid-catalyzed methanolysis of TG is carried out at tem-
peratures at or below that of methanol reflux (65 C). Using sulfuric acid
catalysis under reflux conditions, Harrington and D’Arcy-Evans [30]
first explored the feasibility of in situ transesterification, using homog-
enized whole sunflower seeds as a substrate. Using reflux conditions, a
560-fold molar excess of methanol and a 12-fold molar excess of sulfu-
ric acid relative to the number of moles of triacylglycerol (TAG) were
used. They observed ester production, with yields up to 20% greater than
in the transesterification of preextracted oil, and suggested that this was
an effect of the water content of the seeds, an increased extractability
of some seed lipids under acidic conditions, and also the transesterifica-
tion of seed-hull lipids.
Stern et al. [31] have developed a process to prepare ethyl esters for
use as a diesel fuel substitute from various vegetable oils using hydrated
ethyl alcohol and crude vegetable oil, with sulfuric acid as a catalyst.
Ethyl ester of 98% purity with a very low acidity has been reported.
Schwab et al. [32] have compared acid and base catalysts and con-
firmed that, although base catalysts performed well at lower tempera-
tures, acid catalysis requires higher temperatures. Liu [33] has
compared the influence of acid and base catalysts on yield and purity
of the product, and suggested that an acid catalyst is more effective for
alcoholysis if the vegetable oil contains more than 1% FFA.
Goff et al. [34] have conducted acid-catalyzed alcoholysis of soybean
oil using sulfuric, hydrochloric, formic, acetic, and nitric acids, which were
evaluated at 0.1 and 1 wt% loadings at temperatures of 100 C and 120 C
in sealed ampoules, and observed sulfuric acid was effective. Kinetic
studies at 100 C with 0.5 wt% sulfuric acid catalyst and 9 times methanol
stoichiometry provided more than 99 wt% conversion of TG in 8 h, and
with less than 0.8 wt% FFA concentration in less than 4 h (see Fig. 6.12).
Base catalysts are generally preferred to acid catalysts because they
lead to faster reactions [35]. Base catalysts generally used in transes-
terification reactions are NaOH, KOH, and their alkoxides. KOH is pre-
ferred to other bases because the end reaction mixture can be neutralized
with phosphoric acid, which produces potassium phosphate, a well-known
fertilizer [36].