Double-step esterification
to produce biodiesel from palm fatty
acid distillate (PFAD) was performed by utilizing an ultrasound clamp
reactor. Six pairs of ultrasonic clamps were attached to the left
and right sides of the stainless-steel tube, and each pair was separated
100 mm apart from each other. Therefore, a total of 12 units of ultrasound
clamps distributed 4800 W maximum power (12 × 400 W) throughout
the continuous reactor by an ultrasonic generator. To optimize each
step of the continuous esterification process for producing methyl
ester from PFAD, a response surface methodology was used. The final
93.32 wt % methyl ester purity was attained under a double-step esterification
process. For the first step, a 3.75:1 molar ratio of methanol to PFAD
(46.4 vol % methanol), 6.6 vol % sulfuric acid, and 400 mm length
of ultrasound clamp at 25 L/h PFAD flow rate for converting the PFAD
to 60.24 wt % methyl ester were recommended. For the second step,
the esterification was repeated under a molar ratio of methanol to
the first esterified oil of 2.87:1 (61.6 vol % methanol), 5.6 vol
% of sulfuric acid, and 400 mm length of ultrasound clamp at 25 L/h
esterified oil flow rate. The ultrasonic clamp reactor achieved high
yields of esterified oil and the crude biodiesel in a relatively short
residence period of 32 s. To determine the product yields of a double-step
esterification process, the maximum yields were 103.9 wt % first esterified
oil, 107.6% crude biodiesel, and 98 wt % purified biodiesel when calculated
on the basis of 100 vol % initial PFAD. The average energy consumed
in the production of double-step esterification biodiesel was 0.05796
kWh/L. Therefore, this current approach has a high potential for producing
biodiesel with less energy and requires less time to convert the PFAD
to a high purity of methyl ester.
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