ORIGINAL PAPER
Effect of Ultrasonication on Droplet Size in Biodiesel Mixtures
Peng Wu Æ Ying Yang Æ Jose
´
A. Colucci Æ
Eric A. Grulke
Received: 12 April 2007 / Revised: 16 July 2007 / Accepted: 18 July 2007 / Published online: 7 August 2007
Ó AOCS 2007
Abstract Biodiesel fuels have become more attractive
recently because of their environmental benefits and cost
competitiveness compared to diesel fuel. Many processing
improvements have been proposed to increase the conver-
sion rates and the yields of vegetable oil in order to lower
production costs and improve biodiesel product quality. In
conventional biodiesel production chemistries, alkaline
transesterifications of alcohol/oil dispersions should occur
primarily near the interface. Ultrasonic mixing has already
been shown to increase overall conversion rates for alcohol/
vegetable oil mixtures. Our data show that ultrasonic
mixing produced smaller droplet sizes than conventional
agitation, leading to more interfacial area for the reaction to
occur. Droplet size distributions have been measured for
conventional impeller and ultrasonic mixing systems using
methanol/soybean oil as a model system. The dispersions
were stabilized by surfactant in order to obtain droplet size
distribution for mixture samples. Ultrasonic mixing pro-
duced dispersions with average droplet sizes 42% smaller
than those generated using standard impellers.
Keywords Droplet size distribution Á Dispersions Á
Emulsion stability Á Mechanical agitator Á Ultrasound
Introduction
Biodiesel Processes
Biodiesel, a mixture of alcohol esters of long chain fatty
acids, is biodegradable and non-toxic. It is a diesel fuel
supplement or alternative due to its environmental
benefits [1]. Biodiesel is made by transesterification of
vegetable oils, fats and greases with short chain alcohols.
Acids, bases or enzymes can be used as catalysts. Acid
catalysis is preferred if there are significant quantities
of free fatty acids in the feed stock, but the conversion
rate is low and high levels of alcohols are needed to
force the reaction equilibrium toward the transesterified
products [2]. Ester conversions of 95.1 and 99.7% [3]
have been reported. Lipase can catalyze 98.4% conver-
sion to biodiesel esters [3] but is expensive. Soybean oil
is the most common source for biodiesel production
because the cetane number for biodiesel made from
soybean oil is higher than other oils [3]. Due to the low
cost of methanol and the high rates of base-catalyzed
transesterifications, most commercial biodiesel is made
by the alkali-catalyzed reaction of soybean oil with
methanol.
Alkali Transesterification
Alkali-catalyzed transesterification requires a low molar
ratio of alcohol to oil (6:1) plus low catalyst levels, and
achieves high conversions at low batch times [4]. There are
several disadvantages of this process. Feedstocks with high
levels of free fatty acids (particularly recycled oils and
greases) tend to produce soap byproducts with the alkali
catalysts, reducing conversions, making separations more
P. Wu Á Y. Yang Á E. A. Grulke (&)
Department of Chemical and Materials Engineering,
University of Kentucky, 359 RG Anderson Building,
Lexington, KY 40506-0503, USA
e-mail: egrulke@engr.uky.edu
J. A. Colucci
Department of Chemical Engineering,
University of Puerto Rico, Mayagu
¨
ez Campus,
Mayagu
¨
ez, PR 00681, USA
123
J Am Oil Chem Soc (2007) 84:877–884
DOI 10.1007/s11746-007-1114-9