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Ultrasonics Sonochemistry 22 (2015) 463–473

Contents lists available at ScienceDirect Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson

Mechanistic analysis of cavitation assisted transesterification on biodiesel characteristics

Baharak Sajjadi, A.R. Abdul Aziz ⇑, Shaliza Ibrahim

Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia

article info

Article history:

Received 6 December 2013

Received in revised form 2 June 2014 Accepted 8 June 2014

Available online 20 June 2014

Keywords:

Biodiesel Ultrasound Transesterification Mechanical stirring Biodiesel properties Cavitaion

1. Introduction

Among the biofuels currently in use or researched, Free Fatty Acid Methyl Esters (FAME), which is also known as biodiesel is con- sidered an ideal alternative to diesel due to their similarities and advantages [1]. Biodiesel is commonly generated through a three-step, consecutive and reversible reaction called ‘‘transesteri- fication’’. Low mass transfer due to immiscible nature of reactants is the main weakness of transesterification [2,3]. Recently, ultra- sound assisted transesterification has been confirmed as a green synthesis method that is fast and energy-efficient [4]. It is due to the ultrasound ability to enhance mass transfer between the immiscible reactants. In other words, ultrasound waves are sinu- soidal mechanistic waves consisting of both expansion (negative) and compression (positive) pressure waves. Hence, irradiation of ultrasound waves generates vacuum micro-regions in the liquid called ‘‘sonoluminescence bubble’’ that are filled with reactants

⇑ Corresponding author. Tel.: +60 379675300; fax: +60 379675319. E-mail address: azizraman@um.edu.my (A.R. Abdul Aziz).

http://dx.doi.org/10.1016/j.ultsonch.2014.06.004

1350-4177/Ó 2014 Elsevier B.V. All rights reserved.

abstract

The influence of sonoluminescence transesterification on biodiesel physicochemical properties was investigated and the results were compared to those of traditional mechanical stirring. This study was conducted to identify the mechanistic features of ultrasonication by coupling statistical analysis of the experiments into the simulation of cavitation bubble. Different combinations of operational variables were employed for alkali-catalysis transesterification of palm oil. The experimental results showed that transesterification with ultrasound irradiation could change the biodiesel density by about 0.3 kg/m3; the viscosity by 0.12 mm2/s; the pour point by about 1–2 °C and the flash point by 5 °C compared to the tra- ditional method. Furthermore, 93.84% of yield with alcohol to oil molar ratio of 6:1 could be achieved through ultrasound assisted transesterification within only 20 min. However, only 89.09% of reaction yield was obtained by traditional macro mixing/heating under the same condition. Based on the simu- lated oscillation velocity value, the cavitation phenomenon significantly contributed to generation of fine micro emulsion and was able to overcome mass transfer restriction. It was found that the sonolumines- cence bubbles reached the temperature of 758–713 K, pressure of 235.5–159.55 bar, oscillation velocity of 3.5–6.5 cm/s, and equilibrium radius of 17.9–13.7 times greater than its initial size under the ambient temperature of 50–64 °C at the moment of collapse. This showed that the sonoluminescence bubbles were in the condition in which the decomposition phenomena were activated and the reaction rate was accelerated together with a change in the biodiesel properties.

Ó 2014 Elsevier B.V. All rights reserved.

vapors. Mass transfer goes efficiently inside the micro-fine bub- bles. ‘‘Cavitation’’ happens when bubbles grow (expansion) and collapse (compression) intensively [5]. This phenomenon assists the system to generate fine micro-emulsion through generation of micro streams, micro turbulent eddies and shock waves. Besides, the collapse is extremely energetic, resulting in generation of highly pressurized and over-heated regions called ‘‘hot spots’’ which induce the reaction [6].

Many authors have reported that low frequency ultrasound accelerates reaction rate in which higher conversion is achieved in transesterification within shorter reaction time compared to the other approaches [7–9]. Thermal decomposition can also be carried out in parallel with transesterification within these bub- bles. Highly volatile hydrophobic molecules can easily and directly decompose in hot spots. Generally, biodiesel starts to decompose upon thermal stressing at 275 °C and above. The decomposition mainly involves (i) dimerization or polymerization reactions that form higher molecular weight components; (ii) isomerization reac- tions that transfer unsaturated cis-type FAMEs to trans-type FAMEs and (iii) pyrolysis reactions that break down FAMEs to form lower molecular weight FAMEs and hydrocarbons. These reactions occur

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