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Ultraonication-for-Biodiesel_Biofuels_2012.pdf

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Application of ultrasonication in transesteri cation processes for biodiesel production

Biofuels (2012) 3(4), 479–488

Brian He* & Jon H Van Gerpen

In biodiesel production, adequate mixing is required to create su cient contact between the vegetable oil or animal fat and alcohol, especially at the beginning of the reaction. Application of ultrasonication provides su cient mixing and energy so that the transesteri cation can proceed at a faster rate due to two e ects. First, ultrasonic cavitation and microbubble formation, which are caused by the ultrasonic energy introduced by the sonotrode, greatly improve the interfacial contact between the immiscible methanol and plant oil/ animal fat mixture, thus increasing the reaction rate. Second, the formation and bursting of microbubbles caused by ultrasonic cavitation intensi es the local energy transfer and energizes the reactant molecules, thus enhancing the overall reaction rate. The other possible bene cial aspect of ultrasonication may be ultrasonic energy-induced free radical formation, which initiates chain reactions, as has been observed in other organic systems, although it is not fully understood in transesteri cation yet.

Biodiesel production involves the transesteri cation of plant oils or animal fats with an alcohol to produce alkyl esters. This reaction is reversible and proceeds stepwise. To enhance the completeness of this reaction, it is com- mon to add 1.5- to two-times the stoichiometric amount of alcohol. With methanol as the alcohol, this exceeds the solubility of methanol in the oil or fat, so droplets are formed from the methanol that exceeds the solu- bility limit [1]. These droplets represent a polar phase that captures most of the dissociated sodium methox- ide catalyst. Although the solubility of oil or fat in the methanol phase is low (1–2%), the reaction occurs as the triglycerides diffuse through the droplet interface where they come into contact with methanol and cata- lyst. The reaction rate is widely considered to be lim- ited by this diffusion process [1]. Later in the reaction, glycerol accumulates in this polar phase, maintaining the isolation of the catalyst and, thus, hindering the diffusion processes, which, in turn, limits the reaction rate. In conventional processes, this problem of limited methanol solubility in vegetable oils or animal fat is typically overcome by intensifying mechanical mixing

or simply allowing extended time to let the product esters build up, which helps by improving the methanol solubility in oils or fats. As an attempt to address this issue in an alternative way, application of ultrasonication in this system is studied by many researchers.

Ultrasonic excitation helps increase the liquid–liquid interfacial area through emulsi cation, which is impor- tant for the formation of vapor bubbles and cavita- tion bubbles in viscous liquids, such as plant oils and animal fats. Vapor bubbles within the liquid, such as methanol bubbles generated mechanically or ultrasoni- cally in liquid oils or fats, oscillate and move with the steady currents in the bulk liquid caused by the high- frequency acoustic oscillations or acoustic streaming. This acoustic streaming may also be caused by differ- ences in the ultrasound velocity, which is a property of the materials, traveling in alcohol (e.g., 1182 m/s in ethanol [2]) and in vegetable oils (e.g., 1430 m/s in soybean oil [3]). Simultaneously, the cavitation bub- bling action pushes the liquid towards the interfacial surfaces of the vapor bubbles, where it interacts with the bubbles. This phenomenon enhances the mass transfer

Department of Biological & Agricultural Engineering, University of Idaho, PO Box 442060, Moscow, ID 83844-2060, USA *Author for correspondence: Tel.: +1 208 885 7435; Fax: +1 208 885 8923; E-mail: bhe@uidaho.edu

future science group 10.4155/BFS.12.35 © 2012 Future Science Ltd ISSN 1759-7269 479

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 Supercritical Fluid Extraction Ultraonication-for-Biodiesel_Biofuels_2012.pdf Page 001
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