An analysis of the microwave dielectric properties of solvent-oil feedstock mixtures at 300–3000 MHz Beatrice G. Terigar, Sundar Balasubramanian, Dorin Boldor * Department of Biological and Agricultural Engineering, Louisiana State University Agricultural Center, 149 E.B. Doran Bldg., Baton Rouge, LA 70803, United States article info Article history: Received 25 September 2008 Received in revised form 14 January 2010 Accepted 14 January 2010 Available online 14 April 2010 Keywords: Dielectric properties Microwave Oil extraction Biodiesel feedstock abstract Microwaves can be a more efficient method than traditional thermal treatment to deliver the energy required for heating in solvent-oil extraction due to its volumetric, direct coupling with the material. An understanding of the behavior of dielectric properties of solvent–feedstock mixtures is important for designing and optimizing any microwave-based extraction process. In this study rice bran and soy- bean flour were mixed separately with four different solvents (methanol, ethanol, hexane and isopropa- nol) at different ratios (1:2, 1:1, 2:1 w/w). For the samples mixed with ethanol, the dielectric properties were measured at 23, 30, 40 and 50 °C, while for all other sample-solvent mixtures experiments were performed at room temperature. Dielectric properties were determined using a vector network analyzer and dielectric probe kit using the open-ended coaxial probe method in the frequency range of 300 MHz to 3 GHz. Results from the study indicate that dielectric constants were dependent on frequency and were strongly influenced by temperature, mix ratio and solvent type. The dielectric loss of all mixtures except those with hexane (which were virtually zero) varied with frequency and temperature, solvent type, and mix ratio. Most of the results presented are emphasized at 433, 915 and 2450 MHz, frequencies allocated by the Federal Communication Commission (F.C.C.) for microwave applications. The results of the study, presented here for the first time to our knowledge, will help in selection of appropriate solvent, mixing ratio and frequency for designing microwave-assisted oil extraction systems. Ó 2010 Published by Elsevier Ltd. 1. Introduction Solvent extraction of oil from various biological feedstocks (for human and animal consumption, for biodiesel production, or for value-added purposes) requires energy input in the form of heat to enhance the mass transfer rate of the process. Microwaves can be used for this purpose, with a potential additional advantage of enhanced extraction of bioactive compounds with the oil. These compounds can be easier to separate and purify, especially if the solvent used is relatively inert in relationship to microwaves (Joc- elyn and Bélanger, 1994). The interaction of electromagnetic fields (inclusive in the microwave region) with biological materials is characterized by their dielectric properties. Unlike other variables that influence microwave heating (microwave power level, fre- quency and initial temperature) which can be selected for a partic- ular processing application, dielectric properties are intrinsic/ inherent properties that require empirical measurements, espe- cially for complex materials. The understanding of frequency and/or temperature-dependent dielectric properties of feedstock- solvent mixtures is important both in fundamental studies of microwave processing applications and in assessing their eco- nomic and environmental implications. Thus, dielectric properties are important in both the design calculations for high frequency and microwave heating equipment (Ryynanen, 1995) for extrac- tion and in choosing the appropriate materials for the extraction process. Dielectric properties of materials are defined in terms of their relative complex permittivity (e): e ¼ e 0  je 00 ð1Þ where the real part, e 0 , is the dielectric constant, the imaginary part, e 00 , is the dielectric loss factor. Dielectric constant is associated with the potential for electrical energy storage in the material, while dielectric loss is related to the electrical energy dissipation in the material. The energy of the oscillating electric field is dissipated via two major mechanisms: conduction and dipolar rotation. Both mecha- nisms create inter- and intra-molecular friction which generates heat throughout the volume of the material (Metaxas and Mere- dith, 1983), and the equivalent absorbed energy which is converted into heat can be determined using (Nelson, 1992; Singh and Held- man, 2003): P ¼ rE 2 ¼ 2pf e 0 e 00 E 2 ð2Þ 0960-8524/$ - see front matter Ó 2010 Published by Elsevier Ltd. doi:10.1016/j.biortech.2010.01.097 * Corresponding author. Tel.: +1 225 578 7762; fax: +1 225 578 3492. E-mail address: dboldor@agcenter.lsu.edu (D. Boldor). Bioresource Technology 101 (2010) 6510–6516 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech