Role of Resonances on Microwave Heating of Oil–Water Emulsions Tanmay Basak Dept. of Chemical Engineering, Indian Institute of Technology, Madras, Chennai 600 036, India DOI 10.1002/aic.10207 Published online in Wiley InterScience (www.interscience.wiley.com). A detailed analysis on enhanced heating effects attributed to resonance of microwaves has been carried out to study the efficient heating methodologies for oil–water emulsions. Studies on heating have been carried out for samples incident with microwaves at one side and at both sides. The maxima in average power corresponding to resonances occur at various sample thicknesses for all emulsions and we consider two dominant resonance modes R 1 and R 2 , where the average power at R 1 is larger than that at R 2 . During one side incidence, it is observed that processing rates are greater at the R 2 mode for both oil-in-water (o/w) and water-in-oil (w/o) emulsions, whereas for both-sides incidence, the R 1 mode is favored for o/w emulsion and the R 2 mode is advantageous for w/o emulsion. The greater rates in thermal processing are observed when the emulsions (o/w and w/o) are incident at both sides. Current analysis recommends on the efficient way to use microwaves in a customized plane-wave oven for greater heating effects and determines the optimal sample thicknesses vs. various oil–water contents for thermal processing. The emulsion system is a combination of materials with very low dielectric loss (oil) and high dielectric loss (water), and the effective dielectric response is highly nontrivial to predict the efficient way to heat an emulsion, either o/w or w/o. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2659 –2675, 2004 Keywords: microwave, resonances, oil–water emulsion Introduction Microwave (MW) energy has been widely applied in food and chemical processing for heating, thawing, sintering of ceramics, and many others (Ayappa et al., 1992; Basak and Ayappa, 1997, 2001; Chatterjee et al., 1998; Ratanadecho et al., 2002). Various oil-in-water (o/w) and water-in-oil (w/o) emulsions occur in in- dustrial operations, such as petroleum refining, oil and gas pro- duction, and food processing industries, which include dressings, sauces, butter, mayonnaise, and many more. Efficient heating of emulsions is required for a faster processing based on industrial demand. Microwave heating, because of its volumetric heating effects, offers a faster processing rate. The enhanced processing can be achieved by heating samples in the presence of resonances, which occur as the result of constructive interference of propagat- ing waves within a material for specific sample dimensions, cor- responding to a relationship between sample dimensions, wave- length, and penetration depths, whereas resonances are absent for a very large or very small dimension (Ayappa et al., 1997). A few earlier investigations on resonances were carried out for cylinders and spheres (Massoudi et al., 1979; Ohlsson and Risman, 1978; Weil, 1975). Ohlsson and Risman (1978) car- ried out experimental studies on MW heating at 2450 MHz for spherical and cylindrical samples and they observed resonances in power absorption at the center of the sample for specific radii. Massoudi et al. (1979) observed resonances for multilay- ered samples while studying average power absorption in ho- mogeneous and multilayered cylinders by varying the fre- quency of radiation. Weil (1975) studied average power over the frequency range for spheres of radii 3.3 and 6 cm, where strong resonances were observed when the frequency was varied. Correspondence concerning this article should be addressed to T. Basak at tanmay@iitm.ac.in. © 2004 American Institute of Chemical Engineers AIChE Journal 2659 November 2004 Vol. 50, No. 11