A side-coupled microwave applicator supporting azimuthally symmetric modes with metal stirrer for liquid-phase...

This study presents a side-coupled cylindrical microwave applicator operating in the 2.45 GHz TE011 mode, integrated with a mechanical stirrer for uniform heating. The design was optimized (radius: 90 mm, height: 145 mm) and evaluated using HFSS simulations to examine geometric, dielectric, and coupling effects. TE011-mode excitation remained stable under high-loss loading and was only achieved via vertical waveguide injection. The metallic stirrer caused no measurable perturbation, with resonance frequency and -10 dB bandwidth remaining consistent across 0°-45° blade orientations. Resonance frequency shifted >50 MHz with ±10 mm surface-height changes but was largely insensitive to submersion depth. Experimental validation used a catalyst-free esterification of oleic acid and glycerol (760 ml) at 200 °C and 160 Torr. The microwave system achieved a significantly faster reduction in acid index (AI%) than mantle heating, demonstrating rapid thermal delivery and supporting better energy efficiency under matched thermal profiles. To enhance scalability, a next-generation applicator was developed with bottom waveguide injection and an integrated air buffer. This design preserved TE011-mode excitation across 10-160 mm sample heights and supported stable operation over a broad dielectric range, with S11 < -15 dB and resonance shifts within ±10 MHz. These results establish a robust platform for efficient, solvent-adaptive microwave-assisted chemical processing.