Microwave Heating-Induced Temperature Gradients in Liquid-Liquid Biphasic Systems

Microwaves (MWs) can enable the electrification and intensification of chemical manufacturing. They have been applied to various unit separations, such as drying, distillation, and extraction, entailing gas-liquid and solid-liquid systems. However, a limited quantitative understanding of MW-heated liquid-liquid biphasic systems related to extraction exists. This work measures the temporal and spatial temperature difference between an aqueous and an organic phase in batch and continuous microfluidic modes. We demonstrate permanent temperature differences between phases over 35 degrees C and spatiotemporal periodic and quasiperiodic oscillations modulated by the flow patterns. The temperature differences are primarily driven by the faster absorption rate of MAN irradiation by the aqueous phase versus the slower heat transfer from the aqueous phase to the organic phase. These are amplified by low specific interfacial area and modifications of the electromagnetic field. We employ a multiphysics model to predict the temperature difference in a batch system. The model is in good agreement with the experiments. We demonstrate a strong effect of input power, dielectric properties of organic solvents, the volume of solvents, and the volume ratio between phases on the temperature difference. A simple analytical model describes the temperature difference and provides design principles. The combined approach offers new insights into the design and optimization of the MW-heated biphasic systems.

Automated summary

Microwaves can promote the electrification and intensification of chemical manufacturing. This study focuses on the MW-heated liquid-liquid biphasic systems related to extraction and measures the temperature difference between two phases. The results show permanent temperature differences and periodic oscillations, which are driven by the absorption rate of MW irradiation and the heat transfer between phases. A multiphysics model is also developed to predict the temperature difference in a batch system, and a simple analytical model provides design principles. These findings offer new insights into the design and optimization of MW-heated biphasic systems.