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

Abstract

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 °C and spatiotemporal periodic and quasiperiodic oscillations modulated by the flow patterns. The temperature differences are primarily driven by the faster absorption rate of MW 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.

Microflow chemistry and its electrification for sustainable chemical manufacturing

“…180 However, MW-heated flow reactors are often non-isothermal, as demonstrated using thermal cameras 246,[255][256][257] or location-adjustable temperature-sensing optical fibers. 212,258,259 Of the works that consider the axial temperature distribution, an occasional rate enhancement is reported, 246 but the number of high-quality comparisons of this sort is limited. Despite the insufficient evidence for MW-specific rate enhancements, the rapid heating rates offered by MWs (often reported as more than 5 °C/s; sometimes as high as 50 °C/s) allow for high productivity.…”

“…262 Selective heating and liquid-liquid temperature gradients have been demonstrated without focusing on reaction enhancements. 212,261 In the slurry flow mode, Chemat and coworkers demonstrated an esterification rate enhancement of 50-150% and attributed that to selective heating of the solids (iron(II) sulfate adsorbed onto clay) calculated (but not measured) to be 9-18 °C above the bulk liquid temperature. 220 MW-heated continuous flow packed beds occupy a more significant space in literature.…”

“…180 However, MW-heated ow reactors are oen nonisothermal, as demonstrated using thermal cameras 246,[255][256][257] or location-adjustable temperature-sensing optical bers. 212,258,259 Of the works that consider the axial temperature distribution, an occasional rate enhancement is reported, 246 but the number of high-quality comparisons of this sort is limited.…”

“…Despite the insufficient evidence for MW-specic rate enhancements, the rapid heating rates offered by MWs (oen reported as more than 5 C s À1 ; sometimes as high as 50 C s À1 ) allow for high productivity. 246,260 Relative to a single-phase, a few works exist for liquidliquid 212,261,262 or slurry 180,219,220 ows (excluding nanoparticle synthesis). In the former, MW rate enhancement for tandem diazotization and Heck reactions was reported without an explanation.…”

“…Both systems have a total volume of 6 ml and the biphasic system has a 1 : 1 phase ratio. Adapted with permission from ref 212. Copyright (2022) American Chemical Society.…”