A Novel In Situ High‐Temperature Magnetometry Method for Radiofrequency Heating Applications

Abstract

reactor and heat transfer into a single step. Moving toward renewable electricity as the source of heat for chemical reactions will enable the decarbonization of the chemical process industries as around 60% of process heating at present comes from fossil fuels. [3] Direct RF heating elevates the catalyst temperature above the bulk reactor temperature. This reduces the impact of noncatalytic hot-spots and has been shown to enhance yields for a variety of organic synthesis and pyrolysis reactions. [4][5][6] Lower coke formation is also reported in pyrolysis, [5,7] caused by a rapid quenching of side reactions as products quickly diffuse from the heated catalyst surface to the cooler bulk fluid. [8] RF heating uses lower frequencies than microwave heating, resulting in different advantages and challenges between the two methods. Lab-scale microwave reactions typically operate at 2.45 GHz and are heated through microwave absorption within a high dielectric solvent. [9] Larger microwave systems suffer from non-uniform heating due high absorption and low penetration of microwaves in the reactor solvent and hot-spots caused by constructive interference on the scale of the microwave wavelength, 12 cm for 2.45 GHz. [9] Higher frequencies are used to achieve greater heating power when scaling up microwave heating systems, which lead to lower heating efficiencies of 30% or less. [10] RF heated systems can achieve theoretical efficiencies greater than 80%, and efficiency increases with scale. [11] Finally, the absorption of microwaves in the human body leads to increased Radiofrequency heating of magnetic particles promises highly efficient and direct heating of catalytic reactors for coupling of low carbon electricity with energy intensive chemical transformations. In this work, a novel real-time and in situ magnetometry method is developed to measure minor and major hysteresis loops of soft magnetic nanopowders. It is applied to measure the magnetic properties and hysteresis power absorption of magnetite and maghemite powders up to 500 °C. An arctangent model for saturation magnetization is adapted for minor hysteresis loops. It produces an excellent fit for hysteresis loop power across field strengths up to 18.5 kA m −1 and allows prediction of heating power, remanence, and susceptibility. Samples of magnetite and maghemite are shown to heat rapidly from room temperature at more than 25 °C s −1 , with maghemite giving the strongest heating response. The peak heating power occurs at the transition beyond the ellipsoidal Rayleigh law region. These findings suggest that the properties of magnetic powders, coupled with variable magnetic field strengths and frequencies, can be tuned to optimize the heating power for a variety of applications.

Reflected impedanceometry: a contact-free technique for measuring induced magnetic hysteresis and eddy current heating

“…The heating power of ferri-magnetic maghemite powder was measured by reflected impedanceometry up to 175 °C using a bespoke work coil and heating arrangement (figure 2, refer to SI for experimental details). The synthesis and characterization of the maghemite powder is described in a previous work [7].…”

“…Data was simultaneously collected using the 3-coil pick-up coil magnetometry technique which has been described in a previous work [7] and compared to the reflected impedance results. Due to the intensity of heat transfer into the susceptor material, the temperature rise within the susceptor can exceed the response lag time of a thermocouple measuring the bed temperature.…”

“…Ferri-and Ferro-magnetic materials heat by induced hysteresis at temperatures up to the Curie point, above which they become paramagnetic and thermal energy is sufficient to overcome any internal magnetic energy barriers, reducing the irreversible hysteresis to zero. Recent progress [7] has shown how the application of pick-up coils (magnetometry) can be used to measure the heat generated by magnetic hysteresis but this cannot be extended to eddy current heating. It also requires that the susceptor material or heated catalytic bed are located within the cross section of the pick-up coils which could limit the potential application.…”

A simplified model for minor and major loop magnetic hysteresis and its application for inference of temperature in induction heated particle beds

“…It may not be possible to fully saturate a magnetic material depending on the equipment available, and Noble et al demonstrated that minor hysteresis loops for an induction heated sample may be measured in-situ using pickup coil magnetometry. A simplified model of magnetisation is required for inferring a magnetic sample temperature in realtime due to the rapid heating rates of induction heating of magnetic samples [6] and the need to use low cost, reliable digital signal processing to replicate this technique in the industrial setting of a chemical synthesis process. There are a number of criteria that a model of magnetic hysteresis must satisfy to be practically applied to induction heating hysteresis loops.…”

“…Noble et al [6] established a semi-theoretical arctangent model that successfully models the magnetic hysteresis heating power at applied field strengths well below saturation. This model will be referred to as the LineArc model due to the presence of both linear and arctangent terms.…”