(291g) When Fluidization Meets Microwaves: A Multiphysics Look at the Role of Hydrodynamic Behavior on Microwave Absorption Patterns

Microwave heating plays a crucial role in developing electrically heated fluidized beds. Microwave heating offers distinct advantages over conventional methods by directly transforming electromagnetic (EM) energy into thermal energy within the heated material. This approach offers several key advantages, including rapid, volumetric, and selective heating—enabling heat generation precisely where it is needed, a capability that conventional heating methods cannot achieve. Combining the key benefits of fluidized beds and microwave heating offers a promising pathway for electrifying process heat and advancing reactor design in chemical processes and energy production systems. Recently, several experimental studies have emerged, exploring the use of microwave heating in fluidized bed reactors. For instance, microwave heating of fluidized beds has been successfully utilized to produce clean hydrogen from methane pyrolysis. While experimental studies have been conducted, computational model development remains very limited due to the complex interplay between microwave interactions and the transient hydrodynamics of fluidized beds. It is worth mentioning that due to the significant challenges in spatial temperature and EM field measurements within a fluidized bed during microwave experiments, the thermal and hydrodynamic interaction between microwaves and fluidized beds, as well as the effect of fluidization on microwave-induced hotspots, remains unclear and underexplored. Detailed modeling investigations are needed to bridge the gap and expand the utilization of microwave technology for more commercial and scientific purposes. In a recent study, Salakhi and Thomson demonstrated that microwave energy is converted into thermal energy at the individual particle level within 10,000 micron-sized particles in a fluidized state. Notably, they found that when the ratio of particle radius over its skin depth is less than 5, each particle absorbs microwave power independently. This highlights the importance of coupling CFD-DEM (Computational Fluid Dynamics – Discrete Element Method) with electromagnetics to accurately capture the underlying physics in microwave-heated fluidized bed systems.

Here, we introduce a pioneering CFD-DEM framework coupled with electromagnetics to computationally investigate microwave heating in fluidized beds. This is achieved through a one-way frequency-transient algorithm, where Maxwell’s equations are solved in the frequency domain, while coupling CFD-DEM framework in a two-way transient scheme. In this algorithm, microwave power absorption density, absorption per unit volume of solid particles, is first obtained via Maxwell’s solution for particles within the studied fluidized bed reactor dimension using COMSOL Multiphysics software. This approach removes the need for solid volume fraction data, as microwave absorption is directly resolved at the particle scale. The resulting microwave power absorption density, as a function of particle size and spatial location, is incorporated into the energy equation of each individual particle through a custom-built module in Ansys Rocky. A transient, two-way coupled unresolved CFD-DEM algorithm is then used to simulate the behavior of the microwave-heated fluidized bed reactor. The fluidization hydrodynamics model is validated using experimental pressure drop data, while the accuracy of the microwave heating predictions is assessed by comparing simulated and measured reactor temperatures. After validation, we investigate the microwave heating behavior of a 1-inch-diameter fluidized bed reactor containing 65 grams of carbon particles, with 1,523,021 Geldart B particles, each 350 microns in size.

Results reveal that fluidization hydrodynamics affects microwave power absorption and distribution. For instance, altering the fluidization regime from bubbling to sluggish, or increasing particle size—which inherently modifies the fluidization hydrodynamics—leads to the formation of a new cold spot in the microwave heating distribution within our reactor configuration. In addition, transitioning from a bubbling to a sluggish fluidization regime mitigates hotspot temperature by promoting axial particle mixing between microwave-induced hot and cold spot zones. Notably, the cold spots are heated through convective heat transfer from the gas phase, driven by bubble dynamics, unveiling new physical interactions in microwave heating. Moreover, the negligible temperature difference between particles and surrounding gas indicates rapid thermal equilibration between phases.

We conclude that for temperatures below 500°C, the temperature profile within the bed is governed by a complex interplay of factors, including particle size, fluidization velocity (and associated bubble dynamics), convective heat transfer, and the absorption and distribution of microwave power within both the bed and cavity. These interdependent phenomena collectively shape the thermal behavior of the system. Overall, our findings reveal that microwave heating in fluidized beds provides substantial control over temperature profiles and microwave power absorption dynamics. This high degree of tunability unlocks transformative potential for system optimization, advanced research, and the next generation of microwave-driven technologies. Finally, we emphasize promising opportunities for researchers to further refine and scale this model, enabling substantial advancements in microwave-assisted fluidized bed technologies.