Challenges in the microwave heating of structured carbon adsorbents

Microwave heating is a promising method for achieving energy-efficient electrification of temperature swing adsorption processes. However, to successfully design the adsorbent bed and the regeneration step, it is crucial to analyse the limited penetration depth of microwaves. In this study, we present the first comprehensive investigation of microwave heating applied to large structured activated carbon adsorbents. The experiments were conducted inside a monomode microwave applicator, which consists of a rectangular waveguide operated at a frequency of 915 MHz. monolithic adsorbents of various shapes and sizes (cubic, rectangular cuboid, cylindrical) with dimensions up to 10 cm, were placed inside the cavity and subjected to microwave powers ranging from 10 to 200 W. To maximise microwave absorption by the sample, the geometry and impedance of the cavity were modified. Temperature measurements were taken at different positions within the monoliths to investigate heating homogeneity. The combined influence of reflections and the limited penetration depth results in very different temperature profiles as the size and shape of the monolith vary, even after optimising the cavity. In general, thinner monoliths heat better at the centre, whereas thicker monoliths experience better heating at the point closer to the radiation source. Additionally, these materials were characterized in terms of their dielectric and textural properties and composition, and it was determined that variations in these properties result in significant differences in the heating behavior. The choice of adsorbent bed size and heating time can be adjusted accordingly to maximise energy efficiency and heating homogeneity. Furthermore, the rotation of monoliths within the monomode electromagnetic field to achieve heating homogeneity is examined and a comparison with a multimode device is established.