https://doi.org/10.1016/j.apenergy.2017.02.012
“This is motivated by recent literature highlighting microwave heating as an effect means of regenerating solid adsorbents for CO2 capture and storage [20], [21], [22], [23]. Microwave, or dielectric, heating refers to the direct heating of a sample through interaction with electromagnetic radiation. As such, microwaves offer instantaneous and volumetric heating without [24] heat transfer restrictions associated with conventional conductive or convective heating [25]. For polar solvents, such as water or MEA, microwave heating primarily takes place via reorientation of molecular dipoles in the presence of the rapidly oscillating electric field [24], [25]. A phase lag between the molecular motion and the electric field causes friction between neighbouring molecules, which ultimately leads to dissipation of the electromagnetic energy into heat [26], [27]. Other important loss mechanisms occur through ion conduction in ionic solutions and Maxwell-Wagner polarization, resulting from interfacial phenomena, in inhomogeneous media [27].
The susceptibility of a substance to microwave heating is governed by its frequency-dependent complex permittivity [24]:(1)ε∗(ω)=ε′(ω)-iε″(ω)where the real part, ε′(ω), is a measure of the polarizability of the dielectric by an external field and the imaginary part, ε″(ω) is the dielectric loss factor, which represents the ability to convert absorbed microwave energy into heat [24]. The average power per unit volume absorbed by a sample during microwave heating, Pabs (W), is proportional to its dielectric response and also affected by microwave field properties, specifically the angular frequency, ω (Hz), and the average electric field strength, E (V/m), which is determined by the inlet power and local electric field distribution [28]:(2)Pabs=ωε0ε″(ω)|E|2Vwhere ε0 is the permittivity of free-space (8.854 × 10−12 F/m) and V is the sample volume (m3).
Microwave technology has long been acknowledged as an effective means to intensify chemical processes [24]. Applications have traditionally centred around microwave-assisted organic synthesis and solid adsorption-desorption systems for desiccant dehydration, volatile organic compound (VOC) recovery, air separation and water purification [24], [25], [27]. Many studies have reported more efficient chemical productivity and economic performance with MSR compared to conventional regeneration techniques, owing to the direct and rapid nature of microwave heating. Cherbański et al. compared the desorption kinetics of MSR to conventional temperature swing regeneration (TSR) for acetone and toluene removal from zeolite 13X molecular sieves [29]. They revealed more efficient desorption with MSR due to the direct and instantaneous heating of the adsorbent by microwave radiation, becoming more pronounced for the more polar adsorbate [29]. Hashisho and co-workers developed an MSR system for the adsorptive separation of organic vapours and binary gas mixtures, including CO2/CH4, with activated carbons and titanosilicate Na-ETS-10. Microwave desorption was found to be up to 40 times faster than conductive thermal heating and more energy efficient over multiple regeneration cycles [30], [31], [32], [33], [34], [35], [36]. Ania et al. illustrated the effects of microwave and conventional thermal regeneration on the structure and adsorptive capacity of activated carbons. An inverted temperature gradient during microwave heating encouraged diffusion of the desorbing molecules from the core of the carbon bed towards the surface leading to shorter regeneration times [37]. Polaert and co-workers have made extensive use of MSR for a broad range of solid adsorbents for water and VOC recovery [38], [39]. The absorbed microwave power and dielectric properties were concluded to be the most important parameters in ensuring a favourable economic performance. Optimization of the experimental design lead to important energy savings compared to conventional TSR [38], [39]. Turner et al. investigated the influence of microwave radiation on sorption and competitive sorption of polar and non-polar adsorbates in high-silica zeolites [40]. The microwaves permitted greater sorption selectivity, and also generated interesting surface temperature effects. Due to the extremely low dielectric response of silica, the bulk zeolite couples negligibly to the microwaves, however the surface silanol (hydroxyl) groups possess significant dielectric loss parameters and couple strongly to the microwaves. It was proposed that the surface silanol groups selectively absorb microwave energy and heat the zeolite surface. This lowers the overall energy demand of the desorption process, as energy is targeted at the adsorbed phase without directly heating the bulk. Accordingly, their measurements indicated that less than half the energy was required for microwave regeneration compared to conventional approaches, such as resistive heating or steam stripping [40]. Similarly, Vallee and Conner proposed a localized surface temperature effect for sorption of VOCs on oxides, such as high silica zeolites [41]. These additional contributions unique to microwave heating are so-called “non-thermal” effects, which may influence a chemical or physical process without significantly altering the effective temperature of the whole system [24]. Antonio and Deam suggested that such microwave-specific effects are a consequence of enhanced diffusion properties following dipolar realignment under the applied electric field [42].”