Intensification and electrification of adsorption processes using hybrid structured adsorbents with induction heating as an alternative regeneration method

The ongoing transformation of the chemical industry toward decarbonisation and electrification necessitates the development of energy-efficient and electrified alternatives to conventional thermal processes. A critical area for innovation is the separation and purification stage, which remains one of the most energy-intensive steps in many industrial operations. This research explores Magnetic Induction Swing Adsorption (MISA) as an emerging electrified technology. MISA operates by selectively adsorbing target gases onto a porous solid, followed by electrically induced desorption using an alternating magnetic field. This approach allows for localized, contactless, and rapid heating, representing a significant departure from traditional temperature swing adsorption (TSA) processes reliant on external heating or steam. This PhD research aims to comprehensively investigate various aspects of MISA and to advance the technology toward a higher level of technological readiness, particularly for carbon capture and air drying, through investigation of materials, structured adsorbent design, process configuration, and scalability. The first part of the study (Chapter 4) addresses adsorbent formulation and integration of magnetic susceptors. A variety of adsorbents (zeolite 13X, CALF-20, and Ni-MOF-74) were combined with magnetic materials including Fe3O4, NixZnyFe2O4 ferrites, and steel wool fibres. Fe3O4, widely used for its availability and cost-effectiveness, showed limited thermal durability due to oxidation. Ferrites emerged as a more stable alternative, exhibiting up to threefold higher specific absorption rates (SAR) than Fe3O4 under similar magnetic field conditions. Steel wool fibres provided the highest SAR (nearly 20 times higher than Fe3O4) at low magnetic field intensities but posed challenges in extrusion into structured forms. Several composite adsorbent configurations were fabricated, including extrudates, coated foams, laminates, and monoliths. Fe3O4-based pellets demonstrated rapid heating up to 150 °C in under 60 seconds, with preserved CO2 selectivity and capacity. Coated melamine sponges fabricated via dip-coating could be compressed by over 80% without capacity loss, enhancing heating rates due to improved contact. However, the limited physical attachment and low adsorbent loading restricted their use. Laminates offered excellent mass transfer and facilitated the integration of the high-efficiency steel wool susceptors, but their mechanical fragility limited practical usage. Monolithic structures provided a robust alternative, allowing optimization of channel density to increase surface-area-to-volume ratios and reduce mass transfer resistance, while maintaining reasonable pressure drops. The geometry and performance of monoliths were closely linked to die design, suggesting this as a key area for ongoing development.
The next phase of the research (Chapter 5) investigated several desorption strategies using evacuation and magnetic induction. The application of a short (1-minute) inductive heating pulse, either simultaneously with or just prior to vacuum application, achieved 90% regeneration in under 2 minutes, compared to 46 minutes with vacuum alone. When heating was performed without vacuum, a purge step was required to evacuate CO2, leading to undesired dilution of the CO2 stream. Simultaneous heating and evacuation resulted in smooth desorption profiles, while sequential operation generated sharp peaks with desorption rates as high as 418 mg/(g*min), three times greater than the simultaneous heating and evacuation method. These findings underscore the need for further investigation into the structural impact of rapid thermal cycling on adsorbents. To assess full process viability, a five-step MISA cycle, comprising adsorption, blowdown, inductive heating, evacuation, and cooling, was developed in Chapter 6. Experimental optimization identified the blowdown step (4 seconds) as critical for removing interstitial gas, improving CO2 purity. The optimal condition was achieved using inductive heating at a 100.8 A current for 40 seconds, resulting in 95.9% CO2 purity and 92.1% recovery with a specific thermal energy consumption of only 2.82 MJ/kg CO2. Notably, total cycle time was only 464 seconds, and steady-state operation was achieved after a few cycles, as confirmed by consistent thermal and mass balance data. The process was also adapted to handle humid flue gases by introducing a preceding silica gel bed, which protected the 13X adsorbent and preserved overall performance.
Scalability of the MISA process was examined in Chapter 7. Increasing coil size to accommodate larger bed volumes inadvertently increased inductance and decreased operating frequency, negatively affecting heat generation. Axial temperature gradients were observed in shorter coils, where most heating occurred at the column center. To address this,
stagewise and segmented heating strategies were implemented. In stagewise heating, continuous localized heating was achieved by moving the column through the coil, allowing temperature distribution to be tailored via movement speed. Segmented heating enabled multiple, spatially distributed heating zones. Reverse bidirectional stagewise heating at 136.5 A and 15 s/m column speed achieved a maximum bed temperature of 260 °C and 86% regeneration efficiency in 410 seconds—more than double the efficiency of conventional TSA, which required 2700 seconds and high helium flow for 40% regeneration. The study also investigated the thermal behavior of different column zones during regeneration. Uneven cooling was observed, allowing subsequent adsorption cycles to commence before the entire bed returned to ambient temperature. This property could be exploited to further reduce overall cycle times and enhance productivity. Finally, the versatility of MISA was validated by applying it to air and gas drying in Chapter 8, where silica gel was modified with Fe3O4 to enable inductive heating. Under static conditions (no flow), temperature rise and regeneration efficiency were highly dependent on both Fe3O4 loading and applied current. At 49.4 A and 20 wt% Fe3O4, a 10 °C temperature rise and 42% water removal were recorded in 3600 seconds. At 100.7 A and 40 wt%, the bed reached 250 °C in 180 seconds with 92% water removal. However, temperatures above 150 °C promoted Fe3O4 oxidation to γ-Fe2O3 and degraded silica gel structure. Introducing a purge gas stream during regeneration significantly improved performance and minimized thermal degradation, achieving >97% water removal. The findings of this PhD research demonstrate MISA’s potential for use in a broad range of thermally regenerable adsorption applications.