Removal of VOCs at trace concentration levels from humid air by Microwave Swing Adsorption, kinetics and proper sorbent selection

• Microwave heating accelerates desorption compared to conventional heating.
• Microwave heating has an effect on the desorption order in n-hexane/water mixtures.
• Matching adsorption and dielectric properties is key in the selection of materials for MWSA.

Microwave Swing Adsorption, MWSA, is already established as an alternative way of heating and regenerating solids efficiently in adsorption operations. When zeolites are used for MWSA applications it becomes necessary to tailor the zeolite properties towards microwave heating while at the same time aiming to obtain the highest adsorption capacity and selectivity. These objectives may in some cases be contradictory. In this paper the adsorption of binary mixtures of n-hexane (500 ppm) and water (15,000 ppm) in air, together with the regeneration of the sorbents using microwaves is studied for three commercial, faujasite Y type zeolites: NaY, HY and DAY. In the Henry regime for adsorption of low concentrations of n-hexane, the Na exchanged zeolite shows the higher affinity for the n-hexane. However, when the binary mixtures are considered DAY zeolite, with higher Si/Al ratio, is more selective towards the adsorption of the alkane. During microwave heating of zeolites, the water adsorbed, the mobile cations of the zeolite framework and the hydroxyl groups on the surface of the zeolite contribute to the absorption of microwave energy. Both heating and desorption are considerably faster using MW, compared to conventional heating. The DAY zeolite with a Si/Al = 40 has been found excellent candidate material for MWSA of n-hexane in binary mixtures with water, since it combines good adsorption capacity and efficient interaction with microwaves.

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