Adsorbents regeneration under microwave irradiation for dehydration and volatile organic compounds gas treatment

In adsorption processes, adsorbent regeneration is by far the most time and energy consuming step for which microwave technology offers many advantages. Nevertheless, desorption under microwave is a complex process: electromagnetic energy conversion into heat is strongly linked to several other phenomena such as thermodynamic equilibrium, heat and mass transfers as well as transport. This study points out the key parameters controlling desorption under microwave irradiation. In this paper, an experimental study is presented using several types of adsorbents (silica, activated alumina. NaX and NaY zeolites) and various adsorbates (water, toluene, n-heptane and methylcyclohexane). Theses solids and liquids have been selected for their contrasted dielectric and structural properties. Water desorption from several adsorbents exhibits very different behaviours linked, not only to the evolution of their dielectric properties with temperature and water content, but also to the structure of the adsorbents and more especially to the quantity and position of the exchange cations. Simulated temperature profiles in the solid bed show that, in some cases, desorption can be effective and rapid without reaching very high temperatures in the solid. Results obtained with a zeolite NaX show that desorption rates and effectiveness strongly differ from an adsorbate to another, and seem to be mainly controlled by the microwave absorbed power profile. Morphology-dependent resonances could be responsible for some high rates of heating and accelerating desorption. Microwave heating appears to be a promising technology for adsorbent regeneration processes which is completely dependent on the appropriate choice of the adsorbent-adsorbate couple as well as the reactor's shape and size. (C) 2010 Elsevier B.V. All rights reserved.