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The influence of microwave radiation on sorption and the use of frequency response to study diffusion
The influence of microwave radiation on adsorption selectivity was studied using the sorption of cyclohexane and methanol on high-silica zeolites. The amount of microwave energy absorbed depends on the specific adsorbent/adsorbate system. The adsorbent, a high-silica zeolite, is effectively “transparent” to microwave radiation, while the two adsorbates reflect high (methanol) and low (cyclohexane) absorption of microwave energy. The measured system temperatures required for desorption by microwave energy were tower than those required for conventional heating. Further, microwave radiation can change the sorption selectivity; the adsorbate with the greater microwave absorptivity is selectively desorbed. The conclusion is that the surface and adsorbed species can be selectively heated because the rate of microwave energy absorption can be greater than the rate of heat transfer from the surface. Recent studies suggest that microwave energy can be employed in catalysis and that the results differ from “conventional” heating. This effort studied the influence of microwave energy on automotive exhaust catalysis in the presence and absence of a catalyst poison (SO2). The conclusion is that microwave energy can induce catalyst lightoff (the temperature where 50% of final conversion is achieved) more efficiently than conventional heating and can reverse the poisoning by SO2 for a commercial three-way catalyst. Zeolites have a variety of industrial uses, from catalyst supports to membrane separations. The ability to accurately measure the rate of diffusion in these materials is of great importance. A frequency response device has been designed and constructed to measure diffusion in zeolitic systems. The apparatus was modeled after previous devices constructed by Yasuda, Rees, Meunier, and Grenier (Yasuda 1976; Rees and Shen 1993; Bourdin, Grenier et al. 1995). A detailed description of the apparatus and its improvements are presented. The device was tested using an n-hexane/silicalite and a methanol/silicalite system. The model developed by Yasusda (Yasuda 1982) is used to analyze the frequency response and estimate the diffusion coefficients. This model can describe a system having multiple diffusivities and a surface resistance to diffusion. The results for n-hexane/silicalite and methanol/silicalite agree with those of van den Begin (Begin and Rees 1989) and Nayak (Kayak and Moffat 1988), respectively.
Turner, Michael David, "The influence of microwave radiation on sorption and the use of frequency response to study diffusion" (2000). Doctoral Dissertations Available from Proquest. AAI9988849.