How to deal with heat of adsorption in microscopic mathematical modeling of adsorbent-fluid systems?

Abstract

The location of the heat of adsorption term in microscopic mathematical models is closely related to how we perceive this parameter. The heat of adsorption term may accordingly be included either only in the thermal energy equation or only in the boundary condition at the ambient-adsorbent interface. In this study, adsorbents with different properties, namely, NaA, NaX, SAPO-34 zeolites and a metal organic framework (MOF) were selected to demonstrate the extent of variation of the results obtained when the two different approaches were used, considering adsorption heat pumps utilizing adsorbent coatings. It was determined that using heat of adsorption in mathematical models in different forms had some effects on the results, varying according to the adsorbent used. Relatively long cycle durations due to high coating thicknesses and mass transfer limitations favored the proximity of the results obtained by using different models. Some correlation was present between Lewis/Fourier numbers and the differences between the results obtained by using different models; differences more commonly decreased as Lewis/Fourier numbers increased. A primary factor leading to the differences was related to the boundary condition at the ambient-adsorbent interface, determining the surface temperature and concentration and governing the concentration distribution in the adsorbent layer. Characteristics of water adsorption isotherms of the adsorbents, including their regeneration temperatures, had notable impact on this factor. MOF and SAPO-34 adsorbents, with relatively low regeneration temperatures, exhibited the highest differences between the models, considering the adsorption stage and whole cycle.