Analysis of Non-isothermal Adsorption of Carbon dioxide in Metal Organic Frameworks

Adsorption is fundamentally known to be a non-isothermal process; in which temperature increase is largely significant, causing fairly appreciable impacts on the processkinetics. For porous adsorbent particles like metal organic frameworks (MOFs), silica gel, and zeolite, the resultant relative heat generated is partly distributed within the particle, and the rest is transferred to the surrounding ambient fluid (air). For large step changes in adsorbed phase concentration and fast adsorption rates, especially, the isothermality of adsorption (as in some studies) is an inadequate assumption and inspires rather erroneous diffusivities of porous adsorbents. Isothermal models, in consequence, are insufficient for studying adsorption in porous adsorbents. Non-isothermal models can satisfactorily and exhaustively describe adsorption in porous adsorbents. However, in many of the analyses done using the models, the thermal conductivity of the adsorbent is assumed to be infinite; thus, particle temperature is taken to be fairly uniform during the process—a trend not observed for carbon dioxide (CO2) adsorption on MOFs. A new and detailed analysis of CO2 adsorption in a single microporous MOF-5 particle, assuming a finite effective thermal conductivity along with comprehensive parametric studies for the models, is presented herein. A significant average temperature increase of 5K was calculated using the new model, compared to the 0.7K obtained using the Stremming model. A corresponding increase in diffusivity from 8.17 x 10-13 to 1.72 x 10-11 m2/s was observed, indicating the limitations of both isothermal models and models that assume constant diffusivity.