Numerical Study on the Contribution of Convective Mass Transfer Inside High-Porosity Adsorbents in the VOC Adsorption Process

The transfer mechanism of volatile organic compounds (VOCs) being trapped inside the various types of adsorbents is usually regarded as mere diffusion. This paper investigated the contribution of convective mass transfer inside the adsorbents used for VOC air-cleaning. The adsorbents are typically characterized by their high porosity and thickness which can be as thin as hundreds of microns. By numerical simulation, it was found that the air flow could penetrate the adsorbent matrix when the porosity was high. When the porosity is about 0.7 or even higher, the velocity profile inside the adsorbent and cavity would form, approximately, a consecutive parabola. The convective mass transfer inside the adsorbents would have little impact on the axial VOC transfer but could affect the average adsorption rate significantly at high porosities. The Peclet number Pe which is based on the inlet velocity, the pore diameter and the effective mass diffusion coefficient in the adsorbent could be used to judge the negligibility of the convective mass transfer inside the adsorbent. Pe = 0.52 could be taken as the critical point of neglecting the convective mass transfer inside the adsorbent, with the relative error of average adsorption rate of less than 10% as found by this study.