Numerical Analysis for CO₂ Absorption and Regeneration Behavior in Porous Solid Sorbent by Modified Unreacted-Core Model

Abstract

Lithium ortho-silicate (Li₄SiO₄) is a suitable solid sorbent for capturing CO₂ from solid oxide fuel cells. CO₂ absorption reactors packed with porous-solid spherical pellets of Li₄SiO₄ show unsteady temperature distribution and capture ratio behavior owing to the unsteady CO₂ absorption rate and highly exothermic process. The CO₂ absorption rate of this sorbent reportedly depends on temperature, CO₂ concentration, and CO₂ accumulation, expressed as the weight change of the sorbent. Nevertheless, discussions of detailed mechanisms of CO₂ absorption by this sorbent are rare. In this study, the modified unreacted core model is proposed to explain the mechanism of CO₂ absorption of a porous-solid spherical pellet, and numerical analysis was conducted to simulate the unsteady behavior of the sorbent. Important properties such as the reaction rate constant, the gas film mass transfer coefficient, and the coefficient for effective diffusion through the product layers were empirically derived using thermogravimetry and a diluted packed-bed reactor. Numerical analysis by applying these parameters to the modified unreacted core model adequately explained the complicated CO₂ absorption and regeneration behaviors.