Numerical Analysis for CO<sub>2</sub> 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.