Kinetic Model for CO2 Capture by Lithium Silicates

Lithium-based ceramics, notably Li4SiO4, have been evaluated as potential CO2 sorbents for sorption-enhanced WGS reactions. A challenge in the development of faster, more efficient sorbents is due to the lack of adequate models to reduce kinetic data to physically meaningful parameters. Here, it is shown that a piece-wise fit of the Ishida and Wen (1968) core-shell model yields physically meaningful reaction rate constants and diffusion coefficients when applied to a series of three different Li4SiO4 samples: a citrate sol-gel prepared material, a material from a surfactant-mediated synthesis, and an annealed sample. All samples show an identical, 3-order of magnitude change in diffusivity (from similar to 10(-7) to similar to 10(-10) cm(2)/s) of the rate-limiting process as a function of conversion. We attribute the initial faster diffusion to facile movement of Li+ in the reactant Li4SiO4, which mediates the transport of O-2(-) between silicate and carbonate phases. The effective diffusivity drops with the growth of product in the reactant matrix with increasing average conversion. This result shows that our material synthesis modifications do not change the intrinsic diffusivity as a function of composition in these sorbent materials and that changes in performance are only due to changes in diffusion path length. The novel methodology introduced here may have applicability in the analysis of other diffusion-reaction systems.