A VARIABLE DIFFUSIVITY SHRINKING-CORE MODEL AND ITS APPLICATION TO THE DIRECT SULFATION OF LIMESTONE

A variable diffusivity shrinking-core model that allows for temporal a nd spatial variation of the effective diffusivity in the solid product layer was developed in this study. The model was used to analyze a se t of experimental data for the direct sulfation of three limestones (t hat is, their reaction with SO2 under noncalcining conditions), and a parameter estimation procedure was formulated for extracting the effec tive diffusivity as a distributed parameter from the experimental conv ersion vs. time curves. Very strong effects of temperature, concentrat ion, and depth in the product layer on the effective diffusivity were revealed by the analysis. On the basis of the obtained results, it was concluded that despite being essentially nonporous, uncalcined limest ones can react with SO2 under noncalcining conditions at rates compara ble to those of calcined samples because they present much lower resis tance for diffusion Of SO2 through the product layer than the latter.