Direct sulfation of limestone at elevated pressures

Abstract

Direct sulfation of limestone at elevated pressures has been studied using a pressurized thermogravimetric analyzer. The separate effects of total pressure and the partial pressure of SO 2 on direct sulfation were presented in this paper. It was found that the degree of sulfation decreased with rising total pressure when the partial pressure of SO 2 was kept constant. The present study showed that the reaction order was not of first order with respect to SO 2 at elevated pressures. The effect of temperature on the rate constant of reaction between 750 and 900°C and at a total pressure of 1.3 MPa was correlated by an activation energy of 96.8 kJ/mol. A mathematical model for gas–solid reactions has been suggested to enable the description of the kinetic behavior of direct sulfation of limestone. The effective diffusivity of SO 2 through the product layer of CaSO 4 as a function of the conversion was determined using this model. The effective diffusivity varied noticeably as the reaction proceeded and also in conjuction with changes in total pressure, SO 2 partial pressure and temperature. Furthermore, the activation energy for the effective diffusivity was found to increase with the conversion of CaCO 3 to CaSO 4, in spite of a slight drop in the activation energy beyond a conversion degree of 22%.