Direct sulfation of limestone at elevated pressures

Direct sulfation of limestone at elevated pressures has been studied using a pressurized thermogravimetric analyzer. The separate effects of total pre ssure and the partial pressure of SO2 on direct sulfation were presented in this paper. Tt was found that the degree of sulfation decreased with risin g total pressure when the partial pressure of SO2 was kept constant. The pr esent study showed that the reaction order was not of first order with resp ect to SO2 at elevated pressures. The effect of temperature on the rate con stant of reaction between 750 and 900 degrees C and at a total pressure of 1.3 MPa was correlated by an activation energy of 96.8 kJ/mol. A mathematic al model for gas-solid reactions has been suggested to enable the descripti on of the kinetic behavior of direct sulfation of limestone. The effective diffusivity of SO2 through the product layer of CaSO4 as a function of the conversion was determined using this model. The effective diffusivity varie d noticeably as the reaction proceeded and also in conjuction with changes in; total pressure, SO2 partial pressure and temperature. Furthermore, the activation energy for the effective diffusivity was found to increase with the conversion of CaCO3 to CaSO4, in spite of a slight drop in the activati on energy beyond a conversion degree of 22%. (C) 2000 Elsevier Science Ltd. All rights reserved.