Kinetic modeling of the reaction of HCl and solid lime at low temperatures

Calcium hydroxide is a commonly used sorbent in the dry-scrubbing of hydrogen chloride from flue gases. In this study the kinetic parameters for the reaction between gaseous HCl and solid Ca(OH)(2) have been obtained at low temperatures in a fixed-bed laboratory reactor. The influence of the operating temperature (323-400 K), HCl concentration (150-1000 ppm), and humidity (0-11% M) was studied. The experimental results show that in the first few seconds a very fast chemical reaction occurs, with a reaction rate constant per unit surface area of solid larger than 10(-3) ms(-1). This reaction was found to be first order relative to HCl concentration and its mechanism is apparently independent of the presence of moisture. However, the relative humidity of the gas has a major impact on the progress of the reaction: when no moisture is present the reaction stops after a short period of time (2-3 min), with very low maximum solid reactant conversions (<5%). For the experiments with humidified gas an almost complete conversion of Ca(OH)(2) was obtained after about 40 min of reaction time. For this case the grain model with product layer diffusion limitations is in very good agreement with the experimental results. The diffusion coefficient in the product layer obtained through this model varied from 10(-13) to 10(-11) m(2) s(-1) and the activation energy for this parameter was estimated at approximate to 19 kJ mol(-1) for the range of temperatures studied. A simple linear relationship describes well the effect of relative humidity of the gas on the diffusion coefficient in the solid product layer. In the presence of humidity, the very high conversions of the solid reactant show a good potential applicability to continuous dry-scrubbing of HCl at low temperatures.