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 hydrog en chloride from flue gases. In this study the kinetic parameters for the r eaction between gaseous HCl and solid Ca(OH)(2) have been obtained at low t emperatures in a fixed-bed laboratory reactor. The influence of the operati ng 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 wa s found to be first order relative to HCl concentration and its mechanism i s apparently independent of the presence of moisture. However, the relative humidity of the gas has a major impact on the progress of the reaction: wh en 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 e xperiments with humidified gas an almost complete conversion of Ca(OH)(2) w as obtained after about 40 min of reaction time. For this case the grain mo del with product layer diffusion limitations is in very good agreement with the experimental results. The diffusion coefficient in the product layer o btained through this model varied from 10(-13) to 10(-11) m(2) s(-1) and th e activation energy for this parameter was estimated at approximate to 19 k J mol(-1) for the range of temperatures studied. A simple linear relationsh ip describes well the effect of relative humidity of the gas on the diffusi on coefficient in the solid product layer. In the presence of humidity, the very high conversions of the solid reactant show a good potential applicab ility to continuous dry-scrubbing of HCl at low temperatures.