Kinetic Modeling of the Reaction of HCl and Solid Lime at Low Temperatures Ana M. Fonseca, † Jose ´ J. O Ä rfa ˜ o, ‡ and Romualdo L. Salcedo* ,‡ Universidade Fernando Pessoa, Departamento de Cie ˆ ncia e Tecnologia, Prac ¸ a 9 de Abril, 4100 Porto, Portugal, and Faculdade de Engenharia da Universidade do Porto, Departamento de Engenharia Quı ´mica, R. Bragas, 4099 Porto, Portugal 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 ≈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. Introduction Hydrogen chloride emissions are formed during com- bustion of fossil fuels such as chlorine-rich coals and in incineration of many wastes. Recently, due to increas- ingly severe legal emission limits and high costs associ- ated with traditional scrubbing processes, there has been a great effort in research to develop new processes that are both efficient and cost-effective. Dry-scrubbing of acid gases is a relatively recent technology for the removal of toxic acidic components from gaseous effluents, whereby finely divided dry powder is injected into the flue gas stream. Calcium based sorbents are widely used due to their low cost and high sorbent capacity for removal of SO 2 , HF, and HCl. According to Weinell et al., 1 there is a high potential for dry-scrubbing of HCl using Ca(OH) 2 at low temper- atures and high humidity levels: their experimental results show almost full conversion of lime below 420 K in the presence of moisture. Also, the use of dry- scrubbing at low temperatures allows the recovery of energy from flue gases by lowering temperatures through heat recovery boilers and economizers to 400 K. 2 How- ever, the literature data on the reaction of HCl with Ca(OH) 2 below 400 K is scarce. The kinetics of the reaction was studied by Karlsson et al. 3 in the temperature range 420-670 K in a fixed-bed laboratory reactor where the solid particles of Ca(OH) 2 were dispersed in a sand layer. Their results reveal a high HCl removal ef- ficiency but the Ca(OH) 2 conversions were lower than 40%, thus indicating a poor utilization of this reactant. These authors propose a first-order reaction relative to both reactants. Carminati et al. 4 conducted experiments in a pilot-plant with injection of an aqueous suspension of Ca(OH) 2 in a hot flue gas containing HCl (“spray- dryer” process). Their acid removal efficiencies were higher than 75% for the lowest temperature (450 K) with feed ratios Ca(OH) 2 /HCl between 7 and 8 times the stoichiometric ratio. These authors suggest a complex reaction order relative to HCl, presumably higher than one. Weinell et al. 1 achieved almost complete conversions of Ca(OH) 2 in a differential fixed- bed reactor at low temperatures (330-420 K) in the presence of humidity (4-15% M). According to these authors, the reaction was governed by diffusion in the solid phase, and the high conversions observed in the presence of humidity were due to the formation of a partially liquid product phase. Recently Duo et al. 5 suggest that the rate-limiting step for this reaction is altered with the reaction progress, changing from chemical reaction control to product layer diffusion * To whom correspondence should be addressed. Phone: +351 2 2041644. E-mail: rsalcedo@fe.up.pt. † Universidade Fernando Pessoa. ‡ Faculdade de Engenharia da Universidade do Porto. Ca(OH) 2 (s) + 2HCl(g) f CaCl 2 ‚2H 2 O(s) 4570 Ind. Eng. Chem. Res. 1998, 37, 4570-4576 10.1021/ie980320f CCC: $15.00 © 1998 American Chemical Society Published on Web 11/12/1998