Kinetic Modeling of the Reaction of HCl and Solid Lime at Low Temperatures

Abstract

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 m2 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.

Dry scrubbing of acid gases in recirculating cyclones

“…Previous studies have already proven the high capacity of reverse flow cyclones to remove HCl from a gaseous stream by the injection of solid particles of slaked lime [9,19]. The aim of the present work is to verify if there are significant differences between the behavior of recirculating and simple reverse-flow cyclones for dry cleaning of acid gases.…”

“…The particles were joined together with a gaseous stream of N 2 with a controlled concentration of HCl and humidity level, immediately before the inlet of the cyclone. The effluent gas concentration is measured by bubbling a known fraction of this stream into distilled water by means of an appropriate external circulation airlift absorption vessel [19,21]. The HCl is completely absorbed in the water, and the pH of the resulting solution is monitored on-line with the aid of a computerized data acquisition system.…”

Kinetics of the dissolution of sand into alkaline solutions: application of a modified shrinking core model

“…This model needs some characteristics of the solid particles such as particle radius, molar density and porosity. Experimental data for the reaction between solid lime and gaseous HCl at low temperature have been fitted by Fonseca et al (1998) using the grain model. This model was successfully used for a solid that is made up of small nonporous grains, and each of these grains reacted according to the unreacted core model.…”

Modeling dry-scrubbing of gaseous HCl with hydrated lime in cyclones with and without recirculation

“…A range of 1.0 × 10 −3 to 1.0 × 10 5 m s −1 was set for k s , and 1.0 × 10 −12 to 1.0 × 10 −7 m 2 s −1 for dif. The wide ranges selected for these parameters is related to the variability of values found in the literature for the reaction process consisting of calcium compounds/HCl [9,15,[24][25][26][27][28][29], depending on the system considered and the reaction conditions used.…”