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Ind. Eng. Chem. Res. 1999, 38, 1316-1322
Kinetics of the Reaction of Ca(OH)2 with CO2 at Low Temperature Shin-Min Shih,* Ch’un-Sung Ho, Yuen-Sheng Song, and Jyh-Ping Lin Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan 106, Republic of China
The kinetics of the reaction of Ca(OH)2 with CO2 in humid N2 has been studied at 60-90 °C by using a differential reactor. Ca(OH)2 reacted with CO2 to form CaCO3 only when the relative humidity exceeded 0.08. The reaction ceased before 1 h, and Ca(OH)2 was incompletely converted. The reaction rate and final conversion of Ca(OH)2 were significantly affected by relative humidity, were slightly dependent on temperature, and were zeroth order with respect to gas phase CO2 concentration. The carbonation of Ca(OH)2 was well described by the asymptotic equation which is obtained by assuming chemical reaction control and considering the surface coverage by product. The rate-controlling step might be the dissolution of Ca(OH)2 at the water-adsorbed surface. The reaction behavior of Ca(OH)2 with CO2 was similar to that with SO2. The model derived for the reaction with CO2 may be applied to that with SO2 with proper modifications. The results of this study are useful to the design and efficient operation of the processes using hydrated lime to remove SO2 from the flue gas, in which both SO2 and CO2 are present. Introduction Hydrated lime has been vastly used in dry and spraydrying flue gas desulfurization (FGD) processes for absorbing SO2. Due to the abundance of CO2 (about 12 vol %)1 in the flue gas, the reaction between CO2 and hydrated lime may affect the SO2 removal efficiencies of these processes. There are some reports about the reaction between SO2 and Ca(OH)2 in the presence of CO2 in humid N2,2-4 however, the role of CO2 in the said FGD processes has not been studied. Recently Ho5 and Ho et al.6 found that the presence of CO2 enhanced the reactivity of hydrated lime towards SO2. Hydrated lime has long been known as a good sorbent for absorbing atmospheric CO2 to form calcium carbonate.7 Most of the research on the reaction between CO2 and Ca(OH)2 has been done for the case that CO2 reacts with a slurry of Ca(OH)2 to form small size CaCO3.8,9,10 Few papers on the carbonation of dry Ca(OH)2 have been reported.11,12 Since the basic data about the reaction between CO2 and Ca(OH)2 under conditions similar to a commonly used dry or spray-drying FGD process are so scarce, there is a need to undertake such a study. In the present work, the kinetics of the reaction of Ca(OH)2 powder with CO2 in humid N2 at temperatures of 60-90 °C was studied by using a differential reactor, and the kinetic model was derived. Experimental Section The hydrated lime used was reagent grade Ca(OH)2 powder (Hayashi Pure Chemical Industries). Its specific surface area measured by nitrogen adsorption using BET method was 10.0 m2/g. The true density measured by volume displacement using helium was 2.21 g/cm3. The porosity measured by mercury intrusion was about 49%. The particle size distribution measured by laser diffraction was in the range
