Estudo da captura de dióxido de carbono por misturas de hidróxido de cálcio e óxido de magnésio em baixas temperaturas

Abstract

The present work has studied the carbonation reaction of hydrated lime modified by small percentages of magnesium dioxide (MgO) by the carbon dioxide (CO2) from the exhaust gas of a petrol engine. For this purpose, spherical pellets of solid adsorbents were synthesized and submitted to carbonation essays in a lab-scale fixed bed reactor.The samples conversions were then evaluated and the obtained data were analyzed considering kinetic and diffusion models. A qualitative thermodynamic analysis of the process was also performed aiming to verify the spontaneity condition. The kinetic parameters were determined through the application of Lees model (LEE, 2004), while the thermodynamic parameters were estimated by means of a DTA analysis. The diffusion coefficients and other system parameters were calculated through the application of a simplified model developed based on the general characteristics of the system and Fick´s second law. The results showed that at low temperature carbonation of lime in the presence of steam presents a kinetic behavior similar to the reaction between CO2 and CaO at temperatures above 300ºC. It was also verified that thereaction is kinetic and thermodynamically favored for the conditions studied. The kinetic, however, is strongly influenced by the adsorbent pore size, the calcium active species dispersion on on on the substrate surface and by the additive concentration (which can lead to a structural improvement due to the formation of a microporous structure that disperses the calcium active species on the structure). A balance of these factors indicated the presence of an optimum MgO concentration between 5% and 10% by volume for the conditions studied. This result can be atributted to their higher total capacity of capture of CO2, increased permeation of the gas inside the substrate and a high reactivity presented by these concentrations through a long period of time. The developed model has proved satisfactory in estimating the system parameters, showing correlation coefficients above 0.95 for the estimated conversions. The proposed model has the advantage of being able to predict parameters of any gas-solid reaction with porous adsorbents at low temperature from a simpler formulation that requires lesscomputational cost. Thus, this work represents a simple, fast and efficient methodology to evaluate optimal compositions of mixtures of adsorbent compounds for the capture of any gas with particular attention to carbon dioxide.