The present work examines the chemical absorption of carbon dioxide (CO 2 ) gas in aqueous calcium hydroxide (Ca(OH) 2 ) slurries, which leads to the precipitation of calcium carbonate. Carbonation of the Ca(OH) 2 slurry is an important method for the industrial manufacture of precipitated calcium carbonate (PCC), wherein the hydroxide particles are gradually converted to carbonate. Experimental data have been generated by contacting CO 2 with an aqueous Ca(OH) 2 suspension in a model gas-liquid, stirred contactor, under different operating conditions. It is not possible to distinguish between the different types of particles present in the system (hydroxide, carbonate, mixed particles) "visually". Therefore, the particle size distribution of all the particles present has been obtained using microscopy and optical image analysis techniques. The conversion of hydroxide-to-carbonate is determined by titration. A modeling approach that assumes that the hydroxide particles get converted by the diffusion of dissolved CO 2 into its internal parts (as is observed in a "shrinking core" scenario), coupled with a population balance to describe the size distribution of the unreacted particle cores has been formulated. The experimental and theoretical conversions agree reasonably well.
The problem of gas absorption with reaction, in a slurry containing fine particles, is important in the development of processes for the removal of acidic pollutants such as sulfur dioxide. Typically, lime and lime stone slurries can be used for removal of sulfur dioxide. However, magnesium hydroxide slurries may yield a higher scrubbing capacity as a result of the more soluble reaction product, magnesium sulfite, compared to the corresponding calcium salt. In this study, absorption experiments were carried out in a stirred cell and the resulting data is analyzed using the model proposed by Mehra [Chem. Eng. Sci. 1996, 51, 461-477]. The proposed theory incorporates the process of particle dissolution and the consequent change in particle size near the gas-liquid interface, using Higbie's model of mass transfer with chemical reaction. A population balance approach is used to track the particle size distribution.
The phenomena of precipitation, driven by a reaction, in a gas−liquid system is studied, theoretically as well as experimentally, using the carbon dioxide−calcium hydroxide−calcium carbonate system. Experimental data was generated by contacting carbon dioxide gas with aqueous calcium hydroxide solution in model gas−liquid, stirred contactor, at different operating parameters. The particle-size distribution of the precipitated calcium carbonate particles was found using microscopy and optical image analysis techniques. A modeling approach, based on Higbie's penetration theory, for the diffusion−reaction−precipitation system, combined with population balances for the particles, has been proposed. The model computations and the experimental results agree reasonably well.
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