Process intensification of liquid phase and gas-liquid precipitation of calcium carbonate in narrow channel reactors

Abstract

A fundamental study of liquid phase and gas-liquid precipitation of calcium carbonate in narrow channel reactors is presented. Precipitation is strongly dependent on transport rates, as the final product properties are determined by local values of the concentrations during the reaction. The high transport rates available in narrow channels provided the intensification of a precipitation process in this type of equipment. Liquid phase precipitation of calcium carbonate was studied in two square cross section reactors with diagonal lines of I and 2 mm. Flow rates ranging between 0.15 and I CM3/S were used. Mixing efficiency in the narrow channels was characterized with the iodide-iodate reaction scheme. The results showed that mixing efficiency increased with Reynolds number. The mean particle size of calcium carbonate precipitate was found to range between 3 um and 8 um approximately. The narrow channels allowed production of particles similar or smaller in size than in a well stirred batch reactor. Liquid phase precipitation was ftirther characterized by monitoring the change in electrical conductivity, during precipitation, in a narrow channel reactor with embedded electrodes. This system allowed a preliminary study of precipitation rate and limited data on scaling by solid deposits were also obtained. Gas-liquid precipitation of calcium carbonate was studied in a square cross section 2 mm diagonal line narrow channel reactor. The carbonation reaction between a calcium hydroxide solution or suspension andC02 gas was considered. Liquid flow rates between 0.35 and 1.48 CM3/s and carbon dioxide flow rates between I and 8.22 CM3/S were used. Flow patterns for the water-carbon dioxide system were visually characterized. The experimental volumetric mass transfer coefficients for precipitation from a calcium hydroxide solution were found to vary between 0.33 and 0.8 s-1. Particle size and morphology were characterized on selected calcium carbonate samples obtained in both cases from a calcium hydroxide solution and suspension.