Effect of residence time on the degree of CaCO3 precipitation in the presence of an anti-scalant

The present work examines the effect of residence time on the degree of CaCO3 precipitation in a continuous flow stirred tank reactor in the presence of the inhibiting additive sodium hexametaphosphate (SHMP). The degree of CaCO3 precipitation and the degree of SHMP adsorption on the crystallizing particles were both measured in a series of experiments performed with solutions at the same initial supersaturation level residing in the reaction vessel at various retention times in the range of 20 to 150 min. Variation of the SHMP concentration in the feed gave the anticipated trend of a decrease in the degree of CaCO3 precipitation with an increase in inhibitor concentration. However, surprisingly, increase of the residence time at a constant SHMP dosage level led to a diminished degree of CaCO3 precipitation. The rate of SHMP adsorption on the crystallizing CaCO3 particles was found to vary inversely with the square root of residence time. This result suggests that adsorption of the SHMP inhibitor on the crystallizing CaCO3 particles is a slow, diffusion-controlled process. Precipitation is increasingly inhibited at an increased residence time due to the augmented level of the adsorbed SHMP inhibitor. A diffusion-controlled adsorption model enabled successful correlation of all kinetic data.