Rate models for limestone dissolution: A comparison

Abstract

The use of the film theory model for the dissolution of limestone has been compared with an empirical dissolution model based on the assumption of reaction rate control. Both dissolution models were developed for limestone-water systems. The film theory model consists only of well-known physical constants and bulk concentrations. However, the reaction control model requires three rate constants which must be experimentally determined. The hydrodynamic parameter must be determined for both models. Results of dissolution experiments, performed on a rotating limestone cylinder system, were used to test the ability of these two models to predict the dissolution rate of limestone in aqueous systems, where the mass transfer of the dissolving species is the rate limiting step. Of specific significance was the ability of each model to predict the dissolution rate at different pH-values, CO 2 partial pressures and hydrodynamic conditions at constant temperature. The film theory model describes the rate of limestone dissolution quite satisfactorily within the conditions investigated. Due to the good model-experiment agreement, the dissolution process can be described by a mass transfer model such as the film theory model. In a CO 2-free environment, the comparison reveals that the film theory model predicts the dissolution rate with higher accuracy, closely describing the dependence of pH on the dissolution rate. However, in a CO 2 environment, the models give a similar correlation to experimental data. The good correlation for the reaction control model in a CO 2-atmosphere is highly dependent on the three rate constants which must be experimentally determined. In summary, the film theory model contains only one parameter which must be experimentally determined. This makes the film theory model easier to apply when considering the dissolution rate of limestone in the mass transfer controlled region. The film theory model provides a reasonable mechanism for the dissolution process in the mass transfer controlled region, based only on theoretical considerations.