Integrated model of sulphur dioxide absorption into limestone slurry in an agitated vessel

Abstract

An investigation was undertaken to develop and validate an integrated model for ab-sorption of sulphur dioxide involving a gas-slurry system consisting of limestone in order to contribute to the understanding and modelling of sulphur dioxide absorption in the absorber section of industrial Wet Flue gas desulphurisation processes. The experimentation was executed in a laboratory scale agitated reactor with a very di-lute mixture of sulphur dioxide (2000 to 3000 ppm) with direct on-line measurement of important parameters and on-line sampling for subsequent measurements as a function of time. The research programme consisted of three parts involving sepa-rate experimentation and modelling respectively, with the first two parts confined to the validation of sub-models and the generation of parameters required for the fi-nal integrated modelling.For the third part of the investigation, an integrated model was developed consisting of sulphur dioxide absorption and dissolution, limestone dissolution, calcium sulphite crystallization and carbon dioxide liberation. The modelling results were compared with experimental results for absorption in aqueous solutions and limestone slurries respectively. The model developed here comprises of seven differential equations with two equations for the concentrations of SO2 and CO2 in the exit gas, three equations for the total concentration of sulphur, carbon and calcium in the liquid and two more equations for the solids concentrations of calcium carbonate and calcium sulphite in the slurry phase. The model parameters were fitted to the experimental results and the sensitivity to various phenomena (gas side mass transfer, liquid film transfer, dissolution rate and precipitation rate) were investigated. The estimated parameters were found to agree with results obtained for dissolution of the limestone determined separately (Part 1) and the mass transfer co-efficients determined with aqueous solutions with-out limestone with different initial pH values (Part 2).The agreement of the model with experimental results were found to be satisfactory. An analysis of the occurrence of different mass transfer reaction regimes (gas and liquid mass transfer and dissolution of limestone) during the experimental period with varying pH is also reported.