Assessment of modeling the MgO-CaO-CO₂-SO₂-H₂O-O₂ system using the electrolyte NRTL activity coefficient model

Abstract

This work assesses the thermodynamic modeling of the MgO-CaO-CO2-SO2-H2O-O2 system using the electrolyte NRTL activity coefficient model, primarily focusing on the solubility of potential salts in the system. The assessment includes the SO2-H2O and SO2-Mg(OH)2 vapor–liquid equilibria as well as the precipitation of Mg(OH)2, Ca(OH)2, MgSO3, CaSO3, MgSO4, CaSO4, and their hydrate forms. The analysis covers a temperature range of 0 to 100 °C and focuses on calculations at atmospheric pressure. The performed calculations assess the necessity of defining equilibrium constants Keq as a function of temperature to describe the chemical equilibria accurately. The SO2 solubility in water is studied for a pressure range of 0.545 to 1.788 bar. The SO2-Mg(OH)2 absorption equilibrium is studied for a SO2 partial pressure range of 0.00963 to 1.101325 bar and a MgO concentration of up to 14.5 kg/m3 H2O. The results are evaluated using experimentally determined data from the literature. The study shows that the model recognizes all reported precipitation forms in the correct temperature range in chemically stable systems. The solubility of Mg(OH)2 is calculated with a deviation from literature data of <6% for a temperature range of 70 to 90 °C and a maximum deviation of −40% for temperatures close to 0 °C. The calculated Ca(OH)2 solubility at the complete studied temperature range deviates less than 11% from the literature data. The model also recognizes the pH dependency of the solubility of both hydroxides. The calculated solubility of MgSO3 hexahydrate deviates less than 1% from literature data, and the calculated solubility of MgSO3 trihydrate shows a maximum deviation from literature data of 20% at temperatures up to 90 °C. The solubility of CaSO3 hemihydrate is predicted with a deviation of <1% at a temperature range of 70 to 100 °C and a deviation of <30% for temperatures higher than 20 °C. The model predicts the solubility of MgSO4 monohydrate with a maximum deviation from literature data of 20% and overestimates the solubility of MgSO4 heptahydrate by up to 118% compared to literature data. The solubility of CaSO4 monohydrate is calculated with a deviation from literature data of <2%, and the solubility of CaSO4 dihydrate with a deviation of <7%. The model’s accuracy in predicting the SO2 solubility in water variates strongly depending on which literature data points it is compared to. The smallest deviation from literature data is 1% compared to data measured at 60 °C and a pressure of 1.377 bar. The highest deviation is 60% compared to data measured at 90 °C and atmospheric pressure. The study shows that the experimental data in literature describing the SO2 absorption in an Mg(OH)2 solution are scarce. This leads to a limited ability to evaluate thermodynamic models based on experimental data. The model describes the SO2 absorption in Mg(OH)2 with a deviation between 8 and 52% compared to available literature data.