Modeling the chemistry of corrosive dropout in dense-phase CO2 transport systems using the Mixed-Solvent Electrolyte (MSE) model

Experimental observations have shown that certain impurities in CO2 streams (in particular, NO2, SO2, H2O, H2S, and O2) can cause acid condensation events, leading to potentially severe corrosion of CO2 transportation assets. The formation of such acidic phases is believed to be triggered by reaching certain impurity limits. This can occur under difficult-to-predict conditions, which commonly used thermodynamic frameworks cannot predict. The solubility of impurities in CO2-rich phases is intricately influenced by temperature and pressure, and changes significantly depending on the state of CO2, whether it is in the gas, liquid, or supercritical phase. The formation of sulfate- and ammonium-containing phases can further complicate the solubility behavior of impurities in multiphase CO2-rich environments. The Mixed-Solvent Electrolyte (MSE) framework is utilized in this study to determine the threshold impurity concentrations at which acid dropout may occur, as well as to perform speciation calculations in gas, liquid, or supercritical CO2 phases. The model also predicts the concentration of the acid dropout phases. Additionally, the MSE model predicts the impurity limits for the formation of acid hydrates at low temperatures, which is expected to be particularly important for the transportation of chilled CO2. It has been found that the possibility of formation of ammonium-containing precipitates as a result of the reduction of NOx impurities lowers the NO2 threshold level for acid dropout, necessitating more rigid specifications for CO2 quality.