Modeling the Chemistry of Corrosive Dropout in Dense-Phase CO2 Transport Systems Using the MSE Model

Abstract Experimental observations at the Institute for Energy Technology (IFE) have shown that certain impurities in CO2 streams (e.g., NO2, SO2, H2O, H2S, O2) can cause an acid condensation event, leading to potentially severe corrosion of transportation assets. The formation of such acidic phases is believed to be triggered by reaching a certain impurity limit. 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 it changes significantly depending on the state of CO2—whether it is in the gas, liquid, or supercritical phase. The formation of sulfate/nitrate-containing phases further complicates the solubility behavior of impurities in multiphase CO2-rich environments. The Mixed-Solvent Electrolyte (MSE) framework, which is the only model capable of capturing such complex multiphase behaviors, 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 sub/supercritical CO2 phases. Additionally, the MSE model was used to predict the impurity limit for formation of acid hydrates at low temperatures, which is expected to be particularly important for the transportation of chilled CO2. It was found that the possibility of formation of ammonium-containing precipitations as a result of the reduction of NOx impurities lowered the NO2 threshold level for acid dropout, necessitating more rigid specifications for CO2 quality.