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

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 NO_x impurities lowered the NO₂ threshold level for acid dropout, necessitating more rigid specifications for CO₂ quality.