Models for Calculating Corrosion Rates in Water-Saturated and Under Saturated CO₂ Systems & Water Solubility in CO₂ Systems at Supercritical Conditions

In water-containing dense phase CO₂ environments, which are typical conditions seen in Carbon Capture and Storage (CCS) systems, corrosion rate increases in conjunction with humidity, typically reaching a critical humidity, beyond which corrosion rates become particularly high. In this work, we discuss the development of a model to calculate corrosion rates in CO₂ systems, both pure as well as those containing impurities, with under-saturated water content at dense and supercritical conditions.

A new model is developed to predict corrosion rates in CO₂ systems, both in presence and absence of impurities, with under-saturated water content at supercritical conditions while existing in-house models are used to predict corrosion rates at saturated water content. The model to calculate corrosion rates at under-saturated water conditions uses the solubility of water in CO₂ rich phase at saturated conditions. A Peng-Robinson equation of state, E-PPR78, available in literature, is modified further to match water solubility in CO₂ rich phase in the presence and absence of impurities at supercritical conditions and on both sides of the two phase-region.

We present results from the modified E-PPR78 model, validating calculated water solubility in the CO₂ rich phase against experimental data available in literature for pure CO₂ as well as for CO₂ systems with impurities. The results presented cover a wide range of pressures and temperatures both in sub-critical and supercritical regions. For pure CO₂ with water system, the error in the calculated water solubility in CO₂ rich phase is less than 4% in sub-critical region and is less than 7% in the super-critical region. The presence of impurities in the CO₂ stream influences the solubility of water and hence, the humidity and corrosivity of the CO₂ stream. We present calculated water solubility of water in CO₂ rich phase in CO₂ and N₂ mixture for different N₂ concentrations and validate it against experimental data found in literature.

An empirical correlation was developed based on experimental data to predict corrosion rates in CO₂ systems at under-saturated water conditions in the presence of impurities. The empirical correlation to calculate corrosion rate at water under-saturated conditions uses corrosion rate at water-saturated conditions, calculated from an in-house tool, and the water solubility in CO₂ rich phase, calculated from modified E-PPR78 equation of state model.