Effect of common impurities on the phase behaviour of carbon dioxide rich systems: Minimizing the risk of hydrate formation and two-phase flow

CO₂ produced by carbon capture processes is generally not pure and can contain impurities such as N₂, H₂, CO, H ₂S and water. The presence of these impurities could lead to challenging flow assurance issues. The presence of water may result in ice and/or gas hydrate formation and cause blockage. Reducing the water content is commonly required to reduce the potential for corrosion but for an offshore pipeline system it is also used as a means of preventing gas hydrate problems; however, there is little information on the dehydration requirements. Furthermore, the gaseous CO₂ rich stream is generally compressed to be transported as liquid or dense-phase in order to avoid two-phase flow and increase the density of the system. The presence of the above impurities will also change the system's bubble point pressure, hence affecting the compression requirement. The aim of this communication is to evaluate the risk of hydrate formation in a rich carbon dioxide stream and to study the phase behaviour of CO₂ in the presence of common impurities. An experimental methodology was developed for measuring water content in CO₂ rich phase in equilibrium with hydrates. The water content in equilibrium with hydrates at simulated pipeline conditions (e.g., 4 °C up to 190 bar) as well as after simulated choke conditions (e.g., at -2 °C and around 50 bar) were measured for pure CO₂ and a mixture of 2 mole% H₂ and 98 mole% CO₂. Bubble point measurements were also carried out for this binary mixture for temperatures ranging from -20 °C to 25 °C. A thermodynamic approach was employed to model the phase equilibria. The experimental data available in the literature on gas solubility in water in binary systems were used in tuning the BIPs. The thermodynamic model was used to predict the phase behaviour and the hydrate dissociation conditions of various CO₂ rich streams in the presence of free water and various levels of dehydration (250 ppm and 500 ppm). The results are in good agreement with the available experimental data. The developed experimental methodology and thermodynamic model could provide the necessary data in determining the required dehydration level for CO₂ rich systems, as well as minimum pipeline pressure required to avoid two phase flow, hydrates, and water condensation. © 2009, Society of Petroleum Engineers.