Hydrate and phase behavior modeling in CO₂-rich pipelines

Carbon dioxide transport will be a key part of any Carbon Capture and Storage (CCS) system. Generally for CCS, flow assurance modeling of these pipelines has been restricted to pure CO₂ systems; however CO₂ streams coming from capture processes are not pure and contain various components. These fluids have a certain level of moisture, so dehydration is needed before delivering CO₂ rich fluid to the export pipeline, in order to prevent potential hydrate formation, two-phase flow and corrosion in the export line. In this contribution, the hydrate phase equilibria of the binary CO₂ + (N₂ or CH₄ or O₂ or Ar or CO) systems in the presence of a free water phase were determined as well as direct hydrate dissociation conditions of liquid CO₂ in absence of a free phase. A rigorous and generalized model is presented to predict the phase behavior, hydrate dissociation pressures and the dehydration requirements of CO₂ rich gases. A statistical thermodynamic approach, with the Cubic-Plus-Association equation of state, is employed to model the phase equilibria. The hydrate-forming conditions are modeled by the solid solution theory of van der Waals and Platteeuw. Predictions of the developed model are first validated using simple systems and then for more complicated synthetic multicomponent systems. Showing that accurate phase behavior prediction of CO₂ rich stream is critical for accurate predictions of the hydrate phase behavior of these fluids.