Impact of impurities on the phase behavior of CO₂ in pipeline transportation

Carbon capture, utilization and storage (CCUS) is an important option to improve the climate, for which safe transport is a prerequisite. Pipeline transportation is considered as the optimal form to transport carbon dioxide (CO₂) on land. However, impurities contained in CO₂ pipeline have impact on phase equilibrium and variation of physical parameters of CO₂, thereby the operation of the pipeline. Compared with the known experimental methods, state equations are selected to calculate the phase equilibrium line and physical parameters of multicomponent CO₂ containing impurities and water vapor. A comparative analysis of pure component and multicomponent CO₂ reveals that PR equation is better than other state equations in the aspects of the vapor-liquid phase equilibrium and physical properties of CO₂ and other polar systems. Nonpolar and weakly polar impurities that affect the bubble-point line expand the two-phase region while strongly polar impurities that affect the dew-point line expand the two-phase region. During pipeline transportation, density and viscosity of CO₂ have mutations at a certain temperature and pressure and heat capacity takes on extreme point, while the impurities will change the mutations and the corresponding extreme temperature and pressure. Supercritical state transportation under high-temperature and high-pressure can make it more stable of pipe flow.