Given the high costs of CCUS, using shared CO2 transport and storage infrastructure and CCUS process integration in industrial clusters becomes critical for the economic rollout of CCUS at large-scale. In this context, the C4U project is aimed at developing advanced technologies for CO2 capture from iron and steel plants and optimising integration of the CO2 capture and transport solutions in industrial clusters. In WP4, the CO2 transport infrastructure optimisation is based on whole-system approach, balancing the techno-economic, environmental as well as safety and operability aspects of CO2 transport. In particular, ensuring single-phase flow and preventing formation of solid phase in CO2 transport infrastructure elements are considered critical for their efficient and safe operation. In particular, solid CO2 may form in a transport system as a result of accidental or planned rapid near-isentropic expansion of dense-phase or supercritical CO2. The CO2 solids may obstruct the flow and damage the valves, while the solids accumulated during venting/ discharge process may cause the pressure build-up due to the solid sublimation at later times – posing potential danger for the system safe operation. Assessing the risk of solid formation in CO2 transport systems, e.g., long pipelines, relies on using rigorous and accurate mathematical models.
To this end, the present report describes the development and testing of a pipeline decompression model capable of predicting the fluid phase equilibria involving solid phase for CO2 and its mixtures with impurity components.
The pertinent model utilises a composite equation of state for CO2 and its binary mixtures with non-condensable gases (such as N2, H2, O2, CO, CH4, and Ar) as well as condensable components (such as COS, C10H22 and SO2) that has been developed earlier in the project (D4.2 “CO2 pipeline operability and safety impacts”), to construct phase diagrams and evaluate physical properties of fluid in single-phase and multi-phase states (including the vapour-liquid, vapour-solid and vapour-liquid-solid equilibria) – to avoid time-consuming phase equilibria calculations when solving for the flow model equations. A comprehensive testing of the model also has been performed, including its validation against seven controlled prolonged orifice decompression experiments conducted for liquid-phase and gas-phase CO2 and also dense-phase mixture of CO2 with 3.5% mol N2 by the C4U project partners, DUT (the large-scale test pipeline, 233 mm internal diameter and the length of 257 m) and INERIS (the medium-scale pipeline of 50 mm internal diameter and 40 m long). The initial pressures and temperatures were in the ranges between 37 bar to 91 bar and 8oC to 40oC, respectively. Sensitivity studies were performed to quantify the impacts of the flow conditions and the model parameters, including conditions of heat transfer, the initial fluid temperature, the impurity concentration and the choice of EoS, on the results of simulations by the model. The report discusses in detail the results of the model validation and provides recommendations for future work.
