The Impact of Impurities on Thermophysical Properties of CO₂ Rich Systems

Carbon Capture Utilisation and Storage (CCUS) has attracted much attention as a solution to reduce CO₂ emissions. Employing complementary policies, governments are encouraging industrial decarbonisation worldwide. The number of projects deploying CCUS in different industries, including hydrogen and clean fuels, iron and steel, and cement, is growing1. Over the past decade, the annual capacity of CO₂ capture and storage in operational CCUS projects has increased to around 40 million ton. The main component of the target fluid in all CCUS projects is carbon dioxide. However, the usual presence of impurities (e.g., CH₄, C₂H₆, H₂O, He, O₂, Ar, N₂, H₂, SO₂, H₂S) even traces of them, may alter the thermophysical properties with respect to a pure CO₂ fluid. For some properties (e.g., critical point, phase envelope and hydrate domain), the impact may be significant. As these properties are among the main criteria for design, optimisation and control processes, precise knowledge of these properties is essential for safe and efficient operations. Although thermodynamic models can estimate fluid properties, the lack of experimental data will lead to model predictions below acceptable accuracy for engineering applications. Therefore, more experiments are needed for model tuning in order to address the impact of impurities on the thermophysical properties of CO₂ rich mixtures. This manuscript aims to highlight some experimental and modelling studies conducted as part of an ongoing Joint Industry Project (JIP) to obtain selected thermophysical properties of CO₂ rich mixtures and acid gases. This research project started in 2010 with financial support from some major companies, including TotalEnergies, Linde, Equinor, Petronas, Petrobras, Galp, etc. Most experimental measurements have been conducted at the Hydrate, Flow Assurance and Phase Equilibria research group at Heriot-Watt University, and some measurements have been done at laboratories of the project partner, Mines ParisTech. Within the JIP, a comprehensive literature review was conducted as the first step to check the availability of thermophysical properties of CO₂ rich mixtures. This literature review focused on CO₂ streams in the presence of various impurities. It targeted the following properties of the stream: density, speed of sound, phase equilibrium, viscosity, thermal conductivity, hydrate dissociation condition, interfacial tension, CO₂ freeze-out, solubility of impurities in CO₂ (e.g., methane, water, mercury), heat capacity, and Joule-Thomson coefficient. This review indicated three main issues with literature data. First, there were gaps in literature data for domains of pressure, temperature, and molar fraction that are relevant for CCUS applications. Second, no data was found in the open literature for mixtures of CO₂ with some impurities (e.g., NH₃, Acetaldehyde, Amines) regardless of the target property. Finally, literature data were scarce for some thermophysical properties (e.g., Joule-Thomson, heat capacity) regardless of the impurity type. We concluded our literature review by building a database of all published experimental data. Since then, this database has been updated regularly to include results of measured properties from the JIP together with new experimental data published in open literature. As an essential part of this JIP, experimental measurements have been conducted to obtain the thermophysical properties for various binaries and multicomponent CO₂ mixtures, over a wide range of pressure and temperature. These data have been used to improve the performance of selected cubic and multi-parameter equations of state (EoSs). To facilitate the use of these EOSs, the models and their parameters are implemented in an in-house software called HWPVT. The software provides the user with the opportunity to test various EoSs (i.e., PR, SRK, VPT, PC-SAFT, CPA, sCPA, GERG-2008, HWGERG and many reference EoSs for pure components) and correlations to estimate properties of fluids. Finally, as suggested by some industry partners, the database created in this project was used to benchmark two commercial simulators, Honeywell Unisim® Design R480, and MultiflashTM (developed by KBC), to evaluate the performance of these simulation tools in CCUS applications.