Phase diagrams of CO2 and CO2 -N2 gas mixtures and their application in compression processes

Phase diagrams of carbon dioxide, nitrogen and their mixtures with different amounts of nitrogen (e.g. 5 mol%, 10 mol% N2) were calculated with high accuracy with the NIST Reference Fluid Thermodynamic and Transport Properties database program REFPROP® for up to 200 bar, as well as density-pressure diagrams. Beside the use of carbon dioxide as a solvent for supercritical fluid extraction, increasing interest in physical chemical properties of CO2 exists as Carbon dioxide Capture and Storage (CCS) starts to play an important role to limit anthropogenic emissions of CO2 into the atmosphere. Therefore CO2 processing and pipeline technologies became of considerable commercial importance, especially in the context of future underground storage of CO2. As example the CO2 compression process with gas separation membrane in post-combustion capture was simulated with PRO/II® software. Through adopting several different equations of states, phase diagrams of different CO2 N2 gas mixtures were calculated and compared with the aforementioned accurate thermodynamic and transport properties calculation results. This type of validation is very useful for process engineering analysis. Further investigations related to the compression process were carried out for the different CO2 gas mixtures, which are generated from the gas separation membrane capture process. Specific energy of the compression process was analysed for each gas mixture. The energy consumption and the state of the compressed compounds are strongly influenced by the N2 composition in the mixture. The impurities in multi-component CO2 mixtures vary strongly depending on the different capture technologies. The calculation of reliable phase diagrams of CO2 mixtures provides guidelines for optimization of compression, pipeline transport, and storage processes.