Molecular simulation of CO₂ mixtures

Abstract

Climate change has become one of the most urgent environmental issues due to the rapid reduction of anthropogenic carbon dioxide emissions. Reducing harmful CO2 emissions is possible through carbon capture and storage (CCS). CCS technologies rely on accurate thermodynamic property estimations of carbon dioxide mixtures with impurities to design and operate CO2 transport processes. Consequently, researchers have begun to pay increasing attention to studies related to thermodynamic properties, particularly vapour–liquid equilibrium (VLE) of CO2 mixtures. Conventional methods to obtain these properties, such as experiments or empirical equations of state, however, have many limitations. The phase equilibrium property can be determined using the free energy, which is the criterion for stability. However, it is extremely challenging to calculate the free energies of fluids using computer simulation. A method for obtaining the absolute Helmholtz free energy (F) was developed in this thesis. This thesis aims to use the Density of States (DOS) Partitioning Monte Carlo (MC) method to accurately and unambiguously calculate the free energy to predict the phase coexistence characteristics of binary mixtures of CO2 with SO2, H2S and N2, which are typical impurities in the captured CO2 stream. The computed VLE from the DOS partitioning method is in good agreement with available experimental data, the equation of the state model and MC simulations. Other binary mixtures, especially those containing CO2, can also be explored using this methodology.