Thermodynamic Framework for Cryogenic Carbon Capture

Carbon dioxide capture and storage (CCS) is an important option for climate change mitigation and it has been extensively analysed in recent years to face the climate challenge. A portion of the emitted CO2 comes from fossil fuel power plants. Several post-combustion technologies are available for separating CO2 from the flue gases produced by the combustion of fossil fuels. In recent years, low-temperature/cryogenic technologies have been investigated for this purpose, which rely on the fact that CO2 can be separated out of flue gas by freezing it out. As a consequence, when dealing with the design of this type of processes, it is of paramount importance to be able to satisfactorily predict the thermodynamic phase behaviour of the system of interest, which involves equilibrium conditions also in the presence of solid CO2. The classical approach for phase equilibria calculations involving a solid phase is based on the equality of components' fugacities in the different phases and on the use of an expression for the fugacity of the freezing component in the solid phase that can be derived by relating it to its fugacity in the vapor phase following a proper thermodynamic cycle. This work compares the predictions for solid-vapor equilibria (SVE) conditions of a flue gas mixture that are obtained using such a classical approach with those obtained using the RGibbs calculation block available in the Aspen Plus (R) process simulator. The latter one enables SVE calculations by minimizing the Gibbs energy. The obtained results are useful for determining suitable operating conditions for the separation process, depending on the desired level of CO2 recovery to be achieved.