The design and operation of processes involved in Carbon Capture, Utilisation and Storage, such as cryogenic separation of non-condensable components from CO2 streams, as well as CO2 multistage compression, liquefaction, transport and storage, require using process simulation tools that rely on accurate and robust models predicting vapour-liquid phase equilibria in impure CO2 streams captured from power plants and industrial installations. Of particular interest are the development and validation of short-cut engineering methods that combine simplicity of the mathematical description with the accuracy of most rigorous equations of state models.

In this work, in order to describe the Vapour-Liquid Equilibrium (VLE) in binary mixtures of CO2 with non-condensable impurity gases typically found in industrially captured CO2 streams, a fugacity-activity method utilising Henry’s law is applied. The validity of the method is assessed against predictions using reference equations of state (EoSs) for binary CO2 mixtures with N2, H2, O2, CO, Ar and CH4. Using a simple modelling approach based on Raoult’s and Dalton’s laws was shown to predict the bubble- and dew-point data with low accuracy (less than ca. 60%), even in the infinite dilution limit and at low temperatures. To get more accurate results, a model was constructed where the solvent vapour fugacity coefficient is approximated using the truncated virial EoS and the Poynting correction is applied to the liquid solvent fugacity coefficient. The proposed model enabled predictions of the VLE phase compositions with accuracy ca. ±20% for CO2 mixtures with N2, O2, CO, Ar and CH4 and +10%/–60% for CO2 -H2, in the range of up to 10% solute mole fractions and temperatures below 273 K.

Authors: Sergey Martynov, Richard T. J. Porter and Haroun Mahgerefteh