Soft-SAFT equation of state as a valuable tool for the design of new CO₂ capture technologies

The design, simulation and/or optimization of new processes rely on the availability of robust and accurate models or equations of state (EoS). However, traditional cubic equations of state (EoS's), traditionally used in many process simulators, fail on describing complex polar and associating behavior of some molecules, leading to unreliable results and hence, poor design and optimization. This problem can be overcome with the use of robust, reliable equations of state. This work belongs to a long term project assessing the performance and usefulness of an advanced EoS (soft-SAFT), as a valuable tool for the description of highly non-ideal systems and thus, for the reliable simulation/design of new technologies. We focus here on assessing the validity of soft-SAFT, a molecular-based EoS, for the development of novel technologies for CO₂ capture using polyether blends as solvent. Within soft-SAFT polyether molecules are modeled as chains with end-groups having an association site, explicitly mimicking the hydroxyl end-groups. The study comprises polyethers, including glymes, and their mixtures with CO₂ at different conditions. It is shown that soft-SAFT is able to successfully describe the thermodynamic behavior (e.g. vapor pressures and liquid densities) of these solvents in wide temperature and pressure ranges. Moreover, by explicitly considering the quadrupolar moment of CO₂, and using one, temperature and pressure independent binary interaction parameter, an accurate description of the gas solubilities in several polyethers was achieved. For glymes, which among polyethers exhibit the highest CO₂ solubilities, such parameter was found to correlate with the molecular weight of the solvent. Finally the equation was used to predict the thermal and pressure cycle capacities of the different solvents. Results presented here reinforce the use of soft-SAFT as a reliable tool for solvent screening, offering reliable predictions of phase equilibria and solubility behavior in a wider number of systems.