(123c) Modelling the Phase Behaviour of the CO2+H2O+Amine Mixtures Using Transferable Parameters with SAFT-VR

The reduction in CO₂ emissions from anthropogenic sources has become a topic of widespread interest in recent years. As the power generation sector is by far the largest stationary-point-source of CO₂, being responsible for approximately 35% of total global CO₂ emissions¹ this issue has particular relevance for the energy sector. The current method of choice for large-scale CO₂ capture is amine-based chemisorption; typically in packed columns, with the solvent of choice being a primary alkanolamine: monoethanolamine (MEA). Despite the widespread use if this technique, there is an ongoing debate as to how best to model these systems, owing to the complex reactions and phase equilibrium they exhibit. To capture the interactions that occur in these systems, we use the statistical associating fluid theory (SAFT)². This is a molecular approach, specifically suited to hydrogen-bonding, chain-like fluids. In this contribution we use the SAFT approach for potentials of variable range (SAFT-VR³) to calculate the fluid phase behaviour of amine + H₂O + CO₂ mixtures. The molecules are modelled as homonuclear chains of bonded square-well segments of variable range, and a number of short-ranged off-centre attractive square-well sites are used to mediate the anisotropic effects due to association in the fluids.

We model MEA as a pair of tangentially bonded homonuclear spherical segments. Six distinct association sites are required to mediate the hydrogen bonding interactions exhibited by this molecule. In discriminating between potential models of MEA, we consider both symmetric models - where no distinction is made between the functional groups on MEA ? and asymmetric models of MEA ? where we explicitly consider the multifunctional nature of MEA. We find that despite giving an adequate description of the MEA + H₂O phase behaviour a purely symmetric model of MEA is not suitable for describing either the phase behaviour MEA + CO₂ binary mixture or that of the MEA + CO₂ + H₂O mixture. Further, as the chemistry of this system is well known, we note that it is not possible to preserve the stoichiometry of this mixture with a purely symmetric model of MEA. In order to reduce the number of adjustable parameters required to describe MEA, we exploit the molecular nature of SAFT-VR by transferring parameters from SAFT-VR models of alkanols and alkylamines developed in this work. We select our asymmetric model of MEA, by consideration of its ability to represent vapour-liquid equilibrium (VLE) data as well as enthalpy of vaporisation and interfacial surface tension data. Models for H₂O and CO₂ are taken from previous work⁴,⁵ with two effective "reaction" sites being incorporated in the CO₂ model to mediate the chemical interaction between the MEA and CO₂. In modelling binary mixtures of MEA+H₂O, parameters are transferred from binary mixtures of ethanol+ H₂O and ethylamine+ H₂O. Unlike interaction parameters to describe the MEA+ CO₂ mixture are determined by comparison with ternary experimental data. Our final calculations for the ternary mixture predict the phase behaviour of with an absolute average deviation in the mole fraction of CO₂ in the liquid phase 0.01. The excellent accuracy of the asymmetric MEA model and of the MEA+H2O+CO2 mixture model over a wide range of conditions makes the proposed approach suitable for incorporation in process modelling.

Building on this success, we also investigate alternative solvents. There is a well recognised requirement for an improvement on the MEA based capture methods⁶, with emphasis on increasing CO₂ loading capacity and absorption rate. Recent work⁷ has shown that alkylamines could be ideal candidates to address both of these requirements. However, before these solvents can be deployed in practice, a detailed knowledge of their phase behaviour in aqueous CO₂ mixtures is required. The unlike interaction parameters required to describe aqueous alkylamine mixtures are obtained by comparison to experimental VLE data. Due to the lack of data for the RNH₂ + CO₂ mixture, we transfer binary interaction parameters obtained from NH₃+ CO₂ mixture. Our predictions confirm that ternary mixtures of RNH₂+H₂O+CO₂, for alkyl chains with more than 2 carbon atoms, exhibit liquid-liquid immiscibility, the extent of which increases with alkyl chain length. The consequence of this phenomenon is that the aqueous phase separates into a water-rich and amine-rich region. This phase split results in one phase of essentially pure water, and a concentrated amine-CO₂ phase, containing only a small amount of water. This provides a valuable basis for experimentation and preliminary assessment of alkylamines as solvent alternatives.

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