How Molecular Modelling Tools Can Help …
191
3 Molecular Modelling of New Refrigerants
In 2014, the United States Department of Energy [23] published a roadmap listing
the highest priority for research and development in low-GWP refrigerants, emphasising the need to direct efforts towards investigating and modelling the theoretical
properties of azeotropic blends with low GWP, so that they can be used as pure fluids.
Insights on the phase behaviour along with accurate predictions of the thermophysical
properties of these highly non-ideal fluids are necessary for determining the optimal
compositions of the blends and for the design and evaluation of their performance in
refrigeration cycles. Additionally, transport and interfacial properties of refrigerants
are imperative in describing the two-phase fluid flow and heat transfer [27].
Despite the urgency for developing new low-GWP refrigerants, the amount of
published theoretical work on the molecular modelling of their thermophysical properties remains limited. In early works, a series of authors applied a variety of SAFTbased models to the thermodynamic modelling of third generation refrigerants [56,
85–88]. Few research groups have focussed yet on fourth generation refrigerants
from either molecular theory or molecular simulations. Of special relevance in this
context is the pioneering work of Raabe and co-workers [74–78, 89]. They proposed
new classical force fields for HFOs, which have been applied using molecular simulations to systematically study the thermophysical properties of selected HFOs and
their blends. Regarding molecular theory, Raabe applied PC-SAFT to study the phase
behaviour of binary mixtures containing HFOs [77]. In addition, Lai and co-workers
[90, 91] have predicted thermophysical properties of pure HFO refrigerants utilising
the BACKONE equation of state. Vega and collaborators have also made several
contributions on applying molecular modelling tools to rationally develop new refrigerants with low GWP, initially dedicated to thermophysical properties. Soft-SAFT
[38] and polar PC-SAFT [41] were used in order to study the phase, interfacial
and transport properties of HFCs and HFOs [56, 58, 92–96] as a step towards understanding their performance based on their molecular structure. Besides vapour-liquid
equilibria, calculations included viscosities, thermal conductivities, surface tensions,
heat capacities and other second order thermodynamic derivative properties, all properties of relevance to refrigerants for their final application. Particular attention was
paid to study the impact of the molecular structure of the mixture components on the
phase and interfacial behaviour, in order to find general trends connecting the molecular structure to the macroscopic properties of interest. We summarise next some
of the obtained results to highlight the application of molecular modelling tools for
characterising low-GWP refrigerants and predicting some properties in the absence
of experimental data.
As SAFT is a coarse-grain model, different approximations can be made to represent the molecular features, and provide accurate results as far as the main physics is
captured with the simple models. Within the SAFT approach, HFCs and HFOs can be
modelled as associating fluids, with two associating sites mimicking the dipole, or as
chains explicitly including a dipole placed in one of the spheres, as shown in Fig. 4.
Both approaches have been already used in the literature, leading to very similar
191
3 Molecular Modelling of New Refrigerants
In 2014, the United States Department of Energy [23] published a roadmap listing
the highest priority for research and development in low-GWP refrigerants, emphasising the need to direct efforts towards investigating and modelling the theoretical
properties of azeotropic blends with low GWP, so that they can be used as pure fluids.
Insights on the phase behaviour along with accurate predictions of the thermophysical
properties of these highly non-ideal fluids are necessary for determining the optimal
compositions of the blends and for the design and evaluation of their performance in
refrigeration cycles. Additionally, transport and interfacial properties of refrigerants
are imperative in describing the two-phase fluid flow and heat transfer [27].
Despite the urgency for developing new low-GWP refrigerants, the amount of
published theoretical work on the molecular modelling of their thermophysical properties remains limited. In early works, a series of authors applied a variety of SAFTbased models to the thermodynamic modelling of third generation refrigerants [56,
85–88]. Few research groups have focussed yet on fourth generation refrigerants
from either molecular theory or molecular simulations. Of special relevance in this
context is the pioneering work of Raabe and co-workers [74–78, 89]. They proposed
new classical force fields for HFOs, which have been applied using molecular simulations to systematically study the thermophysical properties of selected HFOs and
their blends. Regarding molecular theory, Raabe applied PC-SAFT to study the phase
behaviour of binary mixtures containing HFOs [77]. In addition, Lai and co-workers
[90, 91] have predicted thermophysical properties of pure HFO refrigerants utilising
the BACKONE equation of state. Vega and collaborators have also made several
contributions on applying molecular modelling tools to rationally develop new refrigerants with low GWP, initially dedicated to thermophysical properties. Soft-SAFT
[38] and polar PC-SAFT [41] were used in order to study the phase, interfacial
and transport properties of HFCs and HFOs [56, 58, 92–96] as a step towards understanding their performance based on their molecular structure. Besides vapour-liquid
equilibria, calculations included viscosities, thermal conductivities, surface tensions,
heat capacities and other second order thermodynamic derivative properties, all properties of relevance to refrigerants for their final application. Particular attention was
paid to study the impact of the molecular structure of the mixture components on the
phase and interfacial behaviour, in order to find general trends connecting the molecular structure to the macroscopic properties of interest. We summarise next some
of the obtained results to highlight the application of molecular modelling tools for
characterising low-GWP refrigerants and predicting some properties in the absence
of experimental data.
As SAFT is a coarse-grain model, different approximations can be made to represent the molecular features, and provide accurate results as far as the main physics is
captured with the simple models. Within the SAFT approach, HFCs and HFOs can be
modelled as associating fluids, with two associating sites mimicking the dipole, or as
chains explicitly including a dipole placed in one of the spheres, as shown in Fig. 4.
Both approaches have been already used in the literature, leading to very similar