Introducing the two-state theory of water in PC-SAFT EoS: water as a non-ideal mixture of high density water and low density water

Nefeli Novak, Georgios M. Kontogeorgis, Xiaodong Liang

Water has more than 50 “anomalous” properties (e.g. density and speed of sound maximum of pure water and hydrocarbon solubility minimum in water). The two-state theory can explain these anomalies and has recently gained significant attention due to experimental evidence published by Pettersson and Nilsson [1]. According to this theory, water is a mixture of low density liquid (LDL) and high density liquid (HDL), which correspond to different types of water clusters. As a result, in subcooled conditions water splits in two phases, one with low-density and one with high-density. At the conditions of practical interest to chemical engineers, water behaves as a homogeneous fluid, but the existence of LDL and HDL, causes the anomalous behaviour.

In our previous work [2], we have introduced the two-state theory of water in the Statistical Associating Fluid Theory (SAFT). We have assumed that water is an ideal mixture of two different types of water molecules, HDW and LDW, which are in chemical equilibrium and have different association schemes and association parameters. Using this approach, a generalized association term for SAFT has been derived and incorporated in PC-SAFT EoS.

In this work we have used different Gibbs energy models to describe the non-ideality of the mixture, including the model by Anisimov [3], 1-parameter Margules, Flory-Huggins and Wilson. The non-ideal mixing shows a slight improvement over the ideal case, especially for the models with an excess enthalpy of mixing. In all cases, several anomalous properties, including density and speed of sound maximum and isothermal compressibility minimum, as well as the minimum in hydrocarbon solubility in water-hydrocarbon LLE, are accurately predicted.

References

[1]      L.G.M. Pettersson, R.H. Henchman, A. Nilsson, Chem Rev. 116 (2016) 7459–7462.

[2]      N. Novak, X. Liang, G. K. Kontogeorgis, doi: 10.1063/5.0186752, Just accepted in Journal of Chemical Physics

[3]      Holten, V., Anisimov, M. A., Scientific Reports 2 (2012), 713