gSAFT application areas

Strongly associating and reacting systems

Strong association, hydrogen bonding and chemical reactions result in highly non-ideal fluid behaviour. Conventional chemical approaches require large numbers of empirical parameters to describe equilibrium schemes for such systems.

H₂O – HF mixture

Study undertaken in conjunction with ICI / INEOS Fluor

SAFT represtation of H₂O and HF molecules

SAFT's association sites make it possible to deal with complicated aggregation or reaction schemes simply and accurately.

Example: Replacement refrigerants – H₂O – HF mixture

An example is the separation of water and hydrogen fluoride, an important process in the production of replacement refrigerants (which are produced by reaction the ozone depleting chlorofluorocarbons with HF).

Owing to the extensive hydrogen bonding, this is an azeotropic mixture which exhibits highly non-ideal behaviour and is difficult to separate. An additional complication is that the experimental data available are rather limited.

The SAFT molecular model is shown on the right. This involves:

2 or 3 association sites for HF
4 association sites for H₂O
asymmetric association between HF and water.

T-x vapour-liquid equilibria at 1 atm

P-x vapour-liquid equilibria at 1 atm

The model parameters were fitted to vapour-liquid equilibrium (VLE) azeotrope point at 1 atm. Because of the versatility of the underlying physical model, this made possible the prediction of behaviour at very different conditions.

Once the thermodynamic properties model had been determined to sufficient accuracy, the properties calculation were integrated in gPROMS in order to model the separation process, as shown below.

Results

T-x and P-x vapour for the system at 1 atm are shown in the diagrams above right; dynamic responses and column profiles are below.

The simulation gave a top product purity of 99.92% HF. CPU time was 7.4s on a Intel Pentium 4, 2.4 GHz machine.

The same treatment was applied to other systems, including replacement refrigerants exhibiting liquid-liquid equilibria (LLE).

Dynamic responses: temperature and HF concentration

Profiles: temperature and HF concentration

References

A. Galindo, P. J. Whitehead, G. Jackson, and A. N. Burgess, Predicting the phase equilibria of mixtures of hydrogen fluoride with water, difluoromethane (HFC-32), and 1,1,1,2-tetrafluoroethane (HFC-134a) using a simpiflied SAFT approach, Journal of Physical Chemistry B, 101, 2082-2091 (1997)

M. Pollock, PhD Thesis, Imperial College London (2009)

N. M. P. Kakalis, PhD Thesis, Imperial College London (2006).