Advanced cubic equations of state

Liquid-Phase Models. Theoretical models of the liquid state are not as well established as those for gases consequently, the development of general equations for the description of liquid-phase equilibrium behavior is not far advanced. Cubic equations of state give a qualitative description of liquid-phase equilibrium behavior, but do not generally yield good quantitative results (3). For engineering calculations, equations and estimation techniques developed specifically for liquids must normally be used. 

It is difficult today to appreciate the magnitude of the advance that the RK equation represented at the time it was developed. To put this into perspective, I have included in Figure 3 and Table III curves and numerical values derived from the equations of van der Waals and Berthelot, two of the then-most-popular cubic equations of state. The results speak for themselves. 

Virtually all modem efforts at devising better versions of the cubic equation of state have been either directly or indirectly inspired by the example of the RK equation. Hence, many experts routinely label any new cubic equation as another variation on the RK equation. Some are, and some are not, but the attitude is expressed often enough that one well might ask if there are possibilities for significant advances within the cubic equation framework. The answer is probably a qualified yes — qualified because one must approach such efforts with an appreciation of what a cubic equation can and cannot do. 

The van der Waals equation does remarkably well at low pressures, say up to 10 bars or so, but is not sufficiently accurate for industrial applications. However it obviously has the right form, as shown by the fact that literally hundreds of modifications have been proposed, and the cubic equation of state is a huge topic in some areas of chemical engineering because of the importance of modeling vapor-liquid equilibria (VLE) in some industries. Walas (1985) provides a good introduction, and more recent advances can be found in Orbey and Sandler (1996) and Anderko (2000). 

Predictions and correlation of VLE and LLE of mixtures containing polar substances with a cubic equation of state requires the use of advanced mixing... 

At 10 MPa and 35 °C, C02 has a density of approximately 700kg/m3. Under these conditions, a cubic meter of sandstone with 10% porosity contains approximately 70 kg of C02 if the pore space is completely filled by C02. However, saturation of C02 is not complete, and some brine remains in the invaded pore spaces (Saripalli McGrail 2002 Pruess et al. 2003). In addition, non-uniform flow of C02 bypasses parts of the aquifer entirely. Darcy-flow based analytical and numerical solutions are used to evaluate some of these effects by simulating the advance of the C02 front over time-scales of decades to hundreds of years and over lateral distances of tens to hundreds of kilometers. To account for the extreme changes in density and viscosity of C02 with pressure and temperature, these models must incorporate experimentally constrained equations of state (Adams Bachu 2002). 

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