Vapour-liquid equilibria, at high pressures

At pressures above a few atmospheres, the deviations from ideal behaviour in the gas phase will be significant and must be taken into account in process design. The effect of pressure on the liquid-phase activity coefficientmustalso be considered. A discussion of the methods used to correlate and estimate vapour-liquid equilibrium data at high pressures is beyond the scope of this book. The reader should refer to the texts by Null (1970) or Prausnitz and Chueh (1968). 

Prausnitz and Chueh also discuss phase equilibria in systems containing components above their critical temperature (super-critical components). 

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The work was continued by Wichterle et al. [83d] and resulted in a generalized method of calculating and predicting vapour-liquid equilibria at high pressures. 

However, it was Soave s modification [30] of the temperature dependence of the a parameter, which resulted in accurate vapour pressure predictions (especially above 1 bar) for light hydrocarbons, which led to cubic equations of state becoming important tools for the prediction of vapour-liquid equilibria at moderate and high pressures for non-polar fluids. 

The wide varity of the properties of chemical compounds does not enable the use of a universal apparatus for the measurement of thermodynamic properties for pure components and mixtures at high pressure. In the case of two-phase equilibria like vapour-liquid equilibria, the typical set of data to be determined is the pressure, the temperature, and the composition of the two phases at equilibrium. Some experimental apparatus also allows the... 

Larousse, B., et al. (2009), Experimental Study of the Vapour-liquid Equilibria of HI-I2-H20 Ternary Mixtures, Part 2 Experimental Results at High Temperature and Pressure , Int. J. Hydrogen Energy, forthcoming. 

Beranek, P. Wichterle, I Vapour-liquid equilibria in the propane - n-butane system at high pressures Fluid Phase Equilib,... 

Section 8.2.1 was concerned with equilibrium between a condensed phase and the vapour. It is often necessary, however, to estimate the effect of pressure on equilibria between two condensed phases. For example, the melting point of sodium at one atmosphere pressure is 97.6°C. Can it be used as a liquid heat transfer medium at 100°C, at a pressure of 100 atm, or will it solidify It is known that the liquid is less dense than the solid, and this argues that high pressures will encourage solidification. This is another aspect of Le Chatelier s work, which we can now quantify. This and similar problems may be solved by the Clapeyron equation, which we shall now derive. 

Systems that establish a functional equilibrium form a highly significant part of our lives and the world around us, from the biochemical reactions that sustain life to the way in which we generate major industrial chemicals. There are many different examples of physical equilibria that one could focus on but one relatively simple situation is the balance between evaporation and condensation that takes place at the surface of a liquid and the concept of the vapour pressure of a liquid. This equilibrium situation can be generalized for any liquid in a sealed container, and becomes ... 

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