Vapour/liquid equilibria correlation

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). 

A good understanding of methods used for correlating vapour-liquid equilibrium data... 

Before beginning the series of runs to determine the relief size, the physical property and kinetic data need to be correlated in the form required, by the code. In some cases, the code may already have the components required on a database. In all other cases, physical property data must be found, estimated or measured and correlated in the appropriate form. Some codes have a front-end program for curve fitting of data. For tempered systems, the vapour/ liquid equilibrium models are of critical importance since errors will cause the code to open the relief system at the wrong temperature and reaction rate. It is therefore worthwhile to spend time to ensure reasonable behaviour of the vapour pressure predictions. Check that all correlations behave sensibly over the entire temperature range of relevance for relief sizing. A good test for the physical property and kinetic data supplied to the code is to first model the (unrelieved) adiabatic calorimetric test which was used to obtain the kinetic data.. . ... 

A good understanding of methods used for correlating vapour-liquid equilibrium data is essential to the understanding of distillation and other equilibrium-staged processes this subject was covered in Chapter 8. 

The description of vapour-liquid equilibrium behaviour can be obtained from analytical equations and generalized correlations. The generalized conelations are generally for the equilibrium ratio, K, and the fugacity coefficients. 

For the analytical equations, there are two methods to compute the vapour-liquid equilibrium for systems. The equation of state method (also known as the direct or phi-phi method) uses an equation of state to describe both the liquid and vapour phase properties, whereas the activity coefficient method (also known as the gamma-phi approach) describes the liquid phase via an activity coefficient model and the vapour phase via an equation of state. Recently, there have also been modified equation of state methods that have an activity coefficient model built into the mixing mles. These methods can be both correlative and predictive. The predictive methods rely on the use of group contribution methods for the activity coefficient models such as UNIFAC and ASOG. Recently, there have also been attempts to develop group contribution methods for the equation of state method, e.g. PRSK. " For a detailed history on the development of equations of state and their applications, as well as activity coefficient models, refer to Wei and Sadus, Sandler and Walas. ... 

Ortega, J. Espiau, F. Dieppa, R. Measurement and correlation of isobaric vapour-liquid equilibrium data and excess properties of ethyl methanoate with alkanes (hexane to decane) Fluid Phase Equilib. 2004,215, 175-186... 

Density, refractive index, viscosity, and isothermal vapour-liquid equilibrium data for hexafluoroacetone-water mixtures, i.e. solutions of the gem-diol (CFa)gC(OH)t, are now available, and so are thermochemical parameters for the addition of water and methanol across the C=0 bond of the ketone to give the hydrate, (CF3)2C(OH)a, and the hemiacetal, (CFa)2C(OH)OMe, respectively. The value for the heat of hydration of hexafluoroacetone [Affr°(soln.) = —93.6 kJ mol" cf. MeCO-CHjCI, -8.4 MeCO CHCl -23.0 MeCHO, -21.3 and CCI.-CHO, -58.5 kJ mol ], taken with that estimated for acetone [-4 4 kJ mol" (too low to have been measured reliably)] and the difference in the heats of hydrogenation of formaldehyde (-92 kJ mol- ) and acetone (—54 kJ mol- ), leads to the conclusion that two CFj groups destabili2e a carbonyl bond by ca. 50 kJ mol-, a factor which correlates with the remarkable reactivity of hexafluoroacetone in carbonyl additions. ... 

Binary variables are used to represent the occurrence of molecular structural groups (e.g. -CH3, -CHO, -OH. ..) found in the group contribution correlations. This allows molecules to be generated according to a set of structural and chemical feasibility constraints. In addition, a variety of pure component physical and environmental property prediction equations, non-ideal multi-component vapour-liquid equilibrium equations (UNIFAC), process operational constraints and an aggregated process model form part of the overall procedure. Finally, the solvent identification task is solved as a mixed integer non-linear programming (MINLP) problem (Buxton et ai, 1999). 

The correlation of solid solubilities in compressed gases using equations of state is not at present in as satisfactory a condition as the correlation of vapour/liquid equilibrium in comparatively low molecular weight systems. 

Clara, R. A. Gomez Marigliano, A. C. Solimo, H. N. Density, viscosity, isothermal (vapour+liquid) equilibrium, excess molar volume, viscosity deviation, and their correlation for chloroform -l- methyl isobutyl ketone binary system. J. Chem. Thermodyn. 2007, 39, 261-267. 

Zielkiewicz, J. Vapour + liquid equilibrium measurements and correlation of the ternary mixture (N-methylacetamide -1- ethanol + water) at the temperature 313.15. J. Chem. Thermodyn. 2000, 32, 55-62. 

Jaques and Furter [95] derived an equation for the salt effect on the water-vapour equilibrium in binary mixtures which correlates the temperature and the liquid concentration of the three components ethanol, water and salt. The equation has 6 constants. The theory of the salt effect has been discussed by Furter and Meranda [96]. On the basis of simplifying assumptions Sada et al. [97] have established a relation for the calculation of vapour-liquid equilibria for non-aqueous binary systems in which the salt is dissolved only in one component e.g., benzene-ethanol with lithium or calcium chloride). 

Adsorption of some organic solvent vapours onto HSZ were studied. Binary adsorption equilibriums except azeotropic mixture-HSZ systems could be correlated by Markham-Benton equation for the whole concentration range, and the break times could be estimated well by using the Extended-MTZ-Method. For azeotropic mixture-HSZ systems, the equilibriums and the break times could be correlated and estimated only for a part of the all concentration range. Then, two azeotropic points appeared in the adsorption equilibriums for IPA-TCE -Y-type system. For this binary systems adsorption equilibrium data could be expressed by proposed equation, similar to liquid-vapour azeotropic equilibrium equation. Breakthrough curve could be simulated using the Stop Go method in the whole range for azeotropic mixture systems as well as for zeotropic systems. 

In an attempt to improve the behaviour of the cubic equation of state, the more elaborate Huron-Vidal mixing rules were used with the Peng-Robinson equation of state.As shown in Figure 4.5 for the (vapour + liquid + liquid) (VLLE) equilibrium of (water + hex-1-ene) the Huron-Vidal mixing rules improved significantly the predicted solubility of hex-1-ene over the standard van der Waals mixing rules however, there was also a significant decrease in the ability of the model to correlate of solubility of water. 

The site-site direct correlation functions of the bulk phases in equilibrium were obtained by solving the RISM-KHM equations (4) and (7) with the parameter t = 0.25 for vapour and T = 0.5 for liquid phases. 

There is no method for theoretical prediction or correlation of VLE data in reacting systems. It has to be remembered that equilibrium in such systems means not only concentration and thermal equilibrium but also absence of chemical change. In other words, the chemical reaction must also be at equilibrium or, if it is irreversible, it must have reached completion. This poses the question of how to express the interphase concentration driving force in, e.g., a packed column in the presence of, say, a slow reaction. Should it be the difference between the local concentration of a component in the vapour phase and a true equilibrium value corresponding to a given liquid concentration of the corai)onent or should a pseudo equilibrium value, which... 

Porter et al. [34] found an equation relating Vjt to the activity coefficient at infinite dilution of the solute, y - This correlation was deduced on the basis of the following simplifying assumptions (a) the column operates under ideal conditions (the equilibrium distribution of the component between the gas and the liquid phase is preserved), the partial pressure of the component obeys Henry s law, the pressure drop along the colunm is zero) (b) the carrier gas and the vapours are ideal (c) the carrier gas is insoluble iu the solvent (d) there are no adsorption effects. 

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