Additional Comments on Applications

There is no simple equation of state that can represent satisfactorily the three conditions of eqn [21] over a wide range of temperature. Experience indicates that equations that could meet the equal fugacity condition as well as vV or vL would be suitable for the intended vapour-liquid equilibrium calculations. Should a situation arise such that the saturated liquid molar volume is required in the estimation, cubic equations containing more parameters may be selected for the representation. Adachi and Lu in 1990 suggested that it is possible to assign different two-parameter or three-parameter cubic equations to different components of the binary mixture under consideration, and then use a four-parameter cubic equation to combine the equations in the vapour-liquid equilibrium calculations for mixtures.

Special case should be applied when equations of state are used to represent the experimental data measured at or near theoretical points of the fluids.

The selection of mixing and combining rules plays a crucial role in the application of equations of state to the correlation and prediction of vapour-liquid equilibria. The importance of selecting an appropriate expression for the excess Gibbs energy cannot be overemphasized.

The prediction of binary vapour-liquid equilibria from pure-component properties by means of the MOSCED method is attractive. However, poor results are obtained for systems where steric considerations are significant. The general applicability of this model is limited due to the difficulties involved in

Table 9 The UNIFAC method

where:

L = -, yjj 1 Xj r = X vfRk, q, = x vkQk kk Rk"volume parameter for group k Qk = surface area parameter for group k vk( )"number of subgroups of type k in molecule of species i ln yiR"q where:

vfQk

«mk = group interaction parameter k

Figure 4 Algorithms for correlating and predicting vapour-liquid equilibrium values.

the determination of pure component parameters. Furthermore, the approach cannot be used for aqueous mixtures, nor for systems with very large yx values.

In general, the accuracy of prediction depends on the availability of some reliable binary data for either the system of interest or another one that is closely related to it.

All the group-contribution methods are approximate. The fundamental assumption involved in the group solution approach is additivity, and the estimated y values are necessarily approximate.

Finally, computer software packages for vapour-liquid equilibrium calculations are available commercially from a number of process engineering software development companies. Typical algorithms for data correlation and prediction are depicted in Figure 4.

See also: II/Distillation: Modelling and Simulation; Multicomponent Distillation; Theory of Distillation; Vapour-Liquid Equilibrium: Theory. III/Physico-Chemical Measurements: Gas Chromatography.

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Solar Panel Basics

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