Vapor pressure, predicting

An afternate method with approximately the same accuracy as the Rackett method is the COSTALD metnod of Hanldnson and Thomson.The critical temperature, a characteristic volume near the critical volume, and an acentric factor optimized for vapor pressure prediction by the Soave equation of state are required input parameters. The method is detailed in the Technical Data Book ... 

It is very satisfying and useful that the COSMO-RS model—in contrast to empirical group contribution models—is able to access the gas phase in addition to the liquid state. This allows for the prediction of vapor pressures and solvation free energies. Also, the large amount of accurate, temperature-dependent vapor pressure data can be used for the parameterization of COSMO-RS. On the other hand, the fundamental difference between the liquid state and gas phase limits the accuracy of vapor pressure prediction, while accurate, pure compound vapor pressure data are available for most chemical compounds. Therefore, it is preferable to use experimental vapor pressures in combination with calculated activity coefficients for vapor-liquid equilibria predictions in most practical applications. 

Group contribution methods can be applied for property prediction of fatty-acid systems. For vapor-pressure prediction the following equation has been recently proposed and tested successfully against experimental data [6, 40] ... 

One of the first group contribution methods used for vapor pressure prediction was described by Ambrose and Sprake [106]. The authors used the group-contribution approach to accurately represent the temperature-dependent vapor pressures of 8 aliphatic alcohols. This study was followed by those of Macknick et al. [107] and Burkhard [108], who analyzed sets with up to several tens of hydrocarbons. Nevertheless, all these studies remain to some extent toy problems due to the limited diversity of compounds. 

Hence, the third parameter, co, implicitly contains information about the vapor pressure, making vapor pressure prediction something like a circular loop. But Soave went beyond this simple observation. Wilson had previously recognized these issues, but his equation met with limited success, especially at low reduced temperatures. Soave was careful to analyze the temperature dependence of his equation of state in great detail at the outset. He achieved this by introducing an adjustable parameter into the attractive contribution of the Redlich-Kwong equation. 

Figure 7.5-2 contains experimental vapor pressure versus temperature data for / -butane, together with vapor pressure predictions from (1) the van der Waals equation ... 

Figure 11.22 Ratio of the vapor pressures predicted ratio of the activity coefficients Kaikane/Kbenzene, and separation factors at 80 C in sulfolane (3) with a mole frac-tion X3 0.8.

Fig. 10.4. Vapor pressures (VP.) of benzer 6 (CeHj)-carbor tetrachloride (CCI,) solutions at 4939 0. The solid curves represent the experimental vapor pressures the dashed lines represent the vapor pressures predicted by Raoult s law.

TABLE 1. Comparison of Vapor Pressure Predictions (Reprinted with Permission from Ind. Eng. Chem. Fundamentals, 15, 293 (1976a). Copyright by The American Chemical Society.)... 

The activity coefficient in convention II is again defined as the ratio of the activity to the mole fraction. It is equal to the actual vapor pressure divided by the vapor pressure predicted by Henry s law and describes how the substance deviates from Henry s law ... 

Vapor pressure predictions can also be obtained through equations of state. Very good results are obtained for nonpolar compounds with the SRK, PR and vdW-711 EoS discussed here, provided that accurate values for Tg, Pg and [Pg.274]

Development of a Cubic Equation of State for Vapor Pressure Prediction... 

We consider, next, the third requirement to be met by these EoS, the prediction of saturation liquid volumes, and then return to discuss vapor pressure predictions. 

To improve PR s vapor pressure predictions, and also avoid the need for two m expressions, see Table 10.2, Magoulas proposes ... 

Figure 10.5 The effect of the uncertainty in the m value on the accuracy of vapor pressure predictions for n-butane with the PR EoS.

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