DePriester correlation

When Raoult s law apphes, this becomes = Pi /P. In general, K-values are functions of T, P, liquid composition, and vapor composition, making their direct and accurate correlation impossible. Those correlations that do exist are approximate and severely hmited in apphcation. The DePriester correlation, for example, gives i< -values for hght hydrocarbons (Chem. Png. Prog. Symp. Sen No. 7, 49, pp. 1 3 [1953]). 

As discussed in Sec. 4, the icomplex function of temperature, pressure, and equilibrium vapor- and hquid-phase compositions. However, for mixtures of compounds of similar molecular structure and size, the K value depends mainly on temperature and pressure. For example, several major graphical ilight-hydrocarbon systems. The easiest to use are the DePriester charts [Chem. Eng. Prog. Symp. Ser 7, 49, 1 (1953)], which cover 12 hydrocarbons (methane, ethylene, ethane, propylene, propane, isobutane, isobutylene, /i-butane, isopentane, /1-pentane, /i-hexane, and /i-heptane). These charts are a simplification of the Kellogg charts [Liquid-Vapor Equilibiia in Mixtures of Light Hydrocarbons, MWK Equilibnum Con.stants, Polyco Data, (1950)] and include additional experimental data. The Kellogg charts, and hence the DePriester charts, are based primarily on the Benedict-Webb-Rubin equation of state [Chem. Eng. Prog., 47,419 (1951) 47, 449 (1951)], which can represent both the liquid and the vapor phases and can predict K values quite accurately when the equation constants are available for the components in question. 

A trial-and-error procedure is required with any K-value correlation that takes into account the effect of composition. One cannot calculate K values until phase compositions are known, and those cannot be known until the K values are available to calculate them. For K as a function of T and P only, the DePriester charts provide good starting values for the iteration. These nomographs are shown in Fig. 13-14/7 andZ . SI versions of these charts have been developed by Dadyburjor [Chem. Eng. Prog., 74(4), 85 (1978)]. 

As discussed in Sec. 4, the K value of a species is a complex function of temperature, pressure, and equilibrinm vapor- and hquid-phase compositions. However, for mixtures of compounds of similar molecular structure and size, the K value depends mainly on temperature and pressure. For example, several major graphical K value correlations are available for light-hydrocarbon systems. The easiest to use are the DePriester chartslChem. Eng. Prog. Symp. Ser. 7,49,1 (1953)],... 

When Raoult s law applies, Eq. (4-326) reduces to K, = P( /P. For modified Raoult s law, K, = yP/ /P. With K) = y,/x, these are alternative expressions of Raoult s law and modified Raoult s law. Were Raoult s valid, K values could be correlated as functions of just T and P. However, Eq. (4-326) shows that they are in general functions of T, P Xi), and y,, making convenient and accurate correlation impossible. Those correlations that do exist are approximate and severely limited in application. The nomographs for K values of light hydrocarbons as functions of T and P, prepared by DePriester Eng. 

This is the most popular QSAR approach among the grid-based QSAR techniques. CoMFA compares the molecular potential energy fields of a set of molecules and searches for differences and similarities that can be correlated with differences and similarities in the property values considered [Cramer III, Patterson et al, 1988 Marshall and Cramer III, 1988 Cramer III, DePriest et al, 1993 Folkers, Merz et al, 1993a, 1993b Kim, 1995a Oprea and Waller, 1997 Martin, 1998 Norinder, 1998 Kubinyi, 2003a]. 

Because of the complex concentration functionality of the m-values, VLE calculations in general require iterative procedures suited only to computer solutions. However, in the case of mixtures of light hydrocarbons, we may assume as a reasonable approximation that both the liquid and the vapor phases are ideal. This allows m-values for light hydrocarbons to be calculated and correlated as functions of T and P. Approximate values can be determined from the monographs prepared by DePriester (1953). The DePriester charts have been fit to the following equation (McWilliams, 1973) ... 

To use the O Connell correlation we need to estimate a and p at the average teirperature and pressure of the column. The column tenperature can be estimated from equilibrium (the DePriester chart). The following values are easily generated from Figure 2-12. 

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