Vapor pressure estimation

Foreman, W. T., Bidleman, T. F. (1985) Vapor pressure estimates of individual polychlorinated biphenyls and commercial fluids using gas chromatographic retention data. J. Chromatogr. 330, 203-216. 

Basak, S. C., Mills, D. Development of quantitative structure-activity relationship models for vapor pressure estimation using computed molecular descriptors. ARKIVOC 2005, 2005(x), 308-320. 

An easily implemented alternative for obtaining better vapor pressure estimates for high boiling chemicals would be to use an experimental value of the boiling point at reduced pressure rather than the normal boiling point. Then, using similar techniques to those described above, we would calculate a smaller change in pressure, so that the numerical uncertainty in vapor pressure at ambient temperature would be much less. 

Fischer R.C., R. Wittlinger, and K. Ballschmiter. 1992. Retention-index based vapor pressure estimation for polychlorobiphenyl (PCB) by gas chromatography. Fres. J. Analyt. Chem. 342 421-425. 

The constants in this equation differ slightly from those in the reference because more data were used to obtain the latter. Vapor pressure estimates do not differ greatly, however 7.39 X 10"3mm. for 2,4-D methyl ester and 1.61 X 10 3 for n-propyl 2,4,5-T. 

It often happens that tabulated vapor pressure data are not available at temperatures of interest, or they may not be available at all for a given species. One solution to this problem is to measure p at the desired temperatures. Doing so is not always convenient, however, especially if a highly precise value is not required. An alternative is to estimate the vapor pressure using an empirical correlation for p" T). Reid, Prausnitz, and Poling summarize and compare vapor pressure estimation methods, several of which are given in the paragraphs that follow. 

EXAMPLE 6.1-1 Vapor Pressure Estimation Using the Clausius-Clapeyron Equation... 

Biomass tars are known to be very complex mixtures which contain significant amounts of polar compounds that can potentially form hydroxyl bonds. This means that current vapor pressure estimation techniques are not only difficult to apply, but are inaccurate for mixtures with significant oxygen functionalities. Therefore we have chosen to approach this problem from an experimental standpoint. 

Using the Antoine equation for vapor pressure, estimate the acentric factor for isobutane from Equation (4-68) and compare it to the value tabulated in Appendix I. 

If the chemical of interest is a solid, it sublimates to pose its vapor pressure. The sublimation process can be viewed as the sum of processes of a hypothetical fusion (melting) to liquid and vaporization from liquid to gas at given temperature and pressure. Such liquid, of course, is hypothetical because it may not exist at the given condition. The vapor pressure of this hypothetical liquid, Pp, rather than the true vapor pressure of the solid, Pg, is often generated by various vapor pressure estimation models. Pg can be converted to liquid vapor pressure Pp using the fugacity ratio F ... 

When a vapor pressure estimate is not available from the literature, it can be estimated approximately using Trouton s rule, which is that the entropy of vaporization is 21.2 times the boiling temperature (K) in cal/mol, i.e., the entropy of vaporation at the normal boiling point is 21.2 cal/mol K. An integrated form of... 

Range Measured vapor pressures Estimated vapor pressures... 

Yalkowsky, S. H. and Mishra, D. S. (1991) Vapor pressure estimations for organic compounds. Sci. Total Environ., 109/110, 243-50. 

Therefore, the average deviations are less important for a vapor pressure estimation method than the number of substances where the method fails completely. In this area, a simple estimation procedure based on the vapor pressure equation of Hoffmann and Florin [34] has proved to be more appropriate than various methods from the literature [35-37] that are based on corresponding states or group contributions. The equation of Hoffmann-Florin is based on the charts developed by Cox [38]. Cox charts are constructed in a way that for some reference fluids the scale of the abscissa is adjusted to make the vapor pressure curve in a log P -T-diagram a straight line. The Hoffmann-Florin equation has two adjustable parameters o and P ... 

Estimation methods for the vapor pressure are still not reliable enough to apply them in process simulations for components whose behavior is decisive. Vapor pressure estimations can be used for components where the order of magnitude of the vapor pressure is sufficient (e.g., to decide whether the component occurs at the top or at the bottom of a column). Even in this case, it depends on the frequency it is used. For the simulation and optimization of a process for basic chemicals it is strongly recommended to have vapor pressure curves based on data points for all the components involved, whereas for the batch production of fine chemicals with a tight timef rame or during process evaluation an estimation might be acceptable. 

This example illustrates the advantages and disadvantages of the C-C equation. Using it, one can very easily make useful vapor-pressure estimates. This example shows a 30-fold extrapolation from the input pressure (100 psia) with only an 11% error. For many applications, this is adequate. 

The accentric factor is used exclusively in making corresponding-states estimates of PvT, EOSs that estimate PvT, and other thermodynamic properties derived from PvT. It is almost never used in making vapor-pressure estimates, and will not be used again in this chapter. 

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