Approach Suitable for Screening Estimation Methods for Critical Properties of Heavy Compounds

Abstract

A simple theoretical-based correlation of the ratio of the critical temperature to the critical pressure (T c /p c ) as a function of the van der Waals surface area (Q w ) has been previously developed based on an extensive database of critical data published prior to 1996. The final equation was the following: T c /p c ) 9.0673 + 0.43309(Q w 1.3 + Q w 1.95 ) where T c is the critical temperature in kelvin and p c is the critical pressure in bar. This correlation is further validated here based on recent experimental data for various families of organic compounds, including some heavy ones (mono-and diacids, alkenes, cyclo/phenylalkanes, and squalane). This and previous validations verify that this correlation has a much broader application range, up to (T c /p c ) ratios of 200, than the data used in its development (compounds with ratios up to 100). It seems that most organic compounds, including the very heavy and complex ones, follow the trend suggested by this equation. This equation can be used for testing existing group-contribution estimation methods. It is shown here that direct comparison of the Joback and Constantinou-Gani methods for two families of compounds (alkenes and carboxylic acids) is in agreement with their validation via the proposed equation. Similar results have been obtained for other compounds. Both group-contribution methods are of equal accuracy for heavy alkenes and acids, provided that experimental boiling point temperatures are available for the Joback method. If such data are not available, e.g., for heavy compounds, the Constantinou-Gani method should be preferred. The proposed correlation does not offer an alternative to group-contribution methods as it only provides a single relationship of the two critical properties. Its universal character, though, and validation for many heavy compounds offer a way to test and compare existing group-contribution methods and, finally, to select the one that best conforms with the proposed correlation. It is recommended that the proposed correlation is indeed used for high molecular weight compounds for which experimental critical properties are typically not available.

Estimation of the critical properties of compounds using volume‐based thermodynamics

“…It is clear that the generalized relation proposed could not be regarded as an alternative to GC methods, as it is essentially an interrelationship between two critical constants. Nonetheless, its universal applicability that has been validated for 1,589 compounds of low to high molecular weights and a wide variety of polarities ensures its utility in comparing and testing existing and under‐development GC methods 29 . Aside from this important screening function, the general trend of compounds belonging to a specific family could also be determined, with the possibility of interpolation and extrapolation.…”

“…Taking a cue from, and concurrently substantiating this supposition, several studies have indicated that the ratio can be correlated much better than either property individually 23‐25 . In particular, expanding upon the study by Bae et al, 2 Kontogeorgis and co‐workers have advocated in a series of publications 26‐29 that it is possible to relate the ratio of high molecular weight compounds to a single parameter, that is, the compound's normalized van der Waals surface area ( Q ) given by Bondi 30 and reported in the UNIFAC GC tables 31 as: …”

“…Basically, having the ratio of offers the possibility of rapid estimation of the missing property if only one of the two critical properties is available. Furthermore, having a generalized equation for this ratio could serve as a suitable tool for screening existing estimation methods of the critical properties 29 …”

Supercritical fluid extraction of minor components of vegetable oils: β-Sitosterol

“…Thus, it is not possible to compare the values estimated by us with those of other authors. Still, an evaluation of the reliability of the properties prediction can be made applying the generalized semitheoretical expression advocated by Zbogar et al (2006), namely:…”

Cubic Equations of State: The Classical Mixing Rules

“…Due to molecular packing (which exists in the liquid as well as the solid state), this co-volume is expected to be higher than the molecular (or vdW) volume. The following equation can be used for screening estimation methods: [19][20][21] T c P c ¼ 9:0673 þ 0:433 09 Q 1:3 w þ Q 1:95 The reason for including b ¼ 1:41 V w in Table 3.4 lies in the well-accepted picture for the liquid state that each molecule has about 10 neighbor molecules, i.e.…”