K-values for hydrocarbons

Values for hydrocarbons other than alkynes and alkadienes can be predicted by the method of Suzuki et al. The best model includes the descriptors T, P, the parachor, the molecular surface area (which can be approximated by the van der Waals area), and the zero-order connectivity index. Excluding alkynes and alkadienes, a studv for 58 alkanes, aromatics, and cycloalkanes showed an average deviation from experimental values of about 30 K. 

A hydrocarbon feed gas is to he treated in an existing foiir-theoretical-tray ahsorher to remove hiitane and heavier components. The recovery specification for the key component, hiitane, is 75 percent. The composition of the exit gas from the ahsorher and the required liqiiid-to-gas ratio are to he estimated. The feed-gas composition and the eqiiilihriiim K values for each component at the temperature of the (soliite-free) lean oil are presented in the following table ... 

Less polar organic compounds such as acetonitrile and tetrahydro-furan have also been used as modulators with nonpolar aMphatic hydrocarbons as eluent. With decreasing polarity of the additive the concentration of modulator in the nonpolar eluent has to be inci ased in order to obtain similar retention values. Approximately the same k values for diphenoxybenzene are achieved with either 0.0S%(v/v)of acetonitrile or 10% (v/v) of dichloromethane in n-hexane 47). As discussed above, the latter case corresponds to an eluent mixture rather than a modulated system. 

The effect of retrograde condensation on a crude oil component such as octane is particularly significant, and small amounts of these heavier components if left in the gas also have a great effect an hydrocarbon dewpoints. Even with extensive gas conditioning cooling it is difficult to remove all crude oil ccanpcnenta from the vapor streams at a pressure of 1500 psl. Note that the K-value for octane at 0°F and 1500 psi is not as low as at 100°F and 1000 psi. 

The great attraction of this equation is that it contains just properties of the pure species and therefore expresses K-values as functions of T and P, independent of the compositions of the liquid and vapor phases. Moreover, and 4> can be evaluated from equations of state for the pure species or from generalized correlations. This allows K-values for light hydrocarbons to be calculated and correlated as functions of T and P. However, the method is limited for any species to subcritical temperatures, because the vapor-pressure curve terminates at the critical point. 

Feed analyses in terms of component compositions are usually not available for complex hydrocarbon mixtures with a final normal boiling point above about 38°C (100°F) (n-pentane). One method of handling such a feed is to break it down into pseudocomponents (narrow-boiling fractions) and then estimate the mole fraction and K value for each such component. Edmister [Ind. Eng. Chem., 47,1685 (1955)] and Maxwell (Data Book on Hydrocarbons, Van Nostrand, Princeton, N.J., 1958) give charts that are useful for this estimation. Once K values are available, the calculation proceeds as described above for multicomponent mixtures. Another approach to complex mixtures is to obtain an American Society for Testing and Materials (ASTM) or true-boiling point (TBP) curve for the mixture and then use empirical correlations to construct the atmospheric-pressure equihbrium flash vaporization (EFV) curve, which can then be corrected to the desired operating pressure. A discussion of this method and the necessaiy charts is presented in a later subsection Petroleum and Complex-Mixture Distillation. 

Gas solubility has been treated extensively (7). Alethods for the prediction of phase equilibria and actual solubility data have been given (8,9) and correlations of the equilibrium K values of hydrocarbons have been developed and compiled (10). Several good sources for experimental information on gas— and vapor—liquid equilibrium data of nonideal systems are also available (6,11,12). 

Flash calculations can also be made for hght hydrocarbons with the data of Figs. 10.13 and 10.14. The procedure here is exactly as described in Ex. 10.5, where Raoult s law applied. With T and P specified, the K-values for hght hydrocarbonsas given by Figs. 10.13 and 10.14 are known, and V, the only unknown in Eq. (10.17), is found by trial. 

The K value for the silver complex of an acetylene, hex-3-yne, as determined by the distribution method 14>, was found to be 19.1, i.e. smaller than those of alkenes such as the pentenes and cyclohexene, but greater than those of aromatic hydrocarbons 1S). A later study of silver-acetylene complexes 16> using the more rapid solubility technique of Andrews and Keefer 1S> gave rise to quasi thermodynamic equilibrium constants , Ka (as opposed to K) for various methyl substituted hex-3-ynes and hept-2-yne. There was good agreement for the K values for hex-3-yne for the two different methods in each case, replacement of an a-hydrogen atom by a methyl group caused a decrease in the value of Ka, similar to that observed in alkenes. Values of AH approximated to 19-21 kJ mole-1. 

Appendix 22 K Values for Light Hydrocarbons (High Temperatures) 1111... 

Available By data on binaries involving hydrocarbons with up to 30 carbon atoms were fitted with Equations 2-7 to determine optimum values for the characteristic constant ky. These results, which will be examined below, have also helped establish trends and even suggest ways of correlating and predicting k /s for hydrocarbon binaries. Before the new information is reviewed, therefore, a few comments are appropriate on what is known and what has been done in attempting to correlate fcy with pure-component properties. 

Figure 5. Hydrocarbon—rich vapor and liquid K— values for water-1 butene system. Calculated with equation-of-state parameters for 1-butene. Experimental data (O), Leland et al. (9) (A), Wehe and McKetta (10).

Some interesting secondary solvent effects have been noted for adsorption of the polycyclic aromatic hydrocarbons and certain of their derivatives on alumina 14). The more nearly linear isomers (e.g., anthracene relative to phenanthrene, 2-bromonaphthalene relative to 1-bromonaph-thalene) are preferentially adsorbed from most solvents, owing to the apparent weak localization of these compounds on linear site complexes see Section 11-2B, There is also a tendency for the preferential adsorption of strong solvent molecules on these same linear site complexes, with the result that strong solvents (or their solutions in weaker solvents) behave as selectively stronger solvents toward the preferentially adsorbed linear aromatics, relative to less linear isomers. As a consequence the ratio of values for two such isomers varies sharply with the solvent used, despite the fact that Eq. (8-3) predicts that this ratio should remain constant for all solvents i.e., A. is generally constant for two or more isomers. Jn extreme cases the ratio of K" values for two isomers of this type can be varied by a factor of 10 or more, depending upon the solvent used (see Table 11-4). 

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