Isobutane equilibrium constant

Figure 4 3. Vapor-solid equilibrium constants for isobutane. (From Gas Processors Suppliers Associotion, Engineering Data Book, 10th Edition.)...

As discussed in Sec. 4, the K value of a species is a complex 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 K-value correlations are available for light-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, n-butane, isopentane, n-pentane, n-hexane, and n-heptane). These charts are a simplification of the Kellogg charts [Liquid-Vapor Equilibria in Mixtures of Light Hydrocarbons, MWK Equilibrium Constants, 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. 

The following is an example of an alkylation reaction that is important in the production of isooctane (2,2,4-trimethylpentane) from two components of crude oil isobutane and isobutene. Isooctane is an antiknock additive for gasoline. The thermodynamic equilibrium constant, K, for this reaction at 25°C is 4.3 X 10, and AH°is -78.58 kj/mol. 

In preparing this table, it was assumed that the concentrations of all components lighter than propane were negligible in the residue and that all components heavier than n-Ci were negligible in the distillate. The isobutane is intermediate to the propane and n-butane and therefore will appear in appreciable quantities in both the distillate and residue. Since the i-C4 is more volatile than n-C4, the following table was prepared on the assumption that 20 per cent of the i-C4 in the feed would appear in the overhead. The volatilities relative to n-C4 are given in Fig. 9-13. These relative volatilities are based on the fugacity data of Lewis and coworkers (Ref. 6). The equilibrium constant K... 

Happel, J., and Mezaki, R., (1973), "Thermodynamic Equilibrium Constants for the Isobutane-Isobutene-Hydrogen System", J. of Chemical and Engineering Data, Vol, p. 152. 

Based on Illustration 8.4-2 this is a problem in vapor-liquid-liquid (3 phase) equilibrium. Also, from Problem 8.9-10, we have that the coexistence pressure is constant over the whole range of average (or total) mole fractions for which two liquid phases exist. From Illustration 8.4-2, one liquid phase is present for isobutane = xi -01128 and xx > 0.9284. For overall mole fractions in the range 0.1128 < Xj <0.9284, two liquid phases exist. To compute the V-L-L coexistence pressure in the one-liquid phase region, we use (neglecting fugacity coefficient corrections) x,7l/ vap + 72/>2vap = P where vap = 490.9 kPa, and... 

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