Hydrocarbon vapor-liquid equilibrium

Commercial visbreaking reactors are operated under non-isothermal conditions, causing variations in temperature inside the reactor, which results in changes in vapor and liquid-phase flowrates, compositions, and thermo-physical properties. To account for the vaporization of hydrocarbons, vapor-liquid equilibrium (VLE) calculations are needed, by means of which their effect on reactor modeling can... 

American Petroleum Institute, Bibliographies on Hydrocarbons, Vols. 1-4, "Vapor-Liquid Equilibrium Data for Hydrocarbon Systems" (1963), "Vapor Pressure Data for Hydrocarbons" (1964), "Volumetric and Thermodynamic Data for Pure Hydrocarbons and Their Mixtures" (1964), "Vapor-Liquid Equilibrium Data for Hydrocarbon-Nonhydrocarbon Gas Systems" (1964), API, Division of Refining, Washington. 

K-factors for vapor-liquid equilibrium ratios are usually associated with various hydrocarbons and some common impurities as nitrogen, carbon dioxide, and hydrogen sulfide [48]. The K-factor is the equilibrium ratio of the mole fraction of a component in the vapor phase divided by the mole fraction of the same component in the liquid phase. K is generally considered a function of the mixture composition in which a specific component occurs, plus the temperature and pressure of the system at equilibrium. 

Multicomponent distillations are more complicated than binary systems due primarily to the actual or potential involvement or interaction of one or more components of the multicomponent system on other components of the mixture. These interactions may be in the form of vapor-liquid equilibriums such as azeotrope formation, or chemical reaction, etc., any of which may affect the activity relations, and hence deviations from ideal relationships. For example, some systems are known to have two azeotrope combinations in the distillation column. Sometimes these, one or all, can be broken or changed in the vapor pressure relationships by addition of a third chemical or hydrocarbon. 

Figure 9-96. Vapor-liquid equilibrium showing X and application cases referred to in the text. Used by permission, Koshy, T. D., and Rukovena, F. Jr., Hydrocarbon Processing V. 85, No. 5 (1986) p. 64 all rights reserved.

GRAYSON, H. G. and STREED, C. W. (1963) Proc. 6th World Petroleum Congress, Frankfurt, Germany, paper 20, Sec. 7, 233. Vapor-liquid equilibrium for high temperature, high pressure hydrogen-hydrocarbon systems. 

The calculations made thus far are of theoretical trays, that is, trays on which vapor-liquid equilibrium is attained for all components. Actual tray efficiencies vary widely with the kind of system, the flow rates, and the tray construction. The range can be from less than 10% to more than 100% and constitutes perhaps the greatest uncertainty in the design of distillation equipment. For hydrocarbon fractionation a commonly used efficiency is about 60%. Section 13.14 discusses this topic more fully. 

Jacoby, R.H., Vapor-Liquid Equilibrium Data for Use of Methanol in Preventing Gas Hydrates, in Proc. Gas Hydrocarbon. Control Conference. University of Oklahoma, Norman, OK (1953). 

Kobayashi, R., Vapor-Liquid Equilibrium in Binary Hydrocarbon-Water Systems, Ph.D. Dissertation, University of Michigan, University Microfilms No. 3521, Ann Arbor, MI (1951). 

Braun, W. G., Fenske, M. R., and Holmes, A. S., Bibliography of Vapor-Liquid Equilibrium Data for Hydrocarbon Systems, American Petroleum Institute, New York, 1963. 

When applying an equation of state to both vapor and liquid phases, the vapor-liquid equilibrium predictions depend on the accuracy of the equation of state used and, for multicomponent systems, on the mixing rules. Attention will be given to binary mixtures of hydrocarbons and the technically important nonhydrocarbons such as hydrogen sulfide and carbon dioxide -Figures 6-7. 

Phase-Equilibrium Data Shaw, D. G., and A. Maczynski (eds.), IUPAC Solubility Data Series, Vol. 81 Hydrocarbons in Water and Seawater—Revised and Updated, published in 12 parts in /. Fhys. Chem. Ref. Data, 2005 and 2006 Gmehling, J., et al, Vapor-Liquid Equilibrium Data Collection Aqueous-Organic Systems, DECHEMA Chemistry Data Series, Vol. I, Part 1-ld, Schon Wetzel GmbH, Frankfurt/Main, Germany, 1988. 

In this section we will examine the effect that hydrocarbons, specifically methane, have on the physical properties of acid gases. The effect on the vapor pressure (i.e., the vapor-liquid equilibrium) will be discussed in a subsequent chapter. 

Table 3A.1 Experimental investigations vapor-liquid equilibrium (non-aqueous) for mixtures containing hydrogen sulfide and light hydrocarbons.

Huron, M.-J., G.-N. Dufour, and J. Vidal. 1978. "Vapor-Liquid Equilibrium and Critical Locus Curve Calculations with the Soave Equation for Hydrocarbon Systems with Carbon Dioxide and Hydrogen Sulphide" Fluid Phase Equil., 1 247-265. 

For azeotropic distillation especially the systems are non-ideal which makes calculating vapor-liquid equilibrium properties more difficult than, for example, in distillation of mixtures of simple hydrocarbons. Work predicting the vapor-liquid equilibrium properties of ternary mixtures of... 

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). 

Blanco, A.M. Ortega, J. Experimental study of miscibility, density, and isobaric vapor-liquid equilibrium values of mixtures of methanol in hydrocarbons (C5, C6). Fluid Phase Equilib. 1996, 122, 207-222. 

Data for methane/hydrocarbon mixtures are not well established. Some results can be generated through commercially available simulators like Aspen or Hysys but are somewhat questionable because of the nonidealities of these mixtures at elevated pressures. Also, the vapor-liquid equilibrium data currently available are very limited and mostly concern blends of methane with n-paraffins, which have very poor ignition characteristics in internal combustion engines. 

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