Hydrocarbon systems

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. 

Figure 4-1. Effect of UNIQUAC equation modification for an alcohol-hydrocarbon system.

The discovery of a significant number of hypercoordinate carboca-tions ( nonclassical ions), initially based on solvolytic studies and subsequently as observable, stable ions in superacidic media as well as on theoretical calculations, showed that carbon hypercoordination is a general phenomenon in electron-deficient hydrocarbon systems. Some characteristic nonclassical carbocations are the following. 

It was the study of hypercarbon-containing nonclassical carboca-tions that allowed us to firmly establish carbon s ability in a hydrocarbon system to bind simultneously with five (or six or even seven) atoms or groups. It should be emphasized that carbocations represent... 

Typical nonsieve, polar adsorbents are siUca gel and activated alumina. Kquilihrium data have been pubUshed on many systems (11—16,46,47). The order of affinity for various chemical species is saturated hydrocarbons < aromatic hydrocarbons = halogenated hydrocarbons < ethers = esters = ketones < amines = alcohols < carboxylic acids. In general, the selectivities are parallel to those obtained by the use of selective polar solvents in hydrocarbon systems, even the magnitudes are similar. Consequendy, the commercial use of these adsorbents must compete with solvent-extraction techniques. 

The principal nonpolar-type adsorbent is activated carbon. Kquilihrium data have been reported on hydrocarbon systems, various organic compounds in water, and mixtures of organic compounds (11,15,16,46,47). With some exceptions, the least polar component of a mixture is selectively adsorbed eg, paraffins are adsorbed selectively relative to olefins of the same carbon number, but dicycUc aromatics are adsorbed selectively relative to monocyclic aromatics of the same carbon number (see Carbon, activated carbon). 

Liquid Third Phase. A third Hquid with coUoidal stmcture has been a known component in emulsions since the 1970s (22) for nonionic surfactants of the poly(ethylene glycol) alkylaryl ether type. It allows low energy emulsification (23) using the strong temperature dependence of the coUoidal association stmctures in the water—surfactant—hydrocarbon systems. 

K. E. Starling, Fluid Thermo fmamic Propertiesfor Eight Hydrocarbon Systems Gulf Pubhshing Co., Houston, 1973. 

For air-hydrocarbon systems, average deviations do not exceed 9 percent. For general nonhydrocarbon gas systems, the average deviation is about 6 percent. 

As discussed in Sec. 4, the icomplex 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 ilight-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, /i-butane, isopentane, /1-pentane, /i-hexane, and /i-heptane). These charts are a simplification of the Kellogg charts [Liquid-Vapor Equilibiia in Mixtures of Light Hydrocarbons, MWK Equilibnum Con.stants, 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. 

Continued) K values (K = y/x) in light-hydrocarbon systems, (h) High-temperatiire range. [DePiiester, Prog. Symp. Sec. 7,49, 1 (1953).]... 

Optimization As stated previously, optimization studies should include the entire system. Such a study was made by Fair and BoUes [Chem. Eng., 75(9), 156 (1968)], using a hght-hydrocarbon system and with the objective of defining optimum reflux ratio. Coolants used were at —87, —40, and +30°C (—125, —40, and +85°F), corresponding to different pressures of operation and associated different condens-... 

It should be remembered that water present in a hydrocarbon system, being immiscible, will add its full vapor pressure to that of the hydrocarbons. The author once wondered why the pressure was so high on a certain overhead accumulator until he noticed the installed bootleg. 

The use of radical cyclizations to make five-membered rings has become a very important tool for synthetic chemists Although there has been a virtual explosion of reports in the literature regarding the cyclization of 5-hexenyl radicals to cyclopentyl carbinyl radicals in all types of hydrocarbon systems [55], the use of this cyclization for the synthesis of fluorme-containing cyclopentanes has been largely ignored... 

Although both predictions of AHflethane) are in reasonable agreement with experiment, they differ from one another by almost 5 kcal-mol. Such a large difference for a simple hydrocarbon system—the sort of species isodesmic reactions are purported to treat well—suggests caution when applying this technique. 

Values of K-equilibrium factors are usually associated with hydrocarbon systems for which most data have been developed. See following paragraph on K-factor charts. For systems of chemical components where such factors are not developed, the basic relation is ... 

Step 3. Calculate the weight average critical temperature and critical pressure for the remaining heavier components to form a pseudo binary system. (A shortcut approach good for most hydrocarbon systems is to calculate the weight average T only.)... 

As previously discussed, the charts of K values are available, but do apply primarily to hydrocarbon systems. Reference 79 presents important other data on K value relationships. See Figures 8 4A and 8-4B for charts with pressure effects included (not ideal, but practical charts). 

Figure 8-40. Hausbrand vapor-pressure diagram for various liquids and at three system steam pressures. A similar diagram can be constructed for other organic/hydrocarbon systems. Used by permission, Ellerbee, R. W., Chem. Eng. Mar. 4 (1974), p. 108.

This is another way of expressing that the heat load from tray to tray is varying in the column to such an extent as to make the usual simplifying assumption of equal molal overflow invalid. The relations to follow do not include heats of mixing. In general they apply to most hydrocarbon systems. 

Figure 8-81. Hydrocarbon systems overhead gas minus lean oil temperature for components absorbed In top theoretical tray (or top actual three trays). Used by permission, Hull, R. J., and Raymond, K., Oil and Gas Jour., Nov. 9 (1953) thru Mar. 15 (1954).

The only available data for correlation is that of Sutherland on air-water [69] and of Myers [47] on two hydrocarbon systems. The latter data being at close tray spacings for laboratory columns. 

Percent open tray areas of 20-30% appear to be optimum for hydrocarbon systems [47]. 

For non-hydrocarbon systems Strigle [82] recommends the general outline to follow, but more detail for a specific design may be obtained from the reference as well as the manufacturer ... 

Figures 9-33A and B illustrate the fit of data taken by FRI on a commercial size column for hydrocarbon systems, using No. 2.5 Nutter rings at three different pressures, and comparing the latest Nutter proprietary correlation previously presented.

When considering pressure drop models based only on water, hydrocarbons system capacity can be significantly overstated. For Nutter random ring packings the pressure drop/capacity models fit the data within +10% over the range of commercial interest, i.e., 0.1 to 1.0 in. water/ft of packing. Pressure drop alues for design operation should... 

Figure 9-33A. Comparison of hydrocarbon systems fit to Nutter Correlation at 165 psia (No. 2.5 Nutter Ring ). Used by permission of Nutter Engineering Co., Harsco Corp. and by special permission of Fractionation Research, Inc. all rights reserved.

Follow the procedures oudined for bare tube equipment, substituting the characteristics of finned tubes where appropriate. The presentation of Wolverine recommends this technique over previous methods. The methods of reference 16 have proven acceptable in a wide number of petrochemical hydrocarbon systems. Figure 10-150 is an example unit in summary form. 

Methane is not used frequently in industrial plants for this service, due to mechanical sealing and safety related problems. Due to the danger of air being drawn into hydrocarbon systems, a positive pressure should always be maintained. 

When water comes in contact with the chloro-fluoro-refrigerants, an acid condition is established. This moisture may be in the form of water vapor coming in with air and is more likely if the suction side is lower than atmospheric pressure. These systems must be checked for leaks and moisture content. The descending order of reactivity with water is refrigerants 11, 12, 114, 22, and 113. Water vapor does not affect ammonia, except to modify the pressure-temperature relationship. When this becomes noticeable, the charge must be dried. Water must be purged from hydrocarbon systems, because emulsions or two-phase conditions may develop. 

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