Qualitative Phase Behavior of Hydrocarbon Systems

The properties of a phase are either intensive or extensive. An intensive property is one which is independent of the total quantity of matter in the system. Examples are density, specific gravity, and specific heat. Properties such as the mass and volume of a system are termed extensive properties since their value is determined by the quantity of matter contained in the system. 

It will he shown that the behavior of heterogeneous systems is influenced by the number of components it contains. A system which consists of a single, pure substance will behave differently from one which is made up of two or more components when the pressure and temperature are such that both a liquid phase and a gas phase are present. Consequently, the discussion of phase behavior will begin with a description of single-component systems. This will be followed by a description of two-component systems. Finally, multicomponent 

Consider a single, pure fluid at constant temperature, in a cylinder fitted with a friotionless piston. If a pressure is applied on the piston which is greater than the vapor pressure of the liquid, the system will consist entirely of liquid when equilibrium is reached. No vapor will be present since at pressures greater than the vapor pressure it condenses into liquid. If, on the other hand, the pressure applied on the piston is less than the vapor pressure of the liquid only vapor will be present at equilibrium. If both liquid and vapor are present in equilibrium with one another, the pressure must be exactly equal to the vapor pressure. Pure substances behave in this manner and liquid and vapor can coexist at a given temperature only at a pressure equal to the vapor pressure. The relative amounts of liquid and vapor that coexist is determined by the volume of the system, and can vary an3nvliere from an infinitesimal amount of liquid to an infinitesimal amount of vapor. 

The upper limit of the vapor pressure line is the point A. This is known as the critical point and the temperature and pressure represented by this point are the critical temperature To and the critical pressure Pc, respectively. At this point the intensive properties of the liquid phase and the vapor phase become identical and they are no longer distinguishable. For a single-component system the critical temperature may also be defined as the temperature above which a vapor cannot be liquefied, regardless of the applied pressure. Similarly, the critical pressme of a single-component system may be 

The lower end of the vapor-pressure line is limited by the triple point O. This point represents the pressure and temperature at which solid, liquid, and vapor coexist under equilibrium conditions. Since the petroleum engineer seldom deals with hydrocarbons in the solid state it will not be necesaaiy to deal with this region of the diagram 

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The qualitative phase behavior of hydrocarbon systems was described in the previous chapter. The quantitative treatment of these systems mil now be discussed and tire methods for calculating their phase behavior presented. It will became apparent that the liquid and vapor phases of mixtures of two or more hydrocarbons are in reality solutions (see below), so that it will be necessary to discuss the laws of solution behavior. Analogous to the treatment of gases, the behavior of a hypothetical fluid known as a perfect, or ideal, solution will be described. This will be followed by a description of actual solutions and tlie deviations from ideal solution behavior that occur. 

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