Binary phase diagrams

Phase diagrams are graphical representations of the liquid, vapor, and solid phases that co-exist at various ranges of temperature and pressure within a reservoir. Binary phase diagrams describe the co-existence of two phases at a range of pressures for a given temperature.

Binary phase diagrams

Fig. 1 is a pressure-composition (p-x-y) phase diagram that shows typical vapor/liquid phase behavior for a binary system at a fixed temperature below the critical temperature of both components. At pressures below the vapor pressure of Component 2, p_v2, any mixture of the two components forms a single vapor phase. At pressures between p_v1 and p_v2, two phases can coexist for some compositions. For instance, at pressure p_b, two phases will occur if the mole fraction of Component 1 lies between x_B and x_E. If the mixture composition is x_B, it will be all liquid; if the mixture composition is x_E, it will be all vapor. At constant temperature and pressure, the line connecting a liquid phase and a vapor phase in equilibrium is known as a tie line. In binary phase diagrams such as Fig. 1, the tie lines are always horizontal because the two phases are in equilibrium at a fixed pressure. For 1 mole of mixture of overall composition, z, between x_B and x_E, the number of moles of liquid phase is

....................(1)

Eq. 1 is an inverse lever rule because it is equivalent to a statement concerning the distances along a tie line from the overall composition to the liquid and vapor compositions. Thus, the amount of liquid is proportional to the distance from the overall composition to the vapor composition, divided by the length of the tie line.

• 

  Fig. 1 – Pressure-composition diagram for a binary mixture at a temperature below the critical temperature of both components.

Phase diagrams such as Fig. 1 can be determined experimentally by placing a mixture of fixed overall composition in a high-pressure cell and measuring the pressures at which phases appear and disappear. For example, a mixture of composition x_B would show the behavior indicated qualitatively in Fig. 2. At a pressure less than p_d (Fig. 1), the mixture is a vapor. If the mixture is compressed by injecting mercury into the cell, the first liquid, which has composition x_A, appears at the dewpoint pressure, p_d. As the pressure is increased further, the volume of liquid grows as more and more of the vapor phase condenses. The last vapor of composition x_E disappears at the bubblepoint pressure, p_b.

• 

  Fig. 2 – Volumetric behavior of a binary mixture at constant temperature that shows a bubblepoint pressure.

If the system temperature is above the critical temperature of one of the components, the phase diagram is similar to that shown in Fig. 3. At the higher temperature, the two-phase region no longer extends to the pure Component 1 side of the diagram. Instead, there is a critical point, C, at which liquid and vapor phases are identical. The critical point occurs at the maximum pressure of the two-phase region. The volumetric behavior of mixtures containing less Component 1 than the critical mixture, x_c, is like that shown in Fig. 2. Fig. 4 shows the volumetric behavior of mixtures containing more Component 1. Compression of the mixture of composition x₂ (in Fig. 3) leads to the appearance of liquid phase of composition x₁ when pressure p_d1 is reached. The volume of liquid first grows and then declines with increasing pressure. The liquid phase disappears again when pressure p_d2 is reached. Such behavior is called "retrograde vaporization" or "retrograde condensation" if the pressure is decreasing.

• 

  Fig. 3– Pressure-composition phase diagram for a binary mixture at a temperature above the critical temperature of Component 1.

• 

  Fig. 4 – Volumetric behavior of a binary mixture at constant temperature showing retrograde condensation.

If the system temperature is exactly equal to the critical temperature of Component 1, the critical point on the binary pressure-composition phase diagram is positioned at a Component 1 mole fraction of 1.0. Fig. 5 shows the behavior of the two-phase regions as the temperature rises. As the temperature increases, the critical point moves to lower concentrations of Component 1. As the critical temperature of Component 2 is approached, the two-phase region shrinks, disappearing altogether when the critical temperature is reached.

• 

  Fig. 5 – Regions of temperature, pressure, and composition for which two phases occur in a binary liquid/vapor system.

Fig. 6 shows a typical locus of critical temperatures and pressures for a pair of hydrocarbons. The critical locus shown in Fig. 6 is the projection of the critical curve in Fig. 5 onto the p-T plane. Thus, each point on the critical locus represents a critical mixture of different composition, although composition information is not shown on this diagram. For temperatures between the critical temperature of Component 1 and Component 2, the critical pressure of the mixtures can be much higher than the critical pressure of either component. Thus, two phases can coexist at pressures much greater than the critical pressure of either component. If the difference in molecular weight of the two components is large, the critical locus may reach very high pressures. Fig. 7 gives critical loci for some hydrocarbon pairs.

• 

  Fig. 6 – Pressure-temperature diagram: a projection of the vapor-pressure p_v1 and p_v2) curves and locus of critical points for binary mixtures. Points C₁ and C₂ are the critical points of the pure components.

• 

  Fig. 7 – Vapor-pressure curves for light hydrocarbons and critical loci for selected hydrocarbon pairs.

The binary phase diagrams reviewed here are those most commonly encountered. However, more complex phase diagrams involving liquid/liquid and liquid/liquid/vapor equilibriums do occur in hydrocarbon systems at very low temperatures (well outside the range of conditions encountered in reservoirs or surface separators) and in CO₂/crude oil systems at temperatures below approximately 50°C. See Stalkup and Orr and Jensen for reviews of such phase behavior.

Noteworthy papers in OnePetro

Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read

External links

Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro

See also

Phase diagrams

Quaternary phase diagrams

Ternary phase diagrams

Phase diagrams for EOR processes

Phase diagrams for reservoir fluid systems

PEH:Phase_Diagrams