It is a matter of common experience that some liquids are immiscible. "Oil and water don't mix." If one of the liquids is also volatile, then we have the situation where three fluid phases co-exist in equilibrium. Fig. 1 depicts the basic problem for a petroleum-water-natural gas system. The equilibrium is between an aqueous liquid, a hydrocarbon-rich liquid (oil or condensate), and a vapour (natural gas).
Fig. 1 Schematic of a Three-Phase Flash
As an introduction to three-phase equilibrium, consider the following experiment. An equimolar mixture of water, ethane and propane is placed in a piston. The initial pressure is so low that the mixture exists as a single phase gas. The mixture is compressed isothermally until the first minute amount of liquid forms. This liquid will be an aqueous liquid and the pressure at which this occurs is the aqueous-dew point. This is the regular, two-phase dew point, that is the first liquid formed. If we continue to compress the mixture, there will be some exchange of the components between the two phases and thus the amounts and compositions of the two phases will change. Finally a point will be reached where a minute amount of a second liquid (hydrocarbon-rich ) begins to form. This is the hydrocarbon-rich-liquid-dew point and is the point where three phases are first encountered. As we continue to compress there is mass transfer between the three phases. Typically, the amount of vapor will decrease and the amount of the second liquid will increase. Finally we reach a point where there is no longer a vapor phase. This is the three-phase bubble point. At higher pressures a vapor phase no longer exists and the equilibrium is between the two liquids.
If we repeat the experiment for several temperatures we will be able to construct a pressure-temperature diagram. The curves on the figure are the dew and bubble point loci. The fields separated by these loci are the phase regions. Although not shown on this diagram, the hydrocarbon-rich liquid-dew point and bubble point loci will intersect at a critical point. Beyond recognizing the existence of such a critical point they will not be discussed here. Note that the pressure-temperature diagram is like a map showing the regions where the various equilibria are encountered.
Fig. 2 Pressure-Temperature Diagram
Showing the Various Phase Loci
(not to scale)
AQUAlibrium can be used to calculate all three loci depicted in Fig. 2. In addition, if the pressure and temperature is such that the fluid is in the aqueous liquid-vapor two-phase, AQUAlibrium provides a flash routine to calculate this equilibrium. Equilibrium in the three-phase region can be calculated using the three-phase flash option in AQUAlibrium. The liquid-liquid equilibrium can be modeled using the liquid-liquid flash module in AQUAlibrium.
It is possible, and in fact common, for these systems to exhibit retrograde behavior. That is, upon further compression it may happen that the amount of the second liquid phase will begin to decrease and the amount of vapor will increase. Thus, as the pressure is increased instead of a bubble-point, a second dew-point, a retrograde dew-point is encountered. This phenomena can be predicted using AQUAlibrium as well. An example of a system exhibiting retrograde behavior is given in the AQUAlibrium manual.