Example 2.4: Phase envelope of a natural gas with retrograde condensation

A natural gas reservoir is characterised by the following molar composition (table 1: compositions under 10^-5 are omitted). The corresponding phase envelope has been generated as shown in figure 1. Conditions in the reservoir are 333.15 K and 15 MPa (point A). In a separator, the gas is first cooled to 298.15 K (point B) and expanded at this temperature down to 9 MPa (point C). Next, the fluid is further expanded at the same temperature down to 1 MPa (point D). Describe the behaviour of the fluid.

Figure 1: Retrograde condensation in a natural gas P – T diagram

Analysis:

The only properties involved are pressure and temperature. Components include hydrocarbons, nitrogen and carbon dioxide. Only the phase behaviour is investigated. States are supercritical and in the vapour-liquid zone. The choice of the model is very important, but in this example the figure is used.

Solution:

Natural gas is fully described by its molar composition. Duhem's phase rule therefore indicates that two properties are sufficient to describe the system. In this case, each state is described by two intensive properties P and T. The different states can be plotted on the phase envelope. This figure also shows the lines of constant liquid fraction.

The first two points (A and B) are clearly located in the supercritical zone: no phase change appears. When the pressure is lowered at constant temperature, the dew line is crossed, meaning that a liquid phase appears. This can be understood physically from the observation that the denser a gas is, the better a solvent it is for the heavy fraction of the fluid. Consequently, decreasing the pressure means decreasing its solvent power. At the temperature considered if the pressure drops below 9.7 MPa, the dew point will be passed and a liquid phase develops. The pressure range where a liquid deposits is called the retrograde region, and the phenomenon is known as retrograde condensation.

The liquid dropout will continue to increase until the pressure reaches 6.5MPa. Further pressure reduction will re-vaporise most of the liquid phase, which is the normal behaviour. At the normal dew pressure, on the bottom of the figure, the fluid is entirely vaporised (point D).