Page 88 - Handbook of Natural Gas Transmission and Processing Principles and Practices

in equilibrium with a bubble of gas, the composition of which is . A dew point is a 2-phase
system of known gas-phase composition: in equilibrium with a drop of liquid having the
composition .
Specifying the composition of one of the two equilibrium phases is equivalent to specify
independent variables. Because the dof is equal to p, another intensive phase variable must also be
specified to make Eq. (2.12) solvable. This can be, e.g., the pressure P or the temperature T.
To sum up, a bubble-point calculation can be performed by specifying the intensive phase
variables:

(2.16)

In such a case, the unknowns are ( unknowns), , and , determined by
solving Eq. (2.12). Once done, the bubble pressure is estimated by application of the EoS:
.
Similarly, a dew point calculation can be performed by specifying:

(2.17)

The unknowns are . Once determined, the dew pressure is obtained from the
equation .
As previously discussed, at a fixed temperature, two dew-point pressures may exist (retrograde
condensation phenomenon). In such a case, the system of equations to solve (Eq. 2.12) will exhibit 2
sets of solutions. As another difficulty, at the maxcondentherm, the dew curve exhibits a vertical
tangent in the P-T plane (Fig. 2.8B). As a consequence, in this area, a small change in the specified
temperature value causes a high variation in the calculated dew point pressure. In such a case, it is
advised to specify the pressure instead of the temperature so that the specified variables become:

(2.18)

The equations to solve are:

(2.19)

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