Page 33 - Handbook of Natural Gas Transmission and Processing Principles and Practices

1.5. Natural Gas Phase Behavior
Natural gas is a naturally occurring hydrocarbon mixture that is found underground and at
elevated conditions of pressure and temperature. Therefore, there is an essential need to know a
priori how the gas fluid will behave under a wide range of pressure and temperature conditions,
particularly in terms of its volumetric and thermophysical properties that are required in simulating
reservoirs, evaluating reserves, forecasting production, designing production facilities, and
designing gathering and transportation systems. In fact, an accurate knowledge of hydrocarbon
fluid phase behavior is crucial in designing and operating the gas-engineering processes efficiently
and optimally. This means, having advanced predictive tools for the characterization of
hydrocarbon phase behavior with the highest accuracy possible is the key to mastering the
economics of natural gas systems.
The natural gas phase behavior is a plot of pressure versus temperature that determines whether
the natural gas stream at a given pressure and temperature consists of a single gas phase or two
phases, gas and liquid. The phase behavior of natural gas with a given composition is typically
displayed on a phase diagram, an example of which is shown in Fig. 1.2. The left-hand side of the
curve is the bubble point line and divides the single-phase liquid region from the two-phase gas–
liquid region. The right-hand side of the curve is the dew point line and divides the two-phase gas–
liquid region and the single-phase gas region. The bubble point and dew point lines intersect at the
critical point, where the distinction between gas and liquid properties disappears. The maximum
pressure at which liquids can form is called the cricondenbar (P CC ), and the maximum temperature
at which liquids can form is called the cricondentherm (T CC ). However, there is something very

interesting going on within the region T < T < T , where we will be moving from a 0%
cc
c
liquid to another 0% liquid condition (both on the dew point curve) in an isothermal compression.
This different behavior of a vapor under compression is called retrograde (contrary to expectation)
condensation. It is also important to see that a similar behavior is to be expected within the region

P < P < P . In this case, we talk about retrograde vaporization since we will be moving
cc
c
from a 100% liquid to another 100% liquid condition (both on the bubble point curve) in an isobaric
heating.

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