Page 79 - Handbook of Natural Gas Transmission and Processing Principles and Practices

2.2.3. Phase Envelopes (or Isopleths) of Binary Systems

A phase envelope (or isopleths or P-T contour plot) is the P-T projection of the phase diagram of a
multicomponent system of fixed composition. It is made up of two parts: (1) the bubble curve and
(2) the dew curve. These two curves join at a critical point characterized by a critical composition
equal to the composition at which the phase envelope is plotted. Fig. 2.8 illustrates the construction

of the phase envelope of a binary system of composition z = 0.5.
1
Fig. 2.8A shows a series of isothermal Pxy projections from low to high temperatures. At the

composition z = 0.5 (see the vertical line) and for a selected temperature T, it is possible to
1
read in panel (A) the corresponding bubble-point pressure (P bubble , symbol: circles) and dew-point
pressure (P dew , symbol: squares). By changing the temperature T at the same composition z , new
1
values of P bubble and P dew are obtained. By reporting the points, the coordinates of which are (P

, T) and (P , T) in a P-T plane, one obtains the phase envelope of the binary system at z =
bubble dew 1
0.5 (see panel B) made up of a bubble and a dew curve. The phase envelope of a mixture of known
composition has generally a loop shape and is rather simply interpreted as for a temperature and a
pressure defining a point inside the loop, the mixture is in a VLE state. Outside the loop, a mixture
located above the bubble curve is in a 1-phase liquid state, whereas a mixture located below the
dew curve is in a 1-phase gaseous state.

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