Bubble, Dew, and Critical Points of Mixtures

Let us now consider a mixture of the two compounds, with a specified composition of component A equal to Z, at a temperature Tj and at some pressure Pj, where the mixture is in the liquid state. We reduce the pressure slowly - at Tj - until the first bubble appears. This represents the bubble point (B.P.) pressure Pj of this mixture at Tj, with a liquid composition of = Zj. If we continue decreasing the pressure, more vapor appears until only one drop of liquid is left. This represents the dew point (D.P.) pressure P for this mixture at 7j, with = Zj. 

Notice that, contrary to our experience with pure compounds, the critical point here does not represent the maximum temperature at which the given mixture can exist in the liquid phase. It does represent, however, the highest temperature where the whole fluid can exist as a liquid. The maximum temperature and pressure that this envelope reaches are referred to as cricondentherm and cr/co/K/e/iZ ar respectively. 

Retrograde condensation is very important in gas condensate reservoirs for the reduction of pressure, as a result of gas recovery, can lead to the condensation - and thus loss - of the very valuable heavy constituents of the gas and in gas distribution systems, where the pressure drop can lead to liquid formation and plugging of the pipes. 

We have already encountered the phenomenon of retrograde condensation in the case of supercritical extraction (Section 11.11), where pressure reduction was used to recover the material dissolved in the gas. 

The locus of the critical points can also be concave upwards, typically in polar mixtures, such as methanol-benzene (McCracken and Smith, 1956). For more complex cases, see van Konynenburgand Scott (1980). 

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