Reverse Condensation

the precipitation near the critical point of the liquid phase in a two-component or multicompo-nent gaseous system after an isothermal pressure drop. The phase diagram of such a system with variable temperature and pressure is presented in Figure 1.

Figure 1. Phase diagram of a two-component system with fixed composition near the liquid-vapor critical point: (C) critical point, (7) temperature, (p) pressure

In contrast to the phase diagrams of pure substances, in which the boiling curve A₁C₁ (ending at the critical point C) is the interface of the liquid and vapor phases, the phase diagram of a mixture has the form of the loop-shaped curve ABC; within the area bordered by ABC the mixture is in both the liquid and vapor states. The boiling curve AC and condensation curve CB of the mixture join at the critical point C, where the difference in the properties of the two phases disappears.

After an isothermal pressure drop, for example, along the isotherm hd in the region of the temperatures T_c and T_m, drops of liquid begin to precipitate from the homogeneous gas phase at point h. The amount of liquid gradually increases to a maximum at point F and then begins to decrease, finally disappearing entirely at point d. This process is called isothermal reverse condensation, or reverse condensation of the first order. The area bounded by CMG, in which the condensate precipitates upon a reduction in p, is called the region of reverse condensation; “reverse” indicates the return of the system to the two-phase state. The phenomenon of reverse isothermal condensation has found broad practical application in the extraction of gas condensate from gas-condensate fields of natural gas.

When the isobaric line intersects the two-phase region in the pressure interval from p_c to p_m for example, along line ab, vapor bubbles begin to appear in a homogeneous liquid mixture at point a. The amount of vapor initially increases with T and then decreases, and the system once again becomes liquid at point b. This process is called reverse evaporation, or reverse condensation of the second order.