Retrograde phenomena condensation

Consider what happens when the system point is at point a in Fig. 13.13 and the pressure is then increased by isothermal compression along line a-b. The system point moves from the area for a gas phase into the two-phase gas-liquid area and then out into the gas-phase area again. This curious phenomenon, condensation followed by vaporization, is called retrograde condensation. 

The culprit is the phenomenon of retrograde condensation, which wa6 previously discussed In connection with hydrocarbon dewpoints. This can best be understood by looking at a graph of equilibrium ratios, commonly called K-values, as shown in Figure 2. We have cross-plotted a limited number of curves, to avoid confusion while Illustrating our point. 

Since in the critical point the bubble point curve (l+g—tf) and the dew-point curve (l+g-+g) merge at temperatures between 7C and 7 , an isotherm will intersect the dew-point curve twice. If we lower the pressure on this isotherm we will pass the first dew-point and with decreasing pressure the amount of liquid will increase. Then the amount of liquid will reach a maximum and upon a further decrease of the pressure the amount of liquid will decrease until is becomes zero at the second dew-point. The phenomenon is called retrograde condensation and is of importance for natural gas pipe lines. In supercritical extraction use is made of the opposite effect. With increasing pressure a non-volatile liquid will dissolve in a dense supercritical gas phase at the first dew point. 

Consider the enlarged nose section of a single PT loop shown in Fig. 12.5. The critical point is at C. The points of maximum pressure and maximum temperature are identified as MP and MT. The dashed curves of Fig. 12.5 indicate the fraction of the overall system that is liquid in a two-phase mixture of liquid and vapor. To the left of the critical point C a reduction in pressure along a line such as BD is accompanied by vaporization from the bubble point to the dew Point, as would be expected. However, if the original condition corresponds to Point F, a state of saturated vapor, liquefaction occurs upon reduction of the pressure and reaches a maximum at G, after which vaporization takes place until the dew point is reached at H. This phenomenon is called retrograde condensation. It is of considerable importance in the operation of certain deep natural-gas wells where the pressure and temperature in the underground forma-... 

The phenomenon of retrograde condensation is also illustrated by Fig. 14-6. As pointed out by Brown,9 if either the bubble-point or dew-point curve is crossed twice while passing through the two-phase region by either isobaric or isothermal paths, retrograde condensation will occur. Lines EF and GH represent isothermal and isobaric paths, respectively, which would produce retrograde condensation. 

In order to explain this phenomenon, we consider a T-s diagram as shown in Fig.4-b. From this figure it can be said that R-12 is not the retrograde substance [4,5] but normal (or intermediate) vapor, and it can be hardly liquefied by merely isentropic compression. Then the nonequilibrium condensation at the cold tube wall seems to be the main origin of the phenomenon. The temperature of the gas adjacent to the cold wall cannot follow Rankine-Hugoniot temperature jump and rapid nonequilibrium condensation is initiated at the wall. 

The phenomenon that a liquid is formed by lowering the pressure at constant temperature or, respectively, by increasing the temperature at constant pressure is called retrograde condensation. 

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. 

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