Condensation retrograde

Condensable Hquids also are recovered from high pressure gas reservoirs by retrograde condensation. In this process, the high pressure fluid from the reservoir produces a Hquid phase on isothermal expansion. As the pressure decreases isotherm ally the quantity of the Hquid phase increases to a maximum and then decreases to disappearance. In the production of natural gas Hquids from these high pressure wells, the well fluids are expanded to produce the optimum amount of Hquid. The Hquid phase then is separated from the gas for further processing. The gas phase is used as a raw material for one of the other recovery processes, as fuel, or is recompressed and returned to the formation. 

A natural gas reservoir in which retrograde condensation will take place as it is depleted and from which liquids can be commercially recovered in surface facilities. 

The hydrocarbon dew point is reduced to such a level that retrograde condensation (i.e., condensation resulting from pressure drop) cannot occur under the worst conditions likely to be experienced in the gas transmission system. Similarly, the water dew point is reduced to a level sufficient to preclude formation of Cl to C4 hydrates in the system. 

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. 

The effect of retrograde condensation on a crude oil component such as octane is particularly significant, and small amounts of these heavier components if left in the gas also have a great effect an hydrocarbon dewpoints. Even with extensive gas conditioning cooling it is difficult to remove all crude oil ccanpcnenta from the vapor streams at a pressure of 1500 psl. Note that the K-value for octane at 0°F and 1500 psi is not as low as at 100°F and 1000 psi. 

Other phenomena can be simply explained by the fact that the critical pressure and temperature for a given mixture is not, as it happens for a pure fluid, the maximum temperature and pressure that allows the coexistence of a vapour and liquid phase in equilibrium. Retrograde condensation phenomena can be easily explained in this way. 

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. 

We will first consider phase diagrams. Then we will define the critical point for a two-component mixture. This will be the correct definition for multicomponent mixtures. Also, we will look at an important concept called retrograde condensation. Then the pressure-volume diagram will be discussed, and differences between pure substances and two-component mixtures in the two-phase region will be illustrated. Finally, the effects of temperature and pressure on the compositions of the coexisting liquid and gas will be illustrated. 

However, consider die isothermal decrease in pressure illustrated by line 123 on Figure 2—18. As pressure is decreased from point 1, the dewpoint line is crossed and liquid begins to form. At the position indicated by point 2, the system is 25 percent liquid by volume and 75 percent gas. A decrease in pressure has caused a change from gas to liquid. This is exactly the reverse of the behavior one would expect, hence the name retrograde condensation. As pressure is decreased from point 2 toward point 3, the amount of liquid decreases, the dew-point line is crossed a second time, and the system again becomes gas. 

The region of retrograde condensation occurs at temperatures between the critical temperature and the cricondentherm. A similar retrograde... 

EXAMPLE 2-5 Determine the temperature range in which retrograde condensation will occur for a mixture of 50.02 mole percent methane and 49.98 mole percent ethane. 

Mixture 2 on Figure 2-37 illustrates a mixture containing a large quantity of the light component. The phase envelope is relatively small and is located at low temperatures. The critical point is located far down the left-hand side of the phase envelope and is fairly close to the critical point of the pure light component. There is a large area in which retrograde condensation can occur. 

Retrograde gases are also called retrograde gas-condensates, retrograde condensate gases, gas condensates, or condensates.1,2 The use of the word condensate in the name of this reservoir fluid leads to much confusion. Initially, the fluid is gas in tire reservoir and exhibits retrograde behavior. Hence, the correct name is retrograde gas. 

Retrograde Condensation For near-ideal mixtures, the intersection of the (p, X2 or y ) isotherm with the critical locus occurs at the maximum in the (p, y2) equilibrium line. For example, an enlargement of the two-phase (p, xi or yi) section for (xi oryi)Ar + (x2 or j2)Kr at T— 177.38 K is shown in Figure 14.12. The point of intersection with the critical locus at point (c) gives rise to an... 

Figure 14.12 The top of the (vapor + liquid) isotherm for ( iAr + Kr) at T = 177.38 K. Point (c) is the intersection with the critical locus. The curve marked g gives the composition of the vapor phase in equilibrium with the liquid curve marked 1. The tubes shown schematically to the right demonstrate the changes in phase when the fluid is compressed at a mole fraction given by (a), or at a mole fraction corresponding to (b) where retrograde condensation occurs. Reprinted with permission from M. L. McGlashan, Chemical Thermodynamics, Academic Press, London, 1979, p. 276.

Retrograde condensation is an example of phenomena that sometimes occur in science that appear to defy human intuition. To quote from M. L. McGlashan... 

The main conclusion to be drawn from the experimental data presented here is that fractionation of residuum through the use of a supercritical fluid system incorporating internal reflux produced by retrograde condensation results in sharper fractions than those obtained by ordinary supercritical extraction. The capability of the FDU to process coal-derived residuum in the... 

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-... 

In other words, retrograde condensation is defined as the formation of liquid by an isothermal decrease in pressure or an isobaric increase... 

Unfortunately, the terminology applied to retrograde phenomena has not yet been standardized in the literature. The phenomena described as retrograde vaporization in the preceding pages are sometimes referred to as retrograde condensation and vice versa. Further-... 

Normal condensation. Retrograde condensation.—The consideration of limiting lines plays an important rdle in all the researches relative to the liquefaction and vaporization of fluid mixtures. The detailed anal3rsis of these researches would exceed the plan of this work so we shall not give it. We shall be content to notice a remarkable consequence of the preceding theories. 

This series of phenomena constitutes retrograde condensation. 

Dew line and line of ebullition for a mixture of given composition, 821.—243. Normal condensation, retrograde condensation, 822. 

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