Retrograde condensation, phase

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. 

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. 

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 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.

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 our heat transfer experiments, bulk fluid conditions were chosen to be just below the phase coexistence curve on either side of the lower consolute point. Thus, on heating at constant pressure, either evaporation of a liquid or condensation (retrograde) of a vapor took place once a small excess of test section surface temperature over bulk fluid temperature occurred. To be specific, the retrograde condensation region of the vapor--liquid phase coexistence curve of Figure 1 is the region of positive slope to the right of the LCST. 

Gregorowicz, J. de Loos, Th.W. de Swaan Arons, J. Unusual retrograde condensation in ternary hydrocarbon systems. Fluid Phase Equilibria 1992, 73, 109-115. 

There are several areas of interest in chemical processing that should be reviewed in this chapter. These areas include retrograde condensation, polymers, and electrolytes. In each case, the material is more extensive than can be covered in any detail in this chapter. Reference should be made to Prausnitz, Lichtenthaler, and Gomes de Acevedo. Furthermore, the principles covered in this chapter form the basis for viewing and calculating the phase equilibrium that applies. The subjects of polymers and electrolytes are so complex that they require a chapter by themselves. However, a brief statement follows describing retrograde condensation. 

One mode of operation is described here. A batch extractor is charged with the mixture to be separated. Gas is passed through the charge and the extract phase passes overhead to a rectifying. The process operates in the retrograde condensation region. Thus, a hot finger causes some of the heavier components to precipitate. Part of the condensate can be withdrawn and the remainder is refluxed back to the extractor. The depleted gas stream is lowered in pressure and temperature to liquid conditions and passed into a separator. The liquefied gas is vaporized and recycled via a compressor and the product is withdrawn. 

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. 

Keywords Phase diagrams Methane Molecular dynamics Clathrate hydrates Retrograde condensation Porosity... 

The phase equilibrium curve is calculated for the methane -i- n-butane mixture at 330 K (Fig. 2). The force field model used reproduces experimental data [3] on methane solubility in liquid butane rather well up to 80 atm. It reproduces the existence of the retrograde condensation region for the mixture under consideration at this temperature. The existence of the region follows from the fact that the phase equilibrium curve does not reach 100 % methane molar fraction. 

The lower limit of the region of retrograde condensation CC is often called critical condensation point. At CC the highest concentration of the low boiler in the vapor phase is obtained in equilibrium with the liquid phase. At this point the dew-point curve runs vertically and thus the slope for a given temperature is... 

Figure 2.11 The influence of curvature on the vapor pressure of a pure substance and the dewpoint pressure of a hydrocarbon mixture (a) system in the single-phase gaseous state. (6) fbr a pure substance, as pressure increases, gas may condense and the condensation will occur first in Tube 1. (c) For a hydrocarbon gas mixture with retrograde condensation behavior, as pressure decreases, liquid may form also in Tube 1 (liquid wets the substrate ...

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. 

J, another dew point at the same temperature. Condensation that takes place with temperature rise or with pressure drop is known as retrograde condensation, and the temperature and pressure conditions under which this occurs define the retrograde region. In such systems, vapor and liquid can coexist at temperatures and pressures above the critical point. The highest temperature and the highest pressure where two-phases can coexist are called, respectively, the cricondentherm and cricondenbar, also shown in Figure 2.1. 

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