Mixtures retrograde condensation

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. 

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. 

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 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... 

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-... 

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. 

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

Experimental measurements of heat transfer coefficients are reported for three binary mixtures near their lower consolute points. Two of these, respectively n-pentane and n-decane in solution with supercritical CO2, involve vapor--liquid equilibrium whereas the third, triethylamine--water, involves liquid--liquid equilibrium. Anomalously high heat transfer coefficients were found for the supercritical mixtures at compositions which condense on heating (retrograde condensation). 

The present invention effects a separation of hydrocarbon mixtures into fractions by taking advantage of the effects sometimes referred to as retrograde vaporization and retrograde condensation. These concepts are useful in explaining the theoretical and scientific background upon which the invention is based and will be discussed in detail hereinafter. ... 

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. 

Figure 2 Bubble-point dew-point locus of a mixture of constant composition. AA and BB are paths along which retrograde condensation and retrograde evaporation take place respectively, C, critical point, D, maxcondentherm-, andE, maxcondenbar...

Since the critical temperature of methane is 190.6 K, methane-containing mixtures at ambient temperatures always have some range of methane concentrations at which the retrograde condensation occurs. This condensation complicates gas field operation, since it results in partial condensation of natural gas near a well bottom. It lowers a well yield and decreases the amount of recoverable hydrocarbons. 

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. 

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 ...

Under certain conditions, gas mixtures exhibit retrograde boiling and condensation, that is, boiling takes place during a temperature decrease or a pressure increase. Similarly, retrograde condensation is the reverse of normal condensation. Follow the vertical broken line in Fig. 3-12 and notice what happens when the pressure changes. 

For a multicomponent mixture, the results may be qualitatively different from a binary case. Let us proceed from a mixture characteristic of a gas-condensate reservoir, whose composition is presented in Table 1. This mixture exhibits retrograde condensation with a dew-point pressure of 200 bar at T = 323 K. Figure 10 shows the nonmonotonous dependence of the surface tension on the distance to the binodal curve, as the molar fractions of Cl and C7 vary. Figure 11 indicates that under varying Cl and CIO, the variation of composition is possible only within a 0.2% region, although the Kelvin radii cover the whole range of macropores and mesopores. These examples show that capillary condensation may produce a rich variety of unusual physical effects in multicomponent mixtures, which are not observed in mixtures with a low mrniber of components. 

The behavior of the mixing volume is illustrated in Fig. 13 for the mixture of methane-n-butane. The phase diagram for this mixture at 300 K, calculated on the basis of the Peng-Robinson equation of state, is shown in Fig. 7b. The critical point corresponds to a pressure of 137 bar and to a molar fraction of methane of 0.77. If the molar fraction of methane exceeds this value, the mixture shows retrograde behavior. The partial volume of butane, Vg 2, is negative in the region of retrograde condensation. 

FIGURE 10.4 Typical experimental result for one specific mixture of two species, showing bubble-point and dew-point curves, critical point, cricondentherm and cricondenbar, and the paths for retrograde condensation or vaporization of the first and second kinds. 

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