Dew-point line

The experiment could be repeated at a number of different temperatures and initial pressures to determine the shape of the two-phase envelope defined by the bubble point line and the dew point line. These two lines meet at the critical point, where it is no longer possible to distinguish between a compressed gas and a liquid. 

It is important to remember the significance of the bubble point line, the dew point line, and the two phase region, within which gas and liquid exist in equilibrium. 

The initial temperature of a gas condensate lies between the critical temperature and the cricondotherm. The fluid therefore exists at initial conditions in the reservoir as a gas, but on pressure depletion the dew point line is reached, at which point liquids condense in the reservoir. As can be seen from Figure 5.22, the volume percentage of liquids is low, typically insufficient for the saturation of the liquid in the pore space to reach the critical saturation beyond which the liquid phase becomes mobile. These... 

Black oils are a common category of reservoir fluids, and are similar to volatile oils in behaviour, except that they contain a lower fraction of volatile components and therefore require a much larger pressure drop below the bubble point before significant volumes of gas are released from solution. This is reflected by the position of the iso-vol lines in the phase diagram, where the lines of low liquid percentage are grouped around the dew point line. 

Figure 2-10 shows a more nearly complete pressure-volume diagram.2 The dashed line shows the locus of all bubble points and dew points. The area within the dashed line indicates conditions for which liquid and gas coexist. Often this area is called the saturation envelope. The bubble-point line and dew-point line coincide at the critical point. Notice that the isotherm at the critical temperature shows a point of horizontal inflection as it passes through the critical pressure. 

The definition of the critical point as applied to a pure substance does not apply to a two-component mixture. In a two-component mixture, liquid and gas can coexist at temperatures and pressures above the critical point, Notice that the saturation envelope exists at temperatures higher than the critical temperature and at pressures higher than the critical pressure. We see now that the definition of the critical point is simply the point at which the bubble-point line and the dew-point line join. A more rigorous definition of the critical point is that it is the point at which all properties of the liquid and the gas become identical. 

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 bubble-point line is also the locus of compositions of the liquid when two phases are present. The dew-point line is tire locus of compositions of the gas when gas and liquid are in equilibrium. The line which ties the composition of the liquid with the composition of gas in equilibrium is known as an equilibrium tie-line. Tie-lines are always horizontal for two-component mixtures. 

The lower line of a saturation envelope is the bubble-point line, and the upper line is the dew-point line. Composition-temperature conditions which plot below the saturation envelope indicate that the mixture is entirely liquid. 

When pressure is less than the critical pressures of both components, the bubble-point and dew-point lines join at the vapor pressures of the pure components at either side of the diagram. When the pressure exceeds the critical pressure of one of the components, the bubble-point line and the dew-point line join at a critical point. For instance, a mixture of 98 mole percent methane and 2 mole percent ethane has a critical temperature of minus 110°F at a critical pressure of 700 psia. 

Tie-lines giving the compositions of the liquid and gas in equilibrium are again horizontal. The bubble-point line gives the composition of the equilibrium liquid, and the dew-point line gives the composition of the equilibrium gas. The lengths of the tie-lines represent the quantities of gas and liquid at equilibrium in the same manner as for the pressure-composition diagram. 

Second, read composition of equilibrium gas at point where the tieline through point 1 connects with dew-point line. See point 2. 

Figure 2-27 gives the saturation envelope for mixtures of methane, propane, and n-pentane at the same temperature as Figure 2-26 but at a higher pressure. The bubble-point and dew-point lines join at a critical point. The critical point gives the composition of the mixture, which has a critical pressure of 1500 psia and a critical temperature of 160°F. 

Consider a pressure above the vapor pressure of n-pentane and below vapor pressure of propane, for instance, 200 psia. See dot 2 on Figure 2-29 and Figure 2-28(2). All mixtures of methane and propane are gas. Both the methane-n-pentane binary and the propane-n-pentane binary are in their two-phase regions. Their bubble-point and dew-point compositions appear along the sides of the ternary diagram as the ends of the bubble-point and dew-point lines of the ternary mixtures. 

At the vapor pressure of propane, 380 psia at this temperature, the bubble-point and dew-point lines of the saturation envelope converge at... 

As the number and complexity of the molecules in a mixture increase, the separation between the bubble-point and dew-point lines on the phase diagram becomes greater. Phase diagrams of several petroleum mixtures are shown in Figures 2-32 through 2-36.8 9,10,11 12... 

Replot two isotherms of the data given in Figure 2-37 as pressure against composition in weight percent. Use temperatures of 75°F and 300°F. This is called a pressure-composition diagram. Label the bubble-point lines and dew-point lines. 2-23. Determine the compositions and quantities of gas and liquid when 10 lb moles of a mixture of 55 mole percent methane, 20 mole percent propane and 25 mole percent n-pentane is brought to equilibrium at 160°F and 1500 psia. 

Now for those of you who may say, This table has its place but what about enthalpy values far to the right of this saturated liquid line or far to the left of this saturated vapor line Chap. 2 addresses this question most specifically. Please consider the fact that pressure increase has little effect on liquid enthalpy at constant temperature. Similarly, notice how pressure lines tend to converge on the vapor dew point line of the phase envelope. This indicates that at constant temperature, increasing the pressure of vapor tends to approach the enthalpy value of the saturation vapor dew point line of the phase envelope of a P vs. H enthalpy figure. See Fig. 2.1 in Chap. 2. 

The concentrations of both the liquid phase, xB, and the vapor phase, yB, are determined by the pressure. The line relating vapor pressure to liquid-phase concentration is called the bubble-point line, because when the pressure on the liquid is reduced to this value, bubbles appear. The lower line, which relates vapor pressure to the vapor-phase concentration, is called the dew-point line because when vapor is compressed to give this pressure, liquid droplets appear on surfaces. 

There will be an isotherm similar to ABCD for each temperature. The complete P-V diagram for the i-pentane, w-heptane S3retem containing 52.4 weight per cent w-heptane is shown in Figure 24. The critical point is the point where the bubble-point line and dew-point line meet. This is equivalent to the statement that the intensive properties of the coexisting liquid and vapor phases are identical at the critical point. Consequently, the liquid and tiie vapor are indistinguishable at the critical pressure and temperature, The critical... 

In the diagram shown in Figure 30 the composition is expressed in weight per cent of the less volatile component. It is to be understood that the composition may equally well be expressed in terms of weight per cent of the more volatile component in which case the bubble-point and dew-point lines have the opposite slope. Furthermore, the... 

A, B, C, and D have been calculated in the preceding example. The point A at 22.75 psia represents the computed bubble-point pressure for a solution whose mole fraction of C4H10 is 0.50. Point B represents the composition of the vapor at the bubble point. Similarly, the points C and D represent the bubble point and composition of the vapor at the bubble point for a solution whose mole fraction of C4Hio>is 0.75. The points E and jP. represent the vapor pressure of pure butane and pure propane, respectively, at 0° F. The line FACE is tile bubble-point line and the line FBDE is the dew-point line. It is obvious that a pressure-composition diagram for any ideal binary system could be calculated in this manner and would serve to describe the phase behavior quantitatively. 

When a binary mixture, whose boiling and dew point lines are shown in Fig. 4.19, condenses on a cooled wall of temperature -j90, a condensate forms, Fig. 4.19b, which is bounded by the vapour. At the phase interface, a temperature i>i develops, which lies between the temperature dc of the vapour far away from the wall and the wall temperature d0. If the vapour is saturated, its temperature is i9a = 0S corresponding to Fig. 4.19a. The temperature profile in the vapour and condensate are illustrated in Fig. 4.19c. 

Fig. 4.19 Temperature-concentration diagram for a binary mixture as well as the temperature and concentration profiles in the vapour and the condensate. Indices 0 cold wall, I interface, G core flow of vapour (G Gas), a boiling and dew point lines b condensate and vapour boundary layer c temperature profile d concentration profile...

The temperature at the phase interface lies, as can be seen in Fig. 4.22, between the temperature on the dew point line (case b) and the temperature on the boiling line (case a). The associated vapour and liquid compositions can be read off the abcissa, points A and B in Fig. 4.22. 

Figure 9.4 shows another phase diagram at constant pressure. The x-axis shows the vapor-liquid mole fraction of the binary mixture. The y-axis shows temperature. The dew point line shows the temperature at which a superheated vapor mixture will begin to condense when cooled for all compositions of the mixture. The bubble point line shows the temperature at which a subcooled mixture will first begin to... 

Fig, 6.2. A mixture of two liquids that follows Raouit s Law. The total vapour pressure of the mixture is plotted as a function of the composition of the liquid (upper line) and of the vapour (lower line). The upper line is sometimes referred to as the bubbte-point line and the lower as the dew-point line. 

NOTE You may be tempted to try and memorize something like the dew point line is on the bottom in a Pxy diagram and on the top in a Txy diagram. This is, however, strongly discouraged, as you will very likely become confused if you depend on this... 

Diagrams for systems that follow Raoult s Law are relatively "nice" it can be shown that they will never have azeotropes, which would be indicated by intersection of the bubble and dew point lines. In addition, since only one parameter in the equation depends on the temperature (the vapor pressure) and the pressure dependence is explicit, the dew and bubble point lines are relatively easy to calculate. 

Fig. 5.1-6 Boiling point and dew point lines of coexisting vapor and liquid phases (left)...

X] aj Azeotropic concentration B Bubble point line D Dew point line... 

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