Vapor/liquid composition diagrams

Figure 13.4. Some vapor-liquid composition diagrams at essentially atmospheric pressure. This is one of four such diagrams in the original reference (Kirschbaum, Destillier und Rektifiziertechnik, Springer, Berlin, 1969). Compositions are in weight fractions of the first-named.

Azeotropes of vapor-liquid composition diagrams that result from this nonideal behavior ... 

It is usually desirable to present the experimental vapor-liquid equilibrium data graphically. A number of methods of presentation have been developed, but the most important are the temperature-composition and the vapor-liquid composition diagrams. 

Figure 11-25. Vapor/liquid composition diagrams for diethyiene glycol and water at 100,300, and 600 mm Hg total pressure. Data of Dow Chemical Company (195Si...

Pressure has a marked effect on the azeotropic composition and vapor-liquid equilibrium diagrams of alcohol-ketone systems (J). This is due to the fact that the slopes of the vapor pressure curves of alcohols are appreciably greater than for ketones it results in an unusually larger change in the relative boiling points of the components of an alcohol-ketone system with change in pressure. 

Figure 1.10 shows the temperature composition and vapor to liquid composition diagrams. Table 1.12 shows the estimated activity coefficients. The azeotropic composition of n-butanol at atmospheric pressure is about 0.247. The region of immiscibility is about x, = 0.02-0.44. 

To have a simple example, we consider an alkane(l) + aromatic(2) mixture, modeled by the Redlich-Kwong equation (8.2.1). Certain vapor-liquid phase diagrams for this mixture were displayed and discussed in 9.3. Here our objective is to compute residual enthalpies for vapor and liquid that coexist in equilibrium in particular, we want to construct an isothermal plot of vs. x and y. (We will call this an hxy diagram, even though it is that is actually plotted.) To do so, we set the temperature, pick a liquid composition Xp and then perform a bubble-P calculation to obtain values... 

A vapor-liquid composition-phase diagram like the one in Figure 15.4 can be used to explain the operation of a fractionating column with an ideal solution of two liquids, A and B. An ideal solution is one in which the two liquids are chemically similar, miscible (mutually soluble) in all proportions, and do not interact. Ideal solutions obey Raoult s Law. Raoult s Law is explained in detail in Section 15.3. 

FIG. 2-10 Enthalpy-concentration diagram for aqueous ethyl alcohol. Reference states Enthalpies of hquid water and ethyl alcohol at 0 C are zero. NOTE In order to interpolate equilihrium compositions, a vertical may he erected from any liquid composition on the hoihug hue and its intersection with the auxihary hue determined. A horizontal from this intersection will estahhsh the equihhrium vapor composition on the dew hue. (Bosnjakovic, Techuische Thermo-dynamik, T. Steinkopff, Leipzig, 1935. )... 

Three types of binary equilibrium cui ves are shown in Fig. 13-27. The y-x diagram is almost always plotted for the component that is the more volatile (denoted by the subscript 1) in the region where distillation is to take place. Cui ve A shows the most usual case, in which component 1 remains more volatile over the entire composition range. Cui ve B is typical of many systems (ethanol-water, for example) in which the component that is more volatile at lowvalues of X becomes less volatile than the other component at high values of X. The vapor and liquid compositions are identical for the homogeneous azeotrope where cui ve B crosses the 45° diagonal. A heterogeneous azeotrope is formed with two liquid phases by cui ve C,... 

Using the vapor-liquid equihbrium data, plot a y-x diagram. Draw a number of operating lines at a slope of 0.75. Note the composition at the 45° intersection, and step off seven plates on each to find the equilibrium value of the bottoms. Some of the results are tabulated in the following table ... 

Since the boiling point properties of the components in the mixture being separated are so critical to the distillation process, the vapor-liquid equilibrium (VLE) relationship is of importance. Specifically, it is the VLE data for a mixture which establishes the required height of a column for a desired degree of separation. Constant pressure VLE data is derived from boiling point diagrams, from which a VLE curve can be constructed like the one illustrated in Figure 9 for a binary mixture. The VLE plot shown expresses the bubble-point and the dew-point of a binary mixture at constant pressure. The curve is called the equilibrium line, and it describes the compositions of the liquid and vapor in equilibrium at a constant pressure condition. 

Quality of a Wet Vapor, in the vapor-liquid region of a pure substance, the composition of a two-phase system (at given T and P) varies from pure saturated liquid at the bubble jjoint M to pure saturated vapor at the dew point N along the line MQN on the P- V diagram (Figure 2-80). For a wet vapor represented by an intermediate... 

FIGURE 8.37 A temperature-composition diagram for benzene and toluene. The lower, blue curve shows the boiling point of the mixture as a function of composition. The upper, orange curve shows the composition of the vapor in equilibrium with the liquid at each boiling point. Thus, point B shows the vapor composition for a mixture that boils at point A. 

The normal boiling point of a binary liquid mixture is the temperature at which the total vapor pressure is equal to 1 atm. If we were to heat a sample of pure benzene at a constant pressure of 1 atm, it would boil at 80.1°C. Similarly, pure toluene boils at 110.6°C. Because, at a given temperature, the vapor pressure of a mixture of benzene and toluene is intermediate between that of toluene and benzene, the boiling point of the mixture will be intermediate between that of the two pure liquids. In Fig. 8.37, which is called a temperature-composition diagram, the lower curve shows how the normal boiling point of the mixture varies with the composition. 

Thus, by knowing aAB from vapor-liquid equilibrium and by specifying xA, A can be calculated. Figure 4.3a also shows a typical vapor-liquid equilibrium pair, where the mole fraction of benzene in the liquid phase is 0.4 and that in the vapor phase is 0.62. A diagonal line across the x-y diagram represents equal vapor and liquid compositions. The phase equilibrium behavior shows a curve above the diagonal line. This indicates that benzene has a higher concentration in the vapor phase than toluene, that is,... 

Solution To determine the location of the azeotrope for a specified pressure, the liquid composition has to be varied and a bubble-point calculation performed at each liquid composition until a composition is identified, whereby X = y,-. Alternatively, the vapor composition could be varied and a dew-point calculation performed at each vapor composition. Either way, this requires iteration. Figure 4.5 shows the x—y diagram for the 2-propanol-water system. This was obtained by carrying out a bubble-point calculation at different values of the liquid composition. The point where the x—y plot crosses the diagonal line gives the azeotropic composition. A more direct search for the azeotropic composition can be carried out for such a binary system in a spreadsheet by varying T and x simultaneously and by solving the objective function (see Section 3.9) ... 

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