Vapor-liquid equilibrium boiling point diagrams

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. 

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. 

A theoretical plate is defined as the degree of separation attained for an infinitesmal vaporization at equilibrium (i.e., the concentration of liquid in a theoretical plate is that of the first bit of vapor to be formed from the liquid in the previous one). Using this definition, approximately how many theoretical plates would be required to achieve a separation into a vapor with xB = 0.1 and a liquid with xB = 0.9 for the system described by the boiling-point diagram in Fig. 9. 

The effect of the solute on the solution boiling point is easy to see from the diagram. Recall that the boiling point of a liquid at a given pressure is the intersection of a horizontal line at that pressure with the vapor-liquid equilibrium curve. At pressure Po, the pure solvent boils at temperature Tbo, while the solution boils at a higher temperature. Tbs. 

As a mixture of volatile liquids is distilled, the compositions of both the liquid and the vapor, as well as the boiling point of the solution, change continuously. At constant pressure, we can represent these quantities in a boiling point diagram, Figure 14-12. In such a diagram the lower curve represents the boiling point of a liquid mixture with the indicated composition. The upper curve represents the composition of the vapor in equilibrium... 

Figure 14-12 A boiling point diagram for a solution of two volatile liquids, A and B. The lower curve represents the boihng point of a liquid mixture with the indicated composition. The upper curve represents the composition of the vapor in equilibrium with the boiling liquid mixmre at the indicated temperature. Pure liquid A boUs at a lower temperamre than pure hquid B hence, A is the more volatile liquid in this illustration. Suppose we begin with an ideal equimolar mixmre = Xg = 0.5) of liquids A and B. The point P represents the temperature at which this solution boils, Tj. The vapor that is present at this equilibrium is indicated by point Q (X = 0.8). Condensation of that vapor at temperature Ti gives a liquid of the same composition (point E). At this point we have described one step of simple distillation. The boiling liquid at point if is in equilibrium with the vapor of composition indicated by point S (X > 0.95), and so on.

Example 18.1. A mixture of 50 mole percent benzene and 50 mole percent toluene is subjected to flash distillation at a separator pressure of 1 atm. The vapor-liquid equilibrium curve and boiling-point diagram are shown in Figs. 18.2 and 18.3. Plot the following quantities, all as functions of f, the fractional vaporization (n) the temperature in the separator, b) the composition of the liquid leaving the separator, and (c) the composition of the vapor leaving the separator. 

Figure 3.1 is a schematic one-component, three-phase equilibrium diagram. The three different phase regions are separated by lines D-TP (solid vapor pressure, or sublimation curve), F-TP (melting point curve), and TP-C (liquid vapor pressure or boiling-point curve). Point C is the critical point where the vapor and liquid phases become indistinguishable and TP is the triple point where solid, liquid, and vapor phases can coexist. There are only two in-... 

Often the vapor-liquid equilibrium relations for a binary mixture of A and B are given as a boiling-point diagram shown in Fig. 11.1-1 for the system benzene (A)-toluene (B) at a total pressure of 101.32 kPa. The upper line is the saturated vapor line (the dew-point line) and the lower line is the saturated liquid line (the bubble-point line). The two-phase region is in the region between these two lines. 

EXAMPLE 11.1-1. Use of Raoult s Law for Boiling-Point Diagram Calculate the vapor and liquid compositions in equilibrium at 95°C (368.2 K) for benzene-toluene using the vapor pressure from Table 11.1-1 at 101.32 kPa. 

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. 

Fig. 3.2. A stylized phase diagram for a simple pure substance. The dashed line represents 1 atm pressure and the intersection with the solid-liquid equilibrium line represents the normal boiling point and the intersection with the liquid-vapor equilibrium line represents the normal boiling point.

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