Boiling-point diagrams

Simple continuous distiUation, also caUed flash distiUation, has a continuous feed to a single equiHbrium stage the Hquid and vapor leaving the stage are considered to be in phase equiHbrium. On the boiling point diagram (Fig. la), the feed is represented by Xj the bottoms Hquid by A and the... 

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. 

Figure 8. Boiling point diagram for binary mixture.

Construct a boiling point diagram at a total pressure of 760 mm Hg, Figure 8-18. 

Figure 8-18. Boiling point diagram for Example 8-5. Benzene-toluene, total pressure = 760 mm Hg. Used by permission of Robinson Gilliland.

Figure 8-20. Boiling point diagram for benzene-toluene mixture using K values, total pressure 760 mm Hg for Example 8-6.

From the boiling point diagram, in Figure 11.10 the liquid temperature when x = 0.42 is seen to be 366.5 K. 

In an analogous manner, the oversimplified boiling-point diagram (7.41c) is replaced by... 

We turn now to the T-xB boiling-point diagram (7.43) at fixed pressure (e.g., P= 1 atm). Under these conditions, the T-xeq curve displays the quantitative variation of the normal boiling point of solution as a function of liquid concentration. The boiling-point diagram therefore provides a basis for exact theoretical analysis of the ancient art of distillation, a staple of chemical laboratory practice. 

To describe the formal theory of fractional distillation, let us consider the boiling-point diagram of a near-ideal A/B binary solution, as shown in Fig. 7.10. The solution is initially at high concentration x of the high-boiling component B. Consider the following four steps, as illustrated in the figure ... 

Figure 7.10 Boiling-point diagram for a binary solution, illustrating two theoretical plates of an idealized fractional distillation process that progressively enriches the distillate in the low-boiling component A (see text).

Fig. 5. Boiling-point diagram of an azeotropic system exhibiting negative azeotropy...

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. 

Figure 7 Benzene-ethyl alcohol boiling-point diagram at 1.0 atm. Data from same source as Fig. 6.

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