Benzene-toluene system

For apphcation to distiUation (a nearly isobaric process), as shown in Figs. 13-8 to 13-13, binary-mixture data are frequently plotted, for a fixed pressure, as y versus x, with a line of 45° slope included for reference, and as T versus y and x. In most binary systems, one of the components is more volatile than the other over the entire composition range. This is the case in Figs. 13-8 and 13-9 for the benzene-toluene system at pressures of both 101.3 and 202.6 kPa (1 and 2 atm), where benzene is more volatile than toluene. 

As the reflux ratio decreases, so does the slope of the upper component balance line, The effect of reflux ratio on the component balance lines is illustrated in Fig, 2,11, using the benzene-toluene system in Exampla 2.1. 

Rgw 5.l Equilibrium and l/(y-.x) plots, Example 5,1, (a) Equi-atrium vapor-liquid compositions, benzene-toluene system, 14.7 psia, a = 2,90. 

FromEq. (5.31) with a = 2.90 for the benzene-toluene system and a = 6.94 for the benzene-p-xylene system,... 

Assuming ideal-system behavior, calculate the K values and relative volatility for the benzene-toluene system at 373 K (212° F) and 101.3 kPa (1 atm). 

Obtain (or plot from data) a phase diagram for the benzene/toluene system. Vapor-liquid equilibrium behavior of binary systems can be represented by a temperature-composition diagram at... 

FIGURE 3.6 The benzene-toluene system at 1000 mmHg (133 kPa) (Example 3.12). 

A feed stream made up of 40% mole benzene and 60% mole toluene is to be separated into benzene-rich and toluene-rich products using a distillation column. The column has ten equilibrium stages including a partial condenser and a partial reboiler and is operated at 172 kPa. The feed stream, with a flow rate of 100 kmol/h, is at its bubble point at 172 kPa and is placed in the fourth stage from the top. It is required to determine the compositions of the two products at different reflux ratios. Vapor-liquid equilibrium data for the benzene-toluene system are provided in Table 5.1 at 172 kPa. 

Although the Murphree model contains an additional assumption (that the liquid leaving plate j is at its bubble-point temperature) over the modified Murphree model, the corresponding values of y 1 predicted by both models on the basis of the same sets coefficients n Li, nGi and points xylf yj+ lf i appear to be in almost perfect agreement for the two examples presented (see Tables 13-2 and 13-3). These examples were taken from Ref. 24. The number of transfer units for each film in these examples was taken to be independent of component identity just as they are for the existing correlations for binary mixtures which are given below. In Example 13-1 (the benzene-toluene system), the vapor and liquid phases closely approximate ideal solutions, but the liquid phase of the ethanol-water system in Example 13-2 is highly nonideal. 

Calculate the effect of the change in slope of the equilibrium line on the local efficiency in a sieve-tray distillation column. Use the benzene-toluene system as an example, and predict / for plates where the mixture is mostly toluene and for those where it is mostly benzene, starting with an estimated value of for the middle of the column. 

Figure 14.11 shows the vapor pressure versus mole fraction of benzene for the benzene-toluene system, which behaves ideally to a good degree of accuracy over the entire range of composition. The partial pressures of benzene and toluene, also shown in the figure, are linear functions of the mole fraction of benzene, since Raoult s law is obeyed. 

Figure 14.11 Vapor pressures in the benzene-toluene system.

The easier of the two diagrams to calculate (but sometimes harder to grasp intuitively) is the Pxy diagram, which is shown below for an idealized Benzene-Toluene system ... 

By holding one variable constant, varying a second, and calculating the other two, it is possible to calculate a phase diagram from Raoult s Law. Typical Pxy and Txy diagrams derived from Raoult s Law were shown in the previous section for the benzene-toluene system. 

Liquid-phase ideality (activity coefficients ](/= ) only occurs when the components are quite similar. The benzene/toluene system is a common example. As shown in Figure 1.5 in the sixth and seventh columns, the activity coefficients of both benzene and toluene are very close to unity. 

Table 11.1-1. Vapor-Pressure and Equilibrium-Mole-Fraction Data for Benzene-Toluene System... 

If the system obeys Raoult s law, such as the benzene—toluene system. 

EXAMPLE 113-1. Relative Volatility for Benzene-Toluene System Using the data from Table 11.1-1, calculate the relative volatility for the benzene-toluene system at 85°C (358.2 K) and 105°C (378.2 K). 

Another common way of representing a binary liquid-vapor equilibrium is through a temperature-composition phase diagram, in which the pressure is held fixed and phase coexistence is examined as a function of temperature and composition. Figure 9.13 shows the temperature-composition phase diagram for the benzene-toluene system at a pressure of 1 atm. In Figure 9.13, the lower curve (the boiling-point curve)... 

Figure 12-4. Benzene-toluene system equilibrium curve. (Reproduced with permission from reference 8. Copyright 1997, American Chemical Society.)...

The benzene-toluene system equilibrium data are given in Figures 12-4, 12-23, and 12-25. 

Let s use the map in Figure 4.67 to analyze benzene/toluene systems. As we did with the absorber, let s begin with a closed system. Consider a two-phase system of benzene and toluene at equilibrium in a closed vessel as shown in Figure 4.69. A known quantity of liquid benzene is added to the vessel. Indicate on the benzene/toluene phase equilibrium map this system before and after the liquid benzene is added. 

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