Benzene/toluene mixtures, phase diagram

Figure 123-1 The liquid-solid phase diagram for ethyl benzene-toluene mixtures.

Figure 7.18 Phase diagrams fet the benzene-toluene mixture t 90 C, calculated using ASPEH PLUS (a) P-x-y diagram (b)i-)i diagram.

Figure 9.13 Temperature-composition phase diagram for the liquid-vapor equihbrium in benzene-toluene mixtures at 1 atm. The boihng points of toluene and benzene are 110.6°C and 80.1°C, respectively.

Because benzene and toluene are common solvents one can find in the literature the thermodynamic data needed to model the flash drum. However, it is instructive to consider how one might measure the requisite data. We need a map of the phases (vapor, liquid, or two phase) for benzene/toluene mixtures. The coordinates of the map should be the composition of the mixture (jc axis) and the pressure of the system (y axis). We dust off the device we used for benzene, diagrammed in Figure 4.3, and measure the phase of the system as a function of pressure, at constant temperature. 

FIGURE 11.18 A phase diagram of temperature versus composition (mole fraction) for a mixture of benzene and toluene. Liquid composition is given by the lower curve, and vapor composition is given by the top curve. The thin region between curves represents an equilibrium between phases. Liquid and vapor compositions at a given temperature are connected by a horizontal tie line, as explained in the text. 

The actual variations in the V and L streams in a distillation column depend on the enthalpies of the vapor and liquid mixtures. The limitations imposed by assuming constant molal overflow can be removed by enthalpy balances used in conjunction with material balances and phase equilibria. The enthalpy data may be available from an enthalpy-concentration diagram, such as the one in Fig. 18.24. Since benzene-toluene solutions are ideal, this diagram was constructed using molar average heat capacities and heats of vaporization. Some... 

Toluene and benzene form liquid mixtures that are practically ideal and closely obey Raoult s law for partial pressure. For the binary system of these components, we can use the vapor pressures of the pure liquids to generate the liquidus and vaporus curves of the pressure-composition and temperature-composition phase diagram. The results are shown... 

The goal Is to create a Txy phase diagram for mixtures of benzene and toluene, where T is the temperature, x is the mole fraction of benzene in the liquid, and y is the mole fraction of benzene In the vapor. A horizontal line drawn for a given T gives the compositions of liquid and vapor in equilibrium at that T. Such diagrams are very useful for distillation calculations... 

Figure 6.11 shows a pressure-composition liquid-vapor phase diagram of ethanol and diethyl ether for a fixed temperature of 20 C. Compare Figure 6.11 with Figure 6.2, which represents the nearly ideal mixmre of benzene and toluene. Figure 6.12 shows the temperature-composition phase diagram of the same mixture for a fixed pressure of 1.84 atm. Compare this figure with Figure 6.3. This system exhibits positive deviation from Raoult s law. The vapor pressure is larger than it would be if the solution were ideal, and the solution boils at a lower temperature than if it were an ideal solution.

Dribicka and Sandall (1979) distilled ternary mixtures of benzene (1), toluene (2), and ethylbenzene (3) at total reflux in a wetted wall column made from stainless steel and of 2.21-cm inside diameter (<7). A schematic diagram of the column is shown in Figure 8.13. Samples of the vapor and liquid phases were taken from various points along the column. During one of their experiments the vapor phase at a height of 300 mm from the bottom of the column had the composition... 

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. 

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