Toluene liquid-vapor pressure-composition

The derivation of this formula is assigned in Problem 6.1. Figure 6.2 shows the liquid-vapor pressure-composition phase diagram of benzene and toluene at a constant temperature of 80°C. The lower curve represents the total pressure as a function of the mole fraction of benzene in the vapor phase at equilibrium with the liquid phase. The area below this curve represents possible equilibrium intensive states of the system when it is a one-phase vapor. The upper curve (a line segment) represents Eq. (6.1-24), giving the total pressure as a function of the benzene mole fraction in the liquid. The area above this line represents possible equilibrium states of the system when it is a one-phase liquid. 

For benzene and toluene, the vapor pressures of the pure components are 118.2 and 36.7 mm Hg, respectively. Prepare a graph like that shown in Fig. 2.6.1 and indicate the approximate compositions at which there is a maximum difference in the composition of the liquid and vapor phase. 

Figure 2 is a boiling point-composition diagram for the cyclohexane-toluene system. If amixture of 75 mole percent toluene and 25 mole percent cyclohexane is heated, we find from Fig. 2 that it boils at 100°C, or point A. Above a binary mixture of cyclohexane and toluene the vapor pressure has contributions from each component. Raoult s law states that the vapor pressure of the cyclohexane is equal to the product of the vapor pressure of pure cyclohexane and the mole fraction of cyclohexane in the liquid mixture ... 

The vapor of the mixture is richer than the liquid in the more volatile component (the component with the greater vapor pressure). Benzene, for instance, is more volatile than toluene, and so we can expect that the vapor in equilibrium with the liquid mixture will be richer in benzene than the liquid is. If we could express the composition of the vapor in terms of the composition of the liquid, we could confirm that the vapor is richer than the liquid in the more volatile component. 

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. 

The solution requires the concentration of the heptane and toluene in the vapor phase. Assuming that the composition of the liquid does not change as it evaporates (the quantity is large), the vapor composition is computed using standard vapor-liquid equilibrium calculations. Assuming that Raoult s and Dalton s laws apply to this system under these conditions, the vapor composition is determined directly from the saturation vapor pressures of the pure components. Himmelblau6 provided the following data at the specified temperature ... 

Although the starting liquid mixture of benzene and toluene has a 1 1 molar composition, the composition of the vapor is not 1 1. Of the 760 mm Hg total vapor pressure for the boiling mixture, 542/760 = 71.3% is due to benzene and 218/760 = 28.7% is due to toluene. If we now condense the vapor, the liquid we get has this same 71.3 28.7 composition. On boiling this new liquid mixture, the composition of the vapor now becomes 86.4% benzene and 13.6% toluene. A third condense/boil cycle brings the composition of the vapor to 94.4% benzene/5.6% toluene, and so on through further cycles until the desired level of purity is reached. 

Toluene 1) and water(2) are essentially immiscible as liquids. Determine the dew-point temperatures and the compositions of the first drops of liquid formed when vapor mixtures of these species with mole fraction i, = 0.23 and r, = 0.77 are cooled at the constant pressure of 101.33 kPa. What is the bubble-point temperature and the composition of the last drop of vapor in each case The vapor pressure of toluene is given by the Antoine equation ... 

Curve ABC in each figure represents the states of saturated-liquid mixtures it is called the bubble-point curve because it is the locus of bubble points in the temperature-composition diagram. Curve ADC represents the states of saturated vapor it is called the dewpoint curve because it is the locus of the dew points. The bubble- and dew-point curves converge at the two ends, which represent the saturation points of the two pure components. Thus in Fig. 3.6, point A corresponds to the boiling point of toluene at 133.3 kPa, and point C corresponds to the boiling point of benzene. Similarly, in Fig. 3.7, point A corresponds to the vapor pressure of toluene at 100°C, and point C corresponds to the vapor pressure of benzene. 

You have been assigned to simulate a flash evaporator that separates a liquid feed stream containing benzene and toluene at temperature Tf ( C) into liquid and vapor product streams m equilibrium at temperature T( C) and pressure P mm Hg). The compositions of the product streams are related by Raoulfs law (Equation 6.4-1), and the component vapor pressures are expressed by the Antoine equation (Table B.4). 

At 100°C cyclohexane has a partial pressure of433 mm and toluene a partial pressure of 327 mm the sum of the partial pressures is 760 mm and so the liquid boils. If some of the liquid in equilibrium with this boiling mixture were condensed and analyzed, it would be found to be 433/760 or 57 mole percent cyclohexane (pointB, Fig. 2). This is the best separation that can be achieved on simple distillation of this mixture. As the simple distillation proceeds, the boiling point of the mixture moves toward 110°C along the line from A, and the vapor composition becomes richer in toluene as it moves from B to 110°C. In order to obtain pure cyclohexane, it would be necessary to condense the liquid at B and redistill it. When this is done it is found that the liquid boils at 90°C (point C) and the vapor equilibrium with this liquid is about 85 mole percent cyclohexane (point D). So to separate a mixture of cyclohexane and toluene, a series of fractions would be collected and each of these partially redistilled. If this fractional distillation were done enough times the two components could be separated. 

The existence of a solid 1 1 and a liquid 2 1 toluene-Cu(AlCl4) molecular complex has been shown by vapor pressure-phase composition studies (2S7). The results indicated that the toluene molecules in the 2 1 complex are not as strongly bonded to the Cu(I) atom as in the 1 1 complex. The toluene could be removed from the 1 1 complex by heating at 50° to 60°C at a pressure of 10 mm, yielding pure CUAICI4. 

A modified local composition (LC) expression is suggested, which accounts for the recent finding that the LC in an ideal binary mixture should be equal to the bulk composition only when the molar volumes of the two pure components are equal. However, the expressions available in the literature for the LCs in binary mixtures do not satisfy this requirement. Some LCs are examined including the popular LC-based NRTL model, to show how the above inconsistency can be eliminated. Further, the emphasis is on the modified NRTL model. The newly derived activity coefficient expressions have three adjustable parameters as the NRTL equations do, but contain, in addition, the ratio of the molar volumes of the pure components, a quantity that is usually available. The correlation capability of the modified activity coefficients was compared to the traditional NRTL equations for 42 vapor—liquid equilibrium data sets from two different kinds of binary mixtures (i) highly nonideal alcohol/water mixtures (33 sets), and (ii) mixtures formed of weakly interacting components, such as benzene, hexafiuorobenzene, toluene, and cyclohexane (9 sets). The new equations provided better performances in correlating the vapor pressure than the NRTL for 36 data sets, less well for 4 data sets, and equal performances for 2 data sets. Similar modifications can be applied to any phase equilibrium model based on the LC concept. 

Since petroleum products are complex mixtures of hundreds of compounds, the compounds characterized by relatively high vapor pressures tend to volatilize and enter the vapor phase. The exact composition of these vapors depends on the composition of the original product. Using gasoline as an example, compounds such as butane, propane, benzene, toluene, ethylbenzene and xylene are preferentially volatilized (Bauman 1988). Because volatility represents transfer of the compound from the product or liquid phase to the air phase, it is expected that the concentration of that compound in the product or liquid phase will decrease as the concentration in the air phase increases. 

Problem Mixtures of benzene and toluene behave almost ideally at 30 C, the vapor pressure of pure benzene is 118.2 mm. and that of pure toluene is 36.7 mm. Determine the partial pressures and weight composition of the vapor in equilibrium with a liquid mixture consisting of equal weights of the two constituents. 

Acrylonitrile is miscible in a wide range of oiganic solvents, including acetone, benzene, carbon tetrachloride, diethyl ether, ethyl acetate, ethylene cyanohydrin, petroleum ether, toluene, some kerosenes, and methanol. Compositions of some common azeotropes of acrylonitrile are given in Table 3. Table 4 presents the solubility of acrylonitrile in water as a function of temperature (6). Vapor—liquid equilibria for acrylonitrile in combination with acetonitrile, acrolein, HCN, and water have been published (6—9). Table 5 gives the vapor pressure of acrylonitrile over aqueous solutions. 

Raoult s law may be used to determine phase compositions for the binary system benzene-toluene, at low temperatures and pressures. Determine the composition of the vapor in equilibrium with a liquid containing 0.4 mole fraction of benzene at 300 K, and calculate the total equilibrium pressure. Estimate the vapor pressure of benzene and toluene at 300 K from the Antoine equation,... 

The equilibrium vapor pressure of a flammable liquid at its closed-cup flash point about equals its LFL in percent by volume. Thus, the vapor pressure of toluene at its closed-cup flash point (4.4°C or 40°F) of 1.2 percent (1.2 kPa) is close to its LFL of 1.1 percent. The composite LFL of a mixture may be estimated by Le Chatelier s Rule ... 

LCSTs can also exhibit VLLE examples include water -i- 2-butanol and water -t 2-butanone. In such cases, VLLE prevents formation of a closed solubility loop, (ii) Many immiscible liquids form homogeneous azeotropes at high pressures, as in Figure 9.15, but some do not. Those without azeotropes include CO2 + long-chain alkanes, such as n-octane and n-decane. (iii) Often VLLE occurs at heterogeneous azeotropes, as in Figure 9.15, and then the vapor-phase composition lies between the compositions of the two liquid phases. However, VLLE also occurs in some mixtures in which the vapor-phase composition does not lie between the compositions of the liquid phases. Examples of the latter include ammonia + toluene and water + phenol. 

In 11.1.7 we noted that the double Rachford-Rice algorithm for VLLE does not apply to binary mixtures. However, in Appendix K we present simple alternatives that usually apply to isothermal VLLE calculations for binary mixtures. To practice the procedure in Appendix K, consider liquid mixtures of toluene(l) and water(2), which are almost completely immiscible at ambient conditions. At 10°C we need the pressure at which this mixture exhibits VLLE, and we need to know the compositions of the three phases. The pure-component vapor pressures can be found from the correlation given in Appendix D. Perform the calculation twice ... 

This linear relationship between the total pressure, P, and the mole fraction, x, of the most volatile species is a characteristic of Raoult s law, as shown in Figure 7.18a for the benzene-toluene mixture at 90°C. Note that the bubble-point curve (P-x) is linear between the vapor pressures of the pure species (at x, = 0, 1), and the dew-point curve (P-yJ lies below it. When the (x, yi) points are graphed at different pressures, the familiar vapor-liquid equilibrium curve is obtained, as shown in Figure 7.18b. Using McCabe-Thiele analysis, it is shown readily that for any feed composition, there are no limitations to the values of the mole fractions of the distillate and bottoms products from a distillation tower. 

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. 

EXAMPLE 11.4-3. Number of Trays in Stripping Tower A liquid feed at the boiling point of 400 kg mol/h containing 70 mol % benzene (A) and 30 mol % toluene (B) is fed to a stripping tower at 101.3 kPa pressure. The bottoms product flow is to be 60 kg mol/h containing only 10 mol % A and.the rest B. Calculate the kg mol/h overhead vapor, its composition, and the number of theoretical steps required. 

Figure 9.12 Vapor-liquid phase equilibrium in a benzene-toluene solution as a function of pressure at 23°C. (a) The total vapor pressure as a function of the mole fraction of benzene in the liquid, (b) The total vapor pressure as a function of the mole fraction of benzene in the vapor, (c) The pressure-composition phase diagram constructed by combining plots (a) and (b). The line/-g is the tie line corresponding to the system at point c.

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)... 

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... 

At some particular temperature, the vapor pressure of pure benzene, QHg, is 0.256 bar and the vapor pressure of pure toluene, QH5CH3, is 0.0925 bar. If the mole fraction of toluene in the solution is 0.600 and there is some empty space in the system, what is the total vapor pressure in equilibrium with the liquid, and what is the composition of the vapor in terms of mole fraction ... 

Both water and toluene will vaporize, but the composition of the vapor will remain constant at j/t 0.442 as long as two phases are present. Since the ratio of the vapor pressures of toluene to water is greater than the ratio in the charge, the toluene liquid phase will disappear first. When all of the toluene has just vaporized, the water vaporized will be... 

A hypothetical solution of a solute B in a solvent A that obeys Raoult s law throughout the composition range from pure A to pure B is called an ideal solution. The law is most reliable when the components of a mixture have similar molecular shapes and are held together in the liquid by similar types and strengths of intermolecular forces. An example is a mixture of two structurally similar hydrocarbons. A mixture of benzene and methylbenzene (toluene) is a good approximation to an ideal solution, for the partial vapor pressure of each component satisfies Raoult s law reasonably well throughout the composition range from pure benzene to pure methylbenzene (Fig. 3.24). 

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