Hexane vapor pressure curves

The Class I binary diagram is the simplest case (see Fig. 6a). The P—T diagram consists of a vapor—pressure curve (soHd line) for each pure component, ending at the pure component critical point. The loci of critical points for the binary mixtures (shown by the dashed curve) are continuous from the critical point of component one, C , to the critical point of component two,Cp . Additional binary mixtures that exhibit Class I behavior are CO2—/ -hexane and CO2—benzene. More compHcated behavior exists for other classes, including the appearance of upper critical solution temperature (UCST) lines, two-phase (Hquid—Hquid) immiscihility lines, and even three-phase (Hquid—Hquid—gas) immiscihility lines. More complete discussions are available (1,4,22). Additional simple binary system examples for Class III include CO2—hexadecane and CO2—H2O Class IV, CO2—nitrobenzene Class V, ethane—/ -propanol and Class VI, H2O—/ -butanol. 

Figure 29 plots vapor pressure curves for pure hexane, for commercial hexane experimentally determined, and for a 10% commercial hexane miscella, both as calculated for an ideal solution and as actually determined. The plots, which are on the conventional basis of log vapor pressure versus reciprocal of the absolute temperature to give straight lines, show that below a temperature of about 93.3°C (200°F)— which corresponds to a /T value of 15.16— the actual vapor pressure... 

Figure 29. Vapor pressure curves A, commercial hexane B, pure hexane C, 10% commercial hexane in mixture with cottonseed oii, ideai curve D, same mixture, actual curve, according to data of Pollard et al. (208).

Figure 32. Vapor pressure curves for low concentrations of hexane in soybean oii (209).

P3.18 At approximately T = 380 K, the vapor pressure curves of water and n-hexane intersect. Calculate the difference in the vapor pressures, if both substances form drops with diameters of d = 2 nm. 

At low temperatures, using the original function/(T ) could lead to greater error. In Tables 4.11 and 4.12, the results obtained by the Soave method are compared with fitted curves published by the DIPPR for hexane and hexadecane. Note that the differences are less than 5% between the normal boiling point and the critical point but that they are greater at low temperature. The original form of the Soave equation should be used with caution when the vapor pressure of the components is less than 0.1 bar. In these conditions, it leads to underestimating the values for equilibrium coefficients for these components. 

Figure 3.10 shows the vapor pressure/composition curve at a given temperature for an ideal solution. The three dotted straight lines represent the partial pressures of each constituent volatile liquid and the total vapor pressure. This linear relationship is derived from the mixture of two similar liquids (e.g., propane and isobutane). However, a dissimilar binary mixture will deviate from ideal behavior because the vaporization of the molecules A from the mixture is highly dependent on the interaction between the molecules A with the molecules B. If the attraction between the molecules A and B is much less than the attraction among the molecules A with each other, the A molecules will readily escape from the mixture of A and B. This results in a higher partial vapor pressure of A than expected from Raoult s law, and such a system is known to exhibit positive deviation from ideal behavior, as shown in Figure 3.10. When one constituent (i.e., A) of a binary mixture shows positive deviation from the ideal law, the other constituent must exhibit the same behavior and the whole system exhibits positive deviation from Raoult s law. If the two components of a binary mixture are extremely different [i.e., A is a polar compound (ethanol) and B is a nonpolar compound (n-hexane)], the positive deviations from ideal behavior are great. On the other hand, if the two liquids are both nonpolar (carbon tetrachloride/n-hexane), a smaller positive deviation is expected.

In a similar manner if. for example, an equimolar vapor mixture of hexane and tri-ethylamine at low pressure and 60°C is isoihermally compressed, at 0.5100 bar the first drop of liquid or dew forms (containing 0.3363 mole fraction hexane). This pressure is called the dew point pressure, and the line of dew point pressure versus composition is the dew point pressure curve. The dew point pressure at, for example, a mole fraction of 0.5 is given by the intersection of the venical 0.5 mole fraction line with the vapor or dew point curve in Fig. 10.1-3. 

Figure 11.3-3 shows the vapor-liquid and liquid-liquid equilibrium behavior computed for the system of methanol and n-hexane at various temperatures. Note that two liquid phases coexist in equilibrium to temperatures of about 43°C. Since liquids are relatively incompressible, the species liquid-phase fugacities are almost independent of pressure (see Illustrations 7.4-8 and 7.4-9), so that the liquid-liquid behavior is essentially independent of pressure, unless the pressure is very high, or low enough for the mixture to vaporize (this possibility will be considered shortly). The vapor-liquid equilibrium curves for this system at various pressures are also shown in the figure. Note that since the fugacity of a species in a vapor-phase mixture is directly proportional to pressure, the VLE curves are a function of pressure, even though the LLE curves are not. Also, since the methanol-hexane mixture is quite nonideal, and the pure component vapor pressures are similar in value, this system exhibits azeotropic behavior. 

It is possible to generate x-y equilibrium curves such as Fig. 3.3 using (3-9) by assuming that the relative volatility is a constant independent of temperature. This is convenient for close-boiling mixtures forming ideal solutions, but can lead to erroneous results for mixtures of components with widely different boiling points because it assumes that both P] and P] are identical functions of T. For example, inspection of the vapor pressure data for the hexane-octane system, Table 3.1, reveals that a varies from 101/16 = 6.3 at G. TC to 456/101 = 4.5 at 125.7°C. Calculation of relative volatilities by more accurate methods will be considered in Chapter 4. 

FIG. 11 Vapor pressures of selected -alkanes. The curves from right to left are for ethane, propane, butane, pentane, hexane, octane, and dodecane. Symbols are the same as for Fig. 10. (From [109], 1999 American Chemical Society.)... 

In order to better demonstrate whether a system follows Raoult s law, a diagram of the phase equilibrium called T-x-y should be plotted. This plot (Figure 2) shows the equilibrium temperatures at which either a liquid solution will start bubbling (bubble curve) or a vapor mixture starts condensing (dew curve). The two systems with their experimental data and the calculation curve of the ideal solution is shown in Figure 2. In Figure 2, the system of hexane-benzene at the pressure of 101.33 kPa [10] and the system of ethylacetate-benzene [11] show negative deviations from RaoulTs law. 

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