Vapor pressure calculations using

VAPOR PRESSURE CALCULATION USING SRK EQUATION Read in properties for Water Tll =6-IT.3 Pt =22u.-1H L-m = LU-1-1... 

As was described in Chapter 6, the solubility parameter, S, can be used as a diagnostic tool for studying molecular association. Table 12.1 shows some of the relevant data for several aluminum alkyls. The solubility parameters were calculated from vapor pressure data using the procedure described in Chapter 6. 

Na (g). We have calculated the heat of sublimation of sodium to form the monatomic gas from the vapor pressure-temperature data, taking due account of the appreciable amount of Na2 molecules contained in the actual vapor at equilibrium. The vapor pressure data used are those of Edmonson and Egerton,1-2 Rodebush and Walters,1 Rodebush,2 Rodebush and de Vries,1 Rodebush and Henry,1 Haber and Zisch,1 Ladenberg and Minkowski,1 and Gibhart.1 See also Kroner,1 Hackspill,1 van Laar,9 and Simon and Zeidler.1 Our value for the heat of sublimation, Na (c) = Na (g), is —25.9 at 18°. Sherman1 calculated —25.8. 

The solubility parameters of many volatile liquids have been calculated directly from their respective heats of vaporization and molar volumes (Eq. 5). Hoy [32] has shown that 8 for relatively non-volatile liquids can be calculated from vapor pressure data using a modification of the Haggenmacher Eq. [33], Large numbers of such data have been reported and these are collected in extensive tables [27, 28, 34],... 

Vapor pressures over AlBrg(cr) and AlBr d) have been measured by Dunne and Gregory ( ), Fischer, Rahlfs and Benze (2) and Sulzmann ( ). The vapor pressures were corrected for vapor non-ideality by means of the equation AG /T=-RlnP-BP/T. The Bertholot equation of state and critical constants T =763 K and P =28.5 atm reported by Johnson, Silva and Cubicciotti (4) were used to calculate B. The corrected vapor pressures were used to calculate Aj,H"(298.15 K) by both 2nd and 3rd law methods. The results of the calculations are as follows with reaction (A) corresponding to 2 AlBrg(cr) Al2Brg(g) and reaction (B) corresponding to 2 AlBr (i)=Al Br (g). 

Equation 1.14 incorporates the definition of the acentric factor and may also be used to predict the vapor pressure, once the acentric factor has been determined. Another route for calculating the vapor pressure is via an equation of state, as described below. In the Soave equation, co is used in formulating the temperature dependency of the parameter a, which may be considered as a function of both P and co. The function fl(P,co) was determined with the objective of fitting vapor pressures calculated by the equation of state to experimental pure component vapor pressure data. 

Use the Wilson equation for activity coefficients and assume ideal gas behavior in the vapor phase, with component vapor pressures calculated by the Antoine Equation 2.19. The constants for this equation are consistent with pressure in kPa and temperature in Kelvin. The following data are given ... 

Streams 1 and 2, defined below, are mixed together in a vessel where the temperature is controlled at 300 K. The mixed stream leaving the vessel must be saturated vapor. What should be the pressure in the vessel Raoult s law may be assumed, with vapor pressures calculated as in Problem 2.22 using the constants given below. 

This equation is implicit in fly, inasmuch as the saturated volumes are dependent on fly, but the equation can be readily solved, for instance, by the method of repeated substitution. By using the solved value of fly in the eos. Equation (4.162), the vapor pressure calculated by the eos simply reproduces the experimental vapor pressure data. Wilson correlated the fly s that are fitted to vapor pressure data of a number of normal fluids to obtain Equation (4.157), the Wilson eos. Vapor pressure calculated by the Wilson eos is improved over that of the RK eos, but the accuracy still leaves something to be desired. Soave correlated the vapor pressure, fitting fly with Equation (4.164). Even better, the Soave eos is useful for the quantitative calculation of vapor pressure. In addition, the Peng-Robinson and the chain-of-rotators eos s provide quantitative calculations of vapor pressure. 

Accurate values of the area (S) of the sample used are necessary, since this factor enters into both the expansion equation and into the calculation of the surface free-energy lowering. The method used was that of Brunauer, Emmett, and Teller (7), and the values obtained are given in Table I. Krypton is omitted from the table, since it has been found (5) that the results for this gas are strongly dependent on the particular vapor-pressure data used. 

Gas adsorption is the preferred method of surface-area determination. An isotherm is generated of the amount of gas adsorbed against gas pressure, and the amount of gas required to form a monolayer is determined. The surface area can tTien be calculated using the cross-sectional area of the gas molecule. Outgassing of the powder before analysis should be conducted very carefully to ensure reproducibility. Commonly, nitrogen at liquid nitrogen vapor pressure is used but, for low surface-area powders, the adsorbed amounts of krypton or xenon are more accurately found. Many theories of gas adsorption have been advanced, but measurements are usually interpreted by using the BET theory [Brunauer, Emmett, and Teller, J. Am. Chem. Soc., 60,309 (1938)]. 

The last term is only used for calculating the vapor pressure of compounds that are solids and would be neglected with compounds that are liquids at ambient temperature. Vapor pressures calculated at 25°C for some compounds listed in Table 2.2 are compiled in Table 2.4. The calculated values are well within the range of the recommended experimental values. It should be emphasized that this relation was developed for relatively nonpolar compounds and is not suited to more polar compounds such as phenols. Other procedures for predicting vapor pressure have been oulined. In addition, procedures are available for calculating boiling points from molecular properties. 

Perform the calculations in TABLES 11.3 and 11.4 including sample calculations for one run on the backs of those tables. TABLE 11.4 requires a careful plot of In P versus l/T on good quality graph paper (at least 10 divisions per inch) followed by calculation of the slope of the resulting line. In addition, if your instructor wants you to calculate maximum error as part of your vapor pressure calculations in TABLE 11.3, use the data from one of your runs in TABLE 11.1 and the procedures explained in APPENDIX B. For maximum error calculations in TABLES 11.4 and 11.5, use estimated errors from TABLE 11.2 and appropriate maximum errors for one run in TABLE 11.3. 

Vapor pressure Is used in NPSH calculations when actually saturation pressure might be more appropriate. A better definition of "vapor pressure" is needed. Be wary of dissolved gases. 

The third method of calculating the vapor pressure is using the Clausius-Clapeyron equation (Appendix D-3). Calculations from this equation provide only a rough estimate of vapor pressure. 

The amount of oil lost with the overhead stream is determined by making an oil vapor pressure calculation, assuming that the oil exerts its full vapor pressure at the temperature of the exit vapor. Maxwell s nomogram is useful in estimating this vapor pressure. The overall material balance should be adjusted to show the amount of oil lost... 

To illustrate calculations for a binary system containing a supercritical, condensable component. Figure 12 shows isobaric equilibria for ethane-n-heptane. Using the virial equation for vapor-phase fugacity coefficients, and the UNIQUAC equation for liquid-phase activity coefficients, calculated results give an excellent representation of the data of Kay (1938). In this case,the total pressure is not large and therefore, the mixture is at all times remote from critical conditions. For this binary system, the particular method of calculation used here would not be successful at appreciably higher pressures. 

The analyst now has available the complete details of the chemical composition of a gasoline all components are identified and quantified. From these analyses, the sample s physical properties can be calculated by using linear or non-linear models density, vapor pressure, calorific value, octane numbers, carbon and hydrogen content. 

---