CORRESPONDENCE Binary Mixtures for Testing Fractionating Columns SIR: The recent article by Bragg and Richards ( 1 ) supplies new data on the vapor-liquid equilibria of benzene-ethylene dichloride mixtures which they interpret as showing deviations from Raoult’s law in a way that is impossible according to thermodynamic theory. Selecting the determinations a t 760 mm., the ratio of the vapor pressures of the pure components taken from the measurements of Smith and Matheson increases from 1.109 to only 1.113 as the mole fraction of benzene rises from zero to unity. Any deviation of the measured relative volatility, a, from the ratio of the vapor pressures may conceivably be due to gas law imperfections or nonideal liquid solutions, Using the equations of Scatchard and Raymond (4) and the gas law deviations correlated by Hirschfelder, McClure, and Weeks (a),the gas law deviations may be shown to lower the relative volatility only 0.3 per cent below the ratio of the vapor pressures, and are thus negligible. Taking the simplest case of nonideal liquid solutions from Equation 11of the paper by the writers (2)’ one may develop the following equation, log
01
= log
Pi/Pz
+ A ( l - 2x1)
Regarding Pl/P* as a constant and A as positive, a plot of log CY us. 2 is a straight line starting a distance A above log P I / P ~ at z = 0. The relative volatility will equal P1/P2a t 2 = 0.5 and will be the distance A lower than log P1/P2 at z = 1.0. Figure 1 shows that the line for Raoult’s law is as good a representation of the data as any line drawn to cross this line. The introduction of more complex liquid-phase deviations, such as predicted by the van Laar equation, will give a curvature and asymmetry to the line, but the plot of log a vs. z will never show a maximum or minimum.
mole fraction. An examination of Bragg and Richards’ refractive index-composition data by plotting deviations from a linear relation between refractive index and mole fraction and a comparison with Pahlavouni’s deviation curve show that the error is probably greater than 0.002. Until the experimental technique is improved-for example, by making the analyses by density determinations, which should reduce the uncertainty in mole fraction to 0.0003-and until any observed deviations from Raoult’s law are shown t o be consistent with thermodynamic theory, the writers recommend the use of the vapor pressure ratio for the relative volatility of this binary mixture. Literature Cited (1) Bragg and Richarda, IND. ENG.CHEM.,34, 1088 (1942). (2) Carlson and Colburn, Ibid., 34, 581 (1942). (3) Hirschfelder, McClure, and Weeks, J . Cham. Phys., 10, 201 (1942). (4) Scatchard and Raymond. J . Am. Cham. SOC.,60, 1278 (1938)
H. C. CARLSON AND A. P. COLBURK D u Pont Company Wilmington, Del.
*....
SIR: Carlson and Colburn have indicated that they recommend the use of the vapor pressure ratio for the relative volatility of the benzene-ethylene dichloride binary mixture. We do not believe that such a practice is advisable. As Carlson and Colburn pointed out, the data of Zawidski were not obtained a t 760 mm.; and while the data were also not isopiestic, nevertheless for comparative purposes they could be placed on the 200-mm. graph, the pressure of Zawidski’s experi1.14 I I I 1 I I I I 1 I ments varying from 236.2 to 268.0 mm. Examination of all the data that have been presented indicates that in every case the data exhibit a definite trend toward a decrease in the value of CY with increasing benzene concentrations up to 60 to 70 mole per cent benzene, and then an increase in the value of a with further increases in the concentration of benzene. The data obtained by the authors a t four separate pressures and the data obtained by three other investigators, all indicate the same trend. The authors can see little reason for ignoring the data and using ideal theoretical results which are well known to be erroneous. I 1 1 I I I I I I .04 1 0.0 0.2 0.4 0.6 0.8 1 .o We all, of course, would like to have data available in which the Mole Fraction of Benzene in Liauid uncertainties in the value of the mole fraction were of the order FIGURE1. RELATIVEVOLATILITY OB BENZENE-ETHYLENE of magnitude of 0.0003 or less; but unfortunately the methods of AT 760 MM. DICHLORIDE MIXTURES obtaining determinations of the composition a t present available, 0 Bragg and.Richards that are capable of providing such accuracy, require the use of X Pahlavouni p Rosanoff and Easley large samples which the usual experimental procedures cannot Raoult’s law -- Bragg and Richards’ curve provide. Consequently, it is felt that, a t least for the present, we must rely upon the conventional refractive index determinations, which provide a direct uncertainty in the value of the mole fraction of the order of magnitude of *0.002. It is, of course, Values of the relative volatility at 760 mm. are plotted in obvious that other errors of technique or observation may result Figure 1, omitting Zawidski’s determinations, which were not in greater uncertainty in the value of the mole fraction correisopiestic but were isothermal a t 50’ C. Bragg and Richards’ sponding to a given value of relative volatility than the indicated values lie both above and below the ratio of the vapor pressures. *0.002. The majority of values determined by all investigators lies below P1/Pp near x = 0.5. We conclude that the experimental values L. B. BRAGG A. R. RICHARDS are in error. Bragg and Richards have pointed out that the Foster Wheeler Corporation Trinidad Leaseholds Limited Carteret, N. J. Pointe-&-Pierre,Trinidad. E. W. I. analysis by refractive index results in an uncertainty of 0.002
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