Relative volatility modification

Winn [99] proposes a modification to recognize temperature variation effects on relative volatility. The method does not apply to mixtures forming azeotropes or at conditions near the critical. Kister [94] proposes ... 

Empirical Efficiency Prediction Two empirical correlations which have been the standard of the industry for distillation tray efficiency prediction are the Drickamer and Bradford, in Fig. 14-46 [Trans. Am. Inst. Chem. Eng. 39, 319 (1943)] and a modification of it by O Connell [Trans. Am. Inst. Chem. Eng. 42, 741 (1946)], in Fig. 14-47. The Drickamer-Bradford plot correlates efficiency as a function of liquid viscosity only, which makes it useful for petroleum cuts. O Connell added the relative volatility to the x axis. 

Winn s modification. In an attempt to account for temperature variation of the relative volatility, Winn (35) proposed the equation... 

The predictive techniques are rather accurate. However, significant errors have been observed in few cases (4, 13, 27, 40). No direct comparison between the three predictive methods is available. The authors of the parachor method (27) claim that their method yields equal or better results than the PDD method for the cases considered in their study it is believed (42), however, that the latter is more reliable and it is recommended. The Weimer-Prausnitz method probably gives less accuracy than the PDD method, but it is more general. For example, Hanson and Van Winkle (40) report that their data on the hexane-hexene pair were not successfully correlated by the WP method. The Helpinstill-Van Winkle modification is recommended over the WP method. Recently, Null and Palmer (43) have presented a modification of the WP method which provides better accuracy but it is less general. The PDD method should be used cautiously when extrapolation with respect to temperature is used (27). When the GLC method is used, reliable results are expected. Evaluation of infinite dilution relative volatilities is recommended (36). 

Accurate use of the Fenske equation obviously requires an accurate value for the relative volatility. Smith (1963) covers in detail a method of calculating a by estimating tenperatures and calculating the geometric average relative volatility. Winn (1958) developed a modification of the Fenske equation that allows the relative volatility to vary. Wankat and Hubert (1979) modified both the Fenske and Winn equations for nonequilibrium stages by including a vaporization efficiency. 

The approach of the relative volatility to unity at the critical means that the compositions of the vapor and liquid are identical. Thus the K values for all components equal 1. The temperature and pressure at which these values become unity are functions of the other components present. Thus, a mixture of butane and ethane would have a certain critical temperature and pressure, while a mixture of butane with hexane would have different critical temperature and pressure, but under both conditions the K value for butane would have to be equal to unity. Thus, the values given in Table 3-2, which were taken to be independent of the character of the other components and a function of the temperature and pressure only, cannot apply in the critical region. In most cases, these effects of the critical region are not serious at total pressures less than 0.5 to 0.7 of the critical pressure. Modifications of the method of utilizing the K values in the critical... 

Separation of Binary Azeotropic Mixtures. A large number of two-component systems form azeotropic mixtures, and it is frequently necessary to separate them into their components. Regular fractional distillation will not separate such mixtures into the components in high purity, but by suitable modifications it is frequently possible to obtain the desired separation. At the azeotropic composition the relative volatility is unity, and rectification is not possible. The methods employed for separating such systems involve using either (1) distillation plus other separation processes to get past the azeotropic composition or (2) a modification of the relative volatility. 

Modification of Relative Volatility. The two most common methods of modifying the relative volatility bf azeotropic mixtures involve (1) changing the total pressure and (2) adding other components to... 

The molecular and bulk properties of the halogens, as distinct from their atomic and nuclear properties, were summarized in Table 17.4 and have to some extent already been briefly discussed. The high volatility and relatively low enthalpy of vaporization reflect the diatomic molecular structure of these elements. In the solid state the molecules align to give a layer lattice p2 has two modifications (a low-temperature, a-form and a higher-temperature, yS-form) neither of which resembles the orthorhombic layer lattice of the isostructural CI2, Br2 and I2. The layer lattice is illustrated below for I2 the I-I distance of 271.5 pm is appreciably longer than in gaseous I2 (266.6 pm) and the closest interatomic approach between the molecules is 350 pm within the layer and 427 pm between layers (cf the van der Waals radius of 215 pm). These values are... 

Dissociation of the neutral acid in water necessitates modifications for air-sea exchange in the model, which is based on Henry s law. Other possible pathways, e.g. sea spray, are neglected. Henry s law is restricted to concentrations of physically solved, non dissociated substances. Since only the non-dissociated acid is volatile, it is important to correct the air-water partition coefficient as to reflect the relative proportions of volatile and non-volatile components. The corrected parameter is the effective Henry s law coefficient, which is related to the Henry s law coefficient as a function of pH (modified Henderson-Hasselbalch equation) ... 

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],... 

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