Ws ' V i

! 1 j ; 1

<« Mol Fraction C4 In Liauid

1 Hi. 1-6 KIToct. of pruuurv un vapor-lniuid oquiliUriu.

<« Mol Fraction C4 In Liauid

1 Hi. 1-6 KIToct. of pruuurv un vapor-lniuid oquiliUriu.

(C.S. Robinson and E.R. Gilliland, "Elements of Fractional Distillation", 19 50, by courtesy of McGraw-Hill Book Company.)

EFFECT OF PRESSURE At a given composition, the higher the saturation temperature the higher the saturation pressure. For this reason, the effect of saturation temperature discussed, above can be considered as !the effect of saturation pressure.

'A study of boiling point data in the pressure range of 1 to 10 j atmospheres ana a wide temperature range (9) led to a simple and rough , approximation of the relationship between saturation pressure and | saturation temperature for common substances:

Where 0.08 <_ q <_ 0.11 with a mean value of 0.09.

Although this approximation is rough, it illustrates the above principle.

| The effect of temperature (or pressure) on relative volatility is J illustrated in Figure 1.6 (7).

| EFFECT OF COMPOSITION The main effect of composition on K values and i relative volatilities is a result of the effect of composition on the ! liquid activity coefficient. Composition also has an effect on the ! fugacity coefficient. The latter effect is harder^ to analyze, but is | generally small at low pressures.

j Activity coefficients are classified according to the nature of their j deviation from Raoult's law. This depends on the heat of mixing of I the components. If heat needs to be added to the mixture to achieve | solution, i.e. the components "prefer to be alone" than in solution, j the deviations are positive (Figure 1.7). Positive deviations give j activity coefficients greater than unity, and higher K-values than ! those predicted from Raoult's law. If heat is evolved on solution, ! the reverse applies (Figure 1.8); the deviations are negative and i K-values are lower than those obtained from Raoult's -law.

The magnitude of the deviations from Raoult's law increases with the difference in nature between the components. For instance, the normal propanol - water system (Figure 1.7) and the acetone - chloroform system (Figure 1.8) show large activity coefficients, the highest being 13. On the other hand, the highest activity coefficient in a mixture of isobutane-normal butane, which are similar to each other, is smaller than 1.1 (at about 100 psi).

Figures 1.9 and 1.10 show the effect of composition on the activity coefficient ratio. According to equation 1-9, this ratio represents the main effect of composition on relative volatility. When a system shows positive deviations, relative volatility decreases as the concentration of the more volatile component increases. The reverse applies for negative-deviation systems.

'G 02 04 0.6 08 1.0 Mole fraction n.proDono! in hgu'd ha. ^Typical variation of liquid-phase activity coefficients with composition in a positive-deviation svstem 5 vet em is n-propanol -water at ] aon Points are observed data of Gadw. WIT Thesis. 1936 . curves are calculated fron the van Laar equation ,Eq> 13-51e and li-alh j with A.. = 1.13 and A,. = 0 4V

(R.H. Perry, "Chemical Engineers Handbook", 1973, by courtesy, McGraw-Hill Book Company.)

2.— 8 Typical variation of liquid-phase activity coefficients with composition in a negative-deviation svstem System is acetooe-chloroform Points are observed data of Zawidski \l. phWik Own, 35. 119 2S0011 at 35'C Cunts are calculated from the van Laar equation [Eqs (13-510) and (13-51*»] with A,, » -0.44 and A„ m -0.34.

(R.H. Perry, "Chemical Engineers Handbook", 1973, bv courtesy, McGraw-Hill Book Company.)



1.1.6 Calculation of Bubble-Points and Dew-Points

The bubble point of a mixture, is calculated from:

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