Distillation positive deviations from

Deviations from Raoult s law can make it impossible to separate liquids by distillation. The temperature-composition diagrams for mixtures of ethanol and benzene and of acetone and chloroform show why. A positive deviation from Raoult s law means that the attractive forces between solute and solvent are lower than those between the molecules of the pure components. As a result, the boiling point of the mixture is lower than that predicted by Raoult s law. For some pairs of components, the boiling point of the mixture is in fact lower than the boiling point of either constituent (Fig. 8.41). A mixture for which the lowest boiling temperature is below... 

If, for example, a mixture of ethanol and water is distilled, the concentration of the alcohol steadily increases until it reaches 96 per cent by mass, when the composition of the vapour equals that of the liquid, and no further enrichment occurs. This mixture is called an azeotrope, and it cannot be separated by straightforward distillation. Such a condition is shown in the y — x curves of Fig. 11.4 where it is seen that the equilibrium curve crosses the diagonal, indicating the existence of an azeotrope. A large number of azeotropic mixtures have been found, some of which are of great industrial importance, such as water-nitric acid, water-hydrochloric acid, and water-alcohols. The problem of non-ideality is discussed in Section 11.2.4 where the determination of the equilibrium data is considered. When the activity coefficient is greater than unity, giving a positive deviation from Raoult s law, the molecules of the components in the system repel each... 

Variation of liquid and vapor compositions during distillation of a system with small positive deviations from Raoult s law. 

Miscibility with the Feed Components. In a solute—solvent system exhibiting strong positive deviations from Raoult s Law, the solute has only limited solubility in the solvent. Above a certain solute concentration, two liquid phases are formed. The presence of two liquid phases on the plates of a distillation column leads to instability and operational problems. It is therefore necessary to ensure that the solute concentration in the liquid phase never exceeds its solubility limit. This solubility limit gives a minimum feasible solvent concentration in the section below the solvent addition plate of the primary column. 

If the vapor pressures of the two pure constituents are close together, then any appreciable positive deviation from Raoult s law will lead to a maximum in the total vapor pressure curve similarly, a negative deviation will, in the same circumstances, be associated with a minimum in the curve. In any event, even if the vapor pressures of the pure constituents are appreciably different, marked positive or negative deviations can lead to a maximum or a minimum, respectively, in the total vapor pressure curve. Such maxima and minima are the cause of the formation of the familiar constant boiling mixtures or azeotropic mixtures. A liquid mixture having the composition represented by a maximum or a minimum will distil without change of composition, for the proportions of the two constituents are then the same in the liquid and vapor phases. That this must be the case will be shown in the next section. 

The impact of inaccurate -model parameters can be very serious. The parameters have a major influence on the investment and operating costs (number of stages, reflux ratio), The influence of the -model parameters on the results is especially large if the separation factor is close to unity. Poor parameters can either lead to the calculation of nonexisting azeotrojjes in zeotropic systems (see Section 11.1) or the calculation of zeotropic behavior in azeotropic systems. Poor parameters can also lead to a miscibility gap which does not exist." In the case of positive deviation from Raoult s law a separation problem often occurs at the top of the column, where the high boiler has to be removed, since at the top of a distillation column the most unfortunate separation factors are obtained. 

B = 4.0 X 10 mmHg. 9.22 Ethanol water mixture is a nonidetil solution that exhibits positive deviations from Raoult s law. Therefore, this system has a minimum boihng point and will form a low boiling azeotrope that cemnot be separated by fractioneil distillation. 9.24 1.3X10 g. 9.26 128 g. 9.28 0.59 m. 

Such a process depends upon the difference in departure from ideally between the solvent and the components of the binary mixture to be separated. In the example given, both toluene and isooctane separately form nonideal liquid solutions with phenol, but the extent of the nonideality with isooctane is greater than that with toluene. When all three substances are present, therefore, the toluene and isooctane themselves behave as a nonideal mixture and then-relative volatility becomes high. Considerations of this sort form the basis for the choice of an extractive-distillation solvent. If, for example, a mixture of acetone (bp = 56.4 C) and methanol (bp = 64.7°Q, which form a binary azeotrope, were to be separated by extractive distillation, a suitable solvent could probably be chosen from the group of aliphatic alcohols. Butanol (bp = 117.8 Q, since it is a member of the same homologous series but not far removed, forms substantially ideal solutions with methanol, which are themselves readily separated. It will form solutions of positive deviation from ideality with acetone, however, and the acetone-methanol vapor-liquid equilibria will therefore be substantially altered in ternary mixtures. If butanol forms no azeotrope with acetone, and if it alters the vapor-liquid equilibrium of acetone-methanol sufficiently to destroy the azeotrope in this system, it will serve as an extractive-distillation solvent. When both substances of the binary mixture to be separated are themselves chemically very similar, a solvent of an entirely different chemical nature will be necessary. Acetone and furfural, for example, are useful as extractive-distillation solvents for separating the hydrocarbons butene-2 and a-butane. 

Robbins chaii. Selecl candidate solvents from groups in the Robbins Chart (Table 13-15) that tend to give positive (or no) deviations from Raoult s law for the key component desired in the distillate and negative (or no) deviations for the other key. 

The distillation process becomes more complex as deviations from ideality become more extreme. For the case (7.57) of extreme positive deviations (shown in brackets below), the T-xB diagram has the form shown at the right ... 

B. Distillation and Purging. The great efficiency of fractional distillation for the removal of water from hydrocarbon and chlorocarbon solvents is often not well appreciated. The physical origin of this good separation is in part the large positive deviation of the water vapor pressure from Raoult s law because of the lack of affinity of water for these liquids. Typically, the distillation of a simple hydrocarbon solvent with a column of 100 plates and discarding the first two... 

Deviations from Raoult s law lead to the formation of azeotropes, constant boiling mixtures that cannot be separated by distillation, making industrial separations difficult. For components A and B, there is a positive deviation if the A-B attraction is less than A-A and B-B attractions (A and B reject each other), and a negative deviation if the A-B attraction is greater than A-A and B-B attractions. If the A-B attraction is nearly equal to the A-A and B-B attractions, the solution obeys Raoult s law. Explain... 

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