Systems, azeotropic

Convergence is usually accomplished in 2 to 4 iterations. For example, an average of 2.6 iterations was required for 9 bubble-point-temperature calculations over the complete composition range for the azeotropic system ehtanol-ethyl acetate. Standard initial estimates were used. Figure 1 shows results for the incipient vapor-phase compositions together with the experimental data of Murti and van Winkle (1958). For this case, calculated bubble-point temperatures were never more than 0.4 K from observed values. 

Binary azeotropic systems are reported for all three derivatives (9). The solubiHties of benzyl chloride, benzal chloride, and ben zotricbl oride in water have been calculated by a method devised for compounds with significant hydrolysis rates (10). 

Fig. 4. Boiling poiat (a) and phase diagram (b) for a minimum boiling biaary azeotropic system at constant pressure. B and C, D are representative...

In the example, the minimum reflux ratio and minimum number of theoretical plates decreased 14- to 33-fold, respectively, when the relative volatiHty increased from 1.1 to 4. Other distillation systems would have different specific reflux ratios and numbers of theoretical plates, but the trend would be the same. As the relative volatiHty approaches unity, distillation separations rapidly become more cosdy in terms of both capital and operating costs. The relative volatiHty can sometimes be improved through the use of an extraneous solvent that modifies the VLE. Binary azeotropic systems are impossible to separate into pure components in a single column, but the azeotrope can often be broken by an extraneous entrainer (see Distillation, A7EOTROPTC AND EXTRACTIVE). 

Acetic acid-water-vinyl acetate Pinched, azeotropic system Self-entraining ... 

In normal applications of extractive distillation (i.e., pinched, closeboiling, or azeotropic systems), the relative volatilities between the light and heavy key components will be unity or close to unity. Assuming an ideal vapor phase and subcritical components, the relative volatility between the light and heavy keys of the desired separation can be written as the produc t of the ratios of the pure-component vapor pressures and activity-coefficient ratios whether the solvent is present or not ... 

An important system in distillation is an azeotropic mixture. An azeotrope is a liquid mixture which when vaporized, produces the same composition as the liquid. The VLE plots illustrated in Figure 11 show two different azeotropic systems one with a minimum boiling point and one with a maximum boiling point. In both plots, the equilibrium curves cross the diagonal lines. 

These are azeotropic points where the azeotropes occur. In other words, azeotropic systems give rise to VLE plots where the equilibrium curves crosses the diagonals. Both plots are however, obtained from homogenous azeotropic systems. An azeotrope that contains one liquid phase in contact with vapor is called a homogenous azeotrope. A homogenous azeotrope carmot be separated by conventional distillation. However, vacuum distillation may be used as the lower pressures can shift the azeotropic point. Alternatively, an additional substance may added to shift the azeotropic point to a more favorable position. When this additional component appears in appreciable amounts at the top of the column, the operation is referred to as an azeotropic distillation. When the additional component appears mostly at the bottom of the column, the operation is called extractive distillation. 

Short-cut methods for non-ideal and azeotropic systems are given by Featherstone (1971) (1973). 

Featherstone, W. (1971) Brit. Chem. Eng. Proc. Tech. 16 (12), 1121. Azeotropic systems, a rapid method of still design. 

By contrast with nonazeotropic systems, for azeotropic systems there is a maximum reflux ratio above which the separation deteriorates16. This is because an increase in reflux ratio results in two competing effects. Firstly, as in nonazeotropic distillation, the relative position of the operating surface relative to the equilibrium surface changes to improve the separation. This is countered by a reduction in the entrainer concentration, owing to dilution by the increased reflux, which results in a reduction in the relative volatility between the azeotropic components, leading to a poorer separation16. 

So far, the separation of azeotropic systems has been restricted to the use of pressure shift and the use of entrainers. The third method is to use a membrane to alter the vapor-liquid equilibrium behavior. Pervaporation differs from other membrane processes in that the phase-state on one side of the membrane is different from the other side. The feed to the membrane is a liquid mixture at a high-enough pressure to maintain it in the liquid phase. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, maintaining it in the vapor phase. Dense membranes are used for pervaporation, and selectivity results from chemical affinity (see Chapter 10). Most pervaporation membranes in commercial use are hydrophyllic19. This means that they preferentially allow... 

Pressure shift should always be explored as the first option when separating an azeotropic system. Adding extraneous components to a separation should always be avoided if possible. Unfortunately, most azeotropes are insensitive to change in pressure, and at least a 5% change in composition with pressure is required for a feasible separation using pressure shift1. 

TABLE 13.7. Examples of Extractive Distillation Processes for the Separation of Ideal, Nonideal, end Azeotropic Systems... 

AZEOTROPIC SYSTEM. A system of two or more components that has a constant boiling point at a particular composition. If the constant boiling point is a minimum, the system is said to exhibit negative azeotropy, if it is a maximum, positive azeotropy. 

The vapor curve KLMNP gives the composition of the vapor as a function of the temperature T, and the liquid curve KKMSP gives the composition of die liquid as a function of die temperature. These two curves have a common point M. The state represented by M is that in which the two states, vapor and liquid, have the same composition xaB on die mole fraction scale. Because of die special properties associated with systems in this state, the Point M is called an azeotropic point and the system is said to form an azeotrope. In an azeotropic system, one phase may be transformed to the other at constant temperature, pressure and composition without affecting the equilibrium state. This property justifies the name azeotropy, which means a system diat boils unchanged. 

Azeotropic Systems. An azeotropic system is one wherein two or more components have a constanl boiling point at a particular composition. Such mixtures cannot be separated by conventional distillation methods. If rhe constant boiling point is a minimum, the system is said lo exhibit negomv azeotropy if it is a maximum, positive azeotropy. Consider a mixture of water and alcohol in the presence of the vapor. This system of two phases and two components is divarianl. Now choose some fixed pressure and study the composition of the system at equilibrium us a function of temperature. The experimental results arc shown schematically in Fig. 5. 

Because a number of Industrially important liquid mixtures are azeotropic systems, means had to be found whereby they may be separated by distillation. Two approaches, extractive distillation and azeotropic distillation, are used. In either case, a separating agent is added to the column so as to alter favorably the relative volatilities of the... 

Fig. 5. Boiling-point diagram of an azeotropic system exhibiting negative azeotropy...

GIBBS-KONOVALOV THEOREMS. Consider a binary system containing two phases (e.g.. liquid and vapor). Both components can pass from one phase lo another. The Gibbs Konovalov theorems refer to the properties of the phase diagrams of such systems (see also Azeotropic System). The lirst theorem is At constant pressure, the temperature of coexistence passes through tin extreme value (maximum, minimum or inflexion with a horizontal value), if the comfutsirlon of the two phases is the same. Conversely, al a point at winch the temperature passes through an extreme value, the phases have the same composition. The second theorem is similar. It refers lo the coexistence pressure at constant temperature. 

Graphical Method for Predicting Effect of Pressure on Azeotropic Systems, and "Graphical Method for Predicting Azeotropism and Effect of Pressure on Azeotropic Constants. ... 

Graphical Method for Predicting Effect of Pressure on Azeotropic Systems... 

A rapid and easily applicable method has been found for indicating the effect of pressure on the composition and boiling point of an azeotropic system. The method is based on the use of the Cox vapor pressure chart (I) on which the log of vapor pressure is plotted as a function of l/(t° C. + 230) to give a straight line over a wide range of pressures. 

While the agreement between predicted and experimental values is far from perfect, the method has served as a valuable guide in estimating effect of pressure on azeotropic systems. 

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