Vapor-liquid equilibrium azeotropic mixtures

Multicomponent distillations are more complicated than binary systems due primarily to the actual or potential involvement or interaction of one or more components of the multicomponent system on other components of the mixture. These interactions may be in the form of vapor-liquid equilibriums such as azeotrope formation, or chemical reaction, etc., any of which may affect the activity relations, and hence deviations from ideal relationships. For example, some systems are known to have two azeotrope combinations in the distillation column. Sometimes these, one or all, can be broken or changed in the vapor pressure relationships by addition of a third chemical or hydrocarbon. 

Example 4.5 2-Propanol (isopropanol) and water form an azeotropic mixture at a particular liquid composition that results in the vapor and liquid compositions being equal. Vapor-liquid equilibrium for 2-propanol-water mixtures can be predicted by the Wilson equation. Vapor pressure coefficients in bar with temperature in Kelvin for the Antoine equation are given in Table 4.113. Data for the Wilson equation are given in Table 4.126. Assume the gas constant R = 8.3145 kJ-kmol 1-K 1. Determine the azeotropic composition at 1 atm. 

Pervaporation. Pervaporation differs from the other membrane processes described so far in that the phase-state on one side of the membrane is different from that on the other side. The term pervaporation is a combination of the words permselective and evaporation. The feed to the membrane module is a mixture (e.g. ethanol-water mixture) at a pressure high enough to maintain it in the liquid phase. The liquid mixture is contacted with a dense membrane. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, thus maintaining it in the vapor phase. The permeate side is often held under vacuum conditions. Pervaporation is potentially useful when separating mixtures that form azeotropes (e.g. ethanol-water mixture). One of the ways to change the vapor-liquid equilibrium to overcome azeotropic behavior is to place a membrane between the vapor and liquid phases. Temperatures are restricted to below 100°C, and as with other liquid membrane processes, feed pretreatment and membrane cleaning are necessary. 

Membranes can also be used to alter the vapor-liquid equilibrium behavior and allow separation of azeotropes. The liquid mixture is fed to one side of the membrane, and the permeate is held under conditions to maintain it in the vapor phase. Most separations use hydrophyllic membranes that preferentially pass water rather than organic material. Thus, pervaporation is commonly used for the dehydration of organic components. 

Vapor-liquid equilibrium data at atmospheric pressure (690-700 mmHg) for the systems consisting of ethyl alcohol-water saturated with copper(II) chloride, strontium chloride, and nickel(II) chloride are presented. Also provided are the solubilities of each of these salts in the liquid binary mixture at the boiling point. Copper(II) chloride and nickel(II) chloride completely break the azeotrope, while strontium chloride moves the azeotrope up to richer compositions in ethyl alcohol. The equilibrium data are correlated by two separate methods, one based on modified mole fractions, and the other on deviations from Raoult s Law. 

There is a considerable amount of experimentaldata for properties of mixtures wherein toluene is a principal constituent. Compilations and bibliographies exist for vapor—liquid equilibrium measurements (9,10), liquid—liquid equilibrium measurements (11), and azeotropic data (12,13). 

Related Calculations. These calculations show how to use vapor-liquid equilibrium data to obtain parameters for activity-coefficient correlations such as those of Van Laar and Wilson. (Use of liquid-liquid equilibrium data for the same purpose is shown in Example 1.20.) If the system forms an azeotrope, the parameters can be obtained from a single measurement of the azeotropic pressure and the composition of the constant boiling mixture. If the activity coefficients at infinite dilution are available, the two parameters for the Van Laar equation are given directly, and the two in the case of the Wilson equation can be solved for as shown in the example. 

For example, suppose there is a stream in the process that is a binary mixture of chemical components A and B. If these components obey ideal vapor-liquid equilibrium behavior, we can use a single distillation column to separate them. If they form an azeotrope, we may have to use a two-column separation scheme. If the azeotropic composition... 

For azeotropic distillation especially the systems are non-ideal which makes calculating vapor-liquid equilibrium properties more difficult than, for example, in distillation of mixtures of simple hydrocarbons. Work predicting the vapor-liquid equilibrium properties of ternary mixtures of... 

This study was undertaken to obtain the necessary vapor-liquid equilibrium data and to determine the distillation requirements for recovering solvent for reuse from the solvent-water mixture obtained from adsorber regeneration. Previous binary vapor-liquid equilibrium data (2, 3) indicated two binary azeotropes (water-THF and water-MEK) and a two phase region (water-MEK). The ternary system was thus expected to be highly nonideal. 

EPAR ATION and purification processes account for a large portion of the design, equipment, and operating costs of a chemical plant. Further, whether or not a mixture forms an azeotrope or two liquid phases may determine the process flowsheet for the separations section of a chemical plant. Most separation processes are contact operations such as distillation, gas absorption, gas stripping, and the like, the design of which requires the use of accurate vapor-liquid equilibrium data and correlating models or, in the absence of experimental data, of accurate predictive methods. Phase behavior, especially vapor-Uquid equilibria, is important in the design, development, and operation of chemical processes. 

Various processes are used for separating components that are difficult or impossible to be separated by conventional distillation. Whether the difficulty of separation arises from the components close boiling points or their tendency to form azeotropes, the separation processes must take into account the complex vapor-liquid equilibrium relationships of the system. The system to be considered involves both the components to be separated and the separating agent that, in one way or another, enhances the desired separation. The vapor-liquid equilibria of such mixtures is highly nonideal, and it is precisely this nonideality that is capitalized on to bring about the separation. 

There are several ways to separate an azeotropic mixture into two components of the desired purities, and these are discussed in. other chemical engineering courses. However, one method will be mentioned here, and it is based on the fact that in general the two components will not have the same heat of vaporization, so that by the Clausius-Clapeyron equation the temperature dependence of their vapor pressures will be different. Since the dominant temperature dependence in vapor-liquid equilibrium is that of the pure component vapor pressures, the azeotropic composition will also change with temperature (and pressure). Therefore, what can be done is to use two distillation columns operating at different pressures. 

The point to note here is the important role of thermodynamics, in that the schematic design of the process to separate the feed mixture into two strearns of specified purity was based completely on vapor-liquid equilibrium. The fact that an azeotrope occurred dictated that the desired separation could not be obtained with a single distillation column. Then how the azeotropic composition changed with temperature was the basis for whether a second column of higher or lower pressure should be used. This example is just one illustration of the very central role of thermodynamics in general, and vapor-liquid equilibrium in particular, in the design of separations processes in chemi-. cal engineering. 

As another example of low-pressure vapor-liquid equilibrium, we consider the n-pentane-propionaldehyde mixture at 40.0 C. Eng and Sandler took data on this system using the dynamic still of Fig. 10.2-5. The x-y-P-T data in Table 10.2-1 and Fig. 10.2-8fl and b were obtained by them. (Such data can be tested for thermodynamic consistency see Problem 10.2-12.) As is evident, this system is nonideal and has an azeotrope at about 0.656 mole fraction pentane and 1.3640 bar. We will use these data to test the UNIFAC prediction method. 

Figure 11.3-3 shows the vapor-liquid and liquid-liquid equilibrium behavior computed for the system of methanol and n-hexane at various temperatures. Note that two liquid phases coexist in equilibrium to temperatures of about 43°C. Since liquids are relatively incompressible, the species liquid-phase fugacities are almost independent of pressure (see Illustrations 7.4-8 and 7.4-9), so that the liquid-liquid behavior is essentially independent of pressure, unless the pressure is very high, or low enough for the mixture to vaporize (this possibility will be considered shortly). The vapor-liquid equilibrium curves for this system at various pressures are also shown in the figure. Note that since the fugacity of a species in a vapor-phase mixture is directly proportional to pressure, the VLE curves are a function of pressure, even though the LLE curves are not. Also, since the methanol-hexane mixture is quite nonideal, and the pure component vapor pressures are similar in value, this system exhibits azeotropic behavior. 

Benzene and cyclohexane are very similar compounds, but their mixture is sufficiently nonideal that an azeotrope is formed in vapor-liquid equilibrium, At 40 C, the vapor pressure of cyclohex-ane is 0.246 bar, and that of benzene is 0.244 bar. At this temperature, the azeotrope occurs at 0.51 mole fraction of cyclohexane and a total pressure of 0.2747 bar. 

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