Azeotropic Temperature

The azeotropic temperature determines the temperature difference between the top and the bottom of the column. A large delta T of the azeotropic temperature to the pure acetic acid temperature implies easy separability. Fewer column stages will be needed for specific product purity specifications. In this regard, ethyl acetate is the best entrainer. This interpretation is confirmed by Table 9.13 because the ethyl acetate system requires the least total number of stages for the same separation. 

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If a binary system forms an azeotrope, the activity coefficients can be calculated from a knowledge of the composition of the azeotrope and the azeotropic temperature. At the azeotropic point the composition of the liquid and vapour are the same, so from equation 8.31 ... 

Vapor-liquid equilibrium compositions, K-values, activity coeff., etc., azeotrope temperature and composition, enthalpy and heat capacity, heats of vaporization... 

THE.8.1. Prigogine, Sur la variation de la temperature azeotropique sous I influence de la pression (On the variation of the azeotropic temperature under the influence of pressure), Bm//. Soc. Chim. Belg. 52, 165-166 (1943). 

We can estimate the change in the entropy of vaporization at azeotropic temperature when the heat flow is known at azeotropic pressure... 

In these equations T is the azeotropic temperature TJ and are the temperatures at which the two pure substances exist in equilibrium in the two phases at the given pressure. 

As an example let us consider a mixture of ethanol and carbon tetrachloride which forms an azeotropic mixture.f At a pressure of one atm. the azeotropic temperature is 65 08 °C. 

Now consider the system at constant pressure. To employ equations (28.18) to calculate the azeotropic temperature T, we must know the effect of temperature on the coefficient a2. As a first approximation we may use equation (21.52 ) in which 0C2 is assumed to be inversely proportional to T that is we assume the solutions are regular. 

Fig. 28.1. Azeotropes of systems containing acetone. Component 1 is in each case that whose boiling point is nearer to the azeotropic temperature.

In an ethyl acetate (l)-ethyl alcohol (2) separation process, it is required to determine if this mixture forms an azeotrope. If it does, it is required to determine the azeotropic temperature and composition as a function of the azeotropic pressure between 70 and 120 kPa at 10 kPa intervals. Solve for the composition and temperature by doing a single computation operation at each pressure value. Additionally, for any one of the pressure points determine if the azeotrope is minimum- or maximum-boiling. Use the van Laar equation for the liquid activity coefficients and assume ideal gas behavior for the vapor phase. The following data are given ... 

The formation of azeotropes due to deviations from Raoult s law was discussed in Section 1.3. An azeotrope is a mixture that, at a given pressure (the azeotropic pressure), boils at a constant temperature (the azeotropic temperature) and has the same composition (the azeotropic composition) in the equilibrium vapor and liquid phases. Homogeneous azeotropes are those that form one liquid phase at equilibrium with the vapor heterogeneous azeotropes are those that form two liquid phases at equilibrium with each other and the vapor. 

Although the azeotrope temperatures are still not too far apart, distillation can be done with adequate reflux ratio and number of stages. 

Fig. 2.3.3-5 Example for the determination of the azeotropic temperature for the system 2-ethylhexa-...

Liquid mixtures having an extremum (maximum or minimum) vapor pressure at constant temperature, as a function of composition, are called azeotropic mixtures, or simply azeotropes. Mixtures that do not show a maximum or minimum are called zeotropic. Azeotropes in which the pressure is a maximum are often called positive azeotropes, while pressure-minimum azeotropes are called negative azeotropes. The coordinates of an azeotropic point are the azeotropic temperature t, pressure, and liquid-phase composition, usually expressed as mole fractions. At the azeotropic... 

This is a remarkable result, for it is merely the Clapeyron equation (8.2.27) extended from pure substances to azeotropic mixtures. The derivative on the Ihs represents the slope of the locus of azeotropes on a PT diagram. We can use (9.3.21) as a basis for correlating azeotropic temperatures and pressures, just as we used it in 8.2.6 for correlating pure-component vapor pressures. We obtain the same generalized form of the Clausius-Clapeyron equation. 

Homogeneous azeotropes occur in a great many binary mixtures, and tables of azeotropic temperatures, pressures, and compositions can be found in the compilations by Horsley [6] and by Gmehling et al. [7]. Such azeotropes occur when one vapor phase is in equilibrium with one liquid phase. In addition, extrema in isothermal Pxy and isobaric Txy diagrams occur in some three-phase VLLE situations for binary mixtures. These are called heterogeneous azeotropes. But at heterogeneous azeotropes the composition of the vapor is rarely the same as that of either liquid these situations are discussed in 9.3.7. 

This means that at any given pressure, such as P2 in Figure 9.15, three-phase VLLE occurs at only one temperature. That temperature identifies a heterogeneous azeotrope at the pressure P2 the azeotropic temperature is the lowest at which vapor can exist in equilibrium with liquid. At this T, boiling of the two-phase liquid will produce a vapor of fixed composition, regardless of the overall composition of the system. 

Absolute pressure (muiHg) Azeotropic temperature (°C) Acid (%w/w)... 

The effects of these and other guidelines must be considered as each separator is designed and as the separation sequence evolves. More recently, Peterson and Partin (1997) showed that temperature sequences involving the boiling points of the pure species and the azeotrope temperatures can be used to effectively categorize many kinds of residue curve maps. This classification simplifies the search for an entrainer that has a desirable residue curve map, for example, that does not involve a distillation boundary. 

Selecting the analysis type as Txy, specifying the system pressure, specifying Valid Phases as Vapor-Liquid and clicking the Go button at the bottom of the dialog box produce the Txy plot shown in Figure 2.2. Tabulated binary analysis results like activity coefficients and K values are also shown. From the example Txy plot of isopropanol-water system, we can see that this system contains an azeotrope at isopropanol mole fraction between 0.6 and 0.8 and azeotropic temperature of about 80°C. To hnd the exact prediction of the azeotropic composition and temperature, the azeotrope search tool can be used. The use of this tool will be explained later in Section 2.5.2. 

One way to separate a mixture containing an azeotrope is to add a light entrainer into the system so that an additional azeotrope(s) can be formed that helps in the separation. In order to make the separation feasible, there are two important eharacteristics of this additional azeotrope (or one of the additional azeotropes). First, the azeotropic temperature of one additional azeotrope should be the minimum temperature of the whole ternary system. Second, this azeotrope should be heterogeneous so that natural liquid-liquid separation, without energy requiremenf can be performed in a decanter at the top of the column. 

As shown in Fig. 8.5, a pyridine-water mixture has an azeotrope with composition of around 77 mol% H2O and azeotropic temperature of around 95°C. By adding toluene... 

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