Azeotropic points composition

The more stmcturaHy, chemically similar components are, the less likely that the separation will be improved by azeotropic distillation (if an MSA-key component azeotrope is being used to alter the RCM) any azeotropes formed between one component and another similar component tend to have similar boiling points, compositions. 

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 ... 

Advantageously, the monomer feed is adjusted to the azeotropic point of the pair of monomers, so that the polymer has the same composition as the monomer. Azeotropic points are shown in Table 10.3. 

Actually, most SAN types have a composition near the azeotropic point, just for the reasons explained above. 

Figure 2.2-1. Two phase equilibria three cases were the composition of the two phase are equal, a Pure component boiling point, b Azeotropic point, c Critical point.

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. 

From the isothermal vapor-liquid equilibrium data for the ethanol(l)/toluene(2) system given in Table 1.11, calculate (a) vapor composition, assuming that the liquid phase and the vapor phase obey Raoult s and Dalton s laws, respectively, (b) the values of the infinite-dilution activity coefficients, Y and y2°°, (c) Van Laar parameters using data at the azeotropic point as well as from the infinite-dilution activity coefficients, and (d) Wilson parameters using data at the azeotropic point as well as from the infinite-dilution activity coefficients. 

Process Applications The prodnction of esters from alcohols and carboxylic acids illustrates many of the principles of reactive distillation as applied to equilibrium-limited systems. The true thermodynamic equilibrium constants for esterification reactions are nsnally in the range of 5 to 20. Large excesses of alcohols mnst be nsed to obtain acceptable yields, resulting in large recycle flow rates. In a reactive distillation scheme, the reaction is driven to completion by removal of the water of esterification. The method used for removal of the water depends on the boiling points, compositions, and liquid-phase behavior of any azeotropes formed between the prodncts and reactants and largely dictates the structure of the reactive distillation flow sheet. 

Not all liquids form ideal solutions and conform to Raoult s law. Ethanol and water are such liquids. Because of molecular interaction, a mixture of 95.5% (by weight) of ethanol and 4.5% of water boils below (78.15°C) the boiling point of pure ethanol (78.3°C). Thus, no matter how efficient the distilling apparatus, 100% ethanol cannot be obtained by distillation of a mixture of, say, 75% water and 25% ethanol. A mixture of liquids of a certain definite composition that distills at a constant temperature without change in composition is called an azeotrope 95% ethanol is such an azeotrope. The boiling point-composition curve for the ethanol-water mixture is seen in Fig. 4. To prepare 100% ethanol the water can be removed chemically (reaction with calcium oxide) or by removal of the water as an azeotrope (with still another liquid). An azeotropic mixture of 32.4% ethanol and 67.6% benzene (bp 80.1 °C) boils at 68.2°C. A ternary azeotrope (bp 64.9°C) contains 74.1% benzene, 18.5% ethanol, and 7.4% water. Absolute alcohol (100% ethanol) is made by addition of benzene to 95% alcohol and removal of the water in the volatile benzene-water-alcohol azeotrope. 

The trajectories along which two (or more) species have the same volatility have been called isovolatility curx es. Based on the ability to trace out such curves in the composition space, we can outline a simple algorithm to robustly determine all azeotropic points that are predicted for a multispecies mixture ... 

The state represented by M is that in which the two states, vapour and Uquid have the same composition on the mole fraction scale. Because of the 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. 

Thus, if an azeotrope point is reached during rectification, this mixture boils at constant temperature and the composition of the vapour is identical to that of the liquid in equilibrium yj = Xj... 

In the systems that we have examined so far, the bubble point and the dew point of the mixture vary monotonically with the composition. This is the case for ideal systems. However, for very non-ideal systems, there may be a maximum or a minimum in the bubble and dew point curves. This is the case for azeotropic systems. An example of a system that exhibits a low-boiling azeotrope is a mixture of 77-heptane and ethanol, which is shown in Figure 3.5. For this type of system, both the bubble and dew point temperature curves have a local minimum at the same composition. At this composition, these two curves meet. This point is known as the azeotrope. At the azeotrope, the composition of the coexisting liquid and vapor phases are identical. In this case at the azeotrope, the boiling temperature... 

Here, too, the deviations from Raoult s law are positive for both components over the entire composition range at 45°C (Figures 1.7 and 1.8). The partial pressure deviations are large enough to result in a maximum in the total pressure (Figure 1.7). The binary thus forms a maximum pressure, or minimum temperature, azeotrope, a phenomenon discussed in more detail later in this section. In this binary, the concentration of ethanol is higher in the vapor than in the liquid below the azeotrope and is lower in the vapor than in the liquid above the azeotrope. At the azeotropic point, the concentrations in the liquid and vapor are identical (Figure 1.9). 

Example 1.9 shows that, for certain systems, deviations from Raoult s law can cause a maximum or a minimum in the vapor pressure to exist at a certain temperature and composition. At constant pressure, the boiling point or bubble point temperature curve could have a maximum or a minimum. Liquid mixtures whose vapor pressure curve or surface exhibits a maximum or a minimum are said to form azeotropes. The composition at which the azeotrope occurs is the azeotropic composition. Binaries are likely to form azeotropes if they deviate from Raoult s law and if their boiling points are not too far apart (within about 8°C). Azeotropes caused by positive deviations from Raoult s law are minimum boiling, that is, the azeotrope boils at a... 

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