Azeotropy

The instantaneous copolymer composition X generally doesn t coincide with the monomer feed composition x from which the copolymer was produced. Such a coincidence X = x can occur only under some special values of monomer feed composition x, called azeotropic . According to definition these values can be calculated in the case of the terminal model (2.8) from a system of non-linear algebraic equations  

In the case of the terpolymerization one can reduce the non-linear set of equations (4.13) to a single equation of fourth degree [121-123], for which a rigorous explicit solution is obtained and the conditions of its existence are analyzed [121]. 

In addition to a true (triple) azeotrope, some authors suggested for m = 3 to consider limited (unitary) [123-125] and partial (binary) [126,123] azeotropes. Both of them in contrast to the former one are located along certain azeotropic 

Note that the expressions (4.15) can be obtained even without the formulae (4.11), if we employ an algorithm similar to the one used for the derivation of formulae (4.11). Actually, the proper diverging from the point i directed tree corresponds to each of the items in the expression for Af. A weighting factor o ay corresponds to each of the arcs in there digraphs that leaves point i and enters point j. The sum of all so weighted trees directed from a root of type i directly gives an expression similar to expressions (4.15) for the value of Af, which is equal (when all coj are positive) within the accuracy of the normalizing factor A to the component xf = Xf = Af/A of azeotropic composition under the copolymerization of an arbitrary number (m) of monomers. 

As long ago as 1960, Tarasov et al. [121] presented some examples of the concrete three-component systems for which the existence of the azeotropic composition had already been predicted theoretically. The list of such systems was widened substantially after publication of the important paper [125], where a set of the known tabulated values of 653 pairs of reactivity ratios for a computer search of the possible multicomponent azeotropes was employed. For this aim one should, at first, reveal all the completely characterized multicomponent systems for which the values of reactivity ratios of all monomer pairs are tabulated. This problem can be formalized by reducing it to the search on the graph with 653 lines of a 

Owing to the non-ideality of binary or multicomponent mixtures, the liquid phase composition is often identical with the vapor phase composition. This point is called an azeotrope and the corresponding composition is called the azeotropic composition. An azeotrope can not be circumvented by conventional distillation since no enrichment of the low-boiHrig component can be achieved in the vapor phase. Separating azeotropic mixtures therefore requires special processes, e.g. azeotropic or extractive distillation or pressure swing distillation. Azeotropic information is available in literature (Gmehling et al., 2004). 

For a better understanding of the separation of multicomponent mixtures, the application of residue curves is helpful. Residue curves were introduced by Schrei-nemakers in 1901 (Schreinemakers, 1901a, 1901b) and are applied to distillation processes by Doherty and coworkers (Doherty and Malone, 2001). Residue curves describe the change of composition in the reboiler over time for open vaporization. 

Schneider, Z. Alwani, W. Heim, E. Horvath, and E. U. Franck, Chem.-Ing.-Tech., 1967, 

Prausnitz and P. L. Chueh, Computer Calculations for High-pressure Vapor-Liquid 

Equilibria , Prentice-Hall, Englewood Cliffs, New Jersey, U.S.A., 1968. 

We shall now apply the ideas of this chapter to the azeotropes introduced in Chapter 7. For an azeotrope in equilibrium with its vapor, the composition of the liquid and the vapor phases are the same. At a fixed pressure, a liquid mixture is an azeotrope at a particular composition called the azeotropic composition. An azeotropic transformation is one in which there is an exchange of matter between two phases without change in composition. In this regard, an azeotrope is similar to the vaporization of a pure substance. This enables us to obtain the activity coefficients of azeotropes just as for a pure substance. 

Let us consider a binary azeotrope. As we have seen in secion 8.1, the chemical potentials of the components can be written in the form p, Xk) = p)- - RT a.y,pck, in which activity coefficient y is a 

For the activity coefficient of the vapor phase, if we use the ideal gas approximation y g = 1. This gives us an explicit expression for the activity coefficient of the liquid phase  

This expression allows the activity coefficient to be calculated for a component of an azeotrope, and it gives a simple physical meaning to the activity coefficient. More on azeotropes can be found in [3]. 

Laidler, K. J., The World of Physical Chemistry. 1993, Oxford Oxford University Press. 

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Design and Operation of Azeotropie Distillation Columns Simulation and design of azeotropic distiUation columns is a difficult computational problem, but one tnat is readily handled, in most cases, by widely available commercial computer process simulation packages [Glasscock and Hale, Chem. Eng., 101(11), 82 (1994)]. Most simida-... 

When an equilibrium reaction occurs in a vapor-hquid system, the phase compositions depend not only on the relative volatility of the components in the mixture, but also on the consumption (and production) of species. Thus, the condition for azeotropy in a nonreactive system = x, for all i) no longer holds true in a reactive system and must be modified to include reaction stoichiometry ... 

The transformed variables describe the system composition with or without reaction and sum to unity as do Xi and yi. The condition for azeotropy becomes X, = Y,. Barbosa and Doherty have shown that phase and distillation diagrams constructed using the transformed composition coordinates have the same properties as phase and distillation region diagrams for nonreactive systems and similarly can be used to assist in design feasibility and operability studies [Chem Eng Sci, 43, 529, 1523, and 2377 (1988a,b,c)]. A residue curve map in transformed coordinates for the reactive system methanol-acetic acid-methyl acetate-water is shown in Fig. 13-76. Note that the nonreactive azeotrope between water and methyl acetate has disappeared, while the methyl acetate-methanol azeotrope remains intact. Only... 

In our discussion of (vapor + liquid) phase equilibria to date, we have limited our description to near-ideal mixtures. As we saw in Chapter 6, positive and negative deviations from ideal solution behavior are common. Extreme deviations result in azeotropy, and sometimes to (liquid -I- liquid) phase equilibrium. A variety of critical loci can occur involving a combination of (vapor + liquid) and (liquid -I- liquid) phase equilibria, but we will limit further discussion in this chapter to an introduction to (liquid + liquid) phase equilibria and reserve more detailed discussion of what we designate as (fluid + fluid) equilibria to advanced texts. 

The Vinyloop process is based on the selective dissolution of PVC used in composites applications like cable insulation, flooring, tarpaulins, blisters, etc. After removal of insoluble parts like metals, rubber or other polymers, the PVC is reprecipitated with all additives by introduction of a non-solvent component whieh will form with the seleetive solvent an azeotropie mixture. By using typical conditions, the process is able to reeover a pure PVC eompound powder ready for use without any additional treatment like melt filtration or a new pelletisation (speeific characteristics of the powder are average diameter of 400 microns and bulk density above 600 kg/ eub.m). All the solvents used are eompletely reeyeled and reused. PVC compounds recovered in the Vinyloop process can be reused in a closed-loop recycling scheme... 

The authors suggested that a combination of pervaporation with a conventional separation technique such as a hybrid distillation-pervaporation system could be useful economically to break the azeotropy. 

Aucejo, A., Loras, S., Munoz, R., Wisniak, J., and Segura, H. Phase equilibria and multiple azeotropy in the associating system methanol + diethylamine, J. Chem. Eng. Data, 42(6) 1201-1207, 1997. 

This equation contains the counterion concentration Cj + which depends on the total surfactant concentration. It follows that x would depend on c j+ and hence would vary above the cmc. This contradiction implies that azeotrope micellization cannot occur if = J3(x). Of course, if c c, the C + would be constant and azeotropy can again occur. If d fdx = 0, azeotropy can be also possible. For ySj = / 2 = 0.7, Cj = 0, c /cj = 3.0, and w(x) = A + B ( 2x-l), which is the Redlich-Kbter expansion (12), with A = -3 and B = 0, one finds from Equation 21 that 0.8113. No value of Qj can be calculated if = 0.7, / 2 = 0.3, and / (x) = iX + /32(1 x). Figures 1 and 2 illustrate this point showing monomer and micelle concentrations (or inventories) for a = 0.8113. In the ionic/ionic case, the micelle composition x and the ratio Cj/c2 are constant above the cmc. In the ionic/nonionic case (Figure 2) the micelle composition varies with total surfactant concentration. Osborne-Lee and Schechter (22) have found evidence of azeotrope micellization for... 

Many other azeotropic separations are known. Butadiene, styrene, benzene, and xylenes are examples of compounds that may be segregated from refinery streams by this means. In fact, any separation of nonaromatic from aromatic hydrocarbons lends itself to this method, but requires the selection of the proper azeotrope former and processing conditions. In bench scale operations, azeotropy has been applied up to and including the lubricating oil range. 

Many azeotropic combinations exist among hydrocarbons themselves. Aromatic hydrocarbons, for example, are almost always found in petroleum fractions distilling below the true boiling point of the aromatics. Marschner and Cropper (41) accurately delineated the limits of azeotropy for benzene and toluene with saturated hydrocarbons, and Denyer et al. (11) did the same for the thiols. Consideration of such data is desirable in the design and operation of equipment for the distillation of gasoline fractions to produce specialized products. 

Azeotropic distillation was briefly described in Vol 1 of Encycl, p 318, under "Azeotropy (Ref 20)... 

Edit, " Fundamentals of Chemical Engineering Operations , Prentice-Hall, Englewood Cliffs, New Jersey (1958), pp343-424 (Distillation) 20) Encyclopedia of Explosives, PATR 2700, Vol 1 (I960), p A518(Azeotropy) 21) Perry, 4th Edit (1963), pp 13-2 to 13-55 22) Kirk... 

Azeotropy (l Azeotropisme, in Fr). Azeotropy may be defined as the capability of a liquid to form with other liquids, some mixts, which possess constant boiling points (max or minim). An azeotropic mixt resembles a chemical individual in boiling without undergoing change in compn, but differs from it in losing this characteristic as soon as the pressure is altered... 

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. 

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

Compared to Fig. 5.9 the line of reactive azeotropy is now shifted to higher concentrations for the cases in Fig. 5.11(b) and (c). Consequently, for a sufficiently low feed concentration, total conversion with pure products B and C is possible in all three cases. Theoretical limitations only occur for high feed concentration. In many practical applications the theoretical limitation illustrated in Fig. 5.11b, c is beyond the solubility limit of the reactants, and has then no practical significance. 

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