States, azeotropic

According to this equation the maximum number of phases that can be in equilibrium in a binary system is = 4 (F= 0) and maximum number of degrees of freedom needed to describe the system = 3 (n=l). This means that all phase equilibria can be represented in a three-dimensional P,T,x-space. At equilibrium every phase participating in a phase equilibrium has the same P and T, but in principle a different composition x. This means that a four-phase-equilibrium (F=0) is given by four points in P, 7, x-space, a three-phase equilibrium (P=l) by three curves, a two-phase equilibrium (F=2) by two planes and a one phase state (F= 3) by a region. The critical state and the azeotropic state are represented by one curve. 

The data for furan/carbon tetrachloride at 30°C shown by Fig. 12.9c provide an example of a system that exhibits small positive deviations from Raoult s law. Ethanol/toluene is a system for which the positive deviations are sufficiently large to lead to a maximum in the Px curve, as shown for 65°C by Fig. 12.9d. Just as a mimimum on the Px curve represents an azeotrope, so does a maximum. Thus there are minimum-pressure and maximum-pressure azeotropes. In either case the vapor and liquid phases at the azeotropic state are of identical composition. 

We shall therefore limit ourselves for the rest of this chapter to a consideration of systems of uniform composition. The relationship between these and the occurrence of azeotropic transformations is so close that we shall call such systems azeotropic states. The study of the conditions relating to the rates of transfer will be deferred until the last volume of this work, when it will be shown that these conditions correspond to the minimum rate of entropy production. 

The activity coefficients in azeotropic states are therefore easily evaluated. If, for a system possessing an azeotrope, an empirical formula is used to represent the activity coefficients e.g, 5, of chap. XXI), then two of the parameters can be calculated from a knowledge of the position of the azeotropic point. ... 

We have shown above that the activity coefficients of the components of an azeotropic mixture may be calculated from a knowledge of the properties of the pure components, and the temperature and pressure of the azeotropic state. In this paragraph we shall investigate the prediction of states of uniform composition from a knowledge of the behaviour of the activity coefficients as functions of temperature and pressure. 

On the other hand when (28.40) is negative the azeotropic state corresponds to a maximum in T and a minimum in p this state is called negative azeotropy. If the azeotropy is negative, an increase in temperature decreases the mole fraction of the component with the higher heat of evaporation. 

Because y = Xf for the azeotropic state, tti2 = 1. Substitution for the two ratios by Eq. (4-307) provides an equation for calculation of tti2 from the thermodynamic functions ... 

These azeotropy expressions 2.19 state that in the space of transformed composition variables the bubble-point and dew-point surfaces are tangent at an azeotropic state (Barbosa and Doherty, 1988a), allowing the azeotropes to be found easily by visual inspection in the reactive phase diagram for the case of Uc — rirx < 3 (figure 2.4). For systems beyond this space, a graphical determination of azeotropes might not be feasible. 

Revised material in Section 5 includes an extensive tabulation of binary and ternary azeotropes comprising approximately 850 entries. Over 975 compounds have values listed for viscosity, dielectric constant, dipole moment, and surface tension. Whenever possible, data for viscosity and dielectric constant are provided at two temperatures to permit interpolation for intermediate temperatures and also to permit limited extrapolation of the data. The dipole moments are often listed for different physical states. Values for surface tension can be calculated over a range of temperatures from two constants that can be fitted into a linear equation. Also extensively revised and expanded are the properties of combustible mixtures in air. A table of triple points has been added. 

Most, if not all, of the acetonitrile that was produced commercially in the United States in 1995 was isolated as a by-product from the manufacture of acrylonitrile by propylene ammoxidation. The amount of acetonitrile produced in an acrylonitrile plant depends on the ammoxidation catalyst that is used, but the ratio of acetonitrile acrylonitrile usually is ca 2—3 100. The acetonitrile is recovered as the water azeotrope, dried, and purified by distillation (28). U.S. capacity (1994) is ca 23,000 t/yr. 

Polyethers such as monensin, lasalocid, salinomycin, and narasin are sold in many countries in crystalline or highly purified forms for incorporation into feeds or sustained-release bolus devices (see Controlled-RELEASE technology). There are also mycelial or biomass products, especially in the United States. The mycelial products are generally prepared by separation of the mycelium and then drying by azeotropic evaporation, fluid-bed driers, continuous tray driers, flash driers, and other types of commercial driers (163). In countries allowing biomass products, crystalline polyethers may be added to increase the potency of the product. 

Gas Fluxing. The methyl borate azeotrope is used as a gaseous flux for welding and brazing. The Gas Flux Co., Elyria, Ohio, manufactures the methyl borate azeotrope for their own use. The azeotrope acts as a volatile source of boric oxide and is introduced directly into the gas stream as a flux for the surfaces to be joined in the welding process. The European automobile industry is the primary user of this process, though there may be some usage for this purpose in the United States. 

The two degrees of freedom for this system may be satisfied by setting T and P, or T and t/j, or P and a-j, or Xi and i/i, and so on, at fixed values. Thus, for equilibrium at a particular T and P, this state (if possible at all) exists only at one liquid and one vapor composition. Once the two degrees of freedom are used up, no further specification is possible that would restrict the phase-rule variables. For example, one cannot m addition require that the system form an azeotrope (assuming this possible), for this requires Xi = i/i, an equation not taken into account in the derivation of the phase rule. Thus, the requirement that the system form an azeotrope imposes a special constraint and reduces the number of degrees of freedom to one. 

Consider azeotropic distillation to dehydrate ethanol with benzene. Initial steady-state conditions are as shown in Fig. 13-108. The overhead vapor is condensed and cooled to 298 K to form two hquid phases that are separated in the decanter. The organic-rich phase is returned to the top tray as reflux together with a portion of the water-rich phase and makeup benzene. The other portion of the water-rich phase is sent to a stripper to recover organic compounds. Ordinarily, vapor from that stripper is condensed and recycled to the decanter, but that coupling is ignored here. 

FIG. 13-108 Initial steady state for dynamic azeotropic distillation of ethanol-water with benzene. 

To a solution of vanillin in toluene is added nitroethane, butylamine and glacial acetic acid. The mixture is refluxed and the water of reaction is steadily azeotropically removed by distillation. After the theoretical amount of water is distilled out, distillation Is continued to remove excess reactants. The last trace of excess reactants is then removed at room temperature under a vacuum. The product is then triturated with a hydrocarbon solvent such as Skellysolve B and is thus obtained in a crystalline state. In general, however, it is preferred to dissolve the residue directly In toluene for use in the next step, without isolating the 1-(2-nitropropen-1-y I )-4-hydroxy-3-methoxy benzene. 

The submitters state that the solution need not be dried, since water is removed by azeotropic distillation as the chloroform is evaporated. However, the checkers dried the chloroform solution with anhydrous magnesium sulfate prior to evaporation. 

Volkov (1994) has given a state-of-the-art review on pervaporation. A number of industrial plants exist for dehydration of ethanol-water and (.vwpropanol-water azeotropes, dehydration of ethyl acetate, etc. There is considerable potential in removing dissolved water from benzene by pervaporation. The recovery of dis.solved organics like CH2CI2, CHCI3, CCI4, etc. from aqueous waste streams also lends itself for pervaporation and pilot plants already exist. 

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