Heat of azeotropic vaporization

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. 

Azeotropic transformations in systems in which chemical reactions may take place in addition to transfers from one phase to another, or which have more than two phases, are not necessarily associated with states of uniform composition but with the more general class of indifferent states. A study of systems of uniform composition is a natural introduction to the more general question of indifferent states. 

We shall not repeat here any discussion of the properties already established for states of uniform composition. It will be recalled that a state of uniform composition corresponds to an extreme value (maximum, minimum, or inflexion with a horizontal tangent) of the equilibrium pressure at constant temperature, or of the equilibrium temperature at constant pressure (Gibbs-Konovalow theorems, chap. XVIII, 6 and 9). 

Consider an equilibrium azeotropic transformation, that is to say an equilibrium transformation as defined in 1 of chapter XIX but during which, in addition, the composition remains constant. We now proceed to show that a transformation of this kind must take place at constant temperature and pressure. 

In an equilibrium transformation, the affinities of transfer must all remain zero thus 

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This gives the heat received by the system per mole of mixture evaporated azeotropically at constant T and p. It is called the latent heat of azeotropic vaporization, which we shall denote by AJi ... 

Vinyl acetate is a colorless, flammable Hquid having an initially pleasant odor which quickly becomes sharp and irritating. Table 1 Hsts the physical properties of the monomer. Information on properties, safety, and handling of vinyl acetate has been pubUshed (5—9). The vapor pressure, heat of vaporization, vapor heat capacity, Hquid heat capacity, Hquid density, vapor viscosity, Hquid viscosity, surface tension, vapor thermal conductivity, and Hquid thermal conductivity profile over temperature ranges have also been pubHshed (10). Table 2 (11) Hsts the solubiHty information for vinyl acetate. Unlike monomers such as styrene, vinyl acetate has a significant level of solubiHty in water which contributes to unique polymerization behavior. Vinyl acetate forms azeotropic mixtures (Table 3) (12). 

Ease of recovery. It is always desirable to recover the solvent for reuse. This is often done by distillation. If this is the case, then the solvent should be thermally stable and not form azeotropes with the solute. Also, for the distillation to be straightforward, the relative volatility should be large and the latent heat of vaporization small. 

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

We may estimate the heat of vaporization for azeotropic mixtures from the Lee-Kesler correlation, with some suitable mixing rules... 

Table I shows that, as the boiling point of the hydrocarbon used as the entrainer increases so does that of the azeotrope with water and the percent of water therein. A high percentage of water in the azeotrope is desired for the heat required for the distillation, which is mainly that of the latent heat of the water plus that of the entrainer. Sufficient entrainer should be available in the azeotrope for reflux to the column although this requirement is not large. Also, the solubility or dilution effect is better with lower-boiling hydrocarbons. Thus there are several factors to be balanced in choosing the azeotrope. The effect of relative boiling points, vapor pressures, and amounts of different entrainers in their azeotropes with water has been discussed as affecting the choice of entrainers for separating water from acetic acid (5). However, that represents a much more difficult selection because there the quantity of reflux is important and also the solvent characteristics of the entrainer for the acetic acid also control the choice.

Given in the literature are vapor pressure data for acetaldehyde and its aqueous solutions (1—3) vapor—liquid equilibria data for acetaldehyde—ethylene oxide [75-21-8] (1), acetaldehyde—methanol [67-56-1] (4), sulfur dioxide [7446-09-5]— acetaldehyde—water (5), acetaldehyde—water—methanol (6) the azeotropes of acetaldehyde—butane [106-97-8] and acetaldehyde—ethyl ether (7) solubility data for acetaldehyde—water—methane [74-82-8] (8), acetaldehyde—methane (9) densities and refractive indexes of acetaldehyde for temperatures 0—20°C (2) compressibility and viscosity at high pressure (10) thermodynamic data (11—13) pressure—enthalpy diagram for acetaldehyde (14) specific gravities of acetaldehyde—paraldehyde and acetaldehyde—acetaldol mixtures at 20/20°C vs composition (7) boiling point vs composition of acetaldehyde—water at 101.3 kPa (1 atm) and integral heat of solution of acetaldehyde in water at 11°C (7). 

In Example 8.3.2 we determined the rates of mass transfer in diffusional distillation, a process described by Fullarton and Schliinder (1983) for separating liquid mixtures of azeotropic composition. Estimate the heat flux through the gas/vapor mixture under the conditions prevailing in the experiment described in Example 8.3.2. 

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. 

Dipole moment 1.83-1.90. Dielectric constant (25 ) 51.7. Latent heal of fusion (mp) 3.025 kcal/mole latent heat of vaporization (bp) 9760 kcal/mole (calc), Crit temp 38Diacidic base. K, (25°) about 9 X 10 7. Forms salts with inorganic acids. Highly polar solvent. Powerful reducing agent. Dissolves many inorganic substances. Misc with water, methyl, ethyl, propyl, isobutyl alcohols. Forms an azeotropic mixture with water, bp 40 120.3°, which contains 55 mole-% (68.5 weight-%) NjH,. LD,g in mice (mg/kg) 57 i.v. 59 orally (Witkin),... 

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