Absolute pressure azeotrope composition

Adsorption is an effective technique to break an azeotrope. The separation makes use of molecular sieve adsorption, and can take place either in vapour or in liquid phase. Unlikely a process using a MSA that could contaminate the final product, here absolute purity product is obtained. Figure 9.33 depicts a process for pharmaceutical-grade ethanol (Stichlmair and Fair, 1999). After pre-concentration in the column C-1 operating at a pressure slightly above normal, the vapour distillate with the azeotropic composition is fed to the adsorption device. Here the water is retained, producing a vapour that consists of pure ethanol. This may be used to heat the second distillation... 

The result of a diminution in pressure is frequently that the azeotropic composition becomes richer in the low-boiling constituent. A reduced pressure may thus be reached at which the special point vanishes. As examples, the mixtures ethanol-water and water-phenol may be taken (see Fig. 226). By distilling dilute alcohol at 70 mm, absolute alcohol may be prepared without the use of anentrainer. The azeotropic point water-phenol is eliminated at 32 ram. The shift in the azeotropic point has been determined by Sheinker and Peresleni [45] for two other systems ... 

Here is the azeotropic pressure at absolute temperature T, while A and B are parameters whose values are obtained by fits to azeotropic data. This correlation works well for both positive and negative azeotropes, as shown in Figure 9.12. The principal drawback to (9.3.22) is that the azeotropic compositions remain implicit to find those compositions, we must solve the phase-equilibrium problem. 

The composition of an azeotrope varies with absolute pressure. In water/solvent mixtures, where this effect is industrially important, the water content of the azeotrope increases with increasing pressure. Thus, if two columns at different pressures are run in series (Fig. 7.6), a dry solvent can be made without the need for an entrainer. This can also be done on a batch still but for both continuous and batch operation the equipment is specialized and the hazard of handling flammable solvents at high pressure must be borne in mind. 

As stated in Chapter 7, the composition of an azeotrope varies with the distillation pressure. Thus, the azeotrope between ethanol and water contains 89.4 mol% ethanol at atmospheric pressure, 90.2 mol% ethanol at 380 mm Hg, and 92.0 mol% ethanol at 190 mm Hg absolute pressure [6]. Similarly, the azeotrope between methyl ethyl ketone and water contains 65.4 mol% MEKat atmospheric pressure, 70.0 mol% MEKat 350 mm Hg, and 72.2 mol% MEKat 200 mm Hg absolute pressure. Because MEK and water form two immiscible liquid phases upon condensation of the overhead, these two compounds can be separated by distilling each of these two phases separately. Because the azeotrope of ethanol and water is a single-phase liquid, separation of these two components can be achieved by distillation in two columns operating at different pressures in order to shift the azeotropic composition. 

Vacuum fractionation may be useful also in the separation of components of azeotropic mixtures, for the composition of the azeotropic mixture will change with pressure, and a better separation may be possible at reduced pressure. Thus, absolute alcohol may be obtained from 96% alcohol by first fractionating at pressures below atmospheric, followed by fractionating at atmospheric pressure, because the azeotropic mixtures at lower pressures are richer in alcohol. 

Lastly, we mention that ternary fluid mixtures can exhibit homogeneous azeotropes. These are vapor-liquid equilibrium states at which both phases have the same composition and the pressure is an absolute maximum or minimum with respect to composition at fixed temperature [6]. 

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