Azeotropic mixtures heterogenous

Fig. 13. Schematic isobatic phase diagrams for binary azeotropic mixtures (az). (a) Homogeneous azeotrope (b) heterogeneous azeotrope.

Fig. 17. Column sequence for separating a binary heterogeneous azeotropic mixture, and B, where represents the process feed mole fraction, (a)...

The simplest case of combining T E and LLE is the separation of a binaiy heterogeneous azeotropic mixture. One example is the dehydration of 1-butanol, a self-entraining system, in which butanol (117.7°C) and water form a minimum-boiling heterogeneous azeotrope (93.0°C). As shown in Fig. 13-69, the fresh feed may be added... 

If the system forms azeotropes, then the azeotropic mixtures can be separated by exploiting the change in azeotropic composition with pressure, or the introduction of an entrainer or membrane to change the relative volatility in a favorable way. If an entrainer is used, then efficient recycle of the entrainer material is necessary for an acceptable design. In some cases, the formation of two liquid phases can be exploited in heterogeneous azeotropic distillation. 

At atmospheric pressure, the n-butanol-water system exhibits a minimum boiling azeotrope and partial miscibility, and hence a binary heterogeneous azeotrope. Figure 1.8 shows the Tyx and Pyx phase diagrams for l-propanol(l)-water(2) azeotropic mixture obtained from the Aspen Plus simulator using the NRTL activity coefficient model. 

Example 1.17 Heterogeneous azeotrope The heterogeneous mixture -butanol( 1 )-water(2) exhibits a minimum boiling azeotrope. The temperature composition data at 1 atm is shown in Table 1.11. 

The condensate that collects on the cold surface is usually a completely homogeneous, or miscible, mixture of components. In general, the relative composition of the liquid components in the condensate is different from the composition in the vapor phase (except for an azeotropic mixture, where the condensate has the same exact molar concentration ratio as the vapor phase) [194]. The film that forms is not necessarily smooth but may show the appearance of streamers (or rivulets), waves, or droplets, depending on the particular mixture and its surface tension (which depends on the local wall temperature) [25,195,196]. If the condensate mixture is heterogeneous, or immiscible (as can occur when one component, for example, is aqueous and the other is organic), the pattern can be quite complex, looking somewhat like dropwise condensation [25,193,197]. These different condensate patterns affect the resulting fluid flow and heat transfer. 

It may be observed that the ternary azeotrope m falls Inside the heterogeneous region. Thus, an overhead vapour of this composition splits by decantation in two phases, one rich in entrainer o other in water w the ethanol being distributed in both. Moreover, o, and w, are in different distillation regions. By clever mixing with other streams, these streams can produce feasible feeds for ethanol and water recovery columns, by overcoming the constraints of the distillation boundaries. Hence, liquid-liquid decantation creates opportunities for the separation of an azeotropic mixture. 

FIG. 13-57 (Continued) Schematic isobaric-phase azeotropic mixtures, (c) Heterogeneous azeotrope. 

When a multicomponent fluid mixture is nonideal, its separation by a sequence of ordinaiy distillation columns will not be technically and/or economically feasible if relative volatiK-ties between key components drop below 1.05 and, particularly, if azeotropes are formed. For such mixtures, separation is most commonly achieved by sequences comprised of ordinary distillation columns, enhanced distillation columns, and/or liquid-liquid extraction equipment. Membrane and adsorption separations can also be incorporated into separation sequences, but their use is much less common. Enhanced distillation operations include extractive distillation, homogeneous azeotropic distillation, heterogeneous azeotropic distillation, pressure-swing distillation, and reactive distillation. These operations are considered in detail in Perry s Chemical Engineers Handbook (Perry and Green, 1997) and by Seader... 

On the base of various heterogeneous azeotropic mixtures, as acetone/toluene/water or 1-propanol/l-butanol/water, the distillation behaviour and its modelling in a packed column is discussed. A detailed description of the experimental equipment and the results is presented in (Repke and Wozny, 2002) and in (Repke, 2002), but a brief overview is listed in (Table 1). 

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