Heterogeneous azeotropes

Fig. 15. Isobaric vapor—liquid—liquid (VLLE) phase diagrams for the ethanol—water—benzene system at 101.3 kPa (D-D) representHquid—Hquid tie-lines (A—A), the vapor line I, homogeneous azeotropes , heterogeneous azeotropes Horsley s azeotropes, (a) Calculated, where A is the end poiat of the vapor line and the numbers correspond to boiling temperatures ia °C of 1, 70.50 2, 68.55 3, 67.46 4, 66.88 5, 66.59 6, 66.46 7, 66.47, and 8, the critical poiat, 66.48. (b) Experimental, where A is the critical poiat at 64.90°C and the numbers correspond to boiling temperatures ia °C of 1, 67 2, 65.5 3, 65.0 ...

The separation of mixtures that form azeotropes often requires the addition of a third component called a solvent or an entrainer. These processes, described in Chapters 2 and 10, may result in the formation of multi-component azeotropes or heterogeneous azeotropes. Multi-component azeotropes, usually ternary, are mixtures whose equilibrium vapor and liquid phases have the same composition. An activity coefficient equation that can predict binary azeotropes could, in its generalized multi-component form, predict ternary azeotropes. Heterogeneous azeotropes are those forming two equilibrium liquid phases and are discussed in the next section. 

The first successful appHcation of heterogeneous azeotropic distillation was in 1902 (87) and involved using benzene to produce absolute alcohol from a binary mixture of ethanol and water. This batch process was patented in 1903 (88) and later converted to a continuous process (89). Good reviews of the early development and widespread appHcation of continuous azeotropic distillation in the prewar chemical industry are available (90). 

Historically azeotropic distillation processes were developed on an individual basis using experimentation to guide the design. The use of residue curve maps as a vehicle to explain the behavior of entire sequences of heterogeneous azeotropic distillation columns as weU as the individual columns that make up the sequence provides a unifying framework for design. This process can be appHed rapidly, and produces an exceUent starting point for detailed simulations and experiments. 

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)...

Fig. 18. Separation of ethanol from an ethanol—water—benzene mixture using benzene as the entrainer. (a) Schematic representation of the azeo-column (b) material balance lines where I denotes the homogeneous and the heterogeneous azeotropes D, the end points of the Hquid tie-line and A, the overhead vapor leaving the top of the column. The distillate regions, I, II, and III, and the boundaries are marked. Other terms are defined in text.

An example of heterogeneous-azeotrope formation is shown in Fig. 13-13 for the water-normal butanol system at 101.3 kPa. At liquid compositions between 0 and 3 mole percent butanol and between 40 and 100 mole percent butanol, the liquid phase is homogeneous. Phase sphtting into two separate liquid phases (one with 3 mole percent butanol and the other with 40 mole percent butanol) occurs for any overall hquid composition between 3 and 40 mole percent butanol. A miuimum-boihug heterogeneous azeotrope occurs at 92°C (198°F) when the vapor composition and the over l composition of the two liquid phases are 75 mole percent butanol. 

FIG. 13-13 Vap or-liqiiid equilibrium data for an n-biitanol-water system at 101.3 kPa (1 atm) phase splitting and heterogeneous-azeotrope formation. 

Three types of binary equilibrium cui ves are shown in Fig. 13-27. The y-x diagram is almost always plotted for the component that is the more volatile (denoted by the subscript 1) in the region where distillation is to take place. Cui ve A shows the most usual case, in which component 1 remains more volatile over the entire composition range. Cui ve B is typical of many systems (ethanol-water, for example) in which the component that is more volatile at lowvalues of X becomes less volatile than the other component at high values of X. The vapor and liquid compositions are identical for the homogeneous azeotrope where cui ve B crosses the 45° diagonal. A heterogeneous azeotrope is formed with two liquid phases by cui ve C,... 

FIG. 13-27 Typical binary eqiiilihriiim curves. Curve A, system with normal volatility. Curve B, system with homogeneous azeotrope (one liquid phase). Curve C, system with heterogeneous azeotrope (two liquid phases in eqiiilih-riiim with one vapor phase). 

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... 

FIG. 13-78 Flowsheet for exters which form a heterogeneous minimiim-hoihng azeotrope with water. 

Figure 8-7. System with heterogeneous azeotrope-two liquid phases in the equilibrium with one vapor phase. Used by permission. Smith, B.D., Design of Equilibrium Stage Processes, McGraw-Hiii, New York (1963), all rights reserved.

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