Heterogeneous azeotropic system

Converging the recycle in this system is easy. In general, this will not be true. The heterogeneous azeotropic system studied in the next section illustrates some of these difficulties. A more robust method for converging recycle will be discussed. 

Figure 8-2. Heterogeneous azeotrope system, n-butanol and water at 1 atmosphere tChii et al.. 1950). (A) Plotted as butanol mole fractons. (B) Plotted as water mole fractions.

F. Generalize. Since the solubility of organics in water is often very low, this type of heterogeneous azeotrope system requires only one distillation column. 

Cl. Derive Eq. (8zZ) for the two-column, binary, heterogeneous azeotrope system. 

The multivessel column is superior to the cyclic column, in terms of batch time (energy) consumption, for all separations studied here. A modified multivessel column without vapor bypass is proposed for the separation of zeotropic systems. However, the conventional multivessel configuration with vapor bypass is proposed for the separation of heterogeneous azeotropic systems. 

We use the w-butanol-water system in this chapter as an example of this type of binary heterogeneous azeotropic system. A straight-forward two-column distillation system can be used to easily achieve high-purity products. A simple control structure is developed that is capable of handling very large disturbances in throughput and feed composition. The control... 

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 ...

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... 

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.

Some systems form two-liquid phases for certain compositions and this can be exploited in heterogeneous azeotropic distillation. The use of liquid-liquid separation in a decanter can be extremely effective and can be used to cross distillation boundaries. 

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. 

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