Breaking an azeotrope

To extract TMB from TMB-methanol mixtures it is necessary to find a solvent that is relatively immiscible in methanol yet is miscible with TMB at the same conditions. TMB is very soluble in benzene, hexane, heptane, nonane, and carbon tetrachloride indicating that it exhibits very lipophilic characteristics (Plank and Christopher, 1976 Niswonger, Plank, and Laukhuaf, 1985 Schmidt, Plank, and Laukhuf, 1985 Munster et al., 1984). Hence, TMB should be soluble in the more common supercritical fluid solvents such as ethane and carbon dioxide. Methanol is moderately miscible with xenon, ethane, ethylene, and carbon dioxide since a single phase is obtained at pressures of less than —200 bar at temperatures between the respective critical temperatures of the binary components (Brunner, 1985). To obtain quickly an estimate of the distribution coefficient for TMB in carbon dioxide, ethane, and ethylene, rapid screening experiments were performed with a dynamic flow apparatus at temperatures ranging from 0 to 55°C at a number of pressures. From this preliminary study it was found that carbon dioxide does not 

and Franck, 1981) and the TMB-methanol systems at 35°C (Unterreiner, McHugh, and Krukonis, 1991). 

Since the binodal curves intersect the TMB-methane axis, it is possible to obtain pure TMB using a countercurrent extraction process with reflux. If the system pressure were increased above the TMB-methane critical pressure at 35°C, a closed-dome, two-phase region would exist and it would not be possible to obtain pure TMB with a countercurrent process. 

Radically different binary phase behavior is found for the methane-TMB and the methane-methanol systems. This suggests that TMB can be extracted from methanol. To verify this conjecture experimental information was obtained on the TMB-methanol-methane system to ascertain whether the weak TMB-methanol complex can be broken by nonpolar methane. Interestingly, carbon dioxide, ethane, and ethylene, all much better supercritical solvents than methane, dissolve both methanol and TMB to such a large extent that they are not selective for either component. But with methane, the interactions between methane and TMB are strong enough to maintain a constant concentration of TMB in the extract phase as TMB is removed from the methanol-rich liquid phase. This means that the distribution coefficient for TMB increases as the concentration in the liquid phase decreases. We know of no other system that exhibits this type of distribution coefficient behavior. 

The examples in this chapter highlight many facets of the development of SCF processes and applications that need to be addressed, understood, and solved before scaleup is considered. The same procedure is necessary for any kind of separation process. Supercritical fluid extraction has the potential to reduce energy costs but that does not necessarily mean it will be less expensive overall. Frequently, capital and other operating costs can more than offset decreased energy costs. Subsequent chapters discuss developments in the application of supercritical fluid solvents to the solution of technically and economically difficult separation problems in which the improved performance of the processed materials can support the processing cost. 

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Operational Factors In industrial use, peiwaporation is a continuous-flow single-stage process. Multistage cascade devices are unusual. Peiwaporation is usually an adjunct separation, occasionally a principal one. It is used either to break an azeotrope or to concentrate a minor component. Large stand-alone uses may develop in areas... 

The combination with molecular-sieve adsorption or membrane permeation allows breaking an azeotrope without the need of a contaminating solvent. This is advantageous when separating ultrapure components, such as medical grade ethanol from its azeotrope with water. More details can be found elsewhere [5, 10, 14]. 

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 three methods used to break an azeotrope by means of distillation are azeotropic distillation, extractive distillation, and pressure swing distillation. 

Pressure Swing Distillation The third method to break an azeotrope is pressure swing distillation, which relies on the fact that the location of the azeotropic point depends in some cases relatively strongly on the total pressure. In contrast to extractive or azeotropic distillation, the azeotrope is not broken by a solvent, but is jumped over by altering the pressure, and thus the distillation can continue. For example, the system tetrahydrofuran (TH F) and water is at first distilled at 1 bar up to approximately 80% THF (Figure 3.3.22a), that is, just before the azeotropic point is reached. The distillate rich in THF with a composition close to the azeotropic... 

To break an azeotrope, azeotropic, extractive, and pressure swing distillation can be used. 

Azeotropic mixtures require special methods for their separation, which usually consist of adding a third component that has the ability to "break" the azeotrope. Perhaps tire most famous case is that of ethanol-water, which has an azeotropic mole fraction in ethanol of 0.8943 at atmospheric pressure (Table 6.8). Here the added component can be benzene and results, on distillation, in the recovery of pure ethanol and a ternary azeotrope containing benzene. That mixture, on condensation, results in two immiscible aqueous and organic layers, which are separated and further processed by distillation. Changing the operating pressure Pj is yet another potentially useful way of breaking an azeotrope. This is taken up in Illustration 6.14. 

Illustration 6.13 The Effect of Total Pressure on a. Part 2 Breaking an Azeotrope... 

Although we achieved our goal of raising a above 1, the low value of a would probably make the process unattractive for large-scale applications. We have, however, confirmed the feasibility of breaking an azeotrope with a modest change in operahng pressme. 

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