Azeotrope pseudo

Residue curve maps exist for mixtures having more than three components but cannot be visualized when there are more than four components. However, many mixtures of industrial importance contain only three or four key components and can thus be treated as pseudo-temary or quaternary mixtures. Quaternary residue curve maps are more compHcated than thek ternary counterparts but it is stiU possible to understand these maps using the boiling point temperatures of the pure components and azeotropes (31). 

Drawing pseudo-binaryjy—x phase diagrams for the mixture to be separated is the easiest way to identify the distillate product component. A pseudo-binary phase diagram is one in which the VLE data for the azeotropic constituents (components 1 and 2) are plotted on a solvent-free basis. When no solvent is present, the pseudo-binaryjy—x diagram is the tme binaryjy—x diagram (Eig. 8a). At the azeotrope, where the VLE curve crosses the 45° line,... 

The mass action model (MAM) for binary ionic or nonionic surfactants and the pseudo-phase separation model (PSM) which were developed earlier (I EC Fundamentals 1983, 22, 230 J. Phys. Chem. 1984, 88, 1642) have been extended. The new models include a micelle aggregation number and counterion binding parameter which depend on the mixed micelle composition. Thus, the models can describe mixtures of ionic/nonionic surfactants more realistically. These models generally predict no azeotropic micellization. For the PSM, calculated mixed erne s and especially monomer concentrations can differ significantly from those of the previous models. The results are used to estimate the Redlich-Kister parameters of monomer mixing in the mixed micelles from data on mixed erne s of Lange and Beck (1973), Funasaki and Hada (1979), and others. 

The required information about the distillation boundary is obtained from the pinch distillation boundary (PDB) feasibility test [8]. The information is stored in the reachability matrix, as introduced by Rooks et al. [9], which represents the topology of the residue curve map of the mixture. A feasible set of linear independent products has to be selected, where products can be pure components, azeotropes or a chosen product composition. This set is feasible if all products are part of the same distillation region. The singular points of a distillation region usually provide a good set of possible product compositions. The azeotropes are treated as pseudo-components. 

A feed stream at the rate of 100 kmol/h contains 50% mole acetone and 50% mole chloroform. The two components form a maximum boiling azeotrope which prevents their separation by conventional distillation. It is proposed to separate them by extractive distillation using benzene as a solvent, at a rate of 800 kmol/h. Both the main feed and the solvent are at 75 C and 110 kPa, and the column pressure is assumed uniform, also at 110 kPa. A total condenser is used, with a reflux ratio of 4. The distillate composition is specified at 95% mole acetone and the bottoms at 5% mole acetone on a solvent-free basis. Using the pseudo-binary... 

Figure 12.18 shows how a complex system differs from a multicomponent system. The complex system exhibits a smooth TBP curve, because even the high separating power of the TBP apparams cannot resolve all the many close-boiling constiments. The multicomponent system, on the other hand, is easily resolved by the powerful TBP apparams, although any azeotropes will be distilled over as though they were pure (pseudo) components. 

In a pseudo azeotropic blend the relative evaporation rates of both components are the same, or ji.Ri = y2-R2- When R and R2 are known, it can be determined with the help of the graphs at which composition yi and 72 fulfil the above requirement. Such calculations for mixtures of the aromatic hydrocarbon solvents toluene, xylene and SHELLSOL A with n-propanol and n-butanol give the result in Table 2.25. 

Product Vol% alcohol in pseudo-azeotropic mixture with hydrocarbon ... 

From Figure 2.16 it is clear that at a hexane/MEK 61.5/38.5 ratio it is predicted that the blend becomes rich in MEK during evaporation. The 75/25 blend is so close to the pseudo-azeotrope that in the beginning hardly any change in composition is expected. When approaching the end of the evaporation a gradual enrichment of hexane has been calculated. 

Pilavakis (20, 29) investigated the esterification of methanol by acetic acid in a packed column. He assumed the reaction to be pseudo-first-order with respect to either methanol or acid over certain specified concentration ranges and incorporated the effect of heat of reaction not only in the enthalpy balances but also in the flux equations. The column was calculated by numerical solution of a set of differential equations. The top product was an azeotropic mixture of methanol and ester which could, however, be broken by introduction of acetic acid high up in the column rather than further down as a mixed feed with methanol. Consequently, in practice such a column will consist of a rectifying section, an extractive distillation section with acetic acid as the extractive solvent and a distillation reactor section. Good agreement was obtained between theory and experiment which, however, suffered from the fact that the hold-up of liquid in the column was small in comparison to the reboiler hold-up so that most of the reaction occurred in the latter location. 

Figure 5.6. Quaternary and pseudo-azeotropes in synthesis of MTBE at an operating pressure of ff TO Pa. Remark transformed compositions (equation 5.8) are used.

Figure 5.8. Residue curve map and separation sequence for zone b in the synthesis of MTBE by reactive distillation. Remark high purity MTBE (pseudo azeotrope) is recovered at the bottom of the column in an indirect separation operating pressure is set at 11-10 ...

In other cases the y,x curve may not cross the diagonal in the two-phase region, and such mixtures do not form pseudo-azeotropes, but they may form, and usually do, true azeotropes in one of their singlephase regions. 

If these data are replotted as mol fraction of ether in the vapor vs. mol fraction of ether in the liquid, assuming that the vapors obey the perfect-gas law, one obtains the results given in Fig. 4-11. The value of the mol fraction of ether in the vapor increases very rapidly with the mol fraction in the liquid and becomes constant at 0.916 in the two-phase-liquid region. The vapor-liquid curve crosses the 45° line at a composition of 0.915 mol per cent ether. The mixture of this composition is a pseudo-azeotrope. For mol fractions of ether greater than... 

---