Azeotropic entrainer

The majority of successful processes are those in which the entrainer and one of the components separate into two Hquid phases on cooling if direct recovery by distillation is not feasible. A further restriction in the selection of an azeotropic entrainer is that the boiling point of the entrainer is 10—40°C below that of the components. 

Data of Azeotropes. The choice of azeotropic entrainer for a desired separation is much more restricted than that of solvents for extractive distillation, although many azeotropic data are known. The most extensive compilation is that of Ogorodnikov, Lesteva, and Kogan (Handbook of Azeotropic Mixtures (in Russian), 1971). It contains data of 21,069 systems, of which 1274 are ternary, 60 multicomponent, and the rest binary. Another compilation Handbook of Chemistry and Physics, 60th ed., CRC Press, Boca Raton, FL, 1979) has data of 685 binary and 119 ternary azeotropes. Shorter lists with grouping according to the major substances also are available in Lange s Handbook of Chemistry... 

Accordingly, the n-butyl acetate is the most thermally efficient of these agents. The n-propyl acetate has been used in large installations, in the first stage as solvent for extraction of acetic acid and then as azeotropic entrainer to remove the accompanying... 

In the primary extraction, however, a hydrocarbon solvent is added. This allows a relatively sharp separation between the middlings and the aqueous bottom layer. A fraction with boiling range of octane is suitable, since it then becomes the azeotropic entrainer for the water. An aromatic solvent—e.g., toluene—might be better, but it may require another separation later. 

Ordinary rectification for the dehydration of acetic acid requires many trays if the losses of acid overhead are to be restricted, so that azeotropic processes are used exclusively. Among the entrainers that have been found effective are ethylene dichloride, n-propyl acetate, and n-butyl acetate. Water contents of these azeotropes are 8, 14, and 28.7 wt %, respectively. Accordingly, the n-butyl acetate is the most thermally efficient of these agents. The n-propyl acetate has been used in large installations, in the first stage as solvent for extraction of acetic acid and then as azeotropic entrainer to remove the accompanying... 

Since the overhead ternary azeotrope is heterogeneous, it may be opportunistically decanted, producing the exact composition in the organic layer previously assumed for the azeotropic entrainer as well as an aqueous layer as determined by the liquid-liquid tie line through the ternary azeotrope (Fig. 26). By mass balance it can be shown that the total amount of azeotropic entrainer required to be mixed into the column has not yet been generated by the decant, so the recycle problem is not quite completely solved. 

No ternary azeotrope is likely in a system unless all three possible binaries are also present. If an added azeotropic entrainer has to be used it should be chosen ideally on the basis that there is so little contamination of the aqueous phase that water can be disposed of without being recycled and the loss of entrainer and wet solvent would be acceptably low. However, it should be remembered that the water phases are quite small compared to the entrainer-rich phases and in Fig. 7.4 it is recycled to the enriching column. 

A valve is required on the reflux return line to the column to hold up the light fronts or the azeotrope entrainer at the end of each batch of a series. 

While the effect maybe so powerful that the relative volatility of the two solvents can be reversed (e.g. it is possible using highly polar water as the entrainer to make ethanol more volatile than methanol) this is seldom done in industrial practice because, unlike the choice of azeotropic entrainers which must lie... 

Benzene is most useful as an azeotropic entrainer for drying ethanol, IPA and ACN, but in all these cases there are alternatives and it will be seldom that the problems of handling benzene do not cause one of them to be preferred. 

The alcohols isopropanol, ferf-butanol and n-propanol all display this phenomenon with some potential azeotropic entrainers (Fig. 16.4). The barrier of about 17% n-propanol in water represents only a... 

MCB has little to recommend it as an azeotropic entrainer for water since it is of similar flammability... 

As the VLB diagram (Fig. 16.28) shows, the water/ dioxane azeotrope is separated easily from both water and from the solvent. However, the laboratory techniques used for drying (molecular sieves, barium oxide, magnesium sulphate and potassium hydroxide) are all rather expensive without a recovery system. Chloroform is an effective azeotropic entrainer and its toxicity is not an automatic disqualification because dioxane itself needs to be handled with very great care. 

Provided that a powerful enough column is available to separate methylene chloride from methyl acetate, the former can be used as an azeotropic entrainer for water, provided that traces of methylene chloride are acceptable in the finished dried product. 

However, the recovery of pyridine from water is expensive in heat and plant time if it is done by distillation because of the very high water content (43% w/w) of the pyridine/water azeotrope (Fig. 16.35 and Table 16.23). The traditional azeotropic entrainer for drying pyridine in the coal-chemical industry, where much of it is made, is benzene, since it is easy to remove from pyridine once the pyridine is dry. Toluene is more economical as an entrainer but harder to remove from dry pyridine because of a pyridine/toluene azeotrope. The presence of inorganic salts reduces the mutual solubility of water and pyridine and about half the water in the azeotrope can be salted out with sodium sulphate. 

A characteristic of ACN is that it forms azeotropes with most organics that are not miscible with water and boil below ACN (Table 16.25). This means that it is difficult to recover the azeotropic entrainers once they have been used to remove water from ACN. This is not a problem if long-term drying of an ACN stream is involved and for this benzene, trichloroethylene and diisopropyl ether can be used. 

Entrainment, azeotropic s. Azeotropic entrainment Enzymes as reagents 26,105 Epichlorhydrin as hydrogen bromide scavenger 26, 208 Epimerization... 

Anhydrous p-to uenesulfonic acid, excess n-butyl ether, and tetrachlorethane heated 10 hrs. at 160° with azeotropic entrainment of the resulting water n-butyl p-toluenesulfonate. Y 74%. F. e. s. D. Klamann and P. Weyerstahl, B. 98, 2070 (1965). 

Azeotropic distillation is only of minor importance in the production of BTX aromatics the most important azeotropic entrainers are methyl ethyl ketone and methanol. Figure 4.13 shows the production of toluene by azeotropic distillation with methanol. 

Approaches to the selection of an extractive distillation solvent are discussed by Beig," Ewell et al. and Tassios. In general, selection criteria follow those for azeotropic entrainers ... 

Acetals, also cyclic acetals—m-Dioxanes—Azeotropic entrainment of ethanol s. 18, 297... 

Figure 8.29. Phase equilibria map and sequence for extractive distillation of a binary azeotropic mixture (1,3) with a recycle of azeotropic entrainer (azeotrope 12) spht 1,2 3 in the first column.

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