Boiling points acetonitrile azeotropes

Table 11.2 presents fundamental physical properties for the key components implied in separations. The difference in the boiling points favors the separation, except of acrylonitrile and acetonitrile. The differences in the freezing point are also sensitive, but they did not justify the investment in a separation by crystallization. It remains that distillation-based separation methods should be tried in the first place. However, the formation of azeotropes of components with water will present difficulties. 

The selection of acetonitrile versus methyl alcohol has several considerations even though their flammabilities and toxicities are relatively close. The advantages of using acetonitrile over methanol are (1) its lower UV absorbance cut-off. (2) its lower viscosity, (3) and its smaller viscosity dependence on temperature. The advantages of using methanol over acetonitrile are (1) its lower cost and lower cost fluctuation in the market place, and (2) its ease to recycle with water as a co-solvent (e.g. in reversed-phase chromatography) since methanol-water does not have an azeotrope as does acetonitrile-water. At a boiling point of 76.5°C, the azeotrope composition of acetonitrile-water is 83.7 16.3 [78]. 

One of the first processes employed to separate butadiene from a C4 hydrocarbon stream was an azeotropic distillation which used liquid ammonia as the solvent. A description of this process has been presented by Poffenberger et al.,24 who also gave a typical analysis of the C4 stream together with the boiling points of hydrocarbons and their azeotropes. Other solvents such as furfural and acetonitrile are presently employed to effect this separation.12... 

Purification. For polymerization, butadiene that is at least 99 mol% pure is required. Although alkynes are the most troublesome impurities, separation of the butadiene from other C4 products is also necessary. Simple fractional distillation is effective for removing the light (C3) and heavy (C5) ends from butadiene, but not for removing the various C4 species because of the closeness of the boiling points to each other and to butadiene. Further complicating purification, butadiene forms azeotropes with re-butane and 2-butene. The most widely used recovery systems are extraction with aqueous cuprous ammonium acetate (CAA) and solvent extractions with furfural, acetonitrile, dimethylformamide, dimethylacetamide, or AT-methylpyrrolidinone (65,66). 

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