Zeotropic mixture

A mixture of chemical substances is known as azeotrope, if the composition of liquid and vapor in the vapor-liquid equilibrium is always different. This Taukurve and boiling curve touch at any point. Mixtures whose dew and boiling curves touch at least one point, and thus the composition in the vapor and liquid is the same, hot azeotropic mixtures.

Vapor pressure and boiling point of a zoetrope mixture characterized by the fact that they always lie between the pure component vapor pressures and boiling temperatures, while at azeotropic mixtures a pressure maximum / minimum temperature or pressure minimum / maximum temperature occurs, which is outside the area bounded by the pure substance range of values.

Importance

Zeotropic mixtures can be purified by distillation or rectification, ie repeated evaporation and condensation, separate into separation columns. Your separation factors are never 1, but may be so low that a separation is technically impossible by rectification.

Technically, are more important Zeotropie and azeotropism even with refrigerant mixtures when already in the naming of short character between zoetrope ( R4xx ) and azeotropic mixtures ( R5xx ) is distinguished. Zeotropic mixtures have the disadvantage that their composition changes during evaporation and condensation. This also boiling points and other properties, especially the important in refrigeration caloric quantities such as enthalpy of vaporization and heat capacity change. Azeotropic mixtures, however, behave in the azeotropic composition as pure substances with constant properties. With zeotropic mixtures must therefore be taken to ensure that the differences remain small and thus zeotropic mixtures with small separation factors are preferred.

Occurrence

Zeotropie occurs when the involved components of a mixture are chemically similar or the boiling point difference is very large. Are chemically similar to, for example, mixtures consisting only of alkanes or monofunctional ketones. In contrast, occurs at smaller azeotropism on boiling point differences and for substances with different functional groups, such as the mixture of methanol and chloroform.

A mixture can be both azeotrope and azeotropic because the azeotropic composition is temperature and pressure dependent, sometimes more and sometimes less. For example, the mixture of ethanol and water behaves below about T = 305 K, and P = 12 kPa zeotropic as occurs at higher temperatures and pressures azeotropy. At atmospheric pressure, the composition is about 90 mole % ethanol and 10 mole - % of water. In the limit, however, the differences between the compositions of the vapor and the liquid phase are the same as the relative volatility still very small, so that a technical use of the effect near the azeotropic / azeotrope - transition is hardly possible, but only at significantly lower temperatures and pressures.

Description

Zeotropie the qualitative statement of a mixture at a given pressure and temperature so that the liquid and vapor is always have different compositions. An indication to a particular composition is therefore not useful.

  • Thermodynamics
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