Zeotropic mixture
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A zeotropic mixture (nonazeotropic mixture) is a mixture of substances that have different boiling points.[2] The dew point and bubble point curves in the graph shown do not touch each other over the entire composition range, with the exception of the pure components (the curve end points)[citation needed]. Azeotropic describes mixtures in which the dew point and bubble point curves are touching each other at at least one point – indicating the same composition in the vapour and liquid phase[citation needed].
Vapor pressures and boiling temperatures of a zeotropic mixture are always between the pure component boiling points resp. vapor pressures. Azeotropic mixtures show a pressure maximum/temperature minimum or pressure minimum/temperature maximum that is above/below the values of the pure components[citation needed].
Stating compositions of zeotropic mixtures is common for some commercially available blends but the term zeotropic is valid only for a given temperature and pressure, not for specific compositions[citation needed].
Relevance
Zeotropic mixtures can be separated by normal distillation. The separation factor never becomes exactly 1, but might be very close to 1, making a separation by distillation very difficult or even technically impossible[citation needed].
Some specific zeotropic mixtures are used as refrigerants (Code names are R-4xxx)[citation needed]. Zeotropic mixtures have the disadvantage that their composition changes during the repeated evaporation and condensation processes[citation needed]. This causes a temperature glide – a change in the boiling temperature at constant pressure[citation needed]. Additionally, this leads to changing thermodynamic properties, notably, heat of vaporization and heat capacity which are the most important in the refrigeration process[citation needed]. Azeotropic mixtures on the other hand have constant properties at their azeotropic composition and behave like pure components[citation needed]. If zeotropic blends are used, it is common to select mixtures with small separation factors (so called near-azeotropic mixtures).[3]
Occurrence
Zeotropy occurs when the boiling difference is large, or when the chemicals in a blend are similar, e. g. two ketones or two amines[citation needed]. Azeotropy occurs if the boiling point difference is smaller and the chemicals contain different functional groups (e. g. chloroform and methanol)[citation needed].
A mixture can be both zeotropic and azeotropic because the azeotropic composition changes with temperature and pressure[citation needed]. A good example is the mixture of ethanol and water, which is azeotropic above approximately 305 K (32 °C; 89 °F) and 12 kilopascals (1.7 psi)[citation needed]. Below that temperature and pressure, the mixture is zeotropic[citation needed]. The separation factor at the limiting temperature is still very small, and thus the differences between liquid and vapor composition is too small to be of use for distillation[citation needed].
External links
- Zeotropes as replacement for refrigerants by Laura Atkinson Schaefer
- Statistics about frequency of zeotropic and azeotropic mixtures in current literature (from Dortmund Data Bank)
References
- ^ a b Tamir A., Wisniak J., "Vapor-Liquid Equilibria of Isobutanol-n-Butanol and Isopropanol-sec-Butanol Systems", J.Chem.Eng.Data, 20(4), 391-392, 1975
- ^ Gaspar; Pedro Dinis; da Silva; Pedro Dinho (2015). Handbook of Research on Advances and Applications in Refrigeration Systems and Technologies. IGI Global. p. 244. ISBN 978-1-4666-8398-3.
- ^ Jones J.A., "Near Azeotropic Mixture Substitute for Dichlorodifluoromethane", US-Patent, Pat.No. PCT/US91/01905, 1-13, 1991