Zeotrope
Encyclopedia
A mixture
of chemicals is said to be zeotropic if the composition of the vapor and the liquid phase at the vapor-liquid equilibrium
state are never the same. Dew point and boiling point curve do not touch each other over the entire composition range with the exception of the pure components (curve end points). Mixtures where the both curves are touching each other in at least a single point - indicating the same composition in the vapor and liquid phase - are azeotropic
mixtures.
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 are above/below the values of the pure components.
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 but not for specific compositions.
. The separation factor never become exactly 1 but might be very close to 1 making a separation by distillation very difficult or even technically impossible.
Some specific zeotropic mixtures are used as refrigerant
s (Code names are R-4xxx). Zeotropic mixtures have the disadvantage that their composition is changing during the repeated evaporation and condensation processes. This causes a temperature glide – a change in the boiling temperature at constant pressure. Additionally, this leads to changing thermodynamic properties, notably, heat of vaporization and heat capacity
which are the most important in the refrigeration
process. Azeotropic mixtures on the other hand have constant properties at their azeotropic composition and behave like pure components. If zeotropic blends are used it is common to select mixtures with small separation factors (so called near-azeotropic mixtures).
and Methanol
).
A mixture can be zeotropic and azeotropic because the azeotropic composition changes with temperature and pressure. A good example is the mixture of Ethanol and Water which is azeotropic above approximately T=305 K and P=12 kPa. Below that temperature and pressure the mixture is zeotropic.
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.
Mixture
In chemistry, a mixture is a material system made up by two or more different substances which are mixed together but are not combined chemically...
of chemicals is said to be zeotropic if the composition of the vapor and the liquid phase at the vapor-liquid equilibrium
Vapor-liquid equilibrium
Vapor–liquid equilibrium is a condition where a liquid and its vapor are in equilibrium with each other, a condition or state where the rate of evaporation equals the rate of condensation on a molecular level such that there is no net vapor-liquid interconversion...
state are never the same. Dew point and boiling point curve do not touch each other over the entire composition range with the exception of the pure components (curve end points). Mixtures where the both curves are touching each other in at least a single point - indicating the same composition in the vapor and liquid phase - are azeotropic
Azeotrope
An azeotrope is a mixture of two or more liquids in such a ratio that its composition cannot be changed by simple distillation. This occurs because, when an azeotrope is boiled, the resulting vapor has the same ratio of constituents as the original mixture....
mixtures.
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 are above/below the values of the pure components.
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 but not for specific compositions.
Relevance
Zeotropic mixtures can be separated by normal distillationDistillation
Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
. The separation factor never become exactly 1 but might be very close to 1 making a separation by distillation very difficult or even technically impossible.
Some specific zeotropic mixtures are used as refrigerant
Refrigerant
A refrigerant is a substance used in a heat cycle usually including, for enhanced efficiency, a reversible phase change from a liquid to a gas. Traditionally, fluorocarbons, especially chlorofluorocarbons, were used as refrigerants, but they are being phased out because of their ozone depletion...
s (Code names are R-4xxx). Zeotropic mixtures have the disadvantage that their composition is changing during the repeated evaporation and condensation processes. This causes a temperature glide – a change in the boiling temperature at constant pressure. Additionally, this leads to changing thermodynamic properties, notably, heat of vaporization and heat capacity
Heat capacity
Heat capacity , or thermal capacity, is the measurable physical quantity that characterizes the amount of heat required to change a substance's temperature by a given amount...
which are the most important in the refrigeration
Refrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...
process. Azeotropic mixtures on the other hand have constant properties at their azeotropic composition and behave like pure components. If zeotropic blends are used it is common to select mixtures with small separation factors (so called near-azeotropic mixtures).
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. Azeotropy occurs if the boiling point difference is smaller and the chemicals contain different functional groups (e. g. ChloroformChloroform
Chloroform is an organic compound with formula CHCl3. It is one of the four chloromethanes. The colorless, sweet-smelling, dense liquid is a trihalomethane, and is considered somewhat hazardous...
and Methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
).
A mixture can be zeotropic and azeotropic because the azeotropic composition changes with temperature and pressure. A good example is the mixture of Ethanol and Water which is azeotropic above approximately T=305 K and P=12 kPa. Below that temperature and pressure the mixture is zeotropic.
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.
Web links
- Zeotropes as replacement for refrigerants by Laura Atkinson Schaefer
- Statistics about frequency of zeotropic and azeotropic mixtures in current literature (from Dortmund Data BankDortmund Data BankThe Dortmund Data Bank is a factual data bank for thermodynamic and thermophysical data. Its main usage is the data supply for process simulation where experimental data are the basis for the design, analysis, synthesis, and optimization of chemical processes...
)