Azeotropic distillation

Azeotropic distillation is a set of chemical techniques used to break an azeotrope. An azeotrope is a specific mixture of liquids that cannot be separated by standard distillation. In a normal mixture, different components boil at different temperatures. However, an azeotrope boils as if it were a single substance. This happens because the components reach a point where their concentrations do not change during boiling. Breaking these mixtures is vital in chemical engineering for creating high-purity substances.

One common method involves adding a material separation agent to the mixture. In chemical engineering, if this agent forms azeotropes with more than one component, it is called an entrainer.

When added to the liquid phase, the entrainer changes the molecular interactions within the mixture. It alters the activity coefficient of the various compounds. This change affects the relative volatility, which is how easily one component evaporates compared to another. By increasing deviations from Raoult's law, the entrainer makes it possible to separate the original components.

The process often creates a new, lower-boiling azeotrope that is heterogeneous. A heterogeneous azeotrope is one that produces two immiscible liquid phases after it condenses. Immiscible means the liquids do not stay mixed and instead form separate layers. This allows the components to be separated through a process called decantation. After separation, the entrainer is typically recovered through distillation or decantation. It is then returned near the top of the original distillation column to be used again.

Historically, azeotropic distillation has been used to dehydrate ethanol and water mixtures. To remove the water, a near-azeotropic mixture is sent to a final column. Several different substances can act as entrainers for this specific process. Options include pentane, cyclohexane, hexane, heptane, isooctane, acetone, and diethyl ether.

Benzene and cyclohexane have been used most extensively in the past. However, because benzene was identified as a carcinogen, industry shifted to using toluene instead. This change ensures the dehydration process remains effective while improving safety.

Another method for breaking an azeotrope is pressure-swing distillation. This technique works because an azeotrope is pressure dependent. An azeotrope is not a fixed range of concentrations that cannot be distilled. Instead, it is the specific point where the activity coefficients of the distillates cross one another. By changing the pressure, engineers can "jump over" this point. For example, in an ethanol and water mixture, changing the pressure can cause water to boil out of the ethanol. This method is very robust, but it has higher energy demands. It also requires more investment because the distillation columns must handle high internal pressure.

For mixtures with low boiling azeotropes, distillation might not achieve full separation. In these cases, scientists use molecular sieves. These are materials that can trap specific molecules. A common example is using 3A zeolite, which is a type of 3A molecular sieve. These sieves can dry ethanol to an extremely high purity of 99.999% ABV. After they have captured the water, the sieves can be regenerated using a vacuum oven.

Azeotropic distillation is also important in organic chemistry for managing chemical reactions. Some dehydration reactions are subject to fast but unfavorable equilibria. One example is the formation of dioxolanes from aldehydes. This reaction produces water as a byproduct. By using azeotropic distillation to remove that water, the reaction can proceed. This ability to manipulate chemical equilibrium makes azeotropic distillation a fundamental tool in many scientific fields.