Explain the fractional distillation technique

The word distillation is derived from Latin distillery ("Trickle down"). The process was already known to the ancient Egyptians. The simplest distillation apparatus consisted of a vessel with a lid on which the condensed liquid precipitated when heated. To prevent the liquid from dripping back into the vessel, woolen pods were used to absorb the liquid. In this way, the craftsmen won essential oils early on with the help of a comparatively primitive apparatus.

Apparatus for distilling spirits

The ancient Greeks already knew the distilling helmet (Greek. ambix, lat. alembicus, arab. al-Anbiq). The alchemical term was derived from these names Alembicus from. In this device, the steam came through a thin pipe into a spherical extension, at the bottom of which there was a drain. The steam was cooled by the relatively large surface area of ​​the sphere and the adjacent air.

Alembicus with helmet and drain

The distillation technique was developed further in the first millennium after Christ, especially by the Arabs. They introduced gentle heating in a water bath and also steam distillation for the production of essential oils and scented oils from herbs. In the 10th and 11th centuries, large ovens were used, on which many small distillation devices were placed and could be operated at the same time. Such constructions were called Galley stoves.

Galley stove in the German Museum in Munich

The alchemists of the Middle Ages improved the cooling performance of the devices and developed them Serpente, an apparatus with a very long steam pipe. In the retort the alembic was fused into one piece with the flask and connected to a long cooling tube.

Alchemical distillation apparatus,
partly with long steam pipe (Serpente)

Retort with oven and sand bath
from the Deutsches Museum in Munich

In the case of the improved apparatus with water cooling, the cooling pipe was lengthened and passed through a water barrel or there was a large water bowl around the still. This apparatus was called Mohrenkopfbecause the cooling bowl looked like a turban.

With this type of water cooling, it worked
extended cooling pipe through a water barrel
"Mohrenkopf" with water cooling
around the still helmet

With the discovery of alcohol in the 12th century, a flammable liquid was obtained that was widely used in medicine. To produce high-proof alcohol, wine had to be distilled several times. Later herbs were also added to alcohol distillation and herbal extracts were obtained. The woodcut shows a company in the 16th century for the production of alcohol. On the left, a woman is chopping up herbs, which are then distilled with alcohol. The woman on the right bottles the extracts:

Production of alcohol and herbal essences in the 16th century

Principle of distillation

In the distillation liquid mixtures are separated by heating. The liquid with the lower boiling point evaporates first when heated up and can be collected again by a cooling system after the evaporated gas has condensed. This should be shown using the example of the distillation of wine, the "schnapps distilling" (> student exercise):
Boiling temperature of ethyl alcohol: 78 ° C
Boiling temperature of water: 100 ° C
In our example, the alcohol would theoretically evaporate at 78 ° C, while the water initially remains. In practice, however, some of the water evaporates at 78 ° C, which is why you never get pure alcohol. In addition, it must be taken into account that at the beginning of the distillation, the so-called forerun, methanol also distills over. Methanol has a boiling point of 64.5 ° C and is occasionally found in poorly fermented alcohol. Since methanol is poisonous, the flow must be discarded.
At the boiling point, a liquid changes to the gaseous state, the liquid and the gaseous phase are then in a thermodynamic state of equilibrium. Since the boiling point depends on the external pressure, the values ​​in the table apply to the boiling point of a substance at normal pressure of 1013 millibars. When the boiling point is reached under normal conditions, the vapor pressure of a liquid corresponds to the external pressure of 1013 millibars.

When a liquid is heated, the heat supplied is first used to heat the liquid. The closer the temperature of the liquid approaches the boiling point, the more the heat is used to evaporate the liquid. When the boiling point is reached, the temperature does not rise for a long time, it remains constant. The heat supplied then only serves to evaporate the liquid. When the temperature remains constant you can see that the boiling point has been reached.
Since the boiling point of a substance decreases with decreasing pressure (and therefore also decreases with increasing altitude, see illustration), distillations are also carried out under vacuum. The decrease in pressure lowers the boiling temperature, so that less energy has to be used for heating.

A simple air-cooled distillation apparatus can be built from a still, a heater (burner, hotplate) and an air-cooled pipe. To improve the cooling performance, the Liebig cooler is usually used, a simple jacketed pipe cooled with water. The water inflow always takes place at the lowest point of the cooler (for other cooler designs see> Cooling). The thermometer measures the temperature of the vaporized substance, and the heated vapor condenses back into a liquid in the cooling tube. The distillate obtained is caught in the template. The following equipment is suitable for distilling wine as a student exercise:

Construction of a simple distillation apparatus with an air-cooled cooling tube

In the fractional distillation several components of a liquid mixture can be separated. The prerequisite is that the various liquids have different boiling temperatures. A simple, fractional distillation can be carried out with a grinding apparatus and a so-called "spider" as a template. An alcohol-water mixture is first heated to the boiling temperature of the alcohol, with the alcohol collecting as a distillate in the first round bottom flask of the receiver. Now the spider is turned and heated to the boiling temperature of the water. The water then collects in the second round bottom flask. In this way, four fractions can be separated with the apparatus shown:

Fractional distillation with a rotating “spider” and the possibility of evacuation

In the Vacuum distillation the apparatus is evacuated at the vacuum inlet of the template with the aid of a pump. As the equipment can implode, protective goggles must be worn. In addition, only relatively small round bottom flasks that are specially available for this purpose may be used. Vacuum distillation is also suitable for substances with a high boiling point and for substances that decompose easily at higher temperatures. Lowering the pressure lowers the boiling points of the substances used.

The distillation apparatus is suitable for vacuum distillation.

The distillation apparatuses described so far are not suitable for continuous, fractional distillation. For example, if you want to distill a mixture of many substances such as crude oil, the template would have to be changed and cleaned every time. In the fractional countercurrent distillation Fractions of a certain boiling range collect on so-called "bubble cap trays", which can be continuously removed.

Enlarged view of a bubble tray column

The rising steam bubbles through the bell bottoms, which have a certain temperature. All substances with a higher boiling point condense in the liquid in the bubble cap. The individual fractions can be removed through the taps on the side. Thermometers can be inserted through side-mounted screw caps to measure the temperature of the fractions. When the bell bottom is full, the liquid flows back into the flask through an overflow. Several bubble caps form one column.

Fractional countercurrent distillation with bubble cap columns

This technique is also used on an industrial scale in fractional petroleum distillation. The crude oil is heated to over 400 ° C in a tube furnace and largely evaporated. The rising crude oil vapor bubbles through the bell bottoms in the distillation tower. Different fractions are obtained, all of which are further processed and represent important intermediate products in organic chemistry (> worksheet).

Fractional distillation of petroleum in the distillation tower

During distillation, the volatile components with a low boiling point accumulate in the vapor phase. If the vapor is condensed in a column further up and distilled again, the separation of the volatile constituents improves. This principle can be carried out in a single operation if the rising vapor is allowed to flow in the opposite direction to the condensed liquid. This procedure is called rectification. It comes with a Vigreux column carried out and enables a particularly effective separation of liquid substance mixtures.

Rectification using a Vigreux column

In the Steam distillation the water vapor carries with it other liquids (or even solids) that have a higher boiling temperature or are not soluble in water. This is achieved either by heating a mixture of water and the substance to be separated or by introducing steam into the substance mixture.

Apparatus for simplified steam distillation for schools

A simple steam distillation apparatus consists of a steam developer, the steam line with the steam, a flask for the material to be distilled and a downstream cooler with receiver. The apparatus shown above is suitable for the extraction of essential oils from lavender or rosemary flowers as a student exercise.

Master copies for distilling