What does the distillation curve mean

distillation

Simple distillation on a laboratory scale

distillation (lat. distillare "trickle down") is a thermal separation process to obtain vaporizable liquids or to separate solvents from substances that are difficult to vaporize. Compared to other separation processes, distillation has the advantage that, as a rule, no further substances such as adsorbents or solvents are added.

During the distillation, the starting mixture is first brought to the boil. The resulting vapor, which is made up of the various volatile components of the solution to be separated, is liquefied again by cooling in a condenser. The Liebig cooler is often used on a laboratory scale. The liquid condensate is then collected.

Typical applications of distillation are the burning of alcohol and the distillation (rectification) of petroleum in the refinery or the production of distilled water.

Ideally, the liquid should not decompose during distillation. This is different with so-called "dry distillation": non-evaporable solid substances are broken down into smaller molecules. So you got z. B. in the dry distillation of wood the "wood spirit" (methyl alcohol, methanol). Although a vaporizable liquid is obtained here by condensation, according to today's theory there is no separation by distillation. This is why this process is better known as thermolysis or pyrolysis.

history

Simple schnapps distillation in Lore (East Timor)
Still in a pharmacy around 1900
Old Ukrainian still for vodka

The distillation of pitch and tar for sealing ships, as an adhesive and also as a remedy has been known since the Neolithic; it was accomplished with the simplest means. In ancient times, mainly essential oils, as fragrances and fragrances, were distilled. The oldest depictions of stills found during archaeological excavations come from Mesopotamia, today's Iraq, and are estimated to be over 5500 years old. These first devices consisted of a vessel with a lid on which the distillate precipitated when heated. To prevent this liquid from dripping back into the vessel, sponges or tufts of wool were used in the lid to soak up the liquid. These were then simply squeezed out regularly to preserve the distillate.

Using the same method, Greek sailors produced around 500 BC. Drinking water from sea water. Among other things, Aristotle described in the 4th century BC how sea water can be made potable by distillation. He went on to describe that wines and other liquids can be subjected to the same procedure. Around 200 BC, Alexander of Aphrodisias explained the process of making distilled water.[1]

The distillation was further improved by the chemist Abu Musa Jābir ibn Hayyān around 800 AD. The Persian scientist and doctor Ar-Razi ("Rhases", 865 - 925) also wrote down his knowledge in a number of extensive writings. His most important work is that Kitab sirr al-asrar, the "Book of the Secret of Secrets". Here he describes the distillation of the naft, crude oil, and explains a simple type of cracking process for the purpose of obtaining low-boiling products such as bitumen and the so-called brick oil (oleum laterinum). With the invention of the still helmet, the distillation of alcohol became possible.

When sulfuric and nitric acid and, above all, drinking alcohol (ethanol) were discovered around the turn of the millennium (1000 AD), distillation gained considerably in importance. In the early modern period, distillation began to be used for medicinal purposes. This is how the doctor Hieronymus Brunschwig wrote in 1512 Distilling the book of true art. [2]In addition, distillation became an important tool in alchemy and later in spagyric. In the 17th century, freshwater distillation from seawater for seawater desalination began again.

Principles

The "simple distillation" described above by heating and cooling is based on the evaporation and condensation of volatile substances. However, these are not separated or only incompletely separated. At most, you can collect individual "fractions" with different boiling temperatures.

The separation of mixturesdifferent Vaporizable and mutually soluble substances can often through repeated Distillation can be accomplished. In these cases the separation effect is based on the different composition of the boiling liquid and the vapor. A necessary but not sufficient condition for this are different boiling points of the components to be separated. The techniques developed for this are listed below. These processes are based on the different high boiling points of the liquids involved, more precisely on their different vapor pressures at the same temperature. This is explained using a mixture of two liquid components that can be mixed with one another ("binary mixture"):

If, as in the figure on the right, a mixture of components 1 and 2 is heated, the temperature rises until the boiling curve is reached. The composition of the gas phase above the boiling liquid is that which the dew point curve shows at the same temperature (horizontal line). By condensation, a liquid is obtained whose composition corresponds to that of the gas phase, that is to say contains an increased proportion of the lower-boiling component 1 (vertical line). In fact, the content is lower due to incomplete equilibrium. In addition, the remaining liquid (in technology: the "Distillation sump") over time on the low-boiling component, causing the horizontal line to slide upwards.

Multi-stage distillation and rectification

By repeatedly re-distilling the condensate, one gets closer and closer to the pure substance 1 in the boiling diagram on a zigzag line. In practice, by installing a column between the "still" and "top", a single distillation can achieve a significantly increased separation efficiency. The number of individual distillations required for the same separation performance is referred to as the "theoretical plate number", so called after the process of crude oil distillation in so-called bubble-cap columns. On the surface of the column, the equilibrium between liquid and gas phase is constantly re-established through condensation and evaporation , whereby the proportion of the low-boiling component continues to rise upwards, while the higher-boiling component flows back into the still, the bottom Vigreux column or by filling with Raschig rings greatly increased.

If the substances to be separated form an azeotrope, the boiling point and dew point curve do not only meet with the pure substances. Separation by distillation is then only possible up to this point. However, the azeotropic mixing ratio depends on the pressure, so that a further separation is possible through vacuum or overpressure distillation. The azeotrope between ethanol and water in a ratio of approx. 25: 1 (under ambient conditions) is the basis for the usual commercial mixture of a "96 percent alcohol".

The large-scale implementation of repeated, continuous distillation is also known as rectification. The individual distillation stages take place in a special container called a rectification column. The column consists of several layers of trays through which the steam can rise to the top and the condensate can flow into the sump. Products can be continuously withdrawn and starting material can be topped up.

Fractional distillation

A mixture consisting of several components can be separated by fractional distillation. The container used to collect the distillate is replaced after the lowest-boiling fraction has been separated off. The time to change is indicated by a change in the temperature in the distillation head. In most cases, an intermediate fraction is separated off until the boiling point of the next component is reached, as a mixture often passes over in the transition area and in order to remove residues of the previous fraction from the cooler. If the boiling points are close together, the volume of the unclean intermediate fraction can be kept small by inserting a column.

Note

The terms “fractional distillation” and “rectification” as countercurrent distillation, reflux distillation, column distillation are often used synonymously. In the strict sense, it means that a mixture consisting of several components can be separated by distillation and fractionation. The container used to collect the distillate is replaced after the lowest-boiling fraction has been collected. Fractionation only means collecting several fractions.

Vacuum distillation

The vacuum distillation is a distillation with reduced total pressure in the distillation plant. This lowers the boiling temperature of the mixture to be separated, which enables the distillation of mixtures of substances whose components remaining in the sump are not sufficiently temperature-stable. In laboratory practice, distillation is almost always done in a vacuum. At higher temperatures, catalyst residues or by-products can be present in the bottom or in the rest of the starting material, which lower the yield as a result of undesired reactions.

On an industrial scale, the “bottom product” of the atmospheric distillation in petroleum refining is then subjected to a vacuum distillation. Mainly the base oils for lubricating oil production and so-called vacuum gas oil are to be produced. This also serves as a valuable starting material for a cat cracker or a hydrocracker.

Overpressure distillation

With overpressure distillation, the system is operated with overpressure in order to push the boiling points further apart. The area of ​​application is for substances with very low boiling points that are close together, such as air liquefaction.

Kugelrohr distillation

Distillations in the Kugelrohr are carried out in the laboratory with small amounts of substance. Further details are described under this lemma.

Drag distillation

Here, an additive is used to distill the product, which "carries along" the product. The best-known variant of this type of distillation is steam distillation. When vacuum distillation is not optimal, it is used to distill heat-sensitive substances with low vapor pressure. Examples are the extraction of essential oils from plants or the application in the purification of substituted aromatics.

Azeotropic distillation

Here a component is added which forms an azeotrope with the substance to be separated. For example, in the case of an acid-catalyzed esterification, the water formed can be removed quantitatively as an azeotrope with toluene, which means that the reaction is only complete. Ideally, a heteroazeotrope is formed which, on condensation, breaks down into two phases, which allows the solvent to be recycled.

Short path distillation

Device for short path distillation

Short-path distillation (KWD) is a distillation that is carried out in the fine vacuum range, i.e. in the pressure range between 1 and 0.001 mbar, and in which the gas phase only has to cover a very short path between the receiver and the condenser. It is also known as molecular distillation and is one of the gentlest thermal separation processes. Due to the low working pressure, the distillation takes place at relatively low temperatures. Compared to other distillation processes, thermally sensitive products such as tocopherols, fatty acid esters, monoglycerides, prepolymers, epoxy resins and active pharmaceutical ingredients can be separated very gently. The method is also suitable for molecules that are difficult to evaporate, such as long-chain hydrocarbons from the residues of the mineral oil industry, which are distilled off under fine vacuum. A modified variant is bulb tube distillation. In industry, apparatus similar to plate heat exchangers are used, in which the distance between the evaporator and condenser is only a few millimeters.

Reactive distillation

Esterification by means of reactive distillation

In reactive distillation, the (multi-stage) distillation is combined with a chemical reaction. By combining both mechanisms, advantages can be achieved compared to simple, serial reaction-distillation processes. Reactive distillation is particularly suitable for “equilibrium limited” reactions. By constantly removing a reactant, the equilibrium is set again and again and in this way complete conversion is made possible. On the other hand, azeotropes that occur as a result of the reaction can be broken. In the case of an exothermic reaction, the heat generated is used to separate the substances. The optimal operating conditions and above all the optimal temperature range for reaction and material separation can prevent this method.

The chemical reaction that occurs can be catalyzed both homogeneously and heterogeneously. If a homogeneous catalyst is used, a further separation stage is necessary to separate off the catalyst. In heterogeneously catalyzed reactive distillation, the catalyst is often installed in the distillation column in the form of reactive packings. These are often separating packs in which the mostly spherical catalyst is integrated in small metal bags. Despite intensive research in the last few decades, reactive distillation is only used relatively rarely in industry. However, it is important for potassium production.

Zone distillation

Zone distillation is a distillation process in an elongated container with partial amalgamation of the refined substance in a moving liquid zone and with condensation of the vapor into the solid phase as the condensate leaves the cold area. The process is worked out theoretically.

When the zone heater is moved along the container from top to bottom, a solid condensate can be formed in the container with the even distribution of the admixtures and the purest part of the condensate can be excluded as a product. The process can be repeated several times, for which the previously obtained condensate (without circulation) should be transferred to the lower part of the container at the place of the refined substance. The uneven distribution of the admixtures in the condensate (i.e. the cleaning effect) increases with the number of repetitions of the process.

Zone distillation is a distillative analogue of zone recrystallization. The distribution of the admixtures in the condensate is described by known equations of zone recrystallization with different numbers of passes through the zone - with the replacement of the distribution coefficient k for the crystallization by the separation coefficient α for the distillation.

Reflux

The refluxing allows very long boiling for the reaction with continual recovery of the evaporating solvent by condensation. Reflux is not a separation process in the strict sense because the separation is only temporary. An advanced type of reflux is performed using the Soxhlet attachment.

Individual evidence

  1. : The evolution of the still. In: Annals of Science. 5, No. 3, 1945, p. 185. doi: 10.1080 / 00033794500201451.
  2. ↑ Magnum Opus Hermetic Sourceworks Series

literature

  • Robert J. Forbes, A short history of the art of distillation: from the beginnings up to the death of Cellier Blumenthal. - Repr. D. Ed. 1948. Brill, Leiden, 1970.
  • Organikum. Organic-chemical basic internship, by an author collective, 7th edition, Deutscher Verlag der Wissenschaften, Berlin, 1967 and subsequent editions.
  • Erich Krell, Laboratory distillation manual: with an introduction to pilot distillation. 3rd edition, Hüthig, Heidelberg et al., 1976, ISBN 3-7785-0340-5.
  • Reinhard Billet, Industrial distillation, Verlag Chemie, Weinheim, 1973, ISBN 3-527-25371-8
  • Johann Stichlmair, Distillation, in Ullmann’s Encyclopedia of Industrial Chemistry, Barbara Elven (Edit.), 7th Edit, Vol. 11, p. 425-494, Wiley-VCH, Weinheim, 2011, ISBN 978-3-527-32943-4
  • Kravchenko, A.I., Zone Distillation: A new purification method, on: Problems of atomic science and technology, 2011. N. 6. Series: "Vacuum, pure materials, superconductors" (19), P. 24-26, in Russian, vant.kipt.kharkov.ua

Web links

Template: Commonscat / WikiData / Difference