Distillation ( lat. Distillare "drip down, drip down" , from silent "drop") is a thermal separation process to obtain vaporizable liquids or to separate solvents from substances that are difficult to vaporize and then to collect them through condensation. Compared to other separation processes, distillation has the advantage that, as a rule, no further substances such as adsorbents or solvents have to be 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" ( methanol ). It is true that a vaporizable liquid is obtained here by condensation, but according to today's theory there is no separation by distillation. This is why this process is better known as thermolysis or pyrolysis .
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 were distilled as fragrances and fragrances. 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 seafarers produced around 500 BC. Chr. Drinking water from sea water . Aristotle described, among other things, 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.
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 the Kitab sirr al-asrar , the "Book of the Secret of Secrets". Here he describes the distillation of naft , the 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 at the turn of the first millennium (1000 AD ), distillation gained considerably in importance. In and from Salerno , refined cooling processes were used in the 12th century to separate liquid mixtures according to their different boiling points. In the early modern period, the use of distillation for medicinal purposes began. In 1500 the surgeon Hieronymus Brunschwig wrote his small distilling book . In addition, distillation has been an important tool in alchemy since the 14th century (especially for the production of a quinta essentia that is not only philosophical ) and later in spagyric . The equipment needed for distillation was already available in middle-class households in the 15th century. In the 17th century, freshwater distillation from seawater for seawater desalination began again . The distillation of ethanol is subject to restrictions, controls, and special taxes in many states; for Germany see: Spirits Monopoly Act .
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 mixtures of different vaporizable and mutually soluble substances can often be accomplished by repeated distillation. 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 methods are based on the different 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 a mixture of components 1 and 2 is heated as shown in the figure on the right, 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). Condensation gives a liquid whose composition corresponds to that of the gas phase, that is to say contains an increased proportion of the lower-boiling component 2 (vertical line). In fact, the content is lower due to incomplete equilibrium. In addition, the remaining liquid (in technology: distillation bottoms ) is depleted of the low-boiling component over time, which shifts the horizontal line upwards.
With simple distillation, the sump is heated to the desired temperature with the help of a suitable heat source (e.g. heating mantle or heating bath ) at which the target component begins to boil. When this is reached, the substance rises in gaseous form and condenses again in the cooler. A Liebig cooler is usually used in the laboratory . The head temperature of the vaporized substance can be read on the thermometer and this value can be used to ensure that the desired component is removed or obtained from the mixture. The receiving flask is at the end of the apparatus.
An example of the application of simple distillation is the separation of dissolved impurities (salts or other solids) from a liquid. These cannot be removed by filtration .
Multi-stage distillation and rectification
By repeatedly re-distilling the condensate, one gets closer and closer to the pure substance 1 on a zigzag line in the boiling diagram. In practice, by installing a column between the still and the still head, a single distillation achieves a significantly increased separation efficiency. The number of individual distillations required for the same separation performance is referred to as the “theoretical number of trays”, based on the petroleum distillation process in so-called bubble-cap columns. On the surface of the column, the equilibrium between the liquid and gas phase is constantly re-established through condensation and evaporation, which means that the proportion of the low-boiling component continues to rise upwards, while the higher-boiling component flows back into the still, the bottom. The size of the surface of the column, which in the simplest case consists of a long glass tube, is greatly increased in different variants such as the Vigreux column or by filling with random packings or structured packings .
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 reason 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 refilled.
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, since a mixture often forms 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.
The terms “fractional distillation” and “ rectification ” as countercurrent distillation, reflux distillation and 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 simply means collecting several fractions.
The vacuum distillation is a distillation with reduced total pressure in the distillation plant. This lowers the boiling temperatures of the individual components, which enables the distillation of mixtures of substances whose components remaining in the sump are not sufficiently temperature-stable. 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 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 .
With overpressure distillation, the plant 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 .
Overpressure distillation with superheated steam is also sometimes used for plant material with oils that are difficult to distill. The oil-water ratio in the distillate is more favorable here than at normal pressure.
Distillations in the Kugelrohr are carried out in the laboratory with small amounts of substance. Details are described in this article.
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.
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 only takes place completely. Ideally, a heteroazeotrope forms , which in turn breaks down into two phases on condensation, which allows the solvent to be recycled.
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 the Kugelrohr 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.
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 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. When using a homogeneous catalyst, 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 used relatively rarely in industry. However, it is important for potassium production .
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 has been 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 condensate obtained earlier should be transferred (without circulation) 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 - when the distribution coefficient k for the crystallization is replaced by the separation coefficient α for the distillation.
Unintentional distillation processes
In technical systems, for example in extraction systems for vapors, deposits often occur unintentionally after previous evaporation, in that extracted vapors condense in the suction pipes and these condensates lead to blockages or further caking in the long term. For example, fat deposits in extractor hoods or water condensate in compressed air hoses (which freezes at low temperatures).
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- Compressed gases for extraction and refining page 5