What is reactive distillation and its meaning
The distillation (lat. distillare "trickle down") is a thermal separation process to separate a liquid mixture of different, mutually soluble substances. The boiling points of the individual components are relatively close together. Typical uses of distillation are burning alcohol and distilling petroleum in the refinery.
During the distillation, the starting mixture is first brought to the boil. The resulting vapor, which is made up of the various components of the solution to be separated, is condensed in a condenser (e.g. Liebig cooler on a laboratory scale). The liquid condensate is then collected. The separation effect is based on the different composition of the boiling liquid and the gaseous vapor.
In ancient times, distillation was also used for other applications.
The distillation of pitch and tar to seal ships, as an adhesive and also as a remedy, has been known since the Neolithic and can be accomplished with the simplest means. In ancient times, it was mainly essential oils that were distilled. 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 17th century, freshwater distillation from seawater was also used in important applications.
Distillation is a thermal separation process that has the advantage over others that, as a rule, no other substances such as adsorbents or solvents are added. The process is based on the different boiling points of the liquids involved, more precisely on their different vapor pressures at the same temperature. If, as can be seen in the adjacent figure, a mixture of substances 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 distillation sump becomes depleted of the low-boiling component over time, 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 on a zigzag line in the boiling diagram. In practice, by installing a so-called column between the bottom and the 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 according to the petroleum distillation process (see below), of this column. On the surface of the column, the equilibrium between the liquid and gas phase is constantly re-established through condensation and evaporation, as a result of which the proportion of the low-boiling component continues to rise, while the higher-boiling component flows back into 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 various variants such as the Vigreux column or by filling it with so-called Raschig rings.
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 may be possible after all by vacuum or overpressure distillation. The best-known azeotrope is likely to be ethanol / water in a ratio of approx. 25: 1 (under ambient conditions).
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.
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" ("countercurrent distillation", "reflux distillation", "column distillation") are often used synonymously . In the strict sense, "fractionating" only means collecting several fractions.
Vacuum distillation is distillation at a lower 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, especially those remaining in the bottom, are not sufficiently temperature-stable. In laboratory practice, distillation is almost always done in a vacuum, since at higher temperatures in the sump, the, among other things, May contain remaining starting material, catalyst residues or by-products, undesired reactions can occur which reduce the yield.
On an industrial scale, the bottom product of the atmospheric distillation in petroleum refining is subjected to vacuum distillation, among other things to remove low-boiling residues and to return them to the atmospheric distillation.
The best-known variant of this type of distillation is steam distillation. It is used to distill heat-sensitive substances with low vapor pressure when vacuum distillation does not seem practical, for example for the extraction of essential oils from plants or for the purification of substituted aromatics.
Here, a component is added to the template, 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 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 takes place in the fine vacuum range, i.e. H. is carried out in the pressure range between 0.001 and 1 mbar (abs). It 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 B. tocopherols, fatty acid esters, monoglycerides, prepolymers, epoxy resins and much more can be separated very gently. Likewise, difficult to evaporate molecules, such as. B. very long-chain hydrocarbons are distilled off from the residues of the mineral oil industry under fine vacuum. A modified variant is bulb tube 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. Thus, through the distillative separation of substances superimposed on the chemical reaction, a conversion that is greater than the chemical equilibrium of the reaction can be achieved, which is why reactive distillation is particularly suitable for equilibrium-limited reactions. Conversely, azeotropes that occur as a result of the reaction can be broken. Furthermore, the heat generated during the reaction can be used for the separation of substances. However, the combination of the optimal operating conditions (especially the optimal temperature range for reaction and material separation) can be problematic. 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 has only found relatively little use in industry.
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Categories: Distillation | process technology
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