Extraction (process engineering)
Extraction (from latin extrahere , pull, remove ') is called each separation process , in which by means of a (solid, liquid or gaseous) extractant one or more components from a mixture of materials (from solid, liquid or gaseous individual substances consisting), the extraction material , is released. The extracted substance, even if it is still in solution, is referred to as an extract , more rarely as an extract , infusion or infus or tea (for example worm tea ). If the extracted substance is not changed (for example only dissolved or adsorbed ), then this extraction is a physical process; if the substance enters into a chemical reaction , it is a chemical process.
If a solvent is used as the extraction agent, the substances to be extracted dissolve better in the pure solvent than in a mixture of substances; the solvent draws the substance that is more soluble in it from the mixture. Depending on the solubility of the substances, the inorganic solvents used are, for example, water and steam , acids , bases and liquefied carbon dioxide , and the organic solvents used are, for example, alcohols , terpenes , diethyl ether , vegetable oils , chlorinated hydrocarbons or n- hexane . Pressure and temperature usually significantly increase the solubility of substances, which is why some extractions are carried out with hot solvents and / or under pressure, an example of a cold extraction is the extraction of rock salt during salt extraction in salt domes.
The generic term "extraction" or "extract" is not only used for dissolving processes, just as a magnetic separator can "extract" magnetic substances from a mixture by magnetic separation or a substance is separated and separated by other separation processes such as filtration or precipitation or pervaporation etc. thereby "extracted", even if these methods are not classically counted as extraction .
Extraction process
A distinction is made between various extraction processes, whereby the physical state of the carrier is mentioned first and then that of the solvent:
- Solid-liquid extractions, in which a liquid extractant extracts a substance from a solid mixture, for example the extraction of petroleum with the help of supercritical CO 2 or the preparation of coffee .
- During maceration , substances are soaked in a solvent and left to dissolve,
- Digestion is maceration with the supply of heat, in which liquid and solid are mixed by stirring and usually separated by decanting the insoluble components,
- When leaching or washing out, minerals or pollutants are usually washed out of a deposit .
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Liquid-liquid extractions , in which a liquid extractant extracts a solute from a liquid,
- In liquid membrane permeation (see also permeation ), a multiple emulsion (i.e. a further emulsion phase in an emulsion) is used for extraction, with the extracted substance being stripped from the innermost phase .
- for example shaking it out in the laboratory practice ,
- Liquid-gas extractions, in which a liquid is used to extract a gas from a gas mixture,
- e.g. extraction of carbon dioxide in a gas scrubber or in gas chromatography ,
- Gas-liquid extractions, in which a gas or vapor extracts a substance from a liquid,
- for example in stripping or in gas chromatography.
- Solid -liquid extractions, in which a substance is extracted from a liquid by adsorption on a solid,
- e.g. adsorption of chlorinated hydrocarbons on activated carbon from drinking water,
Furthermore, extractions from different aggregate states are possible without a term:
- an extraction in which a gas extracts a substance that has been adsorbed or chemically bound to a solid (e.g. gas chromatography or the production of espresso with steam)
- an extraction by means of gas from gas, for example when carbon dioxide absorbs water vapor from the air and collects on the ground
- in the steam stripping the value substance or contaminant-containing solution is the one hand, heated by the steam, and brought volatile constituents to evaporate, on the other hand pulls the sparging steam these other vapors in the vapor stream as a blowing agent with or "extracted" it so from the solution, with the entrained substances do not necessarily have to dissolve in the steam or steam condensate.
- an extraction by means of a solid from a gas, in which a substance is extracted from a gas by adsorption or absorption on a solid,
- For example, removal of traces of water vapor from gases (drying) with the help of calcium oxide
- Enfleurage in the perfume industry for the extraction of essential oils and fragrances
- A “solid-solid” extraction would be if a substance diffuses from one solid into another solid, for example if a plasticizer diffuses from a plastic into applied paint layers, the plasticizer is thereby “extracted”.
application
everyday life
The washing of textiles and dishes is characterized by extraction processes, with the help of mostly water and solubilizers , soiling is removed from laundry and dishes (see also washing machine and dishwasher ).
Organic chemical industry
Extraction is always used when distillation or rectification are out of the question for technical reasons or the extraction process is cheaper. This can be the case, though
- the component of value disintegrates before the mixture has reached its boiling point and one does not want to distill at reduced pressure. Therefore, heat-sensitive substances such as antibiotics or natural substances are extracted.
- the mixture has an azeotrope and one does not want to use special rectification processes such as extractive rectification or the two-pressure process . BTX aromatics are therefore often extracted from their mixtures.
- the boiling temperature difference between the or the relative volatility of the components to be separated is very small.
- a small proportion of a high boiler is to be removed from a low boiler.
Inorganic chemical industry
Leaching or metal salt extraction is an important and irreplaceable process, for example to obtain titanium dioxide (TiO 2 ) or to free bauxite ore from iron hydroxide. Bauxite is an ore (natural mixture of substances) that contains a lot of aluminum and iron hydroxides and oxides. The aluminum hydroxide dissolved in caustic soda during ore digestion can then be recovered from the red mud (iron hydroxide) after being purified from the aluminate lye and converted into aluminum by burning and electrolyzing . The digestion and extraction process of the aluminum compounds in the bauxite is coupled here with the chemical reaction of the aluminum hydroxide with the caustic soda to form the aluminate lye ( complex formation reaction ). The reprocessing of reactor fuel elements also uses extraction.
pharmacy
The preparation of drug extracts from pharmaceutical drugs for drugs or cosmetics by extracting the ingredients from medicinal plants is a central area of pharmacy.
Various methods are used in pharmaceutical technology, e.g. B. Maceration , di- or remaceration (double maceration), digestion (maceration at elevated temperature), re- / percolation (extraction by means of seepage, as in the case of filter coffee preparation), Soxhlet method , extraction according to Twisselmann , turbo (vortex) , Ultra-Turrax, ultrasonic, countercurrent extraction and extraction using a centrifugal extractor .
Many herbal medicines only contain dried plant parts or simple extracts from them. The production of a special extract , such as the flavonoid Taxifolin from larch wood , involves a complex, multi-stage extraction and purification process. This removes unwanted ingredients and enriches the desired phytopharmaceuticals that determine the effectiveness . The use of special extracts has the following advantages, among others. In this way, the concentration of active ingredients in the special extract can be increased. Smaller amounts of a substance are required for the same effect. Unwanted by-products are removed during the extraction, the phytopharmaceutical is better tolerated. The composition and amount of the ingredients are standardized. This guarantees consistent quality.
Perfume industry
Fragrances are also obtained by extraction, rose oil, for example, is produced from rose blossoms by steam distillation (with steam as an entrainer ) , fragrant jasmine oil by enfleurage (solid fat as an absorbent from which the fragrances are then extracted).
food industry
The largest area of application in terms of quantity is the production of edible oils from oil seeds by pressing and extraction with the solvent hexane . The edible oil is obtained from the resulting mixture of oil and solvent ( Miszella ) by distilling off the solvent.
A newer extraction method is extraction with supercritical solvents. At high pressures and temperatures, gases and liquids adopt a state in which the physical properties of the solvent neither correspond to those of the gas nor those of the liquid, this area is called supercritical . Supercritical carbon dioxide (CO 2 ) is mainly used here .
Supercritical CO 2 is used to obtain extracts from food or to extract undesirable substances: caffeine from coffee beans, hop resins from hops , nicotine from tobacco, flavors as well as colorings and ingredients from various spices, aromatic plants and natural substances. With supercritical CO 2 , however, the reverse process, impregnation , can also take place. For this purpose, a solid starting material is contacted with the substances dissolved in the supercritical CO 2 , whereby the impregnation substances penetrate into all pores of the solid. As a result of slow relaxation, the supercritical CO 2 loses its ability to dissolve and the impregnating substances remain evenly distributed in the solid. This process is already being used industrially in a large plant in Denmark for impregnating wood (with acetic anhydride ) (see also acetylated wood ). Various research institutions are also concerned with the dyeing of fabrics, for which special dyes have been developed that have good solubility in supercritical CO 2 .
The water extraction of ground roast coffee and the spray-drying or freeze-drying of the extract produce powder coffee or freeze-dried coffee. This process is used on an industrial scale e.g. B. used at Deutsche Extrakt Kaffee GmbH and Nestlé AG.
biotechnology
Since proteins are denatured by most organic solvents , aqueous 2-phase systems are used for extraction in biotechnology. The principle is based on the mixture of two solutions, which form two separate phases. Examples of this are polyethylene glycol (PEG) and dextran solutions or the PEG / salt system. The product dissolves differently in the two phases according to its distribution coefficient.
Soil remediation
In soil remediation , pollutants can be extracted from contaminated soils using different solvents . For this purpose, the high-pressure extraction has also proven to be suitable, in which, similar to the high-pressure extraction of coffee, hops or fats and oils with compressed gases, such as. B. carbon dioxide extracted.
Extraction through dissolution processes
The extraction through solution is based on the different solubilities of the components to be separated and essentially takes place in four steps:
- Mixture and extractant are mixed intensively in order to create the largest possible surface for the mass transfer. In the chemistry laboratory in a Soxhlet apparatus or in a shaking funnel , in technology in a (closed) mixer.
- The valuable component from the mixture partially dissolves in the extractant (establishment of an equilibrium).
- The extractant with the valuable substance dissolved in it is separated from the extracted material using a suitable separation process . In the case of a liquid-liquid extraction, an extraction agent is chosen in which the valuable substance dissolves better and immiscible liquids are separated by allowing the individual phases to settle. The two solvent phases must have a miscibility gap so that a phase separation occurs. The raffinate phase (primary solvent) and the extract phase (secondary solvent), which now contains part of the substance to be extracted, are separated in a separator by their (as large as possible) density difference, in the chemistry laboratory for example in a shaking funnel , due to the often conical glass container a good separation can also be achieved manually in this way. Decanters or centrifuges are also used .
- The extractant enriched with valuable substances is processed (for example by rectification ) in order to obtain the pure valuable substance and to recover the extractant.
Due to the solubility of the component to be extracted in the original substance, a residual amount is always retained. The mechanism for this is described using Nernst's law of distribution . For this reason, for example, a washing machine or a dishwasher can never remove 100% of a contamination with two wash cycles, which in the case of radioactive contamination , for example, leads to extensive radiation of all equipment washed with it.
Technical implementation
In the industrial sector, so-called mixer-settlers ( mixer settlers) are used, with the liquids being transferred to another container after mixing and being able to settle there. This makes it possible to handle different amounts of liquid in one or more steps.
Alternatively, the mixer and settler can be accommodated alternately in one column. Such a mixer-settler column is particularly suitable for applications where many separation stages are required or when the extraction is coupled with a chemical reaction.
The number of theoretical separation stages can be estimated manually with the help of the so-called pole beam method .
In industrial applications, continuous extraction processes or extraction by means of a centrifugal extractor are used.
Influences on the extraction
The efficiency of the extraction depends on many different parameters (pH value, solubility, temperature, grain size of the extract, penetration behavior of the extractant, wettability, etc.).
The material to be extracted should have a surface area that is relatively large in relation to its volume, since it is proportional to the amount of substance that can be extracted. In practice, it therefore proves to be expedient to comminute the extraction material into a fine powder during the solid extraction. With liquid-liquid extraction , a large surface is achieved by intensive stirring, whereby the liquids are broken up into small droplets and thus offer a large surface, or by emulsification with subsequent emulsion splitting .
There should always be a large difference in concentration between the extract in the extraction material and the solvent, since it is the driving force behind the extraction. It is achieved through:
- Frequent replacement of the loaded solvent (solvent) with fresh solvent.
- Rapid removal of the dissolved extract from the surface of the extraction material.
The diffusion resistance counteracts the extraction. It depends on the size and porosity of the particles in the extraction material and should be as low as possible.
If the solubility is better at higher temperatures, a high temperature favors the extraction, since the stronger heat movement and the lower viscosity of the solvent can accelerate the leaching of the extract.
Requirements for the solvent
The solvent for extraction should be chosen selectively. This means that it should mainly only remove the extract from the extraction material. In practice, however, this is only possible with difficulty, and so several components are often extracted, which then have to be separated by subsequent separation processes .
The extractant must be inert to the extract .
In order to allow the extraction to proceed as quickly as possible, the solvent should dissolve the extract quickly and be able to absorb a large amount of the extract.
A low boiling point of the solvent favors its recovery from the extract solution. This keeps the energy consumption low.
The solvent should be non-flammable, toxic, corrosive, or environmentally hazardous.
In the case of liquid-liquid extraction, there should also be a sufficiently high difference in density between the extraction material and the extractant to enable the two phases to be separated. The polarity of the solvent to the extraction material must be different so that the substances do not dissolve in one another.
Web links
Individual evidence
- ↑ Extraction procedure in preparation for gas chromatography .
- ↑ Extract at www.experimente.net
- ↑ Carsten Bloch: The chromatomembrane method as sample preparation for gas chromatography. Dissertation . FU Berlin, 1999, chapter 3.2.
- ↑ Phillip Kurz, Norbert Stock: Synthetic Inorganic Chemistry: Basic Course . 1st edition. De Gruyter, 2013, ISBN 978-3-11-025874-5 , 2.2.2 Cooler, p. 9 .
- ↑ Ultrasonic extraction of caffeine and other active ingredients
- ^ PW Elsinghorst et al.: The thermal and enzymatic taxifolin-alphitonin rearrangement. In: Journal of Natural Products . 74 (10), Oct 28, 2011, pp. 2243-2249; PMID 21992235 .
- ^ I. Reiss, A. Schleußinger, S. Schulz: Soil remediation through high pressure extraction. In: Chemistry in Our Time . 28th Year No. 4, 1994, pp. 189-196, ISSN 0009-2851 .