Liquid-liquid extraction
The liquid-liquid extraction (English abbreviation LLE for liquid-liquid extraction ) is a separation method that uses the different solubilities of substances in two immiscible solvents . A hydrophilic phase (mostly water ) and a hydrophobic organic solvent each serve as the solvent. This type of separation is often used to work up reaction mixtures in organic chemical laboratories.
execution
The solution with the component to be extracted is mixed with the extractant in the separating funnel by vigorous shaking. This leads to an enlargement of the phase boundary between the two solvents and thus to a better extraction of the component into the other phase. In the idle state, two layers form again according to the different densities . After the two phases have been separated, the product can be obtained, for example, by evaporating the solvent.
Mechanisms
According to Nernst's law of distribution , there is a distribution between the two phases, similar to chemical equilibrium . Depending on the size of the equilibrium constant , a certain amount of substance is transferred to the extractant and can be separated off with it. The desired product can be obtained almost completely by repeated addition of the extractant and renewed separation.
Working principle and function calculation
Liquid-liquid extraction consists of transferring the solvate (or several solvates) that is in the primary liquid solution into another, immiscible liquid ( solvent ). The solvent enriched with the solvate is called the extract , the diluted starting solution is called the raffinate .
The primary liquid solution and the solvent are contacted to effect transfer of the solvate. The two liquid phases (extract and raffinate) are separated by static decanting ( mixer-settler ) or centrifugal force .
mixer
In the mixing zone the primary liquid solution is brought into intimate contact with the solvent by means of a mechanical stirrer so that a good solvate transfer takes place. The mechanical stirrer is equipped with an electric motor that drives a mixing and pump turbine. This sucks in the phases from the settlers of the adjacent stages, brings them into contact and feeds the emulsion produced in the mixer back into the settler.
Settlers
Static settling zone between the two phases. Coalescence grids facilitate the separation of the resulting emulsion into 2 phases (heavy and light). The separate phases are transferred by overflowing the weirs. The height of the weir of the heavy phase can be adjusted so that the position of the intermediate phase (heavy / light) is positioned according to the density of each phase.
Centrifugal extractors
In centrifugal extractors, the phase to be extracted, which contains one or more solvates (yellow on the diagram) and a solvent that is not miscible with it (blue on the diagram) with a different density, a mixing chamber located in the lower part of the device, fed.
With the help of a rotating disk, the immiscible liquids are mixed to form a dispersion (green on the diagram). Different stirring discs are used depending on the interfacial tension between the liquids. Effective mixing means an extremely high mass transfer surface between the two liquids. This favors the transfer of the solvate (s).
A turbine located in the lower part of the centrifuge drum pumps and transfers the dispersion into the drum. The liquids are separated by centrifugal force. The heavy phase (yellow) is thrown against the drum wall, the light phase (blue) is positioned in the middle area of the drum.
The weir of the heavy phase stabilizes the position of the phase interface. Interchangeable weirs with different diameters enable a wide range of density ratios to be covered. The heavy phase flows into the lower part of the outer casing of the centrifuge. The light phase reaches the upper part of this outer housing by overflowing.
The two liquids are drained by gravity into the centrifugal extractor attached. For extraction processes that require several successive stages, the single-stage extractors are connected in series or set up as batteries. One unit per level. The two liquids flow in countercurrent into the batteries.
There is no need for feed pumps between the extractors. The external connection lines enable the liquids to be transferred from one extractor to the other or to be diverted from the process (main extraction, washing or re-extraction), as required. This guarantees optimal flexibility.
Liquid-liquid micro-extraction
In the liquid phase micro-extraction ( liquid-phase microextraction, LPME) extraction from a small amount of sample is made at a drop ( single-drop microextraction ) or a solvent-filled hollow fiber ( hollow-fiber LPME ). This method is mostly used to analyze biological samples. The advantages are the small sample volume, the high pre-concentration factor and the simple sample preparation.
Liquid membrane permeation
Liquid membrane permeation is also used to extract traces of heavy metals from wastewater. For example, sulfuric acid is emulsified in an oil phase that contains dissolved chelating agents and this emulsion is in turn emulsified in the waste water. The heavy metals are dissolved in the oil phase by liquid-liquid extraction and then converted into sulfuric acid by liquid-liquid extraction. After the oil phase has been separated off, the acid-in-oil emulsion is split in a high-frequency alternating current field.
See also
Left
Individual evidence
- ↑ Organikum . WILEY-VCH, Weinheim 2004, ISBN 3-527-31148-3 , pp. 58ff.
- ↑ R. Lucena, M. Cruz-Vera, S. Cárdenas, M. Valcárcel, Bioanalysis , 2009, 1, 135-149.
- ↑ Marr, Prötsch, Bouvier, Draxler, Kriechbaumer: Continuous experiments on liquid membrane permeation in a pilot plant , Chemie Ingenieur Technik 55 (1983) 328–329.