The Extraction Liquid -Liquid is, by distillation, the most important basic operation in the separation of homogeneous liquid mixtures. It consists of separating one or several substances dissolved in a solvent by transferring it to another insoluble, or partially insoluble, solvent in the first one. The transfer of matter is achieved through direct contact between the two liquid phases. One of the phases is dispersed in the other to increase the interfacial surface and increase the flow of transferred material.
In a liquid-liquid extraction operation, the solution to which the components are intended to be separated is called the liquid extraction solvent to be used to separate the desired component, refined to the feed already treated and extract to the solution with the solute. recovered.
Summary
[ hide ]
- 1 Reasons to use it
- 2 Applications
- 3 Factors affecting extraction
- 4 Solvent selection criteria
- 5 Ternary equilibrium diagrams.
- 6 Equipment for liquid-liquid extraction
- 7 Source
Reasons to use it
- Other methods are not feasible: Similar or very small volatilities.
- Very high vaporization heat.
- Compounds sensitive to temperature rise.
- As a substitute for chemical separations.
Applications
- Separation of inorganic compounds such as phosphoric acid, boric acid and sodium hydroxide.
- Recovery of aromatic compounds.
- Refining of lubricating and solvent oils
- In the extraction of products containing sulfur.
- Obtaining paraffin waxes
- Desulfurization of petroleum products
- Pharmaceutical products Example in obtaining penicillin
- Food industry
- Obtaining expensive metals, eg uranium– vanadium .
Factors affecting extraction
- Feed composition, temperature, pressure and speed of flow.
- The desired degree of separation.
- Choice of solvent.
- Operating temperature and pressure.
- The formation of emulsions and foams.
Solvent selection criteria
- High separation factor.
- High distribution coefficient.
- Highly insoluble solvents.
- Easy to retrieve.
- Density differences between the phases that form
- High surface tensionto avoid dispersion of the phases.
- Chemically stable and inert with the other components.
- Low Viscosity, Pv, Freezing Point For Easy Handling
- Non-toxic, non-flammable, cheap and easily accessible.
Ternary equilibrium diagrams.
In the design of a liquid-liquid extraction operation, it is usually considered that the refining and the extract are in balance. The balance data to be handled will be at least those corresponding to a ternary system (two solvents and one solute), with two of the components immiscible or partially immiscible with each other.
One of the most common ways of collecting equilibrium data in ternary systems is triangular diagrams. An equilateral triangular diagram is shown in Figure 1. The vertices of the triangle represent pure compounds, a point on one side would correspond to a binary mixture and a point inside the triangle would represent a ternary mixture. The composition of a mixture can be determined by direct reading on the diagram, as shown in Figure 1. The concentration of the components on the diagram is shown as mole fraction or mass fraction.
In the systems of interest for liquid-liquid extraction, the two solvents involved are immiscible or partially immiscible with each other. That is, mixing them in the proper proportions can lead to the formation of two phases. Furthermore, the presence of a solute modifies the solubility of one solvent in another. To represent this behavior, and to be able to know if a certain mixture corresponds to one or two phases, the liquid-liquid triangular diagrams present the so-called binodal or solubility curve (Figure 1). A mixture represented by a point above the binodal curve will consist of a single phase. In contrast, a mixture below the binodal curve has two phases.
The two phases in equilibrium are linked by a distribution line. The distribution line passes through the mixing point and its ends on the binodal curve indicate the concentration of the two phases in equilibrium (Figure 1).
Figure 1. Ternary equilibrium diagram: equilateral triangular diagram
Liquid-Liquid Extraction Equipment
Cross Flow Multi-Stage Extraction
Multi-stage extraction in countercurrent
Teams
- Staged extraction:
- Mixer – settler
- Towers perforated plates
- Tray columns
- Continuous differential contact extraction:
- Spray towers
- Filler towers
- Pulsed columns
- Centrifugal extractors
Mixers-Settlers . This type of equipment can vary from a single tank, with an agitator, that causes the phases to mix and then they are allowed to settle, to a large horizontal or vertical compartmentalized structure. In general, sedimentation is done in tanks, although centrifuges are sometimes used. However, mixing can be done in different ways, such as by impact in a jet mixer, by shear action when both phases are fed simultaneously in a centrifugal pump, by means of injectors where the flow of one liquid is induced by the other, or either through holes or mixing nozzles.
Spray towers . As in gas absorption , dispersion in the continuous phase limits the application of this equipment to cases where only one or two stages are required.
Filler towers. The same types of fillers are used for liquid-liquid extraction as for absorption and distillation . It is preferable to use a material that is preferably wet from the continuous phase. Axial dispersion is a major problem in packing columns and HETP is generally higher than in staged devices.
Plate towers. In this case, perforated plates are preferred. The separation between the plates is much less than in distillation: 10-15 cm for most applications with low interfacial tension liquids. When operating with an adequate flow regime, the extraction rates in columns of perforated plates are high because the drops of the dispersed phase coalesce and re-form at each stage. This favors the destruction of concentration gradients that can be formed when the drops pass undisturbed throughout the entire column. Columns with perforated plates for extraction are subject to the same limitations as distillation columns: flood, drag and drip.
Mechanically assisted gravity equipment . If the density differences between the two liquid phases are low, the forces of gravity are insufficient for an adequate dispersion of the phases and the creation of turbulence. In this case, rotary agitators driven by an axis extending axially along the column are used in order to create mixing zones that alternate with sedimentation zones in the column. A typical example is the RDC (“rotating disc contactor”) which has been used in sizes up to 12m in height and 2.4m in diameter.
The discs, with high speed of rotation, provide the necessary energy for the mixing of the two phases. Glued to the column are perforated metal discs that serve as a separation between each two rotating discs, directing the flow and preventing axial dispersion. Other devices for commercial use are the column-shaped settler-settler cascade developed by Treybal and the pulsed columns, which are perforated plate columns fitted with a plunger pump to promote turbulence and improve efficiency.
Centrifugal extractors. Centrifugal forces, which can be thousands of times greater than gravity, can facilitate separations when emulsification problems occur, density differences are very low, or when very short residence times are required due to rapid deterioration of the product, as in the antibiotic industry. Generally, centrifugal extractors have only one or two stages, although units with four stages have been built.
