Liquid-Liquid Extraction
Liquid-liquid extraction (LLE), also called solvent extraction, is a separation technique that transfers one or more solutes from a feed liquid phase into an immiscible or partially miscible solvent phase based on differences in solute solubility (expressed as a distribution coefficient or partition coefficient). The process is carried out in mixer-settlers, pulse columns, rotating disc contactors, or centrifugal extractors, and is especially valuable when distillation is impractical due to azeotropes, heat-sensitive solutes, or low solute concentrations. LLE is central to hydrometallurgy, pharmaceutical purification, nuclear fuel reprocessing, and edible-oil refining.
Key Formula
K_D = [A]_solvent / [A]_feed; Extraction fraction E = K_D * V_s / (V_s + V_f / K_D)
LaTeX: K_D = \frac{[A]_{solvent}}{[A]_{feed}}, \quad E = \frac{K_D \cdot V_s}{V_s + V_f / K_D}
| Symbol | Meaning | Unit |
|---|---|---|
| K_D | Distribution (partition) coefficient of solute A | dimensionless |
| [A]_{solvent} | Equilibrium concentration of A in solvent phase | mol/L |
| [A]_{feed} | Equilibrium concentration of A in feed phase | mol/L |
| V_s | Volume of solvent phase | L |
| V_f | Volume of feed (aqueous) phase | L |
Worked Example
Problem
An antibiotic is extracted from 1 L of aqueous broth using 0.5 L of ethyl acetate. The distribution coefficient K_D = 8 (solvent/water). Calculate the fraction of antibiotic extracted in a single stage.
Solution
Step 1: Use single-stage extraction formula. Fraction extracted = K_D × V_s / (K_D × V_s + V_f) = 8 × 0.5 / (8 × 0.5 + 1) = 4 / (4 + 1) = 4 / 5 = 0.80 Step 2: Verify interpretation: 80% of the antibiotic transfers to the ethyl acetate layer in one contact.
Answer
80% of the antibiotic is extracted in a single stage.
Liquid-Liquid Extraction Equipment Types
| Equipment | Throughput | Stage Efficiency | Solute Sensitivity | Typical Application |
|---|---|---|---|---|
| Mixer-settler | High | High (>90%) | Low sensitivity | Hydrometallurgy, nuclear |
| Pulsed packed column | Moderate | Moderate | Moderate | Pharma, fine chemicals |
| Rotating disc contactor | Moderate | Moderate | Moderate | Petroleum refining |
| Centrifugal extractor | Low–Moderate | Very high | High sensitivity | Pharmaceuticals, biotech |
| Spray column | High | Low | Low sensitivity | Crude liquid extraction |
Interactive Tools
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Related Terms
Distillation
Distillation is a thermal separation process that exploits differences in the volatility (relative volatility) of mixture components: a liquid feed is partially vaporised, the vapour enriched in the more volatile component rises and is condensed, while the less volatile component concentrates in the liquid bottoms. In a continuous distillation column, repeated vapour-liquid equilibrium stages—either trays or structured packing—progressively sharpen the separation, with the reflux ratio governing the trade-off between product purity and energy consumption. It is the most widely used separation process in the petrochemical, pharmaceutical, and food industries.
Absorption Column
An absorption column (absorber) is a mass-transfer device in which a gas mixture flows upward counter-currently against a descending liquid solvent, causing one or more gaseous components to dissolve into the liquid phase driven by a concentration gradient and governed by vapour-liquid equilibrium. The height of the packed or trayed column is determined by the Number of Transfer Units (NTU) and the Height of a Transfer Unit (HTU), or by the number of theoretical stages. Absorption is widely used to remove acid gases (CO₂, H₂S) from natural gas, SO₂ from flue gas, and ammonia from industrial off-gas streams.
Mass Balance (chemical engineering)
A mass balance (also called a material balance) is the systematic application of the law of conservation of mass to a defined control volume or process unit, accounting for all mass entering, leaving, generated by reaction, and accumulating within the system. It is the foundational tool of chemical process design, enabling engineers to size equipment, determine conversion, specify recycle streams, and detect leaks or unaccounted losses. At steady state with no reaction, the balance simplifies to: mass in = mass out.
From Latin "extractio" (a drawing out), from "ex-" (out) + "trahere" (to drag, pull). The liquid-liquid variant was distinguished from solid-liquid extraction in the early 20th century; systematic use of distribution coefficients was formalised by Walter Nernst in his 1891 distribution law.