Liquid–liquid extraction

Separation process using two immiscible liquids

Liquid–liquid extraction (LLE), also known as solvent extraction, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. It is an important technique in chemical engineering and analytical chemistry.

Principles of Liquid–Liquid Extraction

Liquid–liquid extraction involves the distribution of a solute between two immiscible liquid phases. The process is driven by the difference in solubility of the solute in the two phases. Typically, one phase is aqueous, and the other is an organic solvent. The solute is transferred from one phase to the other, achieving separation.

A typical laboratory setup for liquid-liquid extraction using separatory funnels

The efficiency of the extraction depends on the partition coefficient, which is the ratio of the concentration of the solute in the organic phase to its concentration in the aqueous phase. A higher partition coefficient indicates a more efficient extraction.

Equipment Used in Liquid–Liquid Extraction

Several types of equipment are used in liquid–liquid extraction, ranging from simple laboratory apparatus to complex industrial systems.

Laboratory Equipment

In the laboratory, a common piece of equipment used for liquid–liquid extraction is the separatory funnel. This allows for the manual separation of the two phases after mixing.

Industrial Equipment

In industrial applications, more sophisticated equipment is used, such as mixer-settlers, centrifugal extractors, and column extractors.

Diagram of a mixer-settler system used in industrial liquid-liquid extraction

• Mixer-Settlers: These consist of a mixing chamber where the two liquids are mixed, and a settling chamber where they are allowed to separate.
• Centrifugal Extractors: These use centrifugal force to enhance the separation of the two phases.
• Column Extractors: These are tall columns where the two phases flow counter-currently, allowing for continuous extraction.

Applications of Liquid–Liquid Extraction

Liquid–liquid extraction is used in various industries and applications, including:

• Pharmaceuticals: To purify and isolate active ingredients.
• Petrochemicals: For the separation of hydrocarbons.
• Food Industry: To extract flavors and fragrances.

Continuous separation of oil and water mixture in an industrial setting

Advantages and Disadvantages

Advantages

• High selectivity and efficiency for certain solutes.
• Can be used for heat-sensitive compounds as it operates at ambient temperatures.

Disadvantages

• Requires careful selection of solvents to avoid emulsions.
• Solvent recovery and disposal can be costly and environmentally challenging.

Related Pages

• Distillation
• Chromatography
• Solvent
• Partition coefficient