Introduction to Liquid-Liquid Extraction

Introduction to Extraction

• Liquid-liquid extraction, or solvent extraction, separates solutes from a liquid solution using an insoluble liquid.
• Separation hinges on the solubility differences of solutes in the contacting liquids.
• The feed is the original solution comprising solute A and carrier liquid C, while the solvent S is a pure liquid.
• The result of extraction appears as two phases: the extract (solvent-rich) and the raffinate (residual liquid).

Purpose of Extraction

• Used to separate mixtures with close boiling points that can't withstand high distillation temperatures.
• Examples include recovering penicillin from fermentation broths with butyl acetate and acetic acid from aqueous solutions using ethyl acetate.

Liquid-Liquid Extraction

• Comprises three components: carrier liquid (A), extracting solvent (B), and solute (C).
• Essential for applications requiring solute transfer between two immiscible or partially miscible liquid phases.

Liquid-Liquid Equilibria (LLE)

• Vital for equipping and designing extraction setups.
• Equilibrium data assists in optimizing the selection of extraction systems and solvents.

Classification of Extraction Processes

• Periodical extraction involves shaking separatory funnels.
• Continuous extraction maintains a constant flow for separation.
• Countercurrent extraction optimizes contact between phases for enhanced separation.

Choice of Solvent

• Key factors include selectivity, distribution coefficient, and insolubility.
• Consider recoverability of solute, density differences, interfacial tension, chemical reactivity, cost, viscosity, vapor pressure, flammability, and toxicity.

Advantages of Solvent Extraction

• Allows selective extraction directly from fermentation broths and reaction mediums.
• Reduces product loss due to degradation as solutes transfer to a different phase.
• Adaptable across various operational scales, including packed extraction towers and mixer-settlers.

Application of Extraction

• Widely employed in the petroleum industry for separating heat-sensitive liquid feeds by chemical type.
• Significantly utilized in biochemical and pharmaceutical sectors for antibiotic separation and protein recovery.
• Used in inorganic chemical processes for recovering high-boiling compounds like phosphoric acid and sodium hydroxide.

Distribution Law

• At equilibrium, the solute concentration ratio in two phases relates by the equation CE/CR = K.
• The distribution constant K must exceed unity for extract streams and be less than unity for raffinate streams.

Typical Extraction System in Industry

• Extraction systems necessitate at least one distillation column or another separation process to recover solvents from the extract.
• Additional distillation is required when solvents exhibit some miscibility in the feed to recover solvents from the raffinate.

Classification of Extractors

• Extractors can be categorized as single-stage or multi-stage systems.

Types of Extractor Equipment

• Single-stage extractors include mixer-settlers and decanters.
• Multi-stage extractors involve mixer-settler batteries and mechanically agitated extractors like Scheibel and rotating disk contactors.

Mixer-Settler

• Mixer-settlers are favored for reliability, operating flexibility, and high capacity in handling challenging systems.
• They accommodate viscous liquids and solid-liquid slurries but may require substantial space and material inventory.
• Operations can be continuous or batchwise, with mixer and settler configurations being integral or separate.