7.6 Extraction

Liquid-liquid extraction is a key separation technique in chemical engineering. It uses two immiscible liquids to separate a solute based on its solubility, with applications in purifying products and recovering valuable components from mixtures.

The process's effectiveness depends on factors like solvent choice and distribution coefficient. Engineers design extraction processes using single or multistage approaches, considering equipment selection and operating conditions to maximize efficiency and separation.

Principles of Liquid-Liquid Extraction

Fundamentals of Liquid-Liquid Extraction

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• Liquid-liquid extraction separates a solute from one liquid phase to another immiscible liquid phase based on the solute's relative solubility in each phase
• Involves two immiscible liquid phases, typically an aqueous phase and an organic solvent phase
• The solute distributes itself between the two phases based on its partition coefficient

Applications of Liquid-Liquid Extraction in Chemical Engineering

• Separates valuable components from mixtures
  • Extracts antibiotics from fermentation broths
  • Recovers metals from aqueous solutions
• Purifies products by removing impurities or unwanted compounds
  • Removes organic acids from wastewater
  • Extracts contaminants from oil
• Concentrates dilute solutions by selectively extracting the desired component into a smaller volume of solvent

Factors Affecting the Effectiveness of Liquid-Liquid Extraction

• Choice of solvent
• Distribution coefficient of the solute
• Phase ratio (ratio of the volume of the extract phase to the raffinate phase)
• Number of extraction stages

Distribution Coefficients and Selectivity

Distribution Coefficient (K)

• Measures how a solute distributes itself between two immiscible liquid phases at equilibrium
• Defined as the ratio of the solute concentration in the extract phase to its concentration in the raffinate phase
• Affected by factors such as the solute's relative solubility in each phase, temperature, and the presence of other components in the system

Selectivity (β)

• Measures an extraction system's ability to separate two solutes
• Defined as the ratio of their distribution coefficients (β = K1/K2)
• A higher selectivity indicates a better separation of the desired solute from other components in the mixture

Determination of Distribution Coefficients and Selectivity

• Determined experimentally by equilibrating the solute between the two phases
• Solute concentration in each phase measured using analytical techniques (gas chromatography, UV-vis spectroscopy)
• Used to evaluate the feasibility and effectiveness of an extraction process
• Helps design the required number of extraction stages

Design of Extraction Processes

Single-Stage Extraction

• Involves contacting the feed mixture with the solvent in a single equilibrium stage
• Followed by the separation of the resulting extract and raffinate phases
• Effectiveness depends on the distribution coefficient and the phase ratio

Multistage Extraction

• Repeats the extraction process over multiple equilibrium stages
• Raffinate from one stage serves as the feed for the next stage
• Achieves higher overall extraction efficiencies
• Approaches the theoretical maximum separation based on the distribution coefficient
• Number of stages required determined using graphical methods (McCabe-Thiele method) or mathematical models (Kremser equation, Varteressian and Fenske equation)

Design Considerations for Extraction Processes

• Choice of equipment (mixer-settlers, extraction columns)
• Flow configuration (cross-current, countercurrent)
• Operating conditions (temperature, pressure, mixing intensity)

Factors Affecting Extraction Efficiency

Solvent Selection

• Critical factor in determining extraction efficiency
• Solvent must have high selectivity for the desired solute, low miscibility with the feed phase, and favorable physical properties (density, viscosity, surface tension)
• Should be inexpensive, non-toxic, non-flammable, and easy to recover and recycle
• Common solvents include hydrocarbons (kerosene, hexane), alcohols (ethanol, isopropanol), ethers (diethyl ether, methyl tert-butyl ether), and halogenated hydrocarbons (chloroform, dichloromethane)

Phase Ratio

• Ratio of the volume of the extract phase to the raffinate phase
• Affects the extraction efficiency by determining the amount of solvent required and the concentration of the solute in the extract phase
• Higher phase ratio leads to higher extraction efficiency but increases cost and complexity
• Optimal phase ratio depends on the distribution coefficient, desired recovery of the solute, and economic and environmental considerations

Other Factors Affecting Extraction Efficiency

• Temperature influences the distribution coefficient and the solubility of the solute
• pH affects the ionization state and the partitioning of the solute
• Presence of other components can cause co-extraction or emulsion formation