separation processes unit 6 study guides

Basics of Liquid-Liquid Extraction

• Liquid-liquid extraction (LLE) separates components based on their relative solubilities in two immiscible liquid phases
• Involves transferring a solute from one liquid phase to another (typically an aqueous phase and an organic phase)
• The two liquids must be immiscible or partially miscible to form separate phases
• Solute distributes between the two phases according to its partition coefficient $K = C_{org}/C_{aq}$
  • Higher $K$ values indicate greater affinity for the organic phase
• Extraction efficiency depends on factors such as solvent selection, phase ratio, and number of stages
• LLE can be performed in batch or continuous mode (mixer-settlers, extraction columns)
• Commonly used in industries such as pharmaceuticals, petrochemicals, and food processing

Principles of Phase Equilibrium

• Phase equilibrium governs the distribution of solutes between immiscible liquid phases
• At equilibrium, the chemical potential of each component is equal in both phases
• Partition coefficient $K$ quantifies the ratio of solute concentrations in the two phases at equilibrium
• Distribution ratio $D$ is the ratio of total solute concentrations (including all forms) in each phase
• Nernst distribution law describes the relationship between $K$ and temperature: $\ln K = -\Delta H/RT + \Delta S/R$
  • $\Delta H$ is the enthalpy change of transfer, $\Delta S$ is the entropy change, $R$ is the gas constant, and $T$ is the absolute temperature
• Deviations from ideal behavior can occur due to solute-solvent interactions, leading to non-linear equilibrium curves
• Ternary phase diagrams represent the equilibrium compositions of three-component systems (solvent, carrier, solute)

Equipment and Techniques

• Mixer-settlers consist of a mixing chamber for contacting the two phases and a settling chamber for phase separation
  • Suitable for systems with fast mass transfer and good phase disengagement
• Extraction columns provide countercurrent contact between the two phases
  • Types include packed columns, perforated plate columns, and agitated columns (rotating disc contactors, Karr columns)
• Centrifugal extractors use centrifugal force to enhance phase separation and increase throughput
• Multistage extraction improves separation efficiency by repeatedly contacting the phases
  • Cross-current and countercurrent configurations are common
• Liquid membrane extraction uses a thin liquid film to selectively transport solutes between two phases
• Supercritical fluid extraction employs a supercritical fluid (CO2) as the extracting solvent
• Microfluidic devices enable precise control over interfacial area and mass transfer in microscale LLE

Solvent Selection and Properties

• Solvent selection is crucial for effective LLE, considering factors such as selectivity, capacity, and regeneration
• Selectivity refers to the solvent's ability to preferentially extract the desired solute over other components
• Capacity is the amount of solute that can be extracted per unit volume of solvent
• Solvent should have low solubility in the feed phase to minimize solvent loss and contamination
• Density difference between the solvent and feed phases facilitates phase separation
• Interfacial tension affects droplet size and coalescence behavior, impacting mass transfer and phase disengagement
• Viscosity influences the hydrodynamics and mass transfer rates in the system
• Volatility and boiling point are important for solvent recovery and regeneration
• Safety considerations include flammability, toxicity, and environmental impact

Mass Transfer in Extraction

• Mass transfer in LLE involves diffusion and convection of solutes across the liquid-liquid interface
• Two-film theory describes mass transfer as diffusion through stagnant films on either side of the interface
• Mass transfer rate is proportional to the concentration gradient and interfacial area: $N = k_L a (C^* - C)$
  • $N$ is the mass transfer rate, $k_L$ is the overall mass transfer coefficient, $a$ is the interfacial area, $C^*$ is the equilibrium concentration, and $C$ is the bulk concentration
• Dispersed phase holdup and droplet size distribution affect the available interfacial area for mass transfer
• Continuous phase turbulence enhances convective mass transfer by reducing film thickness and promoting droplet breakup
• Marangoni effects can arise from interfacial tension gradients, causing convective currents that enhance mass transfer
• Extraction rate is influenced by factors such as agitation intensity, phase ratio, and temperature

Calculation Methods and Design

• Stagewise calculations determine the number of theoretical stages required for a desired separation
• Equilibrium stage models assume perfect mixing and equilibrium between phases in each stage
• Graphical methods like the McCabe-Thiele diagram and Ponchon-Savarit method are used for binary systems
• Analytical methods such as the Kremser equation and Smoker equation provide quick estimates of stage requirements
• Rigorous simulation tools (ASPEN Plus, ProSimPlus) solve mass and energy balances for multicomponent systems
• Shortcut design methods estimate the number of stages and solvent flow rate based on key parameters
  • Minimum solvent flow rate is determined from the operating line and equilibrium curve
• Scale-up considerations include hydrodynamics, mass transfer, and equipment selection
• Economic evaluation compares capital and operating costs of different extraction processes and configurations

Industrial Applications

• Hydrometallurgy: Extracting metals (copper, nickel, cobalt) from aqueous solutions using organic extractants
• Nuclear fuel reprocessing: Recovering uranium and plutonium from spent nuclear fuel using tributyl phosphate (TBP)
• Pharmaceuticals: Purifying antibiotics, vitamins, and other bioactive compounds from fermentation broths
• Food industry: Decaffeinating coffee and tea, extracting edible oils, and recovering valuable components (polyphenols, carotenoids)
• Petrochemicals: Removing aromatics from lubricating oils, separating close-boiling hydrocarbons, and purifying solvents
• Environmental remediation: Treating wastewater effluents, removing pollutants (phenols, pesticides), and recovering valuable metals
• Biotechnology: Downstream processing of proteins, enzymes, and other bioproducts from cell cultures
• Analytical chemistry: Sample preparation, preconcentration, and matrix removal for chromatographic analysis

Troubleshooting and Optimization

• Poor phase separation can result from emulsion formation, solvent degradation, or changes in feed composition
  • Strategies include optimizing agitation, adding coalescers, and adjusting pH or temperature
• Insufficient mass transfer may be caused by low interfacial area, poor mixing, or unfavorable equilibrium conditions
  • Increasing agitation, using finer dispersions, and modifying solvent composition can improve mass transfer
• Entrainment of one phase in the other leads to product contamination and solvent losses
  • Minimize entrainment by controlling droplet size, providing adequate settling time, and using phase separation aids (coalescers, demisters)
• Solvent losses can occur due to solubility in the raffinate, volatilization, or degradation
  • Optimize solvent recovery, use less soluble solvents, and control temperature and pressure to minimize losses
• Fouling of extraction equipment by solids, precipitates, or biological growth reduces efficiency and increases maintenance
  • Implement proper pretreatment (filtration, pH adjustment), use compatible materials, and establish cleaning protocols
• Process integration opportunities, such as heat integration and solvent recycling, can improve overall efficiency and economics
• Online monitoring and control of key variables (pH, temperature, interface level) ensure stable and optimal operation
• Pilot-scale testing and simulation studies help identify potential issues and optimize design before full-scale implementation