Liquid-Liquid Extraction

• Liquid-liquid extraction separates components in a liquid (feed) by contacting it with a second liquid phase (solvent).
• The two phases involved must be chemically different.
• The transfer of components from one phase to another is driven by deviation from thermodynamic equilibrium.
• Separation occurs based on differences in physical and chemical properties, like polarity and hydrophobic/hydrophilic character.
• The process yields a solvent-rich stream named the extract and an extracted feed stream, called the raffinate.

Specific Applications include

• Removing acetic acid from water through an organic solvent or distillation for contaminant removal.
• Removing high-molecular-weight fatty acids from vegetable oil using propane as solvent or high-vacuum distillation for desired component recovery.
• Separating penicillin from complex fermentation mixtures for desired component recovery
• Separating metals from aqueous solutions of copper-iron, uranium-vanadium, and tantalum-columbium for contaminant removal.

Solvent-Extraction Equipment

• Vessels with mechanical agitation
• Vessels in which mixing occurs through the flow of fluids themselves.

Definition of Terms

• Feed is the solution containing components to be separated.
• Feed solvent, also called the carrier solvent, is the major liquid component in the feed.
• Solute is the minor component in the solution feed.
• Extraction Solvent or Separating Agent is an immiscible liquid added to extract one or more solutes.
• Extractant can be dissolved in a diluent.
• Modifier may be added to the diluent to increase extractant solubility or enhance effectiveness.
• Theoretical or Equilibrium Stage is a device or setup that intimately mixes two liquid phases until equilibrium concentrations are reached, then physically separating the phases into clear layers.
• Extraction Factor refers to the ratio of the slope of the equilibrium line to the slope of the operating line.
• Separation Factor is comparable to relative volatility in distillation
• Partition Ratio / Distribution Constant / Distribution Coefficient, K represents the solute concentration in the extract phase divided by that in the raffinate phase after equilibrium in a single stage of contacting.

Solvent Properties

• Loading capacity is the maximum solute concentration the extract phase holds before the two liquid phases can no longer coexist, or the solute precipitates as a separate phase.
• Partition ratio Kᵢ = Yᵢ/Xᵢ allows smaller, less costly extraction equipment, low solvent use, and lower solvent recovery and recycle costs.
• Partition ratios should be on the order of Kᵢ = 10 or higher for an economical process.
• Solute selectivity is needed to recover the desired solute from the feed and separate it from other solutes for solute purification.
• The selectivity of a solvent for solute i compared to solute j is characterized by the separation factor αᵢ,ⱼ = Kᵢ/Kⱼ.
• Values must be greater than αᵢ,ⱼ = 1.0 to increase solute purity on a solvent-free basis
• Mutual solubility, low liquid-liquid mutual solubility between feed and solvent phases reduces separation requirements for removing solvents from the extract and raffinate streams
• Stability: Solvents should have little tendency to react with the product solute and form unwanted by-products, causing a loss in yield.
• Solvents should not react with feed components or degrade to undesirable contaminants
• Viscosity: Low viscosity is preferred as higher viscosity increases mass-transfer resistance and liquid-liquid phase separation difficulty
• Extraction processes operate at higher temperatures where viscosity drops for better mass-transfer performance, even when it results in dropping partition ratios
• Recoverability: Economical solvent recovery from the extract and raffinate is critical for commercial success.
• Density difference: A density difference between solvent and feed phases should be on the order of 0.1 to 0.3 g/mL for the best results
• A value that is too low leads to poor liquid-liquid phase separation & may need a centrifuge.
• A value too high hinders the building of dispersed-droplet population density
• Safety: inherently safe solvents with low potential reactive chemistry hazards are preferred.
• Interfacial tension between the feed and extraction solvent phases is between 5 to 25 dyn/cm (1 dyn/cm = 10⁻³ N/m).
• Systems with lower interfacial tension values easily emulsify
• Higher interfacial tension values allow dispersed droplets to coalesce easily, resulting in low mass transfer and a poorly performing extraction, unless mechanical agitation is used.
• Industrial hygiene solvents with low mammalian toxicity and good warning qualities are desired
• Low toxicity and low dermal absorption reduce the chance of acute exposure injury.
• Freezing point: Solvents that are liquids at all anticipated ambient temperatures are more useful
• Multiple uses: Choose an extraction solvent of materials that can serve a number of purposes at the same manufacturing plant
• Environmental requirements: select solvents of physical or chemical properties that allow effective control of emissions from vents and other discharge streams
• Select preferred properties like low aquatic toxicity, low leakage/spill fugitive emission potential, low photoreactivity, and biodegradation
• Materials of construction solvents allow common, inexpensive materials of construction at moderate temperatures and pressures.
• Availability and cost considerations include initial fill cost and the cost for stocking a solvent inventory

Equilibrium Relations

• Equilibrium involves three components -- A, B & C -- and two phases.
• The process parameters are temperature, pressure, and concentrations
• xA + xB + xC = 1.0

Types of Equipment

• Mixer-Settlers are used for Extraction
• Spray-type Extraction Towers
• Packed Extraction Towers, more efficient than spray towers
• Flooding happens when increasing dispersed or continuous phase flowrates causes the extraction phases to leave through the continuous phase outlet
• Pulsed Towers.
• Mechanically Agitated Extraction Towers

Flooding and Design Flowrates

• Design flowrates should be at 50% of the flooding parameters.

Packed Extraction Towers

• Are more efficient than spray towers
• Only work for a few stages.
• Work best at an interfacial tension (σ) of about 10 dyn/cm.
• The choice of packing material depends on the continuous phase while random packing bests structured packing.
• Height Equivalent to a Theoretical Stage (HETS) is greater than mechanically agitated towers.

Spray-Type Extraction Towers

• Used where rapid, irreversible reactions like neutralizing waste acids occur. 
• These towers only have one or two stages, are low-cost, but are not often used.

Pulsed Packed + Sieve-Tray Towers:

• Agitation boosts mass-transfer efficiency.
• Packed towers can lower the height equivalent to a theoretical stage (HETS) by half.
• Packed towers accommodate liquids exhibiting high interfacial tension (30-40 dyn/cm).
• Sieve-Tray Towers boast hole sizes of 0.32 cm, comprise 20-25% free tray space, and spaced around 5.1 cm apart.