Liquid-Liquid Extraction

Liquid-liquid extraction is the separation of the components in a liquid phase (diluent) by treatment with a solvent in which one or more of the desired components are preferentially soluble. Normally, the diluent + remaining solute is called the raffinate phase while the second solvent + the solute is the extract phase.

It is a process that is suitable to separate materials that may decompose at high temperatures.

Below is an example of an extraction process diagram:

This process is commonly used in the separation of hydrocarbons in the petroleum industry like the separation of aromatics from kerosene-based fuels oils (to improve their burning qualities), separation of aromatics from paraffin and naphthenic compounds (to improve the temperature-viscosity characteristics of lubricating oils), to obtain relatively pure compounds (benzene, toluene and xylene) from catalytically produced reformats.

For liquid-liquid extraction, it is vital that the liquid mixture feed and solvent are at-least partially or completely immiscible and three stages are involved:

a) The feed mixture and solvent are made to contact.
b) Separation of the resulting two phases.
c) Removal and recovery of the solvent from each phase.

Stages (a) and (b) can be combined into a single piece of equipment like a column that operates continuously known as differential contacting. Liquid-liquid extraction is also carried out in a stage-wise equipment like a mixer-settler unit. Extraction can either occur through physical operation or a chemical operation. 

Extraction is a preferred alternative to distillation is cases where:

• Distillation would require excessive amounts of heat like when the relative volatility is near unity.
• The components in the feed have very close boiling points
• The components to be separated are very different in nature.
• The formation of azeotropes limits the degree of separation obtainable.
• One of the components is present at a concentration that is too low for distillation to occur.
• Heating must be avoided.

Mass Balance

If we perform an overall mass balance calculation on the above system from stage 1 to stage N, we get:

F + S = E₁ + R_N

Criteria for Selecting a Solvent for Liquid–Liquid Extraction

• Selectivity: A solvent's ability to separate components A and C (desired compound) is determined by comparing the concentration ratio of C to A in the solvent-rich phase with that in the A-rich phase at equilibrium. For extraction to be effective, the selectivity must be greater than one; higher values indicate better separation efficiency. If the selectivity equals one, separation cannot be achieved.
• Recoverability: The chosen solvent should be easy to recover and recycle using methods that are both safe and cost-effective.
• Density: A significant difference in the densities of the two saturated liquid phases is desirable, as it facilitates phase separation.
• Interfacial Tension: High interfacial tension is generally preferred because it promotes the coalescence of emulsified droplets. However, it can also make the dispersion of one liquid into another more difficult.
• Safety & Cost: An ideal solvent should be non-toxic, non-flammable, and economical to use.
• Chemical Stability: The solvent should be chemically stable and non-reactive toward the components being separated as well as the materials used in the equipment.
• Viscosity, Vapor Pressure & Freezing Point: Low viscosity, low vapor pressure, and a low freezing point are advantageous because they simplify handling, storage, and processing operations.

Uses of Liquid-liquid Extraction

• Extraction of valuable products from a fermentation broth.
• Purification of heat sensitive materials like pharmaceuticals, fragrances etc.
• Removal of high boiling organics like phenol, aniline etc., from waste water.
• Recovery of reaction products. 

Limitations of Liquid-liquid Extraction

• Time consuming and laborious.
• Consumes a large amount of organic solvents hence environmentally polluting. 
• Generates a large amount of waste.
• Selectivity is low.
• Can result in the formation of hard to break emulsions.

Differences Between Liquid-liquid Extractions and Distillation.

• Liquid-liquid extraction uses differences in solubilities of solutes in two solvents while distillation uses differences of boiling points of components in a mixture
• Liquid-liquid extraction uses selective solubility as a degree of separation while distillation uses relative volatility as a degree of separation
• Liquid-liquid extraction doesn’t produce pure products while distillation produces almost pure products
• Liquid-liquid extraction uses a separating funnel while a distillation apparatus is used for distillation
• No new phases created during liquid-liquid extraction while new phases are created by addition of heat during distillation
• Liquid-liquid extraction doesn’t require heating and cooling provisions while distillation does