Liquid Extraction(L-L or L-S)

• Distillation based on differences in boiling point (vaporpressure)

• L-L extraction based on chemical differences (L-Ldistribution coefficient)

• Similarities with distillation and gas absorption– Light and heavy phases (e.g. solvent/water)– Ideal stages, efficiency, ratio of fluid flowrates, and equipment size

• L-L extraction preferred for– Thermally sensitive compounds - proteins, antibiotics– Inorganic substances - metals, salts, some acids/bases– Low solute concentrations– Chemicals with similar BPs– Azeotropic mixtures

• Solvent selection is key– Non-toxic, inexpensive, recoverable, inert– Immiscible w/carrier, difference in density, low

viscosity, low surface tension– High solute partition coefficient (KD)– High solute selectivity (α)– High solute capacity

PHASE IISolvent (S)

PHASE ICarrier (C) +Solute (A)

xAI

xAII

xCI

xCII

xSI

xSII

PHASE IISolvent (S)

PHASE ICarrier (C) +Solute (A)

xA

yA

xC

yC

xS

yS

(A) (C) (S)

• Equipment selection and design is also key– STEP 1: Need optimal mixing - high contact area

(area/volume or area/mass)– STEP 2: Need optimal separation - time for S/C phase

separation‒ Both steps are based on system and solvent choice

• Most common equipment– Mixer-Settler– Packed columns– Perforated plate columns (similar to sieve-plate

distillation columns)– Agitated columns or towers– Membrane contactors

Mixer-Settlers

solvent

Mixing disperses one phase intothe other in the form of droplets

f08_04

Packed and Plate Columns• Packed Columns

– Packing materials breaks down droplets and increasescontact area - area/volume

– Packing must allow wetting of continuous phase• Continuous phase contains the dispersed droplets• E.g. Heavy water drops (C) containing (A) falling through a

lighter solvent phase (S)– Must consider flooding conditions

• Perforated Plate Columns– Light phase “acts as gas” in distillation

Agitated Columns

Membrane Hollow FiberContactors (HFC)

• High contact area/volume• No emulsions, no settling needed• No internal moving parts• Continuous, easily scalable• Can handle suspended solids• Membrane supports interface

– Hydrophobic membrane is wetted byhydrophobic solvent

– Density difference doesn’t need tobe as great as other equipment

• Dilute solutions (< 10 wt%): Changes in flow ratescan be neglected and KD’s are constant

• Concentrated solutions: Flow rates and KD’s change

Dilute vs. Concentrated Feeds

Quick Review of Ternary PhaseBehavior

Type I - one immiscible pair(i.e. solvent/carrier), soluteand solvent are infinitely

Type II - two immiscible pairsdepending on composition (i.e.solvent/carrier and solvent/solute)

Reading the Ternary Graph*acetone extraction from waterwith methyl isobutyl ketone at25oC (water/acetone/MIK)

*40/60 A/W with equil mass MIK

Tie lines slope “up towards” MIK phase - solvent richer then carrier,positive KD, lower solvent requirement

Reading the Ternary Graph

*acetone extraction fromwater with MIK at 25oC(water/acetone/MIK)

Tie lines slope “up towards” MIK phase - solvent richer then carrier,positive KD, lower solvent requirement