liquid extraction (l-l or l-s)
TRANSCRIPT
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
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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