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• Pervaporation
- Discovered 1917.
- Only operation with phase change.
- Non-Porous Membranes.
- Mechanism solution-diffusion.
- Driving force: difference in partial pressure.
-Vacuum (<40 mm Hg), dilution (inert gas, N2) or temperature difference.
Membrane Technology
• Pervaporation
Condenser
Pervaporat. module
Feed
Retentate
Vacuum pump
Permeate condensate
Heater
General Pervaporation system.
Membrane Technology
• Pervaporation
- Industrial applications.
- Alternative to distillation when thermodynamic limitations.
Low energy costs. Low investment costs. Better selectivity, without thermodynamic limitations. Clean and closed operation. No process wastes. Compact and scalable units.
- Recovery of volatile compounds at low concentrations.
- Separation of azeotropic mixtures.
Membrane Technology
• Pervaporation.
- Do not mistake with a distillation where a membrane is just separating phases.
- Three steps mechanisms:
Selective absorption on the membrane. Dissolution at the membrane. Diffusion through the membrane.
Membrane Technology
• Pervaporation
- The membrane is active in this process.
- The permeability coefficient (P) of a compound depends on the solubility (S) and the diffusivity (D), in the polymeric phase, of the crossing compound
Pi = Si (ci, cj)· Di (ci, cj)
- Simplificated transport equation:
pioiii
ii pypx
dP
J
Ji: flux of component i d: membrane thickness xi: molarfraction in liquid i: activity coefficientpi
o: vapour pressure yi: molar fraction at permeate pp: pressure at permeate side
Membrane Technology
• Pervaporation
- Main membrane parameters:
- Separation factors - Enrichment factors
B
A
B,fA,f
B,pA,pA,B /CC
/CCα
A,f
A,pA C
C
Membrane Technology
• Pervaporation
Pervaporation process of an ethanol/water mixture with a PVA membrane.
0.00.2 0.4 0.6 0.8 1.0
0.2
0.4
0.6
0.8
1.0
azeotrope
pseudoazeotropeEh
tan
ol a
t p
erm
eate
(v
apou
r)
Ethanol at feed (liquid)
Phase equilibria
pervaporation
0.0
Membrane Technology
Intermediat tank
Condenser
Pervap. unit
Feed
Ethanol >90% w/w
Permeate
Boiler
Water
Distillation column
Ethanol >99.95% w/w
Ethanol 20-80% w/w
Ethanol 15% w/w
• Pervaporation
Combination of distillation and pervaporation for the production of pure ethanol.
Plant for production of ethanol from sugar (Bethéniville, France).