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Discussion TopicsDiscussion Topics
Pervaporation Principles
Model Description
Performance parameters
Influence Parameters
Membranes for Pervaporation
Applications
Modules
ProcessDesign
Process energy requirements
A Little of History
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A Little of History
1
Was discovered in 1917 by Kober.
The first full scale plant was installed inBrazil in 1982 for the production ofethanol.
Appears as a promising and commerciallycompetitive process for separation (morecost effective for some specific problems).
Figure 1. Membrane market
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2
Current position market
Actually there are 120 PV installations in
used world wide.
Le Carbone-Lorraine is a very important
French company that has built many of
them.
Pervaporation still have to compete
against other membrane separation
techniques.
A Little of History
Future potential
Significant energy savings of up to 55%
could be achieved by replacing all the
thermal separation processes in the EU and
Norway by pervaporation.
Because pervaporation systems make use
of more advanced technologies than
conventional separation methods,
investment costs are considered
comparable.
Simple pay back times of less than 1 year
have been reported for pervaporation
installations.
Operation and maintenance costs (O&M)
are expected to be higher than the
conventional separation process.
Market barriers
Lack of information.
Poor availability of investment capital.
Perceived risks associated with the
reliability of the process.
Comparingpervaporation with
distillation.
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Pervaporation Principles
Is the only membrane process wherephase transition occurs.
At least the heat of vaporization have to
be supply.
The mass transport is achieved loweringthe activity of the permeating componenton the permeate side by: gas carrier,vacuum or temperature difference.
The driving force is the partial pressuredifference of the permeate between the
feed and permeate streams.
The permeate pressure has to be lowerthan the saturation pressure of thepermeant to achieve the separation.
Figure 2. Schematic draws of pervaporation processes.3
VacuumPervaporation
Gas carrierPervaporation
Temperaturedifference
Pervaporation
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Mechanism of Transport
Pervaporation involve a sequence ofthree steps:
Selective sorption
Selective diffusion through themembrane.
Desorption into a vapor phase on the
permeate side.
Because of its characteristics, pervaporationis often mistakenly considered as a kind ofextractive distillation but VLE { Solution-Diffution mechanism.
Figure 3. Comparison betweenVLE and pervaporation
4
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Model Description
Solution-Diffusion Model
Membrane permeability is a function ofsolubility and diffusivity:
Diffusivity and solubility are stronglydependent of feed composition.
The liquids have more affinity towardspolymeric membranes than gases (FloryHuggins theory instead Raoults law).
Equation of transport:
jiijiii CCSCCDP ,, !
piiiiii pypxl
PJ !
0K
Thermodynamic accounting approach
Its distinguished two steps:
Equilibrium evaporation.
Membrane permeation of the
hypothetic vapor. Membrane selectivity contribution tooverall separation is showed as a change ofcomposition for the vapor phase lowering thetotal pressure below equilibrium vaporpressure (Thompson diagram).
There are two ways to rationalize the observed separation effects in pervaporation:
Figure 4. Thompson diagram
5
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Activity Profile
Figure 5. Activity profile
The liquid swells the membrane in pervaporation (anisotropic swelling).
The activity of the liquid is equal to the activity on the membrane (Thermodynamicequilibrium).
The concentration of the liquid on the feed side of the membrane is maximum whilst onthe permeate side is almost zero.
Flux equation (pure liquid):
The concentration inside the membrane (cim) is the main parameter, implying thatpermeation rate is mainly determine for the interaction membrane-penetrant.
When concentration inside the membrane increase the permeation rate also increase.
1,0 ! miii
i ckExpl
DJ
Concentration dependance diffusion coef. iiii ckExpDD ! ,0
ki Plasticizing constant,membrane permeant
interaction
6
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Mixture of Liquids
For the transport of liquid mixtures through a polymeric membrane the flux can alsobe described in terms of solubility and diffusivity, then two phenomena must be
distinguished:
Flow coupling: Is described in terms of the non-equilibrium thermodynamics and
accounts for that the transport of a component is affected due to the gradient of
the other component.
Thermodynamic interaction: Is a much more important phenomenon. It accounts for
the interaction of one component over the membrane, it becomes more accessible to
the other component(s) because the membrane becomes more swollen (the diffusion
resistance decrease).
Figures 6. Mixture of liquids
7
Overall
sorption
Overall
Flux
Sorption
selectivity
Pervaporation
selectivity
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Performance Parameters
Some of the most important parameters used to assess the pervaporation process are:
1. Pervaporation selectivity: This parameter compare the analytical compositions ofpermeate and feed. There are two forms:
Separation factor, E
Enrichment factor, F
Feed
Permeate
Feed
Permeateij
cjci
pjpi
cjci
cjci
!
!E
iF
iPi
c
c!F
2. Sorption selectivity: Permeability isfunction of solubility and diffusivity
and both may be selective.Sorption selectivity may or may not beequal to pervaporation selectivity. Dueto contribution of selective diffusivityto the overall separation effect.
SDPV EEE ! Figure 7. Sorption isotherms8
Flory-Huggins Isotherm(Glassy: liquid sorption)
Langmuir Isotherm(Glassy: gas sorption)
Henry Isotherm(Rubbery: liquid and
gas sorption)
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Performance Parameters (2)
3. Evaporation selectivity: The separation factor is considered to be a product ofevaporation separation and membrane separation yields:
Membrane selectivity depends on permeate pressure, while evaporation invariablyenriches the more volatile solution compound.
4. Flux: Denote the amount of permeate per unit membrane area and unit time at givenmembrane thickness. Its a realy important parameter for the operation of the process.
Fj
i
pj
i
Fj
i
Fj
i
MEVPV
pp
pp
cc
pp
v
!! EEE
11 eu MM or EE
Pervaporation favors themore volatile compound
Pervaporation favors the lessvolatile compound
9
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Influence Parameters
1. Feed concentration: Refers to the concentration of the preferentially permeating(usually minor) solution component, being depleted in the process. There are twoaspects to be considered:the activity of the target component in the feed and thesolubility of the target component in the membrane.
Activity coefficient: The activity of a liquid solution component is given by its partialvapor pressure:
The behavior of the liquid solution is determined for the activity coefficient:
Azeotropic mixture: Positive solution non-ideality is asociated with positiveazeotropes, and negative solution non-ideality is asociated with negative azeotropoes.
Pervaporation can separate only positive azeotropes.
Concentration polarization: In pervaporation, a depletion of the preferentiallypermeating species near the membrane boundary is to be expected, limiting its polymersorption. But depends of the concentration dependance and sign of the activitycoefficient of the penetrant species.
00
iiiiiF papxp !! K
11 eu ii or KKPositive deviation
from Raouls lawNegative deviation
from Raoults law
10
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Influence Parameters (2)
2. Membrane thickness:
Refers to dry thickness.
Because flux is inversely proportional to membrane thickness, thin membranes favors
the overall flux but decrease selectivity.
Thin membranes are used for low swelling glassy membranes and thick membranes are
used for high swelling elastomeric membranes to maintain the selectivity.
3. Pemeate pressure:
Permeate pressure provides the driving force in pervaporation.
The permeation rate of any feed component increases as its partial permeate
pressure is lowered. The highest conceivable permeate pressure is the vapor pressureof the penetrant in the liquid feed.
The effect of this parameter on pervaporation performance is dictated by the
magnitude of the vapor pressures encountered, and by the difference in vapor
pressures between them.
The highestvacuum feasible
is 1 atm.11
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Influence Parameters (3)
4. Temperature
Refers to feed temperature or any other representative between feed and retentate
streems.
The feed liquid provided the heat of vaporization of the permeate, and in consequence
there is a temperature loss between the feed and retentate stream where the
membrane act as a heat exchanger barrier.
Temperature affects solubility and diffusivity of all permeants, as well as the extent
of mutual interaction between them. Favoring the flux and having minor effect on
selectivity.
Pervaporationat elevated
feedtemperatures.
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Membranes for Pervaporation
Membrane Polymers:
The choice of the membrane material has direct bearing on the separation effect tobe achieved. Two main kinds of polymers for pervaporation may be identified:
13
Figure 8. Amorphous polymer
1. Glassy (Amorphous polymers): Preferentially
permeates water and follows a Flory-Hugginstype sorption isotherm.
2. Elastomeric: Polymers interact preferentiallywith the organic solution component, the sorptionisotherm is of the Henry type.
Molecular motion isrestricted to molecular
vibrations (no rotation ormove in the space of the
chains)
Polymerssoft andflexible.
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Membranes for Pervaporation (2)
Important remarks for polymer choice:
Glassy polymers may behave as an elastomer when Toperation > Tg (Swelling takesdown Tg).
Its important that membranes dont swells too much because the selectivity willdecrease drastically.
In other hand low sorption or swelling will result in a very low flux.
Crosslinking should be used only when the membrane swells excessively (p.e. Highconcentrated solutions). Because crosslinking has a negative influence on thepermeation rate.
Figure 9. Tensile module vs T. Figure 10. Diffusivity vs degreeof swelling (non porous polymers)
14
glassystate
rubberystate
Log E
Tg T
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Applications
Aqueous mixtures
Removal of water from organic solvents.
Alcohols from fermentation broths(ethanol, butanol, etc..)
Volatile organic contaminants from wastewater (aromatics, chlorinated hydrocarbons)
Removal of flavor and aroma compounds.
Removal of phenolic compounds.
Non-aqueous mixtures
Alcohols/aromatics (methanol/toluene)
Alcohols/aliphatics (ethanol/hexane)
Alcohols/ethers (Methanol/MTBE) Cyclohexane/benzene
Hexane/toluene.
Butane/butene.
C-8 isomers (o-xylene, m-xylene, p-xylene,styrene).
Are found usually on the chemical process industry but there are other areas for isapplication as:
* Food.
* Farmaceutical industries.
* Enviromental problems.
* Analytical aplications.
Since there are a lot of applications there is a classification that can be useful:
DehydrationVolatile organic
compounds from water
{{
Polar/Non polar
}}}} Aromatics/AliphaticsSaturated/Unsaturated
Isomers
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Applications (2)
Pervaporation is used mainly to remove a smallamount of liquid from a azeotropic liquid mixturewhere simple distillation cant make theseparation.
Figure 13. Pervaporation of 50-50azeotropic mixture.
Figure 14. Hybrid process
distillation and pervaporation.
Other common application is when abinary mixture as located theazeotrope somewhere in the middle ofthe composition range, in this casepervaporation dont made thecomplete separation but break theazeotrope.
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Modules
The more suitable modules types are:
Hollow fiber module: This module isused with an insideout configuration toavoid increase in permeate pressurewithin the fibers, but the outsideinconfiguration can be used with short
fibers. Another advantage of theinside-out configuration is that the thintop layer is better protected buthigher membrane area can be achievedwith the outside-in configuration
Plate and Frame: This module is mainlyused for dehydration of organiccompounds.
Figure 15. Hollow fiber module.
Figure 16. Plate and frame module.18
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Modules (2)
Spiral wound module: This module isvery similar to the plate and framesystem but has a greater packingdensity. This type of module is usedwith organophilic membranes toachieved organicorganic separations.
Tubular modules: Inorganic (ceramic)membranes are produced mainly astubes, then the obvious module is thetube bundle for applications that used
this kind of membranes. On the otherhand, for sweep gas pervaporation,tubular membranes conducting the gas-permeate mixture are the only option.
Figure 17. Spiral wound module
Figure18. Tubular module
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ProcessDesign
Pervaporation stage: Pervaporation is a cross flow operation at ambient feed
pressure. The enthalpy of evaporation produces a temperature loss of the feed
stream, suggesting developing the process into individual separation units
interspersed with heat exchangers.
Figure 19. Ethanol dehydration20
The size of the separationunits (membrane area) will
depend on the allowable
temperature drop!
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ProcessDesign
In membrane separation cascades, the permeate of one stage constitutes the feed toa subsequent stage. The characteristics of pervaporation allow the design of
pervaporation cascades for the recovery of the dilute feed components. p.e. Using an
appropiate membrane, the target component is enrich in the permeate in the initial
pervaporation stage and employing a different type of membrane the remaining
solvent is removed from the first stage permeate, recovering the target component
on the retentate of the second stage.
Figure 20. Cascade configuration
21
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Process Energy Requirements
As partial pressure is the driven force for pervaporation and when a
vacuum pump is used to adjust the partial pressure at the permeate side,
then the power required is give by:
There is another need of energy related to the evaporation of the
permeate, here the feed stream is heat up before entering the process
to supply this heat:
!
1
2ln
p
pnRTE
L
vapprffpf HmTTCm (!
yy
,
Molar flow rate
Isothermal efficiency
22
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Summary
23
Advantages
Low energy consumption.
Low investment cost.
Better selectivity without thermodynamiclimitations.
Clean and close operation.
No process wastes.
Compact and scalable units.
Drawbacks
Scarce membrane market.
Lack of information.
Low permeate flows.
Better selectivity without thermodynamiclimitations.
Limited applications:
Organic substances dehydration.
Recovery of volatile compounds at lowconcentrations.
Separation of azeotropic mixtures.
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Membranes: Composite membranes with an
elastomeric or glassy polymeric top layer.
Thickness: } 0.1 to few Qm (for top layer)
Pore size: Non-porous
Driven force: Partial vapor pressure or activity
difference.Separation principle: Solution/Diffusion
Membrane material: Elastomeric and glassy.
Applications: Dehydration of organic solvents. Removal of organic compounds from
water.
Polar/non-polar. Saturated/unsaturated. Separation of isomers.
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Summary (2)