Pervaporation overview

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Pervaporation Overview
Pervaporation Overview
Membrane Separations
Camilo Mancera Arias Ph. D. Student Graduate
Program of Chemical and Process Engineering -
URV. Tarragona - 2004.
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A Little of History
Pervaporation Principles
Model Description
Performance parameters
Discussion Topics
Discussion Topics
Influence Parameters
Membranes for Pervaporation
Applications
Modules
Process Design
Process energy requirements
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A Little of History

• Was discovered in 1917 by Kober.
• The first full scale plant was installed in
  Brazil in 1982 for the production of ethanol.
• Appears as a promising and commercially
  competitive process for separation (more cost
  effective for some specific problems).

Figure 1. Membrane market
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A Little of History
Comparing pervaporation with distillation.

• 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 (OM) are
  expected to be higher than the conventional
  separation process.

• 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.

• Market barriers
• Lack of information.
• Poor availability of investment capital.
• Perceived risks associated with the reliability
  of the process.

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Pervaporation Principles
Vacuum Pervaporation

• Is the only membrane process where phase
  transition occurs.
• At least the heat of vaporization have to be
  supply.
• The mass transport is achieved lowering the
  activity of the permeating component on the
  permeate side by gas carrier, vacuum or
  temperature difference.
• The driving force is the partial pressure
  difference of the permeate between the feed and
  permeate streams.
• The permeate pressure has to be lower than the
  saturation pressure of the permeant to achieve
  the separation.

Gas carrier Pervaporation
Temperature difference Pervaporation
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Figure 2. Schematic draws of pervaporation
processes.
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Mechanism of Transport

• Pervaporation involve a sequence of three steps
• Selective sorption
• Selective diffusion through the membrane.
• Desorption into a vapor phase on the permeate
  side.

Because of its characteristics, pervaporation is
often mistakenly considered as a kind of
extractive distillation but VLE ?
Solution-Diffution mechanism.
Figure 3. Comparison between VLE and
pervaporation
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Model Description
There are two ways to rationalize the observed
separation effects in pervaporation

• Solution-Diffusion Model
• Membrane permeability is a function of
  solubility and diffusivity
• Diffusivity and solubility are strongly
  dependent of feed composition.
• The liquids have more affinity towards polymeric
  membranes than gases (FloryHuggins theory
  instead Raoults law).
• Equation of transport

• Thermodynamic accounting approach
• Its distinguished two steps
• Equilibrium evaporation.
• Membrane permeation of the hypothetic vapor.
• Membrane selectivity contribution to overall
  separation is showed as a change of composition
  for the vapor phase lowering the total pressure
  below equilibrium vapor pressure (Thompson
  diagram).

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Figure 4. Thompson diagram
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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 (Thermodynamic
  equilibrium).
• The concentration of the liquid on the feed side
  of the membrane is maximum whilst on the permeate
  side is almost zero.
• Flux equation (pure liquid)
• The concentration inside the membrane (cim) is
  the main parameter, implying that permeation rate
  is mainly determine for the interaction
  membrane-penetrant.
• When concentration inside the membrane increase
  the permeation rate also increase.

ki Plasticizing constant, membrane permeant
interaction
Concentration dependance diffusion coef.
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Mixture of Liquids

• For the transport of liquid mixtures through a
  polymeric membrane the flux can also be 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).

Overall sorption
Overall Flux
Sorption selectivity
Pervaporation selectivity
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Figures 6. Mixture of liquids
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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 of permeate
  and feed. There are two forms
• Separation factor, a
• Enrichment factor, b

Flory-Huggins Isotherm (Glassy liquid sorption)
Henry Isotherm (Rubbery liquid and gas sorption)
2. Sorption selectivity Permeability is function
of solubility and diffusivity and both may be
selective. Sorption selectivity may or may not
be equal to pervaporation selectivity. Due to
contribution of selective diffusivity to the
overall separation effect.
Langmuir Isotherm (Glassy gas sorption)
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Figure 7. Sorption isotherms
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Performance Parameters (2)
3. Evaporation selectivity The separation factor
is considered to be a product of evaporation
separation and membrane separation
yields Membrane selectivity depends on
permeate pressure, while evaporation invariably
enriches the more volatile solution
compound. 4. Flux Denote the amount of
permeate per unit membrane area and unit time at
given membrane thickness. Its a realy important
parameter for the operation of the process.
Pervaporation favors the more volatile compound
Pervaporation favors the less volatile compound
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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 two aspects to
  be consideredthe activity of the target
  component in the feed and the solubility of the
  target component in the membrane.
• Activity coefficient The activity of a liquid
  solution component is given by its partial vapor
  pressure
• The behavior of the liquid solution is determined
  for the activity coefficient
• Azeotropic mixture Positive solution
  non-ideality is asociated with positive
  azeotropes, and negative solution non-ideality is
  asociated with negative azeotropoes.
  Pervaporation can separate only positive
  azeotropes.
• Concentration polarization In pervaporation, a
  depletion of the preferentially permeating
  species near the membrane boundary is to be
  expected, limiting its polymer sorption. But
  depends of the concentration dependance and sign
  of the activity coefficient of the penetrant
  species.

Positive deviation from Raouls law
Negative deviation from Raoults law
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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 pressure of 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 highest vacuum feasible is 1 atm.
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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.

Pervaporation at elevated feed temperatures.
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Membranes for Pervaporation
Membrane Polymers The choice of the membrane
material has direct bearing on the separation
effect to be achieved. Two main kinds of polymers
for pervaporation may be identified
1. Glassy (Amorphous polymers) Preferentially
permeates water and follows a Flory-Huggins type
sorption isotherm.
Molecular motion is restricted to molecular
vibrations (no rotation or move in the space of
the chains)
2. Elastomeric Polymers interact preferentially
with the organic solution component, the sorption
isotherm is of the Henry type.
Figure 8. Amorphous polymer
Polymers soft and flexible.
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Membranes for Pervaporation (2)

• Important remarks for polymer choice
• Glassy polymers may behave as an elastomer when
  Toperation gt Tg (Swelling takes down Tg).
• Its important that membranes dont swells too
  much because the selectivity will decrease
  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. High
  concentrated solutions). Because crosslinking has
  a negative influence on the permeation rate.

Figure 10. Diffusivity vs degree of swelling (non
porous polymers)
Figure 9. Tensile module vs T.
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Membrane Structure
Non-porous membranes. Anisotropic morphology.
Asymmetric or composite membranes (porous top
layer and open porous sublayer) Pervaporation
membranes should meet Have a proportional
thickness with performance Not pose technical
resistance to withdrawal. Have dimensional
stability under swelling conditions. The
Requirements for the substructure are Open
substructure. A high surface porosity with a
narrow pore size distribution.
Mechanical resistance and swelling
To minimize transport resistance and avoid
capillary condensation
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Figure 12. Non porous asymmetric membrane
Figure11. Non porous composite membrane
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Applications
Are found usually on the chemical process
industry but there are other areas for is
application as Food. Farmaceutical
industries. Enviromental problems. Analytical
aplications. Since there are a lot of
applications there is a classification that can
be useful
Polar/Non polar
Volatile organic compounds from water
Dehydration

• Aqueous mixtures
• Removal of water from organic solvents.
• Alcohols from fermentation broths (ethanol,
  butanol, etc..)
• Volatile organic contaminants from waste water
  (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).

Aromatics/Aliphatics
Saturated/Unsaturated
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Isomers
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Applications (2)
Pervaporation is used mainly to remove a small
amount of liquid from a azeotropic liquid mixture
where simple distillation cant make the
separation.
Figure 13. Pervaporation of 50-50 azeotropic
mixture.
Other common application is when a binary mixture
as located the azeotrope somewhere in the middle
of the composition range, in this case
pervaporation dont made the complete separation
but break the azeotrope.
Figure 14. Hybrid process distillation and
pervaporation.
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Modules
The more suitable modules types are
Hollow fiber module This module is used with an
insideout configuration to avoid increase in
permeate pressure within the fibers, but the
outsidein configuration can be used with short
fibers. Another advantage of the inside-out
configuration is that the thin top layer is
better protected but higher membrane area can be
achieved with the outside-in configuration
Figure 15. Hollow fiber module.
Plate and Frame This module is mainly used for
dehydration of organic compounds.
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Figure 16. Plate and frame module.
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Modules (2)
Spiral wound module This module is very similar
to the plate and frame system but has a greater
packing density. This type of module is used with
organophilic membranes to achieved
organicorganic separations.
Figure 17. Spiral wound module
Tubular modules Inorganic (ceramic) membranes
are produced mainly as tubes, then the obvious
module is the tube bundle for applications that
used this kind of membranes. On the other hand,
for sweep gas pervaporation, tubular membranes
conducting the gas-permeate mixture are the only
option.
Figure18. Tubular module
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Process Design
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.
The size of the separation units (membrane area)
will depend on the allowable temperature drop!
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Figure 19. Ethanol dehydration
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Process Design
In membrane separation cascades, the permeate of
one stage constitutes the feed to a 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
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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
Molar flow rate
Isothermal efficiency
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
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Summary

• Advantages
• Low energy consumption.
• Low investment cost.
• Better selectivity without thermodynamic
  limitations.
• Clean and close operation.
• No process wastes.
• Compact and scalable units.

• Drawbacks
• Scarce membrane market.
• Lack of information.
• Low permeate flows.
• Better selectivity without thermodynamic
  limitations.
• Limited applications
• Organic substances dehydration.
• Recovery of volatile compounds at low
  concentrations.
• Separation of azeotropic mixtures.

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Summary (2)
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