Per Vapor at Ion

8/3/2019 Per Vapor at Ion

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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

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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

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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

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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

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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

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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

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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:

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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)

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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

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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

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Summary

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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)

Per Vapor at Ion

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