chapter 6. membrane process...

Chang-Han Yun / Ph.D.

National Chungbuk University

November 18, 2015 (Wed)

Chapter 6. Membrane Process

(Pervaporation)

2 Chapter 6. Membrane Process(Concentration) Chungbuk University

Contents

Contents Contents

6.5 Other Driving Force

6.4 Concentration Driving Force

6.3 Pressure Driven Force

6.2 Osmosis

6.1 Introduction


6.4 Concentration Driving Force 6.4.3 Pervaporation

Pervaporation

Liquid on feed (upstream side) at atmospheric pressure

Vapor on permeate(downstream side) at low vapor pressure

(Low vapor(partial) pressure coming from carrier gas or a vacuum)

Partial downstream pressure < saturation pressure

Sequence of pervaporation process

① selective sorption into the membrane on the feed side

② selective diffusion through the membrane

③ desorption into a vapor phase on the permeate side

<Figure 6-21> Schematic drawing of the pervaporation process

with a downstream vacuum or an inert carrier-gas.



Mass and Heat transfer occurs simultaneously ⇨ complex process

Phase transition(liquid to vapor) ⇨ Need heat to vaporize the permeate

A kind of extractive distillation process : Membrane acting as the 3rd component

Difference with distillation on the point of separation principle

Pervaporation : differences in solubility and diffusivity

Distillation : vapor-liquid equilibrium

Transport by solution-diffusion mechanism

Selective sorption and/or Selective diffusion

⇨ determine selectivity

<Figure 6-22> Distillation and pervaporation characteristics

for EtOH-water mixture at 20°C.

(membrane : polyacrylonitrile)



Difference with gas separation

Membrane materials : same in principle, but

• Affinity towards polymer : Liquid ≫ Gas ⇨ solubility⇈(∵ vapor, not gas)

Selectivity estimation

• In gas separation, selectivity expected from ratio of P of pure gases

• With liquid mixtures, thermodynamic interactions ⇨ far different from gases

Solubility

• Gases in polymer (at T<Tg , Grassy state) : Low ⇨ Henry's law

• Liquid with polymer : Much high ⇨ Flory-Huggins theory

Diffusivity(D) & Solubility(S)

• Gas = constant ≠ f(composition)

• Liquid ≠ constant = f(composition)



Pi = Di (ci, cj)∙Si (ci, cj) (6-68)

<Example> PVA membrane to separate ethanol-water mixtures

※ PVA membrane : Highly swollen by water, not by organics

• Low alcohol concentration (< 10%) : membrane swelling↑ ⇨ selectivity↓

• Low water concentration (< 10%) : high selectivity towards water with reasonable flux

<Example> Immiscible mixture over the whole composition range (TCE-water)

※ Silicone rubber membrane : Highly swollen by organic, not by water

• Silicone rubber (polydimethylsiloxane) : Removal TCE from water (solvent)

• PVA : Removal trace water from almost pure TEC

(Silicone rubber : swelling↑ for organics ⇨ selectivity↓ and mechanical properties↓)



For single component, simple transport equations(derived from linear flux-force relationships)

(6-69)

where Li = proportionality or phenomenological coefficient

μi = chemical potential = μoi + RT ln(ai) (6-70) with ai = pi / p

oi (6-71)

poi = saturation pressure of component i

pi = vapor pressure

Because (6-72)

Eq(6-69) → (6-73)

dpi/dx ≈ Δpi / Δx where Δx = membrane thickness ℓ and Pi = (Li∙RT) / pi

Eq(6-73) → (6-74) ※ (6-46)

⇨ Basic equation for liquid transport = for gas transport [see Eq(6-46)]

6.4.3.1 Aspects of separation



High interaction between organic liquids ↔ polymer ⇨ Permeability, Pi = f(c, T)

Solubility(S) and Diffusivity(D) = f(c, T)

Important parameters in (6-74)

Permeability coefficient(Pi) : membrane based parameter

Effective membrane thickness(ℓ)

Partial pressure difference(Δpi)

In general, Eq(6-74) can be written as

(6-75)

where xi = mole fraction of component i in the liquid feed

pio = saturation pressure of the pure component at a given temperature

γi = activity coefficient of component i



Feed side

pio from Antoine equation

(6-76)

where P = mmHg and T = °C ※ constant A, B, C : listed at Appendix 2 of Mulder’s book

Activity coefficients(γi) from semi-empirical equations

(van Laar, Margules, Wilson, UNIVAC and UNIQUAC)

Permeate side : <assume> ideal behavior

Partial pressure = pressure × mole fraction

Eq(6-69) & (6-70) → (6-77) & (6-78)

Activity(ai)of component(index i) in membrane(index j) by Flory-Huggins

(6-79)

where ϕi = volume fraction of the liquid inside the polymer

ϕj = volume fraction of polymer, Xij = Flory-Huggins interaction parameter



For an ideal system (Vi = Vj and Xij = 0), differentiation of Eq(6-79) with respect to ϕi gives

(6-81)

Change ϕ to c and combining Eq(6-77), (6-80) and (6-81) gives Fick's law

(6-82)

Swelling during pervaporation

Liquid generally swells the polymer

Liquid concentration • on the feed side of the membrane = maximum ⇨ Maximum swelling

• on the permeate side = almost zero ⇨ swelling = almost zero

<Figure 6-23> Activity profile of a pure liquid

across a membrane.



Concentration dependent diffusion coefficient, Di(c)

Di(c) change quite considerably across the membrane when p2/po → 0

Di = Do,i exp(γ∙ci) (6-83)

where Do,i = diffusion coefficient at c→0

γ = plasticising constant expressing the plasticising action of the liquid on segmental motion

Combining Eq(6-82) and (6-83) and integrating across membrane using BC

BC 1 : ci = ci,1m at x = 0

BC 2 : ci = 0 at x = ℓ (6-84)

『Meaning』

• Express flux of a pure liquid through a membrane

• Do,i, γ and ℓ = constants ⇨ ci,1m = main parameter (concentration of i at feed-side membrane)

• Concentration inside membrane (ci,1m)↑ ⇨ Flux↑

• Interaction between membrane ↔ penetrant ⇨ Determine flux for single liquid transport



Transport of liquid mixtures through a polymeric membrane : generally much more complex

In the case of a binary liquid mixture,

• Flux ∝ (solubility) × (diffusivity)

• Flow coupling & thermodynamic interaction ⇨ Strong influence on each other

Flow coupling :

Thermodynamic interaction :

<Example> Water-Ethanol separation through polysulfone membrane

Water alone : very low permeability ⇨ Much higher permeability in the presence of ethanol

Ethanol : high affinity to polymer ⇨ higher solubility ⇨ much higher permeability

⇨ water transport to permeate↑

Described in terms of non-equilibrium thermodynamics

Gradient of one component ⇨ Affect transport of the other component

Much more important phenomenon

Interaction↑ ⇨ Transport of the other component↑

∵ Swelling of membrane↑ ⇨ diffusion resistances↓



Sorption value ⇨ Indicate overall interaction of liquid mixture with membrane material

<Figure 6-24>

Ethanol concentration in liquid mixture↑ ⇨ overall sorption value↑

Swelling↑ ⇨ transport resistance↓ ⇨ flux(permeability coefficient)↑

※ Interaction↑ ⇨ Solubility↑ ⇨ Swelling↑ ⇨ Diffusivity↑ ⇨ flux↑

Flu

x(k

g/m

2•h

r)

Ov

erall

so

rpti

on

(g/g

)

<Figure 6-24> Overall sorption (left) and pervaporation flux (right) as a function of the

ethanol/toluene feed composition for a PAA-PVA polymer blend with 20% PVA.

Et-OH conc.(wt%) in feed Et-OH conc.(wt%) in feed



Sorption selectivity or preferential sorption

Correlated to the membrane selectivity in a pervaporation

<Figure 6-25>

Ethanol concentration in feed↑ ⇨ Preferential sorption of ethanol ↓(selectivity↓)

Ethanol concentration in feed↑ ⇨ Swelling of polymer↑ ⇨ J↑ and α↓

Et-OH conc.(wt%) in feed Et-OH conc.(wt%) in feed

So

rpti

on

sel

ecti

vit

y

α(p

erv

ap

ora

tion

sel

ecti

vit

y)

<Figure 6-25> Sorption selectivity (left) and pervaporation selectivity (right) as a function of the

ethanol/ Toluene feed composition for a PAA-PVA polymer blend membrane with 20% PVA.



<Example> Removal of TCE from water (<Figure 6-26>)

Membrane : NBR-18(nitrile-butadiene rubber with a 18% nitrile content)

TCE in feed↑ ⇨ Selectivity for TCE↑ exponentially

Sorbed Preferentially ⇨ permeates preferentially

Determining factor in selective transport in pervaporation

Preferential sorption

TC

E c

on

c. i

n p

erm

eate

(wt

%)

Feed(μg/g)

Flux can be correlated to the overall sorption.

<Figure 6-26> Experimental values for the

preferential sorption and pervaporation of

the system TCE/water/NBR-18.

※ NBR : Nitril-butadien rubber



Binary mixture Polymer

water / methanol

water / ethanol

water / propanol

water / butanol

ethanol / l,2-dichloroethylene

ethanol / chloroform

acetic acid / 1,2-dichloroethylene

chloroform / water

trichloroethylene / water

benzene / water

toluene / water

benzene / cyclohexane

benzene / heptane

o-xylene / p-xylene

toluene / methanol

toluene / ethanol

PMG, PDMS

PVA, CA, PAN, PMM

Selemion, PDMS

PDMS

PDMS

PTFE/PVP

PTFE/PVP

PTFE/PVP

SBR, NBR

NBR,BR

NBR

NBR, BR

PMG

NBR

CTP

PAA-PVA

PAA/PVA

PMG: polymethylglutarnate

PDMS: polydimethylsiloxane

PVA: polyvinylalcohol

CA: cellulose acetate

PAN: polyacrylonitrile

PTFE: Polytetrafluoroethylene

PVP: polyvinylpyrrolidone

SBR: styrene-butadiene rubber

NBR: nitrile-butadiene rubber

CTP: cellulose tripropionate

PAA: polyacrylic acid;

[Table 6-14] Literature data relating

to preferential sorption



For pervaporation and gas separation

Nonporous membranes(asymmetric or composite membranes)

Anisotropic(이방성) morphology

Asymmetric structure

Top-layer : dense

Sub-layer : open porous

Requirements for the sub-layer structure (※ Same as for gas separation membranes)

Open structure to minimize resistance to vapor transport and to avoid capillary condensation

High surface porosity with a narrow pore size distribution

Pore size of the sub-layer on the permeate side

Pressure loss↑ ⇨ Partial pressure↑ ⇨ Driving force(Flux)↓

Pore size = too small ⇨ Pressure loss↑ ⇨ occur capillary condensation

Pore size = too large ⇨ direct application of top-layer upon sub-layer = difficult

6.4.3.2 Membranes for pervaporation



3-layer membrane

Manufactured to keep high surface porosity

Very porous sub-layer + Non-selective intermediate layer + Dense top-layer

Methods to deposit thin top-layer upon a sub-layer

※ Same with gas separation and vapor permeation membranes

dip-coating

plasma polymerization

interfacial polymerization

Choice of the polymeric material on the point of permeability(P)

Depend strongly on the type of application

Permeability for elastomer : a little higher than for glassy polymer (not like gas separation)

∵ Much higher affinity of liquids to polymer than that of gas to polymer

In fact, swelling↑ ⇨ Tg↓ ⇨ glassy may behave as elastomer if T > Tg



Choice of the polymeric material on the point of selectivity(α)

Swelling too much ⇨ flux↑, selectivity↓ drastically

Low sorption or swelling ⇨ very low flux

Optimum(rough estimation)

Overall sorption value : 5 ∼ 25 % by weight

Amorphous (glassy or rubbery) = more preferable than crosslinked or crystalline

∵ crosslinked or crystalline ⇨ flux↓

※ Must use crosslinked polymers

When swelling of polymer is excessively high

When crosslinked membrane shows a good performance.

<Example> Separation of low concentrations of chlorinated hydrocarbons(ex, TCE) from water

For extremely low concentrations of organics in water (≈ 10 ppm)

⇨ use uncrosslinked elastomers

For higher concentrations (> 100 ppm) ⇨ use crosslinking polymer to reduce the swelling

and to improve long term mechanical properties.



[Table 6-15]

High selectivity of PVA, PAN and polyacrylamide are originated from

• Interaction : water ↔ polymer > Et-OH ↔ polymer

• Water molecule = small ⇨ molar volume = small ⇨ positive to diffusivity

PVA : used at high water concentrations ⇨ swell too much ⇨ Selectivity decreases drastically

※ use PVA membrane only at low water concentration

Separation of water from organic solvents

Large differences at size(molar volume)

Large difference in chemical properties

(polarity and H-bonding ability)

As components become more similar

⇨ difficulty in separation↑

relatively easy separation ⇨ simple

Polymer Flux(kg/(m2∙hr)) α

polyacrylonitrile(PAN)

polyacrylamide

polyacrylamide (high carboxyl)

poly(vinyl alcohol) (98%)

poly(vinyl alcohol) (100%)

poly(ether sulfone)

polyhydrazide

0.007

0.011

0.100

0.080

0.060

0.072

0.132

12500

4080

2200

350

140

52

19

[Table 6-15] Flux and selectivity of ethanol

/water through homogeneous membranes.

Feed : 90 wt% ethanol

Membrane thickness: 50 μm T : 70°C



Pervaporation

Complex separation process

Separation characteristics may be strongly influenced by composition.

Used mainly to separate a small amount of liquid from a liquid mixture.

Use highly selective membranes ⇨ supply energy to vaporize only pure permeate

Very attractive when the liquid mixture exhibits an azeotropic composition

6.4.3.3 Application

Mixture Azeotrope(wt %)

water/ethanol

water/i-propanol

water/t-butanol

water/tetrahydrofuran

water/dioxan

methanol/acetone

ethanollhexane

n-propanol/cyclohexane

4.4 / 95.6

12.2 / 87.8

11.8 / 88.2

5.9 / 94.1

18.4 / 81.6

12.0 / 88.0

21.0 / 79.0

20.0 / 80.0

[Table 6-16] Azeotropic compositions associated

with some liquid mixtures.



Hybrid system with distillation

Pervaporation is used only to break the azeotrope.

Azeotrope is located at middle range of concentration ⇨ not apply pervaporation

However, a combination of distillation and pervaporation can be applied

Pervaporation shift the composition from the azeotrope

Small difference in relative volatilities of the components(<Figure 6-28>)

Membranes selectivity > VLE ⇨ use hybrid with distillation

Very attractive in case of 'de-bottle-necking' of an existing distillation plant

Application in food and pharmaceutical industries

Concentrate heat sensitive products or remove (concentrate) aroma compounds

Environmental

Remove volatile organic contaminants from waste water



<Figure 6-27> Schematic drawing of a hybrid distillation/pervaporation

for the separation of a 50/50 azeotropic mixture.

<Figure 6-28> Schematic drawing of a hybrid

distillation/pervaporation process for the

separation of close boiling mixtures.



Application for aqueous mixtures

2 cases

• small volume of water removal in solvent

• small volume of organic removal in water

Dehydration : removal of water(even trace of water) from organic solvents

Removal of volatile organic compounds from water

• alcohols from fermentation broths (ethanol, butanol, acetone-butanolethanol(ABE))

• volatile organics from wastewater (aromatics, chlorinated hydrocarbons)

• removal of flavor and aroma compounds

• removal of phenolics



Non-aqueous mixtures

polar / non-polar

• alcohols/aromatics (methanol/toluene)

• alcohols/aliphatics (ethanol/hexane)

• alcohols/ethers (methanol/methyl-t-butylether (MTBE))

aromatics / aliphatics

• cyc1ohexane/benzene

• hexane/toluene

saturated / unsaturated

• butane/butene

isomers

• C-8 isomers (o-xylene, m-xylene, p-xylene, styrene, ethylbenzene)



6.4.3.4 Summary of pervaporation

Items Characteristics

Membranes Composite membranes with an elastomeric or glassy polymeric top layer

Thickness ≈ 0.1 to few μm (for top layer)

Pore sizes Non-porous

Driving force Partial vapour pressure or Activity difference

Separation principle Solution-diffusion

Membrane material Elastomeric and glassy polymers

Applications

• Dehydration of organic solvents

• Removal of organic components from water

(alcohols, aromatics, chlorinated hydrocarbons)

• Polar/non-polar (e.g. alcohols/aliphatics or alcohols/aromatics)

• Saturated/unsaturated (e.g. cyclohexane/benzene)

• Separation of isomers

(e.g. C-8 isomers; o-xylene, m-xylene, p-xylene, ethylbenzene, styrene)

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