chapter 6. membrane process (pressure driving...

Chang-Han Yun / Ph.D.

National Chungbuk University

November 4, 2015 (Wed)

Chapter 6. Membrane Process

(Pressure Driving Force)

2 Chapter 6. Membrane Process(Pressure Driving Force) 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


6.3 Pressure Driven Membrane Process 6.3.4 RO & NF

Cellulose esters(Cellulose diacetate, Cellulose triacetate)

Very suitable for desalination

• High permeability towards water

• Low solubility towards the salt

Stability against chemicals, temperature and bacteria = very poor

Typical operation conditions

• pH : 5 ∼ 7

• Temperature : < 30°C

Biological degradation is a severe problem.

Poor selectivity towards small organic molecules

6.3.4.1 Membrane for RO and NF



Aromatic polyamide – composite type

High selectivity towards salts

Water flux is somewhat lower

pH : 5 ∼ 9

Main drawback : weak against free chlorine Cl2

Hollow fiber type RO membrane

OD < 100 μm

Membrane thickness ≈ 20 μm ⇨ Permeation rate has decreased dramatically

Extremely high membrane surface area = 30,000 m2/m3

Other materials for RO (the 3rd Class of material)

polybenzimidazoles, polybenzimidazolones, polyamidehydrazide and polyimides




Composite membranes

Most of RO and all of NF : composite membranes

Top- & Sub-layer : different polymer ⇨ optimized separately

Porous sub layer

• Criteria of sub-layer : surface porosity and pore size distribution

• Asymmetric UF membranes are often used

Methods for placing a thin dense layer on top of sub layer

• dip coating

• in-situ polymerization

Most composite RO and NF membranes are prepared by interfacial polymerization


• interfacial polymerization

• plasma polymerization



※ Interfacial polymerization

Two very reactive bifunctional monomers (e.g. a di-acidchloride and a di-amine) or

trifunctional monomers (e.g. trimesoylchloride) are allowed to react with each other

at a water/organic solvent interface and a typical network structure is obtained.


[Table 6-6] Example of monomers used for interfacial polymerization



Application of NF/RO

Application for purification

Salty water purifying

• Brackish water : 1,000 ∼ 5000 ppm

• Seawater : about 35,000 ppm

Production of ultrapure water for the semiconductor industry

Application for concentration

Concentration in the food industry (fruit juice, sugar, coffee)

Galvanic industry (concentration of waste streams)

Dairy industry (concentration of milk prior to cheese manufacture)

6.3.4.2 Applications

Purification(where the permeate is the product) : Major

Solute concentration(where the feed is the product)



NF

NF : network structure is more open than RO

Retention for monovalent salts(Na+, Cl－) : much low

Retention for bivalent ions(Ca2+, CO22－) : high

Retention for micro-solutes and low MW organics : high

(herbicides, insecticides, pesticides, dyes, sugars etc)


Solute RO NF

Monovalent ions (Na, K, CI, NO3)

Bivalent ions (Ca, Mg, SO4, CO3)

Bacteria and virus

Microsolutes (Mw > 100)

Microsolutes (Mw < 100)

> 98%

> 99%

> 99%

> 90%

0∼99%

< 50%

> 90%

< 99%

> 50%

0∼50%

[Table 6-7] Comparison of retention characteristics between NF and RO



6.3.4.3 Summary of Nanofiltration(NF)

Items Characteristics

Membranes Composit

Thickness Sub layer ≈ 150 μm, Top layer ≈ 1 μm

Pore sizes < 2 nm

Driving force Pressure(10 ∼ 25 bar)

Separation principle Solution-diffusion

Membrane material polyamide (Interfacial polymerization)

Main applications

Desalination of brackish water

Removal of micropollutents

Water softening

Waste water treatment

Retention of dyes (textile industry)



6.3.4.4 Summary of Revers Osmosis(RO)


Membranes Asymmetric or Composite


Pore sizes < 2 nm

Driving force Pressure : Brackish water 15 ∼ 25 bar(Sea water 40 ∼ 80 bar)


Membrane material Cellulose triacetate, Aromatic polyamide

Polyamide and Poly(ether urea) (Interfacial polymerization)

Main applications

Desalination of brackish and seawater

Production of ultrapure water (electronic industry)

Concentration of food juice and sugars (food industry)

Concentration of milk (dairy industry)


6.3 Pressure Driven Membrane Process 6.3.5

Pressure Retarded Osmosis

(PRO)

PRO

Salt concentration difference(Osmotic pressure) ⇨ generate energy

Flow through semipermeable membrane(dilute → concentrate) by osmotic pressure

Jv = A (Δπ - ΔP) (6-36)

E = Jv∙AP = A (Δπ - ΔP)∙ΔP (6-37)

where E = power, Watt or J/sec

Maximum power (E = Emax) at dE/d(ΔP) = 0 ⇨ ΔP = 0.5 Δπ

Emax = A / (4 Δπ) (6-38)

Emax = about 1.5 W/m2 on the basis of lab experiments

Principle of pressure retarded osmosis.




(PRO)

Practical problems

Osmosis : Concentration of concentrated solution↓ ⇨ π↓

Salt flux : R < 100% ⇨ π↓

Concentration polarization : major problem(See )

Direction of Jv is opposite with Js

cs on concentrate-side membrane surface < in bulk of concentrate

cs on dilute-side membrane surface > in bulk of dilute π↓

Concentration polarization

in pressure retarded osmosis




(PRO)


Membranes Asymmetric or Composite


Pore sizes < 2 nm

Driving force Concentration difference (Osmotic pressure)


Membrane material Cellulose triacetate, Aromatic polyamide

Poly(ether urea) (Interfacial polymerization)

Main applications Production of energy

6.3.5.1 Summary of Retarded Osmosis(PRO)



Piezodialysis

Piezodialysis

Driving force : Pressure

Ionic solutes permeate through membrane rather than solvent

Operated by mosaic membranes

Principle

Pressure applied at one side of the membrane ⇨ generate electromotive force (ΔE)

ΔE = －β ΔP where β = proportionality constant(Electric Osmotic Coefficient) (6-39)

• β < 0 for anion-exchange membranes

• β > 0 for cation-exchange membranes

The transport of ions through a mosaic membrane during piezodialysis



Piezodialysis

Structure of mosaic membranes

Cation-exchange and anion-exchange groups separated by a neutral region

Generation of a current loop

Ion transport > solvent transport ⇨ salt concentration : permeate > feed

Salt enrichment in permeate : about 2

Major factor to increase the flux = ion-exchange capacity of the membrane

※ Although the basic principle has been demonstrated in the laboratory,

it has not been employed on a commercial scale.

Electro-neutralized by simultaneous

passage of both ions through the membrane

Transport of anions through

anion-exchange region

Transport of cations through

cation-exchange region



Piezodialysis

6.3.6.1 Summary of Piezodialysis


Membranes Mosaic membranes

(cation-exchange regions adjacent to anion-exchange regions)

Thickness ≈ few hundred μm

Pore sizes Non-porous

Driving force Concentration difference (Osmotic pressure)

Separation principle Ion transport (Coulomb attraction and electro-neutrality)

Membrane material Cation/anion-exchange membrane

Main applications Salt enrichment

Chang-Han Yun / Ph.D.

National Chungbuk University

November 4, 2015 (Wed)

Chapter 6. Membrane Process

(Gas Separation)


Contents

Contents Contents

6.5 Other Driving Force


6.3 Pressure Driven Force

6.2 Osmosis

6.1 Introduction


Substances diffuse spontaneously from a high to a low chemical potential.

Processes using concentration difference as the driving force

6.4.1 Introduction

Gas separation Driving Force

Partial pressure difference

or Activity difference Vapor permeation

Pervaporation

Dialysis

Diffusion dialysis

Concentration difference

Carrier mediated processes

Membrane contactor



Nonporous membrane

No information about the permeability of a certain species

Difference of permeability of gas between elastomeric and glassy > 105

Glassy state

Presence of a large free volume

Presence of crystallites ⇨ mobility↓

Low MW penetrant ⇨ segmental mobility (or chain mobility)↑

Concentrations of penetrant inside the polymeric membrane↑ ⇨ chain mobility↑

⇨ permeability (or diffusivity)↑

Affinity of penetrant with polymer

Activity of penetrant in feed

6.4.1 Introduction

Synthetic solid membrane

(gas separation, dialysis and pervaporation) On the basis of structure

and functionality Liquid (with or without a carrier) as the membrane

Large differences in segmental motion

Determine concentration of penetrant

inside polymeric membrane



In gas separation with inert gases(He, H2, N2, O2)

No interaction between gas molecule ↔ membrane material

Gas concentration in the membrane : very low at low feed pressures

With liquid penetrant

Solubility in the membrane = appreciably high ⇨ enhanced chain mobility

High interaction between liquid ↔ membrane in dialysis ⇨ high swelling of the polymer

⇨ allows relatively large molecules diffuse through open membrane

※ Swelling of membrane = wt. of penetrant inside membrane / wt. of dry polymer

Diffusion coefficient can vary over the range 1019 to 10－9 m2/sec.

Swelling↑ ⇨ Mobility of the polymer chains↑

※ Diffusion coefficient in liquids is 10－9 m2/sec

6.4.1 Introduction 6.4 Concentration Driving Force


Swelling : very important factor in transport through nonporous membranes

demonstrates

D can change by up to 10 orders of magnitude.

D of benzene in PVA at zero penetrant concentration < 10－19 m2/sec

D of water in hydrogels > 10－9 m2/sec( ≈ self-diffusion coefficient of water)

6.4.1 Introduction

Diffusivity

as f(degree of swelling in nonporous polymer)



6.4.2 Gas Separation

Viscous flow like MF ⇨ no separation

∵ λ of the gas molecules ≪ Pore diameter

Pore diameter↓ ⇨ Mean free path(λ) of gas > Pore diameter ⇨ Knudsen flow

Knudsen flow, (6-40)

where Dk = Knudsen diffusion coefficient, is given by

T and Mw = temperature and molecular weight, respectively

r = pore radius

Flux ∝ [MW]－0.5 ⇨ Separation factor(J1 / J2) ∝ [MW1 / MW2]－0.5 ⇨ low separation factors

For high separation, cascade connection of many module ⇨ economics↓

Commercial application

Enrichment of uranium hexafluoride(235UF6, a very expensive material)

Separation factor for 235UF6 from 238UF6 < 1.0064 (for ideal case)

Plant using porous ceramic membranes operates in France (at Tricastin).

6.4.2.1 Gas separation in porous membranes Gas separation

Membrane : Porous or Nonporous

Transport mechanism : Completely different




Permeability ⇨ determine separation through nonporous membranes

Fick's law : simplest description of gas diffusion through a nonporous structure

(6-41)

where J = flux through the membrane

D = diffusion coefficient

dc/dx = driving force (concentration gradient across the membrane)

By integration of Eq(6-41) under steady-state conditions,

(6-42)

where co,i = ci in membrane on upstream side

cℓ,i = ci in membrane on downstream side

ℓ = thickness of the membrane

6.4.2.2 Gas separation through nonporous membranes

Nonporous membrane separating two gas phases.




Henry's law

ci = Si∙pi (6-43)

where ci = concentration inside the membrane

pi = partial pressure of gas outside the membrane

Si = solubility coefficient of component i in membrane (cm3(STP)/cm3∙bar)

※ Henry’s law is mainly applicable to amorphous elastomeric polymers

※ Solubility behavior : very often much more complex below Tg (Glassy state)

By combining Eq(6-42) with Eq(6-43),

(6-44)

Permeability coefficient, P = D∙S (6-45)

(6-46)

J ∝ (Δp across membrane) and (ℓ)－1

Ideal selectivity (αi/j ideal) ∝ ratio of the permeability coefficients ⇨ (6-47)




Plasticisation

Occur at high partial pressure of permeating gas having high chemical affinity for polymer

By plasticisation ⇨ Real separation factor ≠ Ideal separation factor

In real, Permeability↑ ⇨ Selectivity↓ by plasticisation

Selectivity dependency on partial pressure ratio across membrane

For high pressure ratio (Pℓ / Po → 0) ⇨ Separation efficiency = maximum

Pressure ratio↑ ⇨ Selectivity↑

Schematic drawing of a gas separation process.




Permeability coefficient (P)

Constant : intrinsic parameter of membrane

Unit : Barrer

※ l Barrer=10－10cm3(STP)∙cm/(cm2∙s∙cmHg)=0.76×10－17m3(STP)∙m/(m2∙s∙Pa)

Interactive systems(Henry's law does not apply.)

Permeability coefficient (P) ≠ constant = f(pressure)

2 important parameters relating to the nature of the polymer (chemical structure)

• Glass transition temperature (Tg)

Glass transition temperature (Tg)

Determines whether a polymer is in the glassy or in the rubbery state

Segmental motion

• Limited for an amorphous polymer in the glassy state

• Enough thermal energy to rotate in the main chain in rubbery state

6.4.2.3 Aspects of separation

• Crystallinity




In general([Table 6-8])

Permeability : Rubbery polymer > Glassy polymers

(∵ Higher mobility of the chain segments)

Selectivity : Rubbery polymer < Glassy

Polymer P of CO2(Barrer) P of CO2 / P of CH4

polytrimethylsilylpropyne(PTMSP)

silicone rubber

natural rubber

polystyrene

polyamide (Nylon 6)

poly(vinyl chloride)

polycarbonate (Lexan)

polysulfone

polyethyleneterephthalate (Mylar)

cellulose acetate

poly(ether imide) (Ultem)

poly(ether sulfone) (Victrex)

polyimide (Kapton)

33100

3200

130

11

0.16

0.16

10.0

4.4

0.14

6.0

1.5

3.4

0.2

2.0

3.4

4.6

8.5

11.2

15.1

26.7

30.0

31.6

31.0

45.0

50.0

64.0

[Table 6-8] The permeability of CO2

and CH4 in various polymers




Exception([Table 6-9])

Permeability of glassy polymers > those of elastomers

※ Fractional free volume of the polymer↑ ⇨ permeability↑

[ex, polytrimethylsilylpropyne(PTMSP), polyphenyleneoxide(PPO)]

Polymer Tg(℃) PO2(Barrer) PN2(Barrer) αideal(PO2/PN2)

PPO

PTMSP

ethylcellulose

polymethylpentene

polypropylene

polychloroprene

polyethylene LD

polyethylene HD

210

≈200

43

29

-10

-73

-73

-23

16.8

10,040.0

11.2

37.2

1.6

4.0

2.9

0.4

3.8

6,745.0

3.3

8.9

0.3

1.2

1.0

0.14

4.4

1.5

3.4

4.2

5.4

3.3

2.9

2.9

[Table 6-9] The permeability of O2 and N2 for some elastomers and glassy polymers




Basic concept of gas separation

Governed by permeability coefficient(P) = solubility(S) × diffusivity(D)

Affinity of gas molecule with polymer↑ ⇨ Solubility↑

Solubility of CO2 : in hydrophilic polymers > in hydrophobic polymers

Affinity for polymer : Liquid ≫ Gas ⇨ Solubility of gas = very low(< 0.2%)

Noble gases

• No polymer interaction

• Ease of condensation ⇨ determine solubility

※ Solubility is determined only by their ease of condensation

• Molecule size(or Tc, Tb)↑ ⇨ condensing↑ ⇨ solubility↑

Solubility of noble gases(Ne < Ar < Kr < Xe)

Ne in silicone rubber = 0.04 cm3(STP)/(cm3∙atm)

Kr in silicone rubber = 1.0 cm3(STP)/(cm3∙atm)




Diffusivity : depend on 2 factor(Molecular size of penetrant and membrane)

Size of the gas molecule

Size↓ ⇨ diffusion coefficient↑

MW : O2 > N2

Size : O2 < N2

Thermodynamic diffusion coefficient, (6-48)

where f = frictional coefficient

Relationship between Diffusion coefficient(D) ↔ Molecular size(r)

Stokes' law, f = 6π η∙r (6-49)

Eq(6-48) with Eq(6-49) for ideal systems (DT = D)

⇨ (6-50)

⇨ (Diffusivity) ∝ (Molecular size, r)－1

Small differences in size ⇨ very large effect on D

D of Ne(MW : 20 g/mol) in PMMA = 10－10 m2/sec

D of Kr(MW : 83.8 g/mol) in PMMA ≒ 10－12 m2/sec

Diffusion coefficient : O2 > N2

⇨ Permeability : O2 > N2

Gas Molecule Diameter(Å)

He

Ne

H2

NO

CO2

C2H2

Ar

O2

N2

CO

CH4

C2H4

C3H8

2.6

2.75

2.89

3.17

3.3

3.3

3.4

3.46

3.64

3.76

3.80

3.9

4.3

[Table 6-10] The kinetic diameter

of some gas molecules.




Relationship between D ↔ Nature of polymer

∙ D of Kr in polydimethylsiloxane ≒ 10－9 m2/sec

∙ D of Kr in PVA ≒ 10－13 m2/sec

Separation depend on S/D, not S or D

Cellulose acetate or other ester-containing polymers

Solubility and solubility ratio of CO2 = especially high ⇨ high selectivity(P ratio)

Polymer DCO2/DCH4 SCO2/SCH4 PCO2/PCH4

Cellulose acetate

Polyimide(Kapton)

Polycarbonate

Polysulfone

4.2

15.4

6.8

8.9

7.3

4.1

3.6

3.2

30.8

63.6

24.4

28.3

[Table 6-11] Ratios of the diffusivity(D), solubility(S) and permeability(P)

of CO2 and CH4 in various polymers




Diffusivity or changes in diffusivity : much stronger effect on the selectivity

Polyimide(Kapton) : glassy polymer with a very rigid structure

Diffusivity ratio ⇨ determines selectivity

Very definite pore structure ⇨ exclude larger molecule

Diffusion effect > Specific interaction effect

⇨ Selective diffusion of O2 to N2

⇨ Selectivity for permanent gas

: glassy polymers > elastomers

Component Permeability

(Barrer)

Nitrogen

Oxygen

Methane

Carbon dioxide

Ethanol

Methylene chloride

Carbon tetrachloride

1,2-Dichloroethane

1,1,1-Trichloroethane

Chloroform

Trichloroethylene

Toluene

280

600

940

3200

53,000

193,000

290,000

248,000

247,000

329,000

740,000

1,106,000

[Table 6-12] Permeability of various gases and vapors

in polydimethylsiloxane at an activity of a = 1 (p = po).




Permeability of a gas = f(polymer) strongly

Organic vapors

Difference between Gas ↔ Vapor : Condensable under standard condition(O°C and 1 bar)

Size : Vapor ≫ Permanent gas ⇨ D : Vapor ≪ Permanent gas

But Permeability : Vapor ≫ permanent gas

∵ P = Diffusivity × Solubility ⇨ Solubility : Vapor ≫ permanent gas ⇨ High permeability

∵ Vapor molecules ⇨ plasticising action on the polymer

(make chains more flexible ⇨ free volume↑considerably ⇨ solubility↑)

Exponential relationship from the free volume theory

Empirical relationship : D = Do exp(ϕ∙γ) (6-51)

where Do = diffusion coefficient at zero penetrant concentration

γ = constant related to plasticising effect of penetrant on polymer

ϕ = volume fraction of the penetrant in the membrane

Concentration ↔ diffusion coefficient = not the same for all polymers




Penetrant size

Penetrant shape

Membrane

Penetrant size↑ ⇨ Do↓

Do : methanol in PVA > about 3 orders of magnitude for n-propanol

For a given penetrant, chain flexibility↑ ⇨ Do↑ ⇨ Do : glassy polymer ≪ elastomer

Do of benzene : in PVA ×10 < in polydimethylsiloxane (silicone rubber)

※ Exception : poly(dimethylphenylene oxide) & polytrimethylsilylpropyne :

very high diffusivities ⇨ very high permeability

Solubility prediction

For permanent gases : use Henry's law

For vapor : use Flory-Huggins thermodynamics(as for liquids)

Determine Do




Joule-Thomson effect

Very peculiar phenomenon in gas separation

Occur if a gas is expanded across a membrane(as in the case of a gas permeation)

Adiabatic expansion of a real gas(not ideal gas) ⇨ T↓ ⇨ P↓ & Selectivity↑

He : Adiabatic expansion ⇨ T↑ (∵ Joule-Thomson coefficient < 0)

Gas expansion from the high pressure side (subscript 1) to the low pressure side (subscript 2)

at adiabatic condition(no heat transfer, q = 0)

Internal energy change, ΔU = U2－U1 =－P2V2 + P1V1 (6-52)

or U1 + P1V1 = U2 + P2V2 (6-53)

or H1 = H2 (isenthalpic) (6-54)

6.4.2.4 Joule-Thomson effect

Schematic representation of

the principle of the Joule-Thompson effect.




Joule-Thompson coefficient, μJT = (∂T/∂P)H

H = f(T, P) ⇨ (6-55)

Furthermore

(6-56) and (6-57)

For the enthalpy change of a reversible process,

dH = V dP + T dS (6-58)

differentiation with respect to P at constant temperature

(6-59)

From the Maxwell’s relation,

(6-60)

Eq(6-56), (6-59) and (6-60) → Eq(6-57),

(6-61)

Gas μJT (K/bar)

He

CO

H2

O2

N2

CH4

CO2

-0.06

0.01

0.03

0.30

0.25

0.70

1.11

[Table 6-13] Joule-Thomson coefficient

of various gases at 1 bar and 298 K




Variation of Permeability(P) of gas or vapor

[Table 6-8] ⇨ Permeability variation of a given gas by > 106 in various polymer

[Table 6-11] ⇨ Permeability variation of various gases and vapors : > 106 for a given polymer

『Meaning』Many materials can be used as a membrane depending on the application.

Gas separation : based on permeability and selectivity(ratio of the permeability)

Thin dense top-layer : hydrodynamic resistance = large

Permeability ratio : usually large([Table 6-11])

Choose highly permeable material(such as silicone rubber or natural rubber)

Elastomers : low selectivity, Glassy polymers : much lower permeability

Permeation rate = P/ℓ ∝ (membrane thickness)－1

Minimize effective membrane thickness

Two types of membranes for gas separation :

• Asymmetric membranes

6.4.2.5 Membranes for gas separation

• Composite membranes




Immersion precipitation technique used to manufacture

asymmetric membranes

sub-layer in composite membrane

Top-layer manufacturing technique

dip-coating

interfacial polymerization

plasma polymerization

Requirement for top-layer : absolutely defect-free

Requirement for porous support layer

Provide mechanical support for the top-layer

Open porous network to minimize resistance to mass transfer

No macro-voids (weak spots for high-pressure applications)

Schematic representation of an asymmetric membrane

and the corresponding electrical circuit analogue.




Defect-free thin top-layer from a glassy polymer : Very difficult

Method to make a defect-free asymmetric membrane

Dual bath method

Evaporation method

Deposit a coating of a highly permeable polymer upon

an asymmetric membrane containing some defects.

⇨ Coating surface pores ⇨ Defects free membrane

Flux of gas i, (6-62)

Overall permeability(P), (6-63)

where Rtot = total membrane resistance

For the uncoated membrane (see [Figure 6-18])

Rtot,un = (R2－1 + R3

－1 ) －1 + R4 －1 (6-64)

For the coated layer (see [Figure 6-19])

Rtot,c = R1 －1 + (R2

－1 + R3 －1 ) －1 + R4 (6-65)

Schematics of a coated

Asymmetric membrane and corresponding

electrical circuit analogue.




Resistance of sub-layer (R4) = negligible

Flux of component i across the uncoated (Jun,i)

(6-66)

Flux of component i across the coated membranes (Jc,i)

(6-67)

where ℓ1 = thickness of the coating layer

ℓ2 = thickness of the top-layer in the sub-structure

ε = represents the surface porosity

Selectivity and flux as a function of the surface

porosity for coated and uncoated membranes

on the basis of the resistance model.




[Figure 6-20]

J and αCO2/CH4 vs. Surface porosity for the uncoated and coated membranes

Support membrane : polysulfone

Coating layer : silicone rubber (data given in [Table 6-8])

Top layer thickness of 1 μm (ℓ1 = ℓ2 = 1 μm).

⇨ By coating with a very permeable low-selective polymer

• Very effective in obtaining a defect-free layer

Uncoated membrane : high defect ⇨ selectivity↓

Coated membranes :

No decrease in selectivity up to 10－4 of porosity

No change in permeability up to 10－4 of porosity No defect




Composite membrane

In general, transport through the thin top-layer : rate-determining step

Plugging of defects with a high permeable polymer

⇨ Special type of composite membrane

∵ Support layer(Sub-layer) determines the separation performance

Top-layer material penetrated into sub-layer(pore penetration)

Overall resistance(effective thickness)↑

Glassy top-layers supported by glassy sub-layer(supports)

Not preferable

Double composite membrane

Highly permeable third layer(e.g., polydimethylsiloxane) is used between the sub-layer

and top-layer ⇨ Intermediate layer or 'gutter'.

Surface of sub-layer = very highly porous

(difficult to deposit a thin selective coating directly)

Top-layer = glassy polymer(difficult to obtain defect-free)



※ Balance between permeability ↔ selectivity

High permeable materials are used if high selectivity are not required

Production of O2 enriched air

• medical applications

• combustion processes

• sterile air for aerobic fermentation processes

Separation of organic vapors from non-condensable gases

• High selectivity with highly permeable materials

• Hydrophobic elastomeric polymer : Permeability of N2 and CH4 ≪ of any organic vapor

If a moderate selectivity is required

Use Low permeable materials (glassy polymers)

1) CO2/CH4



• Purification of CH4 from landfill drainage gas

• Purification of CH4 from natural gas

• Recovery of CO2 in enhanced oil recovery



2) H2 or He from other gases

• H2 or He : relatively small molecular sizes

• High selectivity ratios in glassy polymers

• Recovery of H2 from purge gas in NH3 synthesis, petroleum refineries and methanol synthesis

3) H2S/CH4

• Separate H2S(very toxic, highly corrosive gas) in natural gas below 0.2%

4) O2/N2 : (N2 enriched air, 95 ∼ 99.9%)

• Inert gas in the blanketing of fuel tanks, and in storage of food and agricultural products

5) H2O from gases

• Dehydration of natural gas, air conditioning, and drying of compressed air

6) Acid gas(SO2, CO2 and NOx) from smoke or flue gas

• Relatively low concentrations at atmospheric pressures

• not very suitable for pressure driven operations (low driving force)

• prefer to membrane contactor, carrier mediated processes and membrane reactors

6.4.2 Gas Separation 6.4 Concentration Driving Force



6.4.2.7 Summary of gas separation


Membranes Asymmetric or Composite membranes with an elastomeric or glassy

polymeric top layer

Thickness ≈ 0.1 to few μm(top layer)

Pore sizes non-porous(or porous < 1 μm)

Driving force Pressure, upstream to 100 bar or vacuum downstream

Separation principle Solution-diffusion(non-porous membrane)

Knudsen flow(porous membrane)

Membrane material Elastomer : polydimethylsiloxane, polymethylpentene

Glassy polymer : polyimide, polysulfone

Applications

• H2 or He recovery

• CH4/CO2

• O2/N2

• Organic vapors from air

• Dehydration (compressed air, natural gas, air conditioning)

• Acid gases from flue gas


chapter 6. membrane process (pressure driving...

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