Chapter 6. Membrane Process (Pressure Driving Force)

November 4, 2015 (Wed)

Chapter 6. Membrane Process

(Pressure Driving Force)

Contents Contents

6.5 Other Driving Force

6.4 Concentration Driving Force 6.3 Pressure Driven Force

6.2 Osmosis 6.1 Introduction

 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 3^rd Class of material)

 polybenzimidazoles, polybenzimidazolones, polyamidehydrazide and polyimides 6.3.4.1 Membrane for RO and NF

 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 6.3.4.1 Membrane for RO and NF

• 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.

6.3.4.1 Membrane for RO and NF

[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⁺, CO₂^2－) : high

 Retention for micro-solutes and low MW organics : high (herbicides, insecticides, pesticides, dyes, sugars etc)

6.3.4.2 Applications

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)

Items Characteristics

Membranes Asymmetric or Composite

Thickness Sub layer ≈ 150 μm, Top layer ≈ 1 μm Pore sizes < 2 nm

Driving force Pressure : Brackish water 15 ∼ 25 bar(Sea water 40 ∼ 80 bar) Separation principle Solution-diffusion

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)

(PRO)

 PRO

 Salt concentration difference(Osmotic pressure) ⇨ generate energy

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

 J_v = A (Δπ - ΔP) (6-36)

 E = J_v∙AP = A (Δπ - ΔP)∙ΔP (6-37) where E = power, Watt or J/sec

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

E_max = A / (4 Δπ) (6-38)

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

<Figure 6-11> Principle of pressure retarded osmosis.

(PRO)

 Practical problems

 Osmosis : Concentration of concentrated solution↓ ⇨ π↓

 Salt flux : R < 100% ⇨ π↓

 Concentration polarization : major problem(See <Figure 6-12>)

 Direction of J_v is opposite with J_s

 c_s on concentrate-side membrane surface < in bulk of concentrate

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

<Figure 6-12> Concentration polarization in pressure retarded osmosis

(PRO)

Items Characteristics

Membranes Asymmetric or Composite

Thickness Sub layer ≈ 150 μm, Top layer ≈ 1 μm Pore sizes < 2 nm

Driving force Concentration difference (Osmotic pressure) Separation principle Solution-diffusion

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

 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

<Figure 6-13> The transport of ions through a mosaic membrane during 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

6.3.6.1 Summary of Piezodialysis

Items Characteristics

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

November 4, 2015 (Wed)

Chapter 6. Membrane Process

(Gas Separation)

Contents Contents

6.5 Other Driving Force

6.4 Concentration 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

 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

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

Swelling : very important factor in transport through nonporous membranes

 <Figure 6-14> 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)

<Figure 6-14> Diffusivity

as f(degree of swelling in nonporous polymer)

 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 D_k = Knudsen diffusion coefficient, is given by

T and M_w = 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 c_o,i = c_i in membrane on upstream side

c_ℓ,i = c_i in membrane on downstream side ℓ = thickness of the membrane

6.4.2.2 Gas separation through nonporous membranes

<Figure 6-15> Nonporous membrane separating two gas phases.

 Henry's law

 c_i = S_i∙p_i (6-43)

where c_i = concentration inside the membrane p_i = partial pressure of gas outside the membrane

S_i = 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 T_g (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 (α_{i/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 (P_ℓ / P_o → 0) ⇨ Separation efficiency = maximum

 Pressure ratio↑ ⇨ Selectivity↑

<Figure 6-16> 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 (T_g)

 Glass transition temperature (T_g)

 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 T_g(℃) P_O2(Barrer) P_N2(Barrer) α_ideal(P_O2/P_N2) 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↑

<Example> 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 T_c, T_b)↑ ⇨ condensing↑ ⇨ solubility↑

<Example> 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 (D_T = D)

⇨ (6-50)

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

 Small differences in size ⇨ very large effect on D

<Example> 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

<Example>

∙ 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 D_CO2/D_CH4 S_CO2/S_CH4 P_CO2/P_CH4 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

<Assume> 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 = p^o).

 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 = D_o exp(ϕ∙γ) (6-51)

where D_o = 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↑ ⇨ D_o↓

<Example> D_o : methanol in PVA > about 3 orders of magnitude for n-propanol

 For a given penetrant, chain flexibility↑ ⇨ D_o↑ ⇨ D_o : glassy polymer ≪ elastomer <Example> D_o 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 D_o

<Summary>

 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↑

<Exception> 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

<Figure 6-17> Schematic representation of the principle of the Joule-Thompson effect.

Chapter 6. Membrane Process(Pressure Driving Force) 37

Chungbuk University

 Joule-Thompson coefficient, μ_JT = (∂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 μ

(K/bar) He

CO H

O

N

CH

CO

-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)

<Figure 6-18> 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 R_tot = total membrane resistance

 For the uncoated membrane (see [Figure 6-18]) R_tot,un = (R₂^－1 + R₃ ^－1 ) ^－1 + R₄ ^－1 (6-64)

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

R_tot,c = R₁ ^－1 + (R₂ ^－1 + R₃ ^－1 ) ^－1 + R₄ (6-65)

<Figure 6-19> Schematics of a coated

Asymmetric membrane and corresponding electrical circuit analogue.

<Assume> Resistance of sub-layer (R₄) = negligible

 Flux of component i across the uncoated (J_un,i)

(6-66)

 Flux of component i across the coated membranes (J_c,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

<Figure 6-20> 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 α_CO2/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

6.4.2.6 Applications

• 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 NO_x) 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

6.4.2.7 Summary of gas separation

Items Characteristics

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