Four ur Pr Pressure essure Driven riven Mem Membrane brane Pr - - PowerPoint PPT Presentation

Four ur Pr Pressure essure Driven riven Mem Membrane brane Pr Processes cesses

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In the Name of God

Four pressure driven membrane processes

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NF 0.5-2.5 MPa RO >1.5 MPa UF 0.1-1 MPa MF <0.2 MPa

= P

• Microfiltration (MF)
• Ultrafiltration (UF)
• Nanofiltration (NF)
• Reverse osmosis (RO)

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Introduction

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Schematic drawing

• f

water flow as a function of applied pressure (∆P):

Introduction

Introduction

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Introduction

◼ Van΄t Hoff equation (for dilute solutions)

non-ionisable materials: Ionisable salts:

▪ Virial equation:

c iRT( ) M  =

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c RT( ) M  =

2 3

Ac Bc Dc  = + +

) X Ln(γ V RT π

w w w

− =

Microfiltration (MF)

◼ Membranes: (a)symmetric porous ◼ Thickness: 10-150 µm ◼ Pore sizes: 0.05-10 µm ◼ Driving force: pressure difference (< 2 bar) ◼ Separation principle: sieving mechanism

Microfiltration (MF)

:

Membrane materials for MF applications

Hydrophobic polymeric membranes Polytetrafluoroethylene (PTFE, teflon) Poly(vinylidene fluoride) (PVDF) Polypropylene (PP) Polyethylene (PE) Hydrophilic polymeric membranes Cellulose esters Polycarbonate (PC) Polysulfone/poly(ethersulfone)(PSf/PES) Polyimide/poly(ether imide) (PI/PEI) Polyamide (PA) Polyetheretherketone (PEEK) Ceramic membranes Alumina (Al2O3) Zirconia (ZrO2) Titania (TiO2) Silicium carbide (SiC)

Microfiltration (MF)

Some applications:

✓ Juice, wine or beer clarification ✓ Sterilization

✓ Separation of oil-water emulsions

✓ Water and wastewater treatment ✓ Membrane bioreactor

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Ultrafiltration (UF)

◼ Membranes: asymmetric porous ◼ Thickness: 150 µm ◼ Pore sizes: 1-100 nm ◼ Driving force: pressure difference (1-10bar) ◼ Separation principle: sieving mechanism

Ultrafiltration (UF)

• Characteristic parameter: Molecular Weight Cut-Off

MWCO is defined as the molecular weight which is 90% rejected by the membrane.

Relation between MWCO and the pore size for UF Membranes

. 1 Dalton (Da) represents the mass of a hydrogen atom

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Ultrafiltration (UF)

Membrane materials for UF applications:

◼ Polysulfone/ poly (ether sulfone) / sulfonated polysulfone ◼ Poly(vinylidene fluoride) ◼ Polyacrylonitrile ◼ Cellulosics (e.g. cellulose acetate) ◼ Polyimide / poly (ether imide) ◼ Polyamide ◼ Polyetheretherketone ◼ In

addition to such polymeric materials, inorganic (ceramic) materials have also been used for ultrafiltration membranes, especially alumina and zirconia.

Ultrafiltration (UF)

Some applications:

✓ Concentration of milk ✓ Treatment of oil-water emulsion ✓ Water and wastewater treatment ✓ Recovery of whey proteins

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Nanofiltration (NF)

◼ Membranes: composite ◼ Thickness:

sublayer: about 150 µm; toplayer: about 1 µm

◼ Pore sizes: < 10 nm ◼ Driving force: pressure difference (5-25

bar)

◼ Separation principle: solution-diffusion ◼ Membrane material: polyamide

Nanofiltration (NF)

Some applications:

✓ Water softening ✓ Desalination of brackish water (amount of salt: 1000-

5000 ppm)

✓ Wastewater treatment ✓ Removal of micropollutants ✓ Retention of dyes (textile industry)

Still looking for applications. Potentially when UF does not give sufficient rejection and RO is not economical.

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Reverse osmosis (RO)

◼ Membranes: asymmetric ◼ Thickness: sublayer: about 150 µm; toplayer:

about 1 µm

◼ Pore sizes: < 2 nm ◼ Driving

force: pressure difference (brackish water: 15-25 bar and sea water: 40-80 bar)

◼ Separation principle: solution-diffusion ◼ Membrane

material: cellulose triacetate, polyamide and poly(ether urea)

Reverse osmosis (RO)

Some applications:

✓ Production of ultra pure water for electronic

industry

✓ Desalination of brackish or sea water (amount of

salt: 35000 ppm)

✓ Wastewater treatment

✓ Concentration of juices, milk or sugar solutions

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Comparison of retention characteristics between nanofiltration and reverse osmosis:

Reverse osmosis (RO)

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Dialysis and Electrodialysis Processes

In the Name of God

Dialysis

◼ Membranes: homogeneous ◼ Thickness: 10-100 µm ◼ Driving force: concentration difference ◼ Separation principle: difference in diffusion rate,

solution-diffusion

❖CA (cellulose acetate) ❖Regenerated cellulose such as cellophane and cuprophane ❖PVA (poly vinyl alcohol) ❖PAA (polyacrylic acid) ❖PMMA (polymethylmethacrylate) ❖PC (polycarbonates) Membrane materials for dialysis applications:

Dialysis

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Applications:

◼ Hemodialysis (removal of toxic substances from blood) ◼ Alcohol reduction in beer ◼ salt removal in pharmaceutical industry ◼ Alkali recovery in pulp and paper industry

Dialysis

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Hemodialysis (HD)

Artificial kidney

Electrodialysis (ED)

◼ Membranes:

anion-exchange and cation-exchange membranes

◼ Thickness: 100-500 µm ◼ Driving force: electrical potential difference ◼ Separation principle: Donnan exclusion mechanism ◼ Membrane

material: crosslinked copolymers based

• n

divinylbenzene (DVB) with polystyrene or polyvinylpyridine, copolymers

• f

polytetrafluoroethylene (PTFE) and poly (sulfonyl fluoride-vinyl ether)

Electrodialysis (ED)

The basic parameters for a good membrane:

• High selectivity
• High electrical conductivity
• Moderate degree of swelling
• High mechanical strength

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Electrodialysis (ED)

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Electrodialysis (ED)

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Electrodialysis (ED)

• ED with reverse polarization (EDR)
• ED at high temperature
• ED with electrolysis

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Intensity evolution versus applied potential

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Applications:

◼ Desalination of water ◼ Production of salt ◼ Demineralisation of whey ◼ Deacidification of fruit juices ◼ Production of boiler feed water ◼ Removal of organic acids from a fermentation broth ◼ Separation of amino acids from each other

Electrodialysis (ED)

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Electrolytic cell for the production of Cl2 and NaOH with cationic membrane

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Pervaporation, gas separation and liquid membrane

In the Name of God

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Pervaporation

• Operation with phase change
• Composite membranes (0.1 to few µm for top layer)
• Nonporous membranes
• Solution-diffusion mechanism
• Driving force: difference in partial pressure
• Vacuum (<40 mm Hg) and dilution (inert gas, N2)

➢ Overall the vacuum mode is more popular. ➢ The inert gas plays the same role as vacuum in creating a driving

force for transport through the membrane.

Pervaporation

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Pervaporation

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Pervaporation

• Three-step mechanism:
• Selective sorption on the upstream side of the

membrane

• Selective diffusion through the membrane
• Desorption on the permeate side

Pervaporation

Pervaporation has the following advantages:

◼ Low capital and operating cost. ◼ Azeotropes can be readily broken by using an appropriate

membrane.

◼ Easy operation and space saving ◼ Clean operation ◼ Compact and scalable units

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Pervaporation

• Simplified transport equation:

Pi: permeability coefficient of component i

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Pervaporation

• Main membrane parameters:
• Separation factor:
• Enrichment factors:

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Pervaporation of methanol/MTBE mixtures

◼

BP of Methanol=65 °C BP of C5H12O=55 °C

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Pervaporation (applications)

◼ Aqueous mixtures

Two main classes can be distinguished here; either a small amount of water has to be removed from an

• rganic solvent (dehydration) or a small amount of
• rganic solvent has to be removed from water:

 Dehydration

◼ Removal of water from organic solvents. Even traces of

water can be removed (e.g., from chlorinated hydrocarbons and alcohol)

Pervaporation (applications)

Removal of volatile organic compounds from

water

◼ Alcohol from fermentation broths (ethanol, butanol

and acetone-butanol-ethanol (ABE))

◼ Volatile organic contaminants from wastewater

(aromatics, chlorinated hydrocarbons)

◼ Removal of flavour and aroma compounds ◼ Removal of phenolics

Pervaporation (applications)

◼ Non-aqueous mixtures:

 Polar/non-polar

◼ Alcohols/aromatics (methanol/toluene) ◼ Alcohols/ethers (methanol/ methyl-t- butylether (MTBE))

 Saturated/unsaturated

◼ Butane/butene

 Separation of isomers

◼ C-8 isomers

Pervaporation (applications)

◼ Although dehydration has widespread application, the other

applications are not as commercially successful as dehydration.

◼ The process is currently best identified with dehydration of

ethanol, isopropyl alcohol, and ethylene glycol.

◼ The membrane polymer to be selected for a given separation

should have high sorption affinity for the solute to be removed preferentially.

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Hydrophilic Polymers Available for Selective Water Removal Poly(vinyl alcohol) (different cross-linkers) Poly(acryclic acid) (different cross-linkers) Various poly(amide)s Various poly(imide)s Cellulose acetate Other cellulose derivatives/forms

Pervaporation

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Hydrophobic/Organophilic Polymers Available for Selective Removal of Organics from Aqueous Solutions Poly(dimethyl siloxane) (PDMS) Poly(octyl methyl siloxane) (POMS) Ethylene propylene diene rubber (EPDM) Natural rubber (NR) Styrene butadiene rubber (SBR) N-butyl rubber (NBR) Nitrile rubber Poly(ether block amides) (PEBA) Elastomeric poly(urethanes)

Pervaporation

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Gas separation

• Membranes: porous (<1 µm) and non-

porous

• Several possible mechanisms for gas

transport:

✓ Viscous Flow ✓ Knudsen Flow ✓ Solution-diffusion

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Gas separation

i j j i

M M J J 

• Knudsen Flow (porous membranes): When the pore diameter is
• n the range of (comparable to or smaller than) the mean free

path of the molecules.

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Gas separation

• Solution-diffusion (non-porous membranes)- The

permeability coefficient (P) of a substance depends on its solubility coefficient (S) and diffusivity (D) in the membrane:

• The ideal selectivity is given by the ratio of the

permeability coefficients:

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Gas separation

• Driving force: pressure difference
• Working pressure: up to 100 bar
• Asymmetric or composite membranes (0.1 to few

µm for top layer)

• Polymeric membranes:

PDMS, PSf, polymethylpentene (PMP) and PI

• Ceramic membranes (small pores for Knudsen

flow)

• Metallic membranes (Pd and Ag alloys)

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Gas separation

Applications (some examples):

• Enrichment, recovery and dehydration of N2
• H2 recovery in residual flows of processes
• Adjustment of the ratio H2/CO in synthesis gas
• Removal of H2O, CO2 and H2S from natural gas
• Helium recovery from natural gas and other sources
• VOC removal from process streams

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Liquid membrane

• A liquid barrier between two phases
• Driving force: Concentration difference
• Types:

▪ Bulk Liquid Membrane (BLM) ▪ Supported Liquid Membrane (SLM)

▪ Emulsion Liquid Membrane (ELM) ▪ Flowing Liquid Membrane (FLM)

Bulk Liquid Membrane

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Bulk liquid membranes (BLM), usually consist of an aqueous feed and stripping phase, separated by a water- immiscible liquid membrane phase.

Bulk Liquid Membrane

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▪ Advantages: ➢Very simple ➢ Easy operation ▪ Disadvantages: ➢ Low contact surface area ➢ Low permeation rate and separation efficiency

Transport mechanisms

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Simple transport

Transport mechanisms

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Simple transport with chemical reaction in strip solution

Transport mechanisms

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Facilitated or carrier-mediated transport

Transport mechanisms

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Transport mechanisms

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Some carriers

Name Structure Oxime Tertiary amine Crown ether Calixarene

Supported Liquid Membrane

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Supported Liquid Membrane

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Flat- Sheet Supported Liquid Membrane

Supported Liquid Membrane

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Hollow Fiber Supported Liquid Membrane

Supported Liquid Membrane

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◼ The liquid film is immobilized within the pores of a porous

membrane

◼ The porous membrane serves only as a framework or

supporting layer for the liquid membrane

◼ Such membranes can easily be prepared by impregnating a

(hydrophobic) porous membrane with a suitable organic solvent.

◼ Some porous membranes frequently used as support for SLM:

➢ Polypropylene ➢ Polytetrafluoroethylene ➢ Poly(vinylidene fluoride) ➢ polyethylene

Supported Liquid Membrane

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◼ Advantage:

➢ Very

little amount

• f
• rganic

phase is required.

◼ Disadvantage:

➢ Leaching out of organic phase from the pores

Emulsion Liquid Membrane

◼ Two immiscible phases, water and oil for example, are

mixed vigorously and emulsion droplets are formed which are stabilized by the addition of a surfactant. (A water/oil emulsion is obtained in this way.)

◼ This emulsion is added to a large vessel containing an

aqueous phase where a water/oil/water emulsion is now

• formed. The oil phase is the membrane phase.

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Emulsion Liquid Membrane

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Emulsion Liquid Membrane

▪ Advantage: ➢ High contact surface area ▪ Disadvantage: ➢Emulsion swelling (instability problem)

Flowing Liquid Membrane

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A membrane solution is circulated through a thin channel between two hydrophobic microporous membranes separating feed gas (containing ethylene) and sweep gas phases from the flowing liquid membrane phase. Ethylene, first, diffuses through the pores of porous membrane. Then the carrier through the channel absorbs ethylene to form a complex. The complex then is decomposed at the other side to release ethylene and the carrier. Finally ethylene diffuses through the opposite side porous membrane and goes to sweep gas

• phase. FLM solved the stability problem occurring in SLM.

Applications

❖ Removal of specific ions

• Cations (cadmium, copper, nickel and lead)
• Anions (nitrate and chromate)

❖ Removal of gases

• Oxygen/nitrogen separation
• Removal of SO2, CO, NH3, H2S, NOx and CO2 from gases

❖Separation of organic liquids (organic mixtures) ❖Removal of phenol from wastewater

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