ByChanakya Pallem

Membrane Separation Processes

What is a membrane?

It is defined essentially as a

“Barrier which separates 2 phases & restricts

transport of various

molecules in a selective manner”.

Driven by • Pressure• Concentration• Temperature• Electrical

potential Gradients

It can be

• Thick/Thin• Liquid/Solid• Symmetric/

Asymmetric• Natural/Synthetic• Neutral/Charged• Homogeneous/

Heterog-eneous

History: In 1748 - Abbe Jean-Antoine Nollet; French physicist

separated degassed alcohol using pig’s bladder.1824, Rene-Joachim-Henri Dutrochet, French physiologist

introduced “Osmosis”: Movement of water through a biological membrane.

1846 – Discovery of nitrocellulose (gave scope to MF)1855 – Frick discovered Cellulose nitrate membranes.1861- Thomas Graham (Father of Modern Dialysis):

Coined “Dialysis “- Separated Dissolved substances based on mol.wt., n concentration.

1865 – Moritz Traube invented first artificial membrane using copper ferrocyanide precipitates.

1875- Wilhelm Friedrich Philipp Pferrer: Made the membranes to withstand operational pressures.

1906-Bechhold devised a technique to prepare nitrocellulose membranes of graded pore size .

1930’s-Micro porous colloidal membranes became commercially available.

1950’s- Development n Significant use of MF technology in the filtration of drinking water samples at the end of World War II: Research effort was sponsored by US army.

1959- Samuel Yuster made a breakthrough in RO– by the invention of Loeb-Sourirajan membrane at UCLA.

By 1960-Elements of modern membrane science had been developed such as Gas Separation, Membrane Distillation etc.

In the early 80’s- Henis &Tripodi made industrial GS: economically feasible.

Kober and coworkers developed Pervaporation. Later in 2000’s modified for large scale applications.

Carbon Nanotube Membranes

Gas Separation n Pervaporation

Working mechanism:Membrane process: the feed stream is divided into two

streams:

Retentate (concentrate) streamPermeate stream

Either the concentrate or permeate stream is the product of our interest.

IDE

AL

ME

MB

RA

NE

Permeate Feed

Driving Force

RE

AL

ME

MB

RA

NE

Phase 1Phase 2

Sch. Representation of Membrane Separation:

I

What is a Membrane?

First generation membrane processes

Microfiltration (MF)Ultrafiltration (UF)Nanofiltration (NF)Hyper filtration (HF) /Reverse osmosis (RO)Electro dialysis (ED)

Second generation membrane processes

Gas separation (GS)Pervaporation (PV)Membrane Distillation (MD)

Membrane Processes:

Microfiltration (MF): Separates suspended solids and some colloidal materials

(>0.1μ) from a feed stream.

The concentrate requires periodic removal or cleaning to prevent the eventual plugging of membrane feed passage ways.

Pore size : 0.1-10.0 microns Pressure difference : Apprx. 10-500 kPa

Ultrafiltration (UF):Separates colloidal material, emulsified oils, micro

biological materials, and large organic molecules.

Somewhat dependent on charge of the particle, and is much more concerned with the size of the particle.

Pore sizes ranges: 10-1000 A° (103-0.1 microns) :most typical 0.005 μ

Pressure difference : Apprx. 0.1-1.0 MPa

Typically not effective at separating organic streams

Dead end and cross-flow:

Feed

PermeatePermeate

Feed Retentate

1. Dead-end 2. Cross-flow

Nanofiltration (NF): Used when low molecular weight solutes such as inorganic

salts/ small organic molecules (glucose, sucrose) have to be separated.

Uses a membrane that is partially permeable to perform the separation (like in RO), but NF pores >> RO pores

Can operate at much lower pressures, and passes some of the inorganic salts due to larger pore size

Pore size is typically 1 nm

Pressure difference: 10-20 bar

Reverse Osmosis (RO) (Hyper filtration):

Specifically used for the separation of dissolved ions from water (dissolved solids, bacteria, viruses, salts, proteins, and other germs)

Charged ions and all other materials greater than or equal to .001 μ.

Essentially a pressure driven membrane diffusion process for separating dissolved solutes.

Relatively a low energy process. Smallest pore structure, 5-15 A0 (0.5 nm - 1.5 nm)

allows only the smallest organic molecules and unchanged solutes to pass through the semi-permeable membrane along with the water

>95-99% of inorganic salts and charged organics will also be rejected by the membrane due to charge repulsion established at the membrane surface

In the ED process a semi-permeable barrier allows passage of either positively charged ions (cations) or negatively charged ions (anions) while excluding passage of ions of the opposite charge. These semi-permeable barriers are commonly known as ion-exchange, ion-selective or electrodialysis membranes.

Electrodialysis:

Gas Separation (GS): Used for separation of gas mixtures. Separation of gases is due to their different solubility n

diffusivity in the polymer membranes.

Rate of permeation: Proportional to pressure differential across the membrane,

solubility of gas in the membrane, diffusivity of gas through membrane.

Inversely proportional to the membrane thickness.

Driving force: Concentration difference.

Pore size: < 1 nm.

Ex: Palladium membranes –Hydrogen Separation.

Pervaporation (PV): Separation of miscible liquids

Liquid is maintained at atmospheric pressure on the feed side of the membrane, and permeate is removed as a vapour because of a low vapour pressure existing on the permeate side.

Differs from all other membrane processes because of the phase change of the permeate.

Transport is effected by maintaining a vapour pressure gradient across the membrane.

Membranes used: Zeolite n Poly Dimetyl Siloxane

Three steps sequence: Selective sorption of one of the components of the liquid

into the membrane on the feed side Selective diffusion of this component across the

membrane Evaporation, as permeate vapour, into the partial

vacuum applied to the underside of the membrane

Is a process in which two liquid or solutions at different temperatures are separated by a porous hydrophobic membrane.

The liquid/solution must not wet the membrane otherwise the pores will be filled for capillary force.

Membrane distillation is a type of low temperature, reduced pressure distillation due at the use porous hydrophobic polymeric membranes.

Membrane Distillation:

Feed

H2O

T1

Permeate

H2O

T2

Air/vapour

Hydrophobic porous membrane

T1>T2

Liquid water

Liquid water

Schematic representation: Such transport occur in a sequence of three steps:

Evaporation on the high-temperature side.

Transport of vapour molecules through the pores of the hydrophobic porous membrane.

Condensation on the low-temperature side.

It is one of the membrane processes in which the membrane is not directly involved in separation the only function of the membrane is to act as a barrier between the twos phases. Selectivity is completely determined by the vapour liquid equilibrium involves. This means that the component with the highest partial pressure will show the highest permeation rate.

Fractionation by membrane distillation, 1, porous hydrophobic membrane polymer;

2, feed; 3, vapour space; 4, cooling water; 5, chilled wall; 6, condensed droplets.

2

1

3

6

5

4

Materials used:

Synthetic polymeric membranes:

a) Hydrophobic b) Hydrophilic

Ceramic membranes

PolyTetraFluoroEthylene,TeflonPolyVinyliDineFluoridePolyPropylenePolyEthylene

Cellulose estersPolyCarbonatePSf/PESPolyImide/PolyEtherImidePolyEtherEtherKetone

PolyTetraFluoroEthylene,TeflonPolyVinyliDineFluoridePolyPropylenePolyEthylene

Cellulose estersPolyCarbonatePSf/PESPolyImide/PolyEtherImidePolyEtherEtherKetone

Alumina, Al2O3

Zirconia, ZrO2

Titania, TiO2

Silicium Carbide, SiC

Alumina, Al2O3

Zirconia, ZrO2

Titania, TiO2

Silicium Carbide, SiC

26

Modules:A module is the simplest membrane element that can be

used in practice.

Module design must deal with the following issues:

3. Membrane integrity against damage and leaks

3. Membrane integrity against damage and leaks

2. Minimum waste of energy2. Minimum waste of energy

4. Easy egress of permeate

4. Easy egress of permeate

5. Permit the membrane to be cleaned

5. Permit the membrane to be cleaned

1. Economy of manufacture

1. Economy of manufacture

Membrane Modules: Plate-and-frame module

Spiral-wound module

Tubular module

Capillary module

Hollow-fiber module

Membrane module

Membrane area/unit vol. (m2 m-3 )

Membrane

costs

Control ofFouling Application

Plate & frameModule

400 - 800 medium good MF, UF, RO, ED

Spiral-woundmodule

800 - 1200 low good UF, RO, GS

Tubular

module

20 - 100 very high very good MF, UF, RO

Capillary

module

600 - 1200 low very good UF, MF,

Hollow fibermodule 2000 - 5000 very low very poor RO, GS

Membrane Fouling ?

It is a process where solute or particles deposit onto a membrane surface or into membrane pores in a way that degrades the membrane's performance.

Major Foulants:

Organic materials

Biological growth

Colloidal n suspended particles

Soluble salts

Membrane properties

Solution properties

Operating conditions

Influential factors

Methods to reduce fouling:

1. Pre-treatment of the feed solution1. Pre-treatment of the feed solution

2. Membrane properties

2. Membrane properties

3. Module and process conditions

3. Module and process conditions

4. Cleaning4. Cleaning

a. Reducing concentration polarisationa1. Increasing flux velocitya2. Using low flux membranes

a. Reducing concentration polarisationa1. Increasing flux velocitya2. Using low flux membranes

a. Narrow pore size distributionb. Hydrophilic membranes

a. Narrow pore size distributionb. Hydrophilic membranes

a. Heat treatmentb. pH adjustamentc. Addition of complexing agentsd. Chlorinatione. Adsorption onto active carbon

a. Heat treatmentb. pH adjustamentc. Addition of complexing agentsd. Chlorinatione. Adsorption onto active carbon

a. Hydraulic cleaningb. Mechanical cleaningc. Chemical cleaning

a. Hydraulic cleaningb. Mechanical cleaningc. Chemical cleaning

No specific chemical knowledge is needed for operation

No Complex instrumentationBasic concept is simple to understandSeparation can be carried out continuouslyMembrane processes can easily be combined with

other separation processesSeparation can be carried out under mild conditionsMembrane properties are variable and can be

adjustedGreater design flexibility in designing systemsClean technology with operational ease

Advantages:

Membranes are relatively expensiveCertain solvents, colloidal solids, especially

graphite and other residues can quickly and permanently destroy the membrane surfaces

Oil emulsions are not "chemically separated," so secondary oil recovery can be difficult.

Synthetics are not effectively treated by this method

Biofouling/membrane fouling;Low membrane lifetime;Generally low selectivity

Disadvantages:

Concentration: The desired component is present in a low concentration and solvent has to be removed;

Purification: Undesirable impurities have to be removed;

Fractionation: A mixture must be separated into two or more desired components.

Applications:

Development and Advancement of Nano-materials for effective membrane strength n separations.

Over-coming the problem of Membrane Fouling.

To design membranes for high selectivity.

Future Challenges: