membrane separation processe
DESCRIPTION
TRANSCRIPT
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: