Membrane Separation Technology

7/16/2019 Membrane Separation Technology

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Membrane Separation Processes

Neha Kathayat

Rugved PathareDaksh Pratap Singh



Overview

Introduction

Ultrafiltration

Reverse Osmosis

Electrodialysis

Summary



Overview

Introduction

Ultrafiltration

Reverse Osmosis

Electrodialysis

Summary



Overview

Introduction

Ultrafiltration

Reverse Osmosis

Electrodialysis

Summary



Overview

Introduction

Ultrafiltration

Reverse Osmosis

Electrodialysis

Summary



Overview

Introduction

Ultrafiltration

Reverse Osmosis

Electrodialysis

Summary



Overview

Introduction

Ultrafiltration

Reverse Osmosis

Electrodialysis

Summary



Introduction

Strathmann defines a membrane as “aninterphase separating two phases and

selectively controlling the transport of materials between those phases”

Since the 1960s a new technology using

synthetic membranes for processseparations has been rapidly developed



Introduction

Strathmann defines a membrane as “aninterphase separating two phases and

selectively controlling the transport of materials between those phases”

Since the 1960s a new technology using

synthetic membranes for processseparations has been rapidly developed



Advantages offered

Ambient temperature operation

Relatively low capital and running costs

Modular construction



Advantages offered






Advantages offered






General Classification

Most of the membrane processes arepressure driven barring a few like

electro dialysis (ED). Pressure driven process includes micro

filtration (MF), ultra-filtration (UF),reverse osmosis (OS)

Micro filtration, Ultra filtration, Reverse-Osmosis are different with respect topore-size of membrane

















Historical Perspective

Following the end of World War II , the USGovt. became concerned about the shortageof water before the end of century.

The US Dept. set up the Office of Saline

Waters (OSW), and committed substantialfinancial resources to the development of various separation processes for waterdesalination, a significant portion of whichwas dedicated to the development of membranes for desalination, continuing thework up to 2 decades.

Result was development of RO and UF and itsno co-incidence that US is the world leaderon this front.



Ultra filtration

A Pressure Driven

Membrane SeparationProcess



Membrane Structure and Fabrication

The thin skin on thesurface: usually 0.1 to 1 in thickness.

Permits high hydraulicpermeability

The more open poroussubstructure (typically

120 in thickness)provides goodmechanical support.

Virtually eliminates

internal pore-fouling.



Today UF membranes are made from

thermally and chemically stable syntheticpolymers like PVC, PAN, polyimides(PI),polysulphone(PS) ,PVDF.

In addition there are inorganic UF

membranes made from zirconium andaluminium oxides.

Plasticizers are necessary for some

membranes, if they are to be dried, toprevent collapse of the pores duringdrying.



















Modification

Additionalstrength is

provided bycasting themembrane on aspun-bonded

poly-ethylene orpolypropylenebacking.



Pore Size Determination

UF membranes have “diffuse cut-off” characteristics.

The retention R (in percent) may be definedas ,R = 100 ( 1- ( Cuf / CR ) )

The convention states that the molecular

weight cut-off of the membrane is equal to the molecular weight of the globular proteins which are 90% retained by the membrane .



Retention Characteristics

1. Size and shape considerations

2. Adsorption losses

3. Charged membranes

4. Pressure effects

5. Temperature effects



1. Size and Shape considerations

Retention differswith linear or

sphericalstructure of molecules.



2. Adsorption losses

The polymer which makes the UFaffects the retention if it adsorbs the

species on the membrane surface. Retention of membranes are often

measured in stirred cells.

A mass balance on cell, integrated overtime t .

R = 100 * (ln( Cf / Co ) )/ln( Vo / Vf ))



3. Charged membrane

Charged UF membrane reject low conc.Of salts, because the fixed chargedgroup on membrane skin reject ionicsolutes via repulsion of co ions.

Obviously, divalent, trivalent ions arerejected better than monovalent ions.



4. Effect of pressure



5. Effect of temperature

It has been found experimentally for alarge no.of membranes systems and

feed streams that the permeation rateis inversely proportional to fluidviscosity.

Viscosity of water decreases by 2.5%

for every C rise, researchers refer to3% rule that flux increases 3% per 1 C as rule of thumb.



Evaluation of Mass Transfer

Co-efficient To evaluate m.t.c, laminar parabolic velocity profileis assumed to be established at the channelentrance

Leveque’s solution gives, Sh = 1.62 ( Re Sc dh / L)0.33

(For 100< (Re Sc dh /L) <5000) Generally, K = .816 (γ D2 /L )0.33

γ = the fluid shear rate at the membrane surface.= 6U/b for rectangular slits.= 8U/d for circular tubes.

















UF Plant Design

Mode of operation

The arrangement of membrane moduleand their mode of operation can affectthe economics as much as the moduledesign.



Applications

Semi conductor industry

Reclamation of waste lubricating oil

Decontamination of crude oil

Waste treatment



Applications




Waste treatment



Applications




Waste treatment



Applications




Waste treatment



Summarizing U.F

Twenty-five years of invention.

Reliable and economic.

Eliminated severe pollution problems. Recovery of by-products adds to the profit.

Grafting eliminates fouling problems.

Stringent environmental controls may

necessitate UF. Bio-reactors using UF, have tremendous

potential for continuous enzyme reactors.



Reverse Osmosis

A Pressure Driven

Membrane SeparationProcess



Osmosis



Reverse Osmosis



R.O membranes

High resolution electron microscopycannot resolve the extensive pore in the

separating layer of the R.O. membranes Therefore it is generally considered that

they do not contain pores and that they

operate mainly by “solution diffusion” mechanism



Solution diffusion mechanism

Introduced by SOLTANEIH and GILL.

It states that the membrane is non

porous, and that solvent and solutescan only be transported across themembranes by first dissolving in, and

subsequently diffusing through, themembrane



HAASE and BELFORT model

The solvent flux through the membraneis proportional to pressure gradient:

For the solute it is found that:

)(11 P K J

222 C K J



Thus, solvent (water) flow occurs onlywhen | ΔP| > |ΔΠ|, solute flow isindependent of | ΔP|

Hence, increasing the operatingpressure increases the effectiveseparation

Typically for brackish water (1.5-2kg/m3 salts), |ΔΠ| = 0.1- 0.7MPa and| ΔP| = 3-8 MPa













Salt rejection

Rejection of ions at R.O membranesdepends on valence

The rejection of organic moleculesdepends on molecular weights



Membrane modules

There are currently four genericconfigurations for membranes inindustrial use :

1. Tubular modules2. Hollow fiber modules3. Flat plate modules

4. Spiral wound modules.



Tubular module



Hollow fiber modules



Flat- sheet module



Spiral wound modules



Plant configuration

Batch Recirculation

Feed and Bleed configuration

Continuous Single-pass



Plant configuration

Batch Recirculation





Plant configuration

Batch Recirculation





Batch Recirculation



Feed and Bleed Configuration



Single pass configuration



Applications

In some applications the product is theretentate, and the objective is toconcentrate or purify the retained

species and in others the product ispermeate.

Whereas, in some both retentate and

filtrate are important. For example, if avaluable product or by-product is apollutant in a waste stream, recoveryand use of the product will often pay

for pollution abatement.



R.O. in water treatment

R.O. is a well-established large scaleindustrial process for the desalination of

brackish water More than 1000 units in operation, each

capable of producing 105 m3 /day of

drinking water



Membranes employed

The two basic membranes employedin in the commercial R.O. systems are

1. The Thin Film Composite (TFC)membranes

2. Cellulose acetate blend (CAB)

membranes



Membranes employed




membranes



Membranes employed




membranes



Case Study: R.O at MRL

Objective is to reuse sewage water asprocess water for MRL

Spiral wound module employed.

MRL uses advanced composite membrane(ACM) which is in TFC form.

It is made by first casting the porous

polysulfone film on fabric support. The thin(0.05- 0.2 μ) polyamide membrane is thenformed on the film surface by polymerizationof an aromatic diamine and cycloaliphatictricarbonyl chloride



Advantages of ACM

Excellent biological stability

Excellent chemical stability except

toward Cl. Resistant to membrane compaction

Longer life as compared to CAB

membranes.



Advantages of ACM





membranes.



Advantages of ACM





membranes.



Advantages of ACM





membranes.



Thin Film Composites



Process block diagram



Pretreatment

R.O almost always requirespretreatment to control fouling

Pretreatment scheme1. Addition of HCl to control pH

2. Addition of SHMP to avoid calciumsulfate scale

3. Micron cartridge filter to removeparticles greater than 10 μ size



Pretreatment







Pretreatment







Pretreatment







Permeate water characteristics

pH = 7.5

TDS = 400 ppm

Total Hardness as CaCO3= 100ppm

Free ammonia = 0.1 ppm

Nitrates = 1ppm

Silica = 10 ppm

BOD =2ppm COD= 5ppm

total Phosphates = 0.1 ppm



ACM characteristics

Membrane configuration = spiral wound

Material = polyamide

Supplier = dupont

Dimension= 8” dia x 40” long

Rated operating pressure = 200-350 psig

Temperature range = 0-45oC

pH range = 4-11 Membrane SA = 37.2 m2

Cl tolerance = 0.25 ppm (pH>8) ; 0.1ppm(pH<8)



Permeate can be used as

Process water

Landscaping, gardening

Ground water discharge for controllingintrusion of sea water into the groundwater table



Summarizing R.O

Can be widely used as captive wastewater recycle plants in industries

Used for the production of drinkingwater in European countries. Largestsuch plant produces 140000 m3 / daywater for north Paris

In temperate climates nanofiltration ismore economical on this regard.



Electrodialysis

An electrically driven

membrane separationprocess.



Flow-diagram for ED



The membranes

Thin films of polymeric chains containingelectrically charged functional sites.

Anion-exchange membrane (e.g. withquartenary ammonium groups)

Cation-exchange membranes (i.e. withsulfonate groups)

Various methods of producing

Close to 98% efficiency



ED Stack

1: Polypropylene end plate 2: Electrode 3: Electrode chamber 4: spacer-sealing PVC 5: Spacer fabric 6: Screws 7: Steel frame 8: Inlet anode cell 9: Inlet concentrate cell 10: cation exchange membrane

11: AAM 12: Inlet diluate cell 13: Inlet cathode chamber



Process configurations



Typical process configuration



Typical Applications

Demineralization

Concentration of electrolytes

Ion-replacement reactions

Metathesis reactions

Separation of electrolysis products

Fractionation of electrolytes



Electrodialysis reversal (EDR)



Cation-neutral ED



Limiting current density

Transference number is the fraction of current carried by an ion

t+

for cations and t-

for anions ts

- for anion through solution (say .5)

tm- for anion through anion exchange

membrane (say 1.0)

So at membrane there is a depletionlayer set up



Contd…

At a certain current theconc. becomes 0

This is called limiting

current density Beyond this H+ and OH-

is transported acrossthe membrane

Loss of efficiency andppt of salts due to pHchanges

Ilim= limiting currentdensity

D= diffusion coefficient

F= Faraday’s const.

l=Equivalent filmthickness

)(

lim

smi t t l

DF

zC

i



Typical process configuration



Typical Applications

Demineralization

Concentration of electrolytes

Ion-replacement reactions Metathesis reactions

Separation of electrolysis products

Fractionation of electrolytes



Electrodialysis reversal (EDR)

Deals with fouling of the membrane andscale formation

Reversal of polarityseveral times anhour along withalternation of conc.

and demin stream Useful with Ni, Zn,

Fe salts etc



Variations of ED

Cation exchangeand neutralmembrane

Elimination of anionmembrane

Greater flexibility inselecting flow rates

and feed and o/pconc

Electrosorption

Neutral inner layerbetween a cation ex

membrane andanion ex membrane

During normaloperation the

neutral gets loadedand in reverse getsunloaded



Compact water purification electrodialysis units are connected directly to themain water supply system through a flexible hose.

The pressure of 1 –3 atm is quite sufficient,because their operating pressure is within 0.3 –0.4atm. Thus, in comparison with RO there is no need of additional pump of high pressure;

as a result there is another main advantage:consumers have the opportunity to regulate themselves the taste of water by means of a simpleturn of the tap;

residual chlorine in water does not influence thedesalination process, therefore there is no need of cartridges before treatment;

simplicity and slight adjustment of the process,low power consumption and low cost of units promises a great advantage in the future;

when using these units there is no need of

bottle water purchase.



Seawater desalination with ED



Comparison of RO and ED

Membrane Separation Technology

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