lecture on electrodialysiss

7/31/2019 Lecture on Electrodialysiss

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Electrodialysis (ED)

Desalination Division, BARC

12th July, 2011


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Membrane Processes

Driving Force Membrane Process

Pressure Difference Micro, Ultra, Nano-filtration, Reverse osmosis

Concentration Difference Gas Separation, Pervaporation, Dialysis

Temperature Difference Membrane Distillation

Electrical Voltage Difference Electrodialysis , Electrodeionization

A membrane process is capable of performing a certain separation by use ofmembrane

Feed

Flux

Permeate

Membrane

Driving Force


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Ion Exchange Membranes

Cation Exchange Membranes are incorporated with negatively charged groups(i.e. sulfonic & carboxylic acid groups) which will repel anions & only allowtransport of cations

Anion Exchange Membranes are incorporated with positively charged groups (i.e.quantery ammonium salts) which will repel cations & only allow transport of cations

Heterogeneous Ion Exchange Membranes: Heterogeneous Ion Exchange

Membranes are prepared from ion exchange resin & film forming polymers.

Homogeneous Ion Exchange Membranes: In Homogeneous Ion ExchangeMembranes the charged groups are directly attached to the polymer chain.

Desirable characteristics of Ion Exchange Membrane :

1. High Ionic Perm selectivity2. Low Electric Resistance3. Low Diffusion Coefficient4. High Mechanical Strength5. High Chemical Stability6. High Dimensional Stability


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Separation Principle of Ion Exchange Membrane

CationExchangeMembrane

Cation

Anion

Boundarylayer

Donnan Exclusion at Cation ExchangeMembrane Surface

Bulk21

11011 ..ln.. FzaTR

22022 ..ln.. FzaTR

2

121ln.

.

.

a

a

Fz

TRE

don

Donnan Potential

1

c

c

c

cR

From Donnan equilibrium & electro neutrality condition for Cation exchangemembrane it can be shown

Where,c

c

Rc

Concentration of co ions inside the membranes

Concentration of co ions in bulk solution

Concentration of fixed charges inside the membranes

Where,

a= Activity, = Electrical PotentialF= Faradays no, T= Temp

Chemical Potential of Cations


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CationExchange

MembranePore

Donnan Exclusion InsideMembrane Pore

Cathode-

Anode+

Fz

itJ

.

.

Flux in bulk solution

Fz

itJ

.

.

Flux in membrane

where

cmcm

cmt

..

.

cmcm

cm

t ..

.

Transport no of Cation & Anion in solution

Transport no of in membranes

211 tttt

11 ttt

Transport no in cation exchange membrane

11 ttt

Transport no in anion exchange membrane

tt

tt

cmcm

cmt

..

.

Wherem is the mobility of the ion

c is conc. of the ions

Transport Equation of Ion Exchange Membrane


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Electrodialysis

Anode Reaction

HOOH

eClCl

2

22

221

2

2Cathode Reaction OHHeOH 222 22

One Cell Pair consists of1. One Cation Exchange Membrane2. One anion Exchange Membrane

3. One concentrating Chamber4. One Dilution Chamber

ElectrodeChamber

MembraneStack


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Electrical Resistance

A C A

Cell Pair

DiluateChamber

ConcentrateChamber

Ram Rd Rcm Rc

Rcell

Ram = Resistance of Anion Exchange Membrane

Rd = Resistance of Diluate Chamber

Rcm = Resistance of Cation Exchange Membrane

Rc = Resistance of Concentrate Chamber

Overall Resistance of single cell pair

R Cell = Ram + Rd + Rcm +Rc

The Resistance of whole membrane stack consisting of n cell pair is

R membrane stack = n x R Cell


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Current vs. Voltage of Electrodialysis Cell

Three regions are observed

1. Ohmic Region,

icr

dx

dEi

2. Region of limiting Current,

Where

is electrical conductance

3. Region of over limiting current

Water Splitting

HOHOH2


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Concentration Polarization

Cathode-

Anode+


Boundarylayer

BoundaryLayer

CC

CC

Cd

Cd


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Derivation of critical Current Density

Cathode-

Anode+


Boundarylayer

Cd

Cd

Electrical FluxJ1

ElectricalFluxJ3

DiffusiveFluxJ2

x=0

X=

Fz

itJ

.

.1

Fz

itJ

..

2

dx

dCDJ .3

321 JJJ

dx

dCDtt

Fz

i

.

dccxBC :1

dccxBC '0:2

tt

FzccDi dd

.

.).'.(

tt

Fzck

tt

FzcD

tt

FzccDi ddddCcrit

m

...

.

...

.

.).'.(lim 0

After integration:

Critical Current Density:


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Current Efficiency

sequivalentcurrentusedremovedquivalentsalteEfficiencyCurrent

F

In

ZccQ outdindd

.

.,,


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Energy Requirement

EIEelectrical .

1. Electrical Energy

(Ohmic Region)nRIE cellelectrical ..2

FzQcI dd

...

Where,

n (Current Utilization )

FzQcE dd

...


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Application of Electrodialysis

Production of potable water by desalination

Production of salt from sea water

Removal of salt & acid from pharmaceuticals solutions & food

Removal of water & valuable material from industrial effluents


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for your kind attention

lecture on electrodialysiss

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