Dairy Process Engineering

Module 6. Membrane processing

Lesson 28
ELECTRO DIALYSIS

Introduction

● Electro Dialysis (ED) is a membrane process, during which ions are transported through semi permeable membrane, under the influence of an electric potential.

● Electrodialysis is an electrically driven membrane separation process that is capable of separating, concentrating, and purifying selected ions from aqueous solutions (as well as some organic solvents).

History

· The history of electrodialysis goes back to the development of the first multi cell stack.

· However, modern electrodialysis became a practical reality with the development of the first reliable ion-exchange membranes having both good electrolyte conductivity and ion-permselectivity.

· Electrodialysis was first commercially exploited for the desalination of brackish water by Ionics Inc.

· In the early 1980’s a completely new area for the application of electrodialysis was opened up with the introduction of bipolar membranes for the recovery of acids and bases from the corresponding salt.

Fig. 28.1 ED by ion permeable membranes

Principle:

* Electro dialysis stack includes following components:

* Diluate Feed Stream (D)
* Concentrate Stream(C)
* Electrode Stream (E)
* Cell compartment formed by ion exchange membrane.
* Electrodes

* Potential difference created between electrodes attracts oppositely charged ions.
* Cations (positively charged ions) are attracted toward cathode (-) and Anions (negatively charged ions) are attracted toward anode (+).
* Anion exchange membrane (positively charged) allows anions to pass through but checks cations.
* Similarly, cation exchange membrane (negatively charged) allows cation to pass through but checks anion.
* Equal amount of cations and anions get collected in C stream (transferred from D stream).Overall charge balance is maintained.
* The overall result of the electro dialysis process is an ion concentration increase in the concentrate stream with a depletion of ions in the diluate solution feed stream.

Materials and specifications

1. Ionic mobility: The ionic mobility is directly proportioned to specific conductivity and inversely proportioned to number of molecules in solution. ~3-6 x 10² mm/sec.

2. Membranes: Electro dialysis membranes are comprised of polymer chains - styrene-divinyl benzene made anionic with quaternary ammonium groups and made cationic with sulphonic groups.

3. Operating voltage: 1-2 V is applied across each pair of membranes.

4. Membrane arrangement:

a. Anion and cation exchange membranes are arranged alternately in parallel between an anode and a cathode.

b. Distance between membranes is 1 mm or less.

c. A plate and frame arrangement similar to a plate heat exchanger or a plate filter is used.

2. Electrodialysis using Bipolar Membranes

A bipolar membrane in an electric field generates hydrogen ion on one side and hydroxyl ion on the other side. Therefore electrodialysis with bipolar membrane can be used to produce acid and base from salt.

Current efficiency:

Current efficiency is a measure of how effective ions are transported across the ion exchange membranes for a given applied current. Current efficiency is generally a function of feed concentration

Where,

ξ = current utilization efficiency

z = charge of the ion

F = Faraday constant, 96,485 Amp-s/mol

Q_f= diluate flow rate, L/s

= diluate ED cell inlet concentration, mol/L

= diluate ED cell outlet concentration, mol/L

N = number of cell pairs

I = current, Amps.

Advantages

· ED system separates without phase change, which results in relatively low energy consumption.

· When brackish water is desalted by ED system, the product water needs only limited pre-treatment. Typically only chlorination for disinfection is required.

· As ED system removes only ionized species, it is particularly suitable for separating non-ionized from ionized components.

· Osmotic pressure is not a factor in ED system, so the pressure can be used for concentrating salt solutions to 20% or higher.

Limitations

· Organic matter, colloids and SiO₂ are not removed by ED system.

· Feed water pre-treatment is necessary to prevent ED stacks fouling.

· Elaborate controls are required, and keeping them at optimum condition can be difficult.

· Selection of materials of construction for membranes and stack is important to ensure compatibility with the feed stream.

Applications

1. Removal of salts from food.
2. Demineralization of milk products and whey for infant formula and special dietary products.
3. Concentrate salts, acids and bases.
4. Adjustment of pH in food and wine.
5. Large scale brackish and seawater desalination.
6. Drinking water production
7. Pre-demineralization (for process water, chemical manufacturing, boiler makeup)
8. Glycol desalting
9. Glycerin purification