electrodialysis.ppt

DIALYSIS and ELECTRODIALYSIS

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Dialysis What is dialysis? Dialysis is a membrane process where solutes (MW~ Donnan equilibria build up => electrical potential has to be included into the transport (flux) calculation.

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Dialysis Diffusion dialysis Membranes: ion exchange membranes (cation and anion) similar to electrodialsis Thickness: ~few hundreds of mm (100 - 500 mm) Separation principle: Donnan exclusion mechanism Main applications: acid recovery from eaching, pickling and metal refining; alkali recovery from textile and metal refining processes.

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Dialysis Diffusion dialysis Example: HF and HNO3 are often used as etching agents for stainless steel. In order to recover the acid, diffusion dialysis can be applied since the protons can pass the membrane but the Fe3+ ions can not.

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Dialysis Share of the market Although the application range of dialysis is limited and the industrial interest is low, it would be silly to claim that dialysis is not important.

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Dialysis

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ELECTRODIALYSIS (ED) What is electrodialysis? Electrodialysis is a membrane process in which ions are transported through ion permeable membranes from one solution to another under the influence of an electrical potential gradient. First applications in the 30’s.

General Principles • Salts

dissolved in water forms ions, being positively (cationic) or negatively (anionic) charged. • These ions are attracted to electrodes with an opposite electric charge. • Membranes can be constructed to permit selective passage of either anions or cations. 16

ELECTRODIALYSIS (ED) How the process takes place? Electrodialysis cell

Module Hundreds of anionic and cationic membranes placed alternatively

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ELECTRODIALYSIS (ED) Ion Permeable Membranes  Non porous

 Sheets of ion-exchange resins and other polymers  Thickness 100 - 500 mm

Are divided in Anion - exchange Positively charged groups E.g. Quarternary ammonium salts –NR3 or –C5H5N-R

Cation - exchange Negatively charged groups E.g. Sulfonic or carboxylic acid groups - SO3 -

Chemically attached to the polymer chains (e.g. styrene/divinylbenzene copolymers) 23

ELECTRODIALYSIS (ED) Types of Ion - Exchange Membranes Heterogeneous

Ion - exchange resines + Film - forming polymer High Electrical resistance Poor mechanical strenght

Homogeneous

Introduction of an ionic group into a polymer film

Crosslinking

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ELECTRODIALYSIS (ED) Requirements for Ion - Exchange Membranes • High electrical conductivity • High ionic permeability • Moderate degree of swelling • High mechanical strength Charge density 1 - 2 mequiv / g dry polymer Electrical Resistance 2 - 10 W.cm2 Diffusion coefficient 10-6 - 10-10 cm2/s 25

ELECTRODIALYSIS (ED) How the process takes place? Donnan exclusion Electrostatic repulsion

Osmotic flow

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ELECTRODIALYSIS (ED) Equations involve in the process k = m, b

(2)

(1)

In Steady State (3)

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ELECTRODIALYSIS (ED) Equations involve in the process Boundary conditions

Operational i [

i Current density [A/m2 (4)

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ELECTRODIALYSIS (ED) Equations involve in the process Limiting current density

ilim

Cm

0 (5)

Required membrane area (8)

(9)

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ELECTRODIALYSIS (ED) Equations involve in the process Required membrane area (10)

Required energy (15)

P Required power [J/s

[

Rc Total resistance in a cell (W) 33

ELECTRODIALYSIS (ED) Designing of an electrodialysis desalination plant Desalination 142 (2002) 267-286

Parameters: • Stack Construction • Feed and product concentration • Membrane permselectivity • Flow velocities • Current density • Recovery Rates

Optimized in terms of

• Width of the cell • Length of the stack • Thickness of the cell chamber • Volume factor • Shadow effect Safety factor

• Component design and properties • Operating Parameters

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ELECTRODIALYSIS (ED) Electrodialysis desalination costs Costs

Amount of ionic species

Operating costs • Energy consumption • Maintenance

• Electrical energy • Energy for pumps

• Plant size • Feed salinity

Capital costs • Depreciable items (ED stacks, pumps, membranes, etc.) • Non-depreciable items (land, working capital)

Membrane Costs

• Properties • Feed concentration

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ELECTRODIALYSIS (ED) Electrodialysis desalination costs as a function of the limiting current density at a feed solution concentration of 3500 mg/l NaCl

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ELECTRODIALYSIS (ED)

Electrodialysis desalination costs as a function of the Feed solution concentration

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ELECTRODIALYSIS (ED) Applications Potable from brackish water Reduce Electrolyte Content

Food products - whey, milk, soy sauce, fruit juice Nitrate from drinking water Boiler feed water Rinse water for electronics processing Effluent streams Blood plasma to recover proteins Sugar and molasses Amino acids Potassium tartrate from wine Fiber reactive dyes

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ELECTRODIALYSIS (ED) Recover Electrolytes

Pure NaCl from seawater Salts of organic acids from fermentation broth Amino acids from protein hydrolysates HCl from cellulose hydrolysate

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ELECTRODIALYSIS (ED) Electrodialysis Reversal Process (EDR) The polarity of the electrodes is reversed, so the permeate becomes the retentate and viceversa.

Electrodialysis at high temperatures Electrodialysis with electrolysis

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