1. DIALYSIS and ELECTRODIALYSIS
Maretva Baricot
Ronnie Juraske
Course: Membrane Separations
December, 2003

2. Dialysis What is dialysis? General Principles
Dialysis is a membrane process where solutes (MW~<100 Da) diffuse from one side of the membrane (feed side) to the other (dialysate or permeate side) according to their concentration gradient. First application in the 70’s.
General Principles
Separation between solutes is obtained as a result of differences in diffusion rates.
These are arising from differences in molecular size and solubility.
This means that the resistance increases with increasing molecular weight.

3. Dialysis
A typical concentration profile for dialysis with boundary layer resistences
contains low-molecular-weight solute, A
intermediate size molecules, B
, and a colloid, C

4. Dialysis
In order to obtain a high flux, the membrane should be as thin as possible
membrane
Purifed
feed
feed
dialysate
Schematic drawing of the dialysis process

5. Dialysis The solutes separate by passing through the membrane that
behaves like a fibre filter and separation occurs by a sieving
action based on the pore diameter and particle size
(i.e. smaller molecules will diffuse faster than larger molecules).
Transport proceedes via diffusion through a nonporous membranes.
Membranes are highly swollen to reduce diffusive resistence.

6. Dialysis
Transport
Separation of solutes is determined by the concentration of the molecules on either side of the membrane; the molecules will flow from a high concentration to a lower concentration.
Dialysis is a diffusion process and at steady-state transport can be described by :

7. Dialysis Membranes homogeneous Thicknes: 10 – 100 mm
Membrane material: hydrophilic polymers (regenerated cellulose such as cellophane, cellulose acetate, copolymers of ethylene-vinyl alcohol and ethylene-vinyl acetate)
Membrane application: optimum between diffusion rate and swelling

8. Dialysis Applications
Dialysis is used in varying circumstances such as: when a large pressure difference on the sides of the membrane is impractical, in heat sensitive areas, and when organic solvents are not feasible. In areas such as the bloodstream, a pressure difference would rupture blood cells. Dialysis is not a function of pressure; therefore a pressure difference is not needed.
By far the most important application of dialysis is the therapeutic treatment of patients with renal failure. The technique is called hemodialysis and attempts to mimic the action of the nephron of the kidney in the separation of low molecular weight solutes, such as urea and creatinine, from the blood of patients with chronic uremia.

9. Dialysis

10. Dialysis Further applications
Recovery of causic soda from colloidal hemicellulose during viscose manufacture
Removal of alcohol from beer
Salt removal in bioproducts (enzymes)
Fractionation (pharmaceutical industry)

11. Dialysis Diffusion dialysis
Diffusion process in which protons and hydroxyl ions are removed from an aqueous stream across an ionic membrane due to a concentration difference
Similar to dialysis but due to the presence of ions and an ionic membrane => Donnan equilibria build up => electrical potential has to be included into the transport (flux) calculation.

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

13. 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.

14. 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.

15. Dialysis

16. ELECTRODIALYSIS (ED) What is electrodialysis? General Principles
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.

17. ELECTRODIALYSIS (ED) How the process takes place? Electrodialysis cell
Module
Hundreds of anionic and cationic membranes placed alternatively

18. ELECTRODIALYSIS (ED)

19. ELECTRODIALYSIS (ED)

20. ELECTRODIALYSIS (ED)

21. ELECTRODIALYSIS (ED)

22. ELECTRODIALYSIS (ED)

23. ELECTRODIALYSIS (ED) Ion Permeable Membranes Are divided in
Non porous
Sheets of ion-exchange resins and other polymers
Thickness 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)

24. ELECTRODIALYSIS (ED) Types of Ion - Exchange Membranes Crosslinking
Heterogeneous
Ion - exchange resines + Film - forming polymer
High Electrical resistance
Poor mechanical strenght
Homogeneous
Introduction of an ionic group into a polymer film
Crosslinking

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

26. ELECTRODIALYSIS (ED) How the process takes place? Donnan exclusion
Electrostatic repulsion
Osmotic flow

27. ELECTRODIALYSIS (ED) Equations involve in the process In Steady State
k = m, b
(2)
(1)
In Steady State
(3)

28. ELECTRODIALYSIS (ED) Equations involve in the process Operational i
Boundary conditions
Operational i
i Current density [A/m2 [
(4)

29. ELECTRODIALYSIS (ED)
Boundary conditions

30. ELECTRODIALYSIS (ED) Equations involve in the process Cm
Limiting current density
ilim Cm
(5)
Required membrane area
(8)
(9)

31. ELECTRODIALYSIS (ED)
Intensity evolution versus applied potential

32. ELECTRODIALYSIS (ED) Equations involve in the process
Required membrane area
Mass balance
(6)
Charge flow
(7)

33. 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)

34. ELECTRODIALYSIS (ED) Equations involve in the process Required energy
(11)
(12)
Combining (12) and (8)
(13)
Combining (13) and (11)
(14)

35. ELECTRODIALYSIS (ED) c Rc
Ram
Rcm
Rrc
Rfc

36. ELECTRODIALYSIS (ED)
Designing of an electrodialysis desalination plant
Desalination 142 (2002)
Width of the cell
Length of the stack
Thickness of the cell chamber
Parameters:
Stack Construction
Feed and product concentration
Membrane permselectivity
Flow velocities
Current density
Recovery Rates
Volume factor
Shadow effect
Safety factor
Component design and properties
Operating Parameters
Optimized in terms of

37. ELECTRODIALYSIS (ED) Electrodialysis desalination costs Costs
Amount of ionic species
Operating costs
Capital costs
Electrical energy
Energy for pumps
Energy consumption
Maintenance
Plant size
Feed salinity
Depreciable items (ED stacks, pumps, membranes, etc.)
Non-depreciable items (land, working capital)
Membrane Costs
Properties
Feed concentration

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

39. ELECTRODIALYSIS (ED)
Electrodialysis desalination costs as a function of the Feed solution concentration

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

41. ELECTRODIALYSIS (ED)
Pure NaCl from seawater Salts of organic acids from fermentation broth Amino acids from protein hydrolysates HCl from cellulose hydrolysate
Recover Electrolytes

42. 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