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

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Presentation on theme: "1 DIALYSIS and ELECTRODIALYSIS Maretva Baricot Ronnie Juraske Course: Membrane Separations December, 2003."— Presentation transcript:

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

2 2 Dialysis What is dialysis? 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 3 Dialysis A typical concentration profile for dialysis with boundary layer resistencesA typical concentration profile for dialysis with boundary layer resistences contains low-molecular-weight solute, A intermediate size molecules, B, and a colloid, C

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

5 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 6 Dialysis 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 : Transport

7 7 Dialysis homogeneous Thicknes: 10 – 100  m 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 Membranes

8 8 Dialysis ApplicationsApplications 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 9 Dialysis

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

11 11 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. Diffusion dialysis

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

13 13 Dialysis Example: HF and HNO 3 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 Fe 3+ ions can not. Diffusion dialysis

14 14 Dialysis 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. Share of the market

15 15 Dialysis

16 16 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.

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






23 23  Non porous  Sheets of ion-exchange resins and other polymers  Thickness  m Ion Permeable Membranes ELECTRODIALYSIS (ED) Chemically attached to the polymer chains (e.g. styrene/divinylbenzene copolymers) Anion - exchange Positively charged groups E.g. Quarternary ammonium salts –NR 3 or –C 5 H 5 N-R Cation - exchange Negatively charged groups E.g. Sulfonic or carboxylic acid groups - SO 3 - Are divided in

24 24 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

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

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

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

28 28 Boundary conditions Operational i Equations involve in the process ELECTRODIALYSIS (ED) i Current density [A/m 2 [ (4)

29 29 ELECTRODIALYSIS (ED) Boundary conditions

30 30 ELECTRODIALYSIS (ED) Equations involve in the process Limiting current densityi lim CmCm 0 Required membrane areaarea (5)(5) (8)(8)(9)

31 31 ELECTRODIALYSIS (ED) Intensity evolution versus applied potential

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

33 33 ELECTRODIALYSIS (ED) Equations involve in the process Required membrane area Required energyenergy (10) (15) P Required power [J/s [ RcRc Total resistance in a cell (  )

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

35 35 ELECTRODIALYSIS (ED) cc c c Rc RamRcmRrcRfc

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

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

38 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 39 ELECTRODIALYSIS (ED) Electrodialysis desalination costs as a function of the Feed solution concentration

40 40 ELECTRODIALYSIS (ED) Applications Reduce Electrolyte Content 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

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

42 42 Electrodialysis Reversal Process (EDR) ELECTRODIALYSIS (ED) Electrodialysis at high temperatures Electrodialysis with electrolysis The polarity of the electrodes is reversed, so the permeate becomes the retentate and viceversa.

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