DIALYSIS and ELECTRODIALYSIS
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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. 2
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
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Dialysis • In 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 4
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.
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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 :
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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 7
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. 8
Dialysis
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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)
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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.
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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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