DIALYSIS and ELECTRODIALYSIS. Maretva Baricot Ronnie Juraske Course: Membrane Separations December, 2003. Dialysis. What is dialysis?.
DIALYSIS and ELECTRODIALYSIS
Course: Membrane Separations
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.
A typical concentration profile for dialysis with boundary layer resistences
contains low-molecular-weight solute, A
intermediate size molecules, B
, and a colloid, C
In order to obtain a high flux, the membrane should be as thin as possible
Schematic drawing of the dialysis process
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.
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.
Share of the market
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.
Hundreds of anionic and cationic membranes placed alternatively
Anion - exchange
Cation - exchange
Positively charged groups
Negatively charged groups
E.g. Quarternary ammonium salts
–NR3 or –C5H5N-R
E.g. Sulfonic or carboxylic acid groups
Are divided in
Chemically attached to the polymer chains
(e.g. styrene/divinylbenzene copolymers)
Ion - exchange resines + Film - forming polymer
High Electrical resistance
Poor mechanical strenght
Introduction of an ionic group into a polymer film
Charge density 1 - 2 mequiv / g dry polymer
Electrical Resistance 2 - 10 W.cm2
Diffusion coefficient 10-6 - 10-10 cm2/s
k = m, b
In Steady State
i Current density [A/m2
Limiting current density
Required membrane area
PRequired power [J/s
Required membrane area
Rc Total resistance in a cell (W)
Optimized in terms of
Desalination 142 (2002) 267-286
Amount of ionic species
Electrodialysis desalination costs as a function of the limiting current density at a feed solution concentration of 3500 mg/l NaCl
Electrodialysis desalination costs as a function of the Feed solution concentration
Potable from brackish waterFood 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
Pure NaCl from seawater Salts of organic acids from fermentation broth Amino acids from protein hydrolysates HCl from cellulose hydrolysate
The polarity of the electrodes is reversed, so the permeate becomes the retentate and viceversa.