Presentation on theme: "DIALYSIS and ELECTRODIALYSIS"— Presentation transcript:
1DIALYSIS and ELECTRODIALYSIS Maretva BaricotRonnie JuraskeCourse: Membrane SeparationsDecember, 2003
2Dialysis 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 PrinciplesSeparation 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.
3DialysisA typical concentration profile for dialysis with boundary layer resistencescontains low-molecular-weight solute, Aintermediate size molecules, B, and a colloid, C
4DialysisIn order to obtain a high flux, the membrane should be as thin as possiblemembranePurifedfeedfeeddialysateSchematic drawing of the dialysis process
5Dialysis The solutes separate by passing through the membrane that behaves like a fibre filter and separation occurs by a sievingaction 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.
6DialysisTransportSeparation 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 :
7Dialysis 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
8Dialysis 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.
10Dialysis Further applications Recovery of causic soda from colloidal hemicellulose during viscose manufactureRemoval of alcohol from beerSalt removal in bioproducts (enzymes)Fractionation (pharmaceutical industry)
11Dialysis Diffusion dialysis Diffusion process in which protons and hydroxyl ions are removed from an aqueous stream across an ionic membrane due to a concentration differenceSimilar 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.
12Dialysis Diffusion dialysis Membranes: ion exchange membranes (cation and anion) similar to electrodialsisThickness: ~few hundreds of mm ( mm)Separation principle: Donnan exclusion mechanismMain applications: acid recovery from eaching, pickling and metal refining; alkali recovery from textile and metal refining processes.
13Dialysis 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.
14Dialysis 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.
16ELECTRODIALYSIS (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 PrinciplesSalts 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.
17ELECTRODIALYSIS (ED) How the process takes place? Electrodialysis cell ModuleHundreds of anionic and cationic membranes placed alternatively
23ELECTRODIALYSIS (ED) Ion Permeable Membranes Are divided in Non porousSheets of ion-exchange resins and other polymersThickness mmAre divided inAnion - exchangePositively charged groupsE.g. Quarternary ammonium salts–NR3 or –C5H5N-RCation - exchangeNegatively charged groupsE.g. Sulfonic or carboxylic acid groups- SO3 -Chemically attached to the polymer chains(e.g. styrene/divinylbenzene copolymers)
24ELECTRODIALYSIS (ED) Types of Ion - Exchange Membranes Crosslinking HeterogeneousIon - exchange resines + Film - forming polymerHigh Electrical resistancePoor mechanical strenghtHomogeneousIntroduction of an ionic group into a polymer filmCrosslinking
25ELECTRODIALYSIS (ED) Requirements for Ion - Exchange Membranes High electrical conductivityHigh ionic permeabilityModerate degree of swellingHigh mechanical strengthDatos tomados del libroCharge density mequiv / g dry polymerElectrical Resistance W.cm2Diffusion coefficient cm2/s
26ELECTRODIALYSIS (ED) How the process takes place? Donnan exclusion Electrostatic repulsionOsmotic flow
27ELECTRODIALYSIS (ED) Equations involve in the process In Steady State k = m, b(2)(1)In Steady State(3)
28ELECTRODIALYSIS (ED) Equations involve in the process Operational i Boundary conditionsOperational ii Current density [A/m2[(4)
36ELECTRODIALYSIS (ED)Designing of an electrodialysis desalination plantDesalination 142 (2002)Width of the cellLength of the stackThickness of the cell chamberParameters:Stack ConstructionFeed and product concentrationMembrane permselectivityFlow velocitiesCurrent densityRecovery RatesVolume factorShadow effectSafety factorComponent design and propertiesOperating ParametersOptimized in terms of
37ELECTRODIALYSIS (ED) Electrodialysis desalination costs Costs Amount of ionic speciesOperating costsCapital costsElectrical energyEnergy for pumpsEnergy consumptionMaintenancePlant sizeFeed salinityDepreciable items (ED stacks, pumps, membranes, etc.)Non-depreciable items (land, working capital)Membrane CostsPropertiesFeed concentration
38ELECTRODIALYSIS (ED)Electrodialysis desalination costs as a function of the limiting current density at a feed solution concentration of 3500 mg/l NaCl
39ELECTRODIALYSIS (ED)Electrodialysis desalination costs as a function of the Feed solution concentration
40ELECTRODIALYSIS (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 dyesReduceElectrolyteContent
41ELECTRODIALYSIS (ED)Pure NaCl from seawater Salts of organic acids from fermentation broth Amino acids from protein hydrolysates HCl from cellulose hydrolysateRecover Electrolytes
42ELECTRODIALYSIS (ED) Electrodialysis Reversal Process (EDR) The polarity of the electrodes is reversed, so the permeate becomes the retentate and viceversa.Electrodialysis at high temperaturesElectrodialysis with electrolysis