*DIALYSIS and ELECTRODIALYSIS

*DialysisWhat is dialysis?Dialysis is a membrane process where solutes (MW~

*DialysisA typical concentration profile for dialysis with boundary layer resistences

contains low-molecular-weight solute, A intermediate size molecules, B , and a colloid, C

*DialysisIn 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

*Dialysis

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.

*DialysisSeparation 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

*DialysishomogeneousThicknes: 10 100 mmMembrane 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 swellingMembranes

*DialysisApplications

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.

*Dialysis

*DialysisRecovery of causic soda from colloidal hemicellulose during viscose manufactureRemoval of alcohol from beerSalt removal in bioproducts (enzymes)Fractionation (pharmaceutical industry)

Further applications

*DialysisDiffusion 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.

Diffusion dialysis

*DialysisMembranes: ion exchange membranes (cation and anion) similar to electrodialsisThickness: ~few hundreds of mm (100 - 500 mm)Separation principle: Donnan exclusion mechanismMain applications: acid recovery from eaching, pickling and metal refining; alkali recovery from textile and metal refining processes.

Diffusion dialysis

*DialysisExample: 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.

Diffusion dialysis

*DialysisAlthough 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

*Dialysis

*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 30s.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.

*ELECTRODIALYSIS (ED)How the process takes place?Electrodialysis cellModuleHundreds of anionic and cationic membranes placed alternatively

*ELECTRODIALYSIS (ED)

*ELECTRODIALYSIS (ED)

*ELECTRODIALYSIS (ED)

*ELECTRODIALYSIS (ED)

*ELECTRODIALYSIS (ED)

* Non porous Sheets of ion-exchange resins and other polymers Thickness 100 - 500 mmIon Permeable MembranesELECTRODIALYSIS (ED)Chemically attached to the polymer chains (e.g. styrene/divinylbenzene copolymers)Are divided in

*ELECTRODIALYSIS (ED)Types of Ion - Exchange MembranesCrosslinking

*ELECTRODIALYSIS (ED)Requirements for Ion - Exchange MembranesElectrical Resistance 2 - 10 W.cm2 Charge density 1 - 2 mequiv / g dry polymer High electrical conductivity High ionic permeability Moderate degree of swelling High mechanical strengthDiffusion coefficient 10-6 - 10-10 cm2/s

*ELECTRODIALYSIS (ED)How the process takes place?Donnan exclusionElectrostatic repulsionOsmotic flow

*ELECTRODIALYSIS (ED)Equations involve in the processIn Steady State(1)(2)(3)

*Boundary conditionsOperational iEquations involve in the processELECTRODIALYSIS (ED)(4)

*ELECTRODIALYSIS (ED)

*ELECTRODIALYSIS (ED)Equations involve in the processLimiting current densityilimCm0Required membrane area(5)(8)(9)

*ELECTRODIALYSIS (ED)Intensity evolution versus applied potential

*ELECTRODIALYSIS (ED)Equations involve in the processRequired membrane areaMass balanceCharge flow(6)(7)

*ELECTRODIALYSIS (ED)Equations involve in the processRequired membrane areaRequired energy(10)(15)Rc Total resistance in a cell (W)

*ELECTRODIALYSIS (ED)Equations involve in the processRequired energy(12)(11)Combining (12) and (8)(13)Combining (13) and (11)(14)

*ELECTRODIALYSIS (ED)

*ELECTRODIALYSIS (ED)Desalination 142 (2002) 267-286Parameters: Stack Construction Feed and product concentration Membrane permselectivity Flow velocities Current density Recovery RatesDesigning of an electrodialysis desalination plant

*ELECTRODIALYSIS (ED)Costs Energy consumption Maintenance Depreciable items (ED stacks, pumps, membranes, etc.) Non-depreciable items (land, working capital)Electrodialysis desalination costs

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

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

*ELECTRODIALYSIS (ED)ApplicationsReduce 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

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

*Electrodialysis Reversal Process (EDR)ELECTRODIALYSIS (ED)Electrodialysis at high temperaturesElectrodialysis with electrolysisThe polarity of the electrodes is reversed, so the permeate becomes the retentate and viceversa.

****Datos tomados del libro*