Dialysis and electrodialysis
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DIALYSIS and ELECTRODIALYSIS. Maretva Baricot Ronnie Juraske Course: Membrane Separations December, 2003. Dialysis. What is dialysis?.

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Dialysis and electrodialysis

DIALYSIS and ELECTRODIALYSIS

Maretva Baricot

Ronnie Juraske

Course: Membrane Separations

December, 2003


Dialysis

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.


Dialysis1

A typical concentration profile for dialysis with boundary layer resistences

Dialysis

contains low-molecular-weight solute, A

intermediate size molecules, B

, and a colloid, C


Dialysis2

In order to obtain a high flux, the membrane should be as thin as possible

Dialysis

membrane

Purifed

feed

feed

dialysate

Schematic drawing of the dialysis process


Dialysis3

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.


Dialysis4

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 :


Dialysis5

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


Dialysis6

Applications

Dialysis

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.


Dialysis7

Dialysis


Dialysis8

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)


Dialysis9

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.


Dialysis10

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.


Dialysis11

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.


Dialysis12

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.


Dialysis13

Dialysis


Electrodialysis ed

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.


Electrodialysis ed1

ELECTRODIALYSIS(ED)

  • How the process takes place?

Electrodialysis cell

Module

Hundreds of anionic and cationic membranes placed alternatively


Electrodialysis ed2

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

- SO3-

ELECTRODIALYSIS (ED)

  • Ion Permeable Membranes

  • Non porous

  • Sheets of ion-exchange resins and other polymers

  • Thickness 100 - 500 mm

Are divided in

Chemically attached to the polymer chains

(e.g. styrene/divinylbenzene copolymers)


Electrodialysis ed3

Ion - exchange resines + Film - forming polymer

High Electrical resistance

Heterogeneous

Poor mechanical strenght

Homogeneous

Introduction of an ionic group into a polymer film

ELECTRODIALYSIS (ED)

  • Types of Ion - Exchange Membranes

  • Crosslinking


Electrodialysis ed4

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


Electrodialysis ed5

ELECTRODIALYSIS(ED)

  • How the process takes place?

Donnan exclusion

Electrostatic repulsion

Osmotic flow


Electrodialysis ed6

k = m, b

ELECTRODIALYSIS(ED)

  • Equations involve in the process

(2)

(1)

In Steady State

(3)


Electrodialysis ed7

i Current density [A/m2

[

ELECTRODIALYSIS(ED)

  • Equations involve in the process

Boundary conditions

Operationali

(4)


Electrodialysis ed8

ELECTRODIALYSIS(ED)

  • Equations involve in the process

Limiting current density

ilim

Cm

0

(5)

Required membrane area

(8)

(9)


Electrodialysis ed9

PRequired power [J/s

[

ELECTRODIALYSIS(ED)

  • Equations involve in the process

Required membrane area

(10)

Required energy

(15)

Rc Total resistance in a cell (W)


Electrodialysis ed10

  • 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

Optimized in terms of

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


Electrodialysis ed11

Amount of ionic species

  • Electrical energy

  • Energy for pumps

Operating costs

  • Plant size

  • Feed salinity

Capital costs

  • Properties

  • Feed concentration

Membrane Costs

ELECTRODIALYSIS(ED)

  • Electrodialysis desalination costs

Costs

  • Energy consumption

  • Maintenance

  • Depreciable items (ED stacks, pumps, membranes, etc.)

  • Non-depreciable items (land, working capital)


Electrodialysis ed12

ELECTRODIALYSIS(ED)

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


Electrodialysis ed13

ELECTRODIALYSIS(ED)

Electrodialysis desalination costs as a function of the Feed solution concentration


Electrodialysis ed14

ELECTRODIALYSIS (ED)

  • Applications

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

Reduce

Electrolyte

Content


Electrodialysis ed15

ELECTRODIALYSIS (ED)

Pure NaCl from seawater Salts of organic acids from fermentation broth Amino acids from protein hydrolysates HCl from cellulose hydrolysate

Recover Electrolytes


Electrodialysis ed16

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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