DIALYSIS and ELECTRODIALYSIS

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DIALYSIS and ELECTRODIALYSIS

• Maretva Baricot
• Ronnie Juraske
• Course Membrane Separations
• December, 2003

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Dialysis

• What is dialysis?

Dialysis is a membrane process where solutes
(MWlt100 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 70s.

• 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.

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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
Purifed feed
feed
dialysate
Schematic drawing of the dialysis process
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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

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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.
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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 gt Donnan equilibria
  build up gt 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 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.

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ELECTRODIALYSIS (ED)

• How the process takes place?

Electrodialysis cell
Module
Hundreds of anionic and cationic membranes placed
alternatively
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ELECTRODIALYSIS (ED)
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ELECTRODIALYSIS (ED)
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ELECTRODIALYSIS (ED)
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ELECTRODIALYSIS (ED)
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ELECTRODIALYSIS (ED)
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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
Chemically attached to the polymer chains (e.g.
styrene/divinylbenzene copolymers)
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ELECTRODIALYSIS (ED)

• Types of Ion - Exchange Membranes

• 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
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ELECTRODIALYSIS (ED)

• How the process takes place?

Donnan exclusion
Electrostatic repulsion
Osmotic flow
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ELECTRODIALYSIS (ED)

• Equations involve in the process

(2)
(1)
In Steady State
(3)
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ELECTRODIALYSIS (ED)

• Equations involve in the process

Boundary conditions
Operational i
(4)
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ELECTRODIALYSIS (ED)
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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)
Intensity evolution versus applied potential
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ELECTRODIALYSIS (ED)

• Equations involve in the process

Required membrane area
Mass balance
(6)
Charge flow
(7)
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ELECTRODIALYSIS (ED)

• Equations involve in the process

Required membrane area
(10)
Required energy
(15)
Rc Total resistance in a cell (W)
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ELECTRODIALYSIS (ED)

• Equations involve in the process

Required energy
(11)
(12)
Combining (12) and (8)
(13)
Combining (13) and (11)
(14)
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ELECTRODIALYSIS (ED)
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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

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ELECTRODIALYSIS (ED)

• Electrodialysis desalination costs

Costs

• Energy consumption
• Maintenance

• Depreciable items (ED stacks, pumps, membranes,
  etc.)
• Non-depreciable items (land, working capital)

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