Transcript DIALYSIS and ELECTRODIALYSIS

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 (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.
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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
feed
Purifed
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 => Donnan equilibria build up => 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 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.
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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)
Ion Permeable Membranes
 Non porous
 Sheets of ion-exchange resins and other polymers
 Thickness 100 - 500 mm
Are divided in
Anion - exchange
Positively charged groups
E.g. Quarternary ammonium salts
–NR3 or –C5H5N-R
Cation - exchange
Negatively charged groups
E.g. Sulfonic or carboxylic acid groups
- SO3 -
Chemically attached to the polymer chains
(e.g. styrene/divinylbenzene copolymers)
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ELECTRODIALYSIS (ED)
Types of Ion - Exchange Membranes
Heterogeneous
Ion - exchange resines + Film - forming polymer
High Electrical resistance
Poor mechanical strenght
Homogeneous
Introduction of an ionic group into a polymer film
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
k = m, b
(2)
(1)
In Steady State
(3)
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ELECTRODIALYSIS (ED)
Equations involve in the process
Boundary conditions
Operational i
[
i Current density [A/m2
(4)
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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)
Equations involve in the process
Required membrane area
(10)
Required energy
(15)
P Required power [J/s
[
Rc Total resistance in a cell (W)
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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
Optimized in terms of
• 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
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ELECTRODIALYSIS (ED)
Electrodialysis desalination costs
Costs
Amount of ionic species
Operating costs
• Energy consumption
• Maintenance
• Electrical energy
• Energy for pumps
• Plant size
• Feed salinity
Capital costs
• Depreciable items (ED stacks, pumps, membranes, etc.)
• Non-depreciable items (land, working capital)
Membrane Costs
• Properties
• Feed concentration
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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
Reduce
Electrolyte
Content
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
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ELECTRODIALYSIS (ED)
Recover Electrolytes
Pure NaCl from seawater
Salts of organic acids from fermentation broth
Amino acids from protein hydrolysates
HCl from cellulose hydrolysate
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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
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