Electrodeionization (EDI)
Outline
• Process Principles • EDI History • Applications and Benefits • Performance • Feed Water Requirements and Pretreatment
Process Principles
Device
Process Principles
• Cation exchange membrane • Anion exchange membrane • Spacers • Electrodes • Ion exchange resin
HCO3-
- - - - Cathode (-)
+ + + + Anode (+)
SiO2
CO2
Na+ SO4
= Cl- Ca++
Anion-Exchange Membrane
Cation-Exchange Membrane
Process Principles
Feed
Product Concentrate Recycle
Concentrate Make-up
Waste
EDI System
Process Principles
How does EDI work?
Process Principles
• Enhanced transfer regime (high salinity) – Remove strong ions from water
• Electroregeneration regime (low salinity) – Remove ionizable species (weak acids or weak
bases) from water
PROCESS PRINCIPLES
Anion-Exchange Membrane Cation-Exchange Membrane
CH 3
N +
CH 3
CH 3
CH 3
N +
CH 3
CH 3
CH 3
N +
CH 3
CH 3
Cl -
Cl -
Cl -
Mobile Counter Anions
Water-Filled Passage
Fixed Cation Sites
Polymer Support Structure
Mobile Counter Cations
Water-Filled Passage
Fixed Anion Sites
Polymer Support Structure
S O -
O
O
S O -
O
O
S O - O
O Na +
Na +
Na +
Ion-Exchange Membrane
HCO3-
Ca++ Na+
SO4= Cl-
Mg++
OH-
H+
HSiO3- CO3
=
High-Purity Product Water HCO3
-
- - - - Cathode (-)
+ + + + Anode (+)
SiO2
CO2
Na+ SO4
= Cl- Ca++
Anion-Exchange Membrane
Cation-Exchange Membrane
Process Principles
Enhanced transfer regime
Ions are transferred from solution to resins by diffusion. Ions move inside resins by means of electricity Ions reach the membranes and pass to the brine flow.
Process Principles
Polarization
Concentrado
Cationic Membrane
Anionic Membrane
Concentrate
Concentrate
FEED
H+ + HCO-3 CO2 + H2O
H2O H+
OH-
OH- + HCO-3 CO3
-- + H2O CO--
3 + Ca++ CaCO3
Electro regeneration Regime
Process Principles
• Resin in hydrogen and hydroxide forms • Removal of weakly ionized compounds by
ionization reactions – CO2 + OH- ---> HCO3- pKa = 6.4 – HCO3- + OH- ---> CO3= pKa =10.3 – SiO2 + OH- ---> HSiO3- pKa = 9.8 – H3BO3 + OH- ---> B(OH)4- pKa = 9.2 – NH3 + H+ ---> NH4+ pKa = 9.2
Polarization
µ S/cm
EDI History
EDI Technology History
• 1955 - Walters et al, work with periodic electro-regeneration of ion exchange resin in stack.
• 1957 - Kollsman patent on basic EDI method and apparatus.
• 1959 - Gluekauf, early EDI experimental data and theory.
• 1960’s - Early Ionics EDI experiments. • 1987 - Millipore Commercialized EDI - “CDI”.
Applications and Benefits
EDI Applications
• Microelectronics • Power Generation • Boiler Feed • Pharmaceutical • Industrial • Others
Technology Benefits
• Continuous process with constant stable product quality.
• No acid or caustic regenerants required • No upsets or downtime from regeneration • No need for high purity water rinse and
backwash • Produces high purity water with high water
recovery (95%).
Additional Technology Benefits
• No regeneration waste • Eliminates need for a waste treatment plant • Eliminates acid and caustic storage tank • Eliminates acid and caustic dilution skid • Reduced floor space • Safe and reliable (No chemical handling)
Additional Technology Benefits
• Minimizing on-going O&M costs – essentially no chemical usage – minimal maintenance
• Easy to install – lower installation costs than with conventional
ion-exchange – on-line quicker
• Provide consistent high percentage boron rejection
Weakly Ionized Species Removal By EDI
0
0.5
1
1.5
2
2.5
3
0 100 200 300 400 500 600 Operating Hours
Prod
uct C
ondu
ctiv
ity (
S/cm
) RO - MB IX RO - EDI 2 Pass RO
Product Quality Comparison
Boron Removal by EDI
• Boron present at very low concentrations in many waters
• Presence of Boron often unchecked • Boron can cause problems in semiconductor
manufacturing
Boron Removal by RO
• Boron removal by RO is pH dependent • RO does not reject Boron well at neutral pHs
due to: – poor ionization of boric acid – extremely small boric acid molecule
• Typical Boron rejection by RO: – 50% to 80% at neutral pH – >90% when > pH10
Boron Removal by Ion-Exchange • Boron is not well removed by ion-exchange
due to: – low selectivity – poor ionization of boric acid
• Boron is the first to break through ion-exchange
• Studies show Boron breaks through at 20% to 50% of silica run-length
• Boron displacement produces Boron spikes in product water
Performance of Mixed-Bed Ion-Exchange
Undetected Boron Invasion Zone
0 2 4 6 8
10 12 14 16 18 20
Operating Time
Meg
ohm
-cm
0
2
4
6
8
10
12
14
ppb
Effluent Resistivity (monitored) Influent Boron Effluent Boron Effluent Silica (monitored) EDI Effluent Boron
(Boron Break-through could effect wafer yield)
Boron Removal By EDI
• One of the most effective process to removal boron
• Well designed EDI will provide 96% to 99% boron rejection consistently
Feed B(ppb)
Product B(ppb)
B Removalby EDI (%)
B Removalby RO (%)
SemiconductorPlant #2 71 2.75 96.8 39.3
SemiconductorPlant #3 83.5 2.8 96.1 24.1
SemiconductorPlant #4 64.4 0.74 96.6 24.1
SemiconductorPlant #5 2.5 <0.1 >96.0 34.2
Boron Levels and Removal by RO and EDI
EDI Boron Removal at Varying Current Density
70
75
80
85
90
95
100
0 0.2 0.4 0.6 0.8 1 1.2 1.4I/Io
Boro
n Re
mov
al (%
)
Silica Removal by EDI
Silica removal is critical for semiconductor manufacturing
• high silica levels lead to expensive chip failures
• spec. limited by the ability of instrumentation
Silica Levels and Removal by EDI
Feed SiO2
(ppb)Product SiO2
(ppb)SiO2 Removal
(%)SemiconductorPlant #1 642 3.1 99.5SemiconductorPlant #2 181 2.8 98.5SemiconductorPlant #3 393 <2 >99.5SemiconductorPlant #4 274 2.6 99.1SemiconductorPlant #5 650 <5 >99.2
Study 2: EDI Performance at Varying Current Density
90
92
94
96
98
100
0,5 0,7 0,9 1,1 1,3
Sili
ca R
emov
al
(%)
I / Io
Feed Conductivity=20 micros/cm, CO2=6 ppm
Study 3: Current Densities Required to Maintain >98.5% Silica Removal
0
0,5
1
1,5
2
2,5
3
3,5
4
0 50 100 150 200 250
I/Io
Feed$Conduc*vity$(µS/cm)$
Carbon Dioxide Removal by EDI
• Carbon dioxide is often largest load on ion-exchange beds, especially after an RO
• Not effectively removed by other membrane demineralization processes such as RO or ED unless chemical adjustments are made
• Removed >99% by EDI
CO2 Levels and Removal by EDI
Feed CO2
(ppm)Product CO2
(ppb)CO2 Removal
(%)SemiconductorPlant #1 2.80 10 99.6SemiconductorPlant #2 4.48 29 99.4SemiconductorPlant #3 6.52 25 99.6SemiconductorPlant #4 6.05 10 99.8SemiconductorPlant #5 6.50 22 99.7
Summary of Weakly Ionized Species Removal
• > 99% Silica removal
• > 99% CO2 removal
• > 96% Boron removal
• > 98% Ammonia removal
Feed pH
Perc
ent o
f CO
2 or S
iO2 i
n Io
nize
d Fo
rm
% of SiO2 Ionized
% of CO2 Ionized
100
80
60
40
20
14
Resin pH
Feed
/Ani
on R
esin
pH
8
12
10
6
4
0-10 30-40 60-70 90-100
% of Flowpath INLET OUTLET
Process Principles
EDI Performance
EDI Stack Design
EDI Performance and Pretreatment
Performance
• EDI performance effected by: – Feed water quality – Stack current – Water recovery – Flow rate – Temperature
Feed Water Requirement
• Conductivity: < 40 mS/cm • Hardness: < 0.25 ppm as CaCO3 • TOC: < 0.5 ppm • Pressure: 20 to 50 psi • Temperature: 10 to 35oC • pH: 4 to 10 • Chlorine: < 0.1 ppm • Fe, Mn, Sulfide: < 0.01 ppm • CO2 < 8 ppm
Scaling in EDI
Cathode (-)
+ + + + Anode (+) SO4
= Cl- HSiO3-
H+ Ca++ Na+ Mg++
HCO3-
HCO3-
SiO2
CO2
Na+
SO4=
Cl-
Ca++
OH-
Anion-Exchange Membrane
HCO3- + OH- --> CO3
= + H2O Ca++ + CO3
= --> CaCO3
Ca++
Cation-Exchange Membrane
Feed Water Requirement
Non-Scaling Area
Water Recovery Decrease
Total CO2 Concentration
Har
dnes
s
PERFORMANCE
• Stack Current – Controlled by
• Stack voltage • Brine & electrode stream conductivity
– Optimum current • Too high current
– Higher scaling potential – CO2 back diffusion – Higher power consumption – Shorter membrane life
PERFORMANCE
• Temperature
– Better performance at higher temperature
– Stack pressure drop is sensitive to temperature
Feedwater Requirements and Pretreatment
Pretreatment
• Minimum pretreatment – TFC RO is required to minimize:
• Scaling • Organic fouling • Particulate and colloidal plugging • Oxidative attack • Chemical cleaning
Pretreatment
• Optional pretreatment – Multimedia filter – AC filter – Degasification – UF – Softener – UV – Organic scavenger
Testing - Organics • What is removed?
– Organic acids (acetic acid) – TMAH(tetramethyl ammonium hydroxide) – NMP(N-Methyl Pyrollidone)
• What may be removed? – Waco(601?) surfactant
• What isn’t removed? – IPA
• What may not be removed? – Anti-forming agent – Ethylene glycol
EDI Operation and Maintenance
STACK MAINTENANCE
• Routine Maintenance • Scaling and Scaling Control • Fouling and Fouling Control • Bacteria Control • Cleaning-In-Place
STACK MAINTENANCE
• Routine Maintenance – Daily log sheet – Visually inspect the stack weekly – Hose down any chemical buildup outside the
stack – Check the stack torque for the first three month,
and twice a year thereafter
STACK MAINTENANCE
• Stack Scaling
– Increasing stack resistance
– Reducing brine flow rate
– Decreasing silica rejection
– Product resistivity decline
STACK MAINTENANCE
• Scaling Control
– Brine stream pH control • pH3 or lower
– Softener addition ahead of EDI
– Reduce water recovery
– Reliable RO
STACK MAINTENANCE
• Stack Fouling
– Increasing stack resistance
– Decreasing silica rejection
– Product resistivity decline
– Stack pressure drop increase
STACK MAINTENANCE
• Fouling Control
– AC filter
– UV
– Organic scavenger
– Reliable RO
STACK MAINTENANCE
• Bacteria Grow in Brine Stream
– Brine flow decrease
– High bacteria count in brine stream
• Bacteria Control
– Sanitize the brine stream periodically • 0.1 to 0.2 ppm level of chlorine
STACK CLEANING
• Cleaning-in-place (CIP)
– Continuous (in-flight) brine stream acid CIP • pH 1.5 for hours
– Batch brine stream acid CIP • pH1 or lower for more than 30 min.
– Dilute stream salt & caustic CIP • 10% salt • pH11 • 40 C