The Basics of Demineralisation by
Ion Exchange
Raw Water Supply
Water comes into sites from many sources and can be potable
(suitable for drinking), an industrial supply provided by the local
water plc or the clients own supply extracted on site from a river /
borehole.
The drinking water use is sterile and includes all many of the
natural elements we need to sustain a healthy life.
All these ions however cannot be left in the water fed to boilers and to
other processes. They would cause corrosion and deposits affecting
performance and causing premature plant failure.
Raw Water Supply
Water in the UK can come from many sources:
Four principle types of supply are widely encountered
1.
2.
3.
4.
Ground Waters – Pumped from boreholes or wells, these supplies have a
high salts content. From deep boreholes the water quality remains very
constant and it is normally high in hardness (calcium & magnesium) and high
in alkalinity (bicarbonate). Normally dissolved organics are not present.
Surface Waters – Upland sources low in dissolved solids but with a high
proportion of dissolved organics.
Surface Waters – Lower levels with moderate dissolved solids and with
moderate to high organics.
Mix of surface and ground waters of variable quality – (river supplements)
Raw Water Supply
Ions present in all natural waters:
Cations
Sodium (Na)
Calcium (Ca)
Magnesium (Mg)
Potassium (K)
Iron (Fe)
Anions
Bicarbonate (HCO3) / Carbon Dioxide (CO2)
Sulphate (SO4)
Chloride (Cl)
Nitrate (NO3)
Silica (SiO2)
In addition dissolved organics can be present which can be
important on some sites with regard to resin selection.
Typical IEx Plant Designs
To achieve high water quality the majority of plants
employed in the UK fit into the following categories:
1.
2.
3.
4.
Cation – Anion (Main subject for today’s presentation)
Cation – Anion – Polishing Cation
Cation – Anion – Mixed Bed
Reverse Osmosis – Ion Exchange Plant
The cation and anion columns can employ either co-flow or
counter-flow regeneration and in some cases they can also
they employ a Degassing Tower after the cation unit.
Ion Exchange Resin - Properties
Synthetic Ion Exchangers require certain properties
to perform demineralisation. The three main
properties required are:
a. Insoluble in, but permeable by water.
b. An ability to exchange ions, with the different types of ions
commonly encountered in water supplies. Active groups
throughout the beads perform the ion exchange.
c. To allow the passage of water through the resin bed at
optimum rates without undue pressure drop.
Cation Exchange Resins
Two principle types of cation resin:
Weak Acid Cation – with carboxylic group
(Resin – COOH) – Dealkalisation Process
Strong Acid Cation – with sulphonic acid
group (Resin - S03H)
Regeneration is with an excess amount of
dilute acid (sulphuric or hydrochloric acid).
Cation Unit Representation in service and after co-flow regeneration
Raw Water
In Service OperationResin
Resin - SO3H + Na  Resin - SO3Na + H
2Resin - SO3H + Ca  2Resin – SO3Ca + 2H
Calcium
Magnesium
Sodium
H+ (unused)
In Service
Order of Selectivity: Fe > Ca > Mg > K > Na
In Regeneration – (Typically with 5% HCl conc.)
Resin – SO3Na + HCl
 Resin – SO3H + NaCl + Excess Acid
Resin – SO3Ca + H2SO4  Resin – SO3H + CaSO4 + Excess Acid
Treated water contains high concentration of H+ ions so water
exit cation has a low pH.
Anion Resins
Strong base anion resins are employed on all
demineralisation plants for producing high quality
water.
Either in separate anion units and or as the strong
base anion component in mixed beds.
Strong base anion resins will remove all anions
present but require an excess of Sodium Hydroxide
(Caustic Soda) to regenerate them.
Anion Unit Representation in service and after co-flow regeneration
Raw Water
In Service OperationResin
Sulphate
Nitrate
Chloride
Bicarbonate / CO2
Silica
OH- (unused)
In Service
Resin – Amine OH + Cl
 Resin – Amine Cl + OH
2Resin – Amine OH + SO4  2Resin – Amine SO4 + 2OH
Order of selectivity:
SO4 > NO3 > Cl > Bicarbonate / CO2 > Silica
In Regeneration (Typically with 4% NaOH conc.)
Resin – Amine Cl + NaOH  Resin – Amine OH + NaCl + Excess NaOH
Resin – Amine SO4 + NaOH  Resin – Amine + Na2SO4 + Excess NaOH
Treated water now contains OH- ions which combine with H+ ions
to form pure water H2O.
Ion Exchange Resin
Standard grade resins from all manufacturers are typically made 300 to 1200
microns with less than 1% less than 300 microns. Hence internal systems / nozzles
are selected to have a maximum slot / aperture of 200 microns.
In addition resin suppliers also make more uniform and specialist grades.
Ion Exchange Resin Grades
Narrow Uniform Grade Resins
Most Narrow grade resins typically in the
range of 400 – 800 microns (some of these
resins have a very narrow distribution and a
low uniformity coefficient )
Standard grade resins 300 – 1200
microns.
Narrow grade resins can offer:
a. Higher capacity
b. Better Rinse
c. Lower pressure drop
d. Higher breaking weight
e. Are more suitable to some specialist
engineering designs (e.g. Packed
beds)
Ion Exchange Resin Selection
The Six Most Important Factors Affecting Resin Selection:
Raw water quality.
(TDS and other contamiants)
Treated water quality.
(conductivity / silica specification)
Engineering techniques employed.
(co-flow or counter flow regen)
Operating flow rate.
(good kinetics)
Process temperatures.
(anion resins have low maximum temp limits)
Presence of organic foulants.
(anion resin resistant to fouling)
Degassing Towers
Between the cation and anion stage on many large demin plants
there is a degassing tower. (Normally if the bicarbonate content
of the raw water supply is above 50 mg/l).
These are a very efficient way of removing the bicarbonate
present in the water mechanically and cheaply.
When the Ca / Mg associated with the bicarbonate passes
through a cation resin this happens.
Ca(HCO3)2 + Resin-2H+  Resin-Ca + H2CO3 (Carbonic acid)
When the resin releases the H+ ions the water becomes acidic
(pH 2-3 exit SAC). At low pH Carbonic acid is unstable.
H2CO3 at low pH  H2O + CO2.
(forming pure water and carbon dioxide)
Co-flow vs Counter Flow Regeneration (Cation Representation)
Service
flow
Counter flow (Example showing upflow regen.)
Co-flow
Co-flow
Regeneration
After regen:
Counter Flow
Regeneration
After regen:
Ca
Ca Ca Ca
Mg Mg
Mg Mg
Na Na
Na Na
Na Na Na
Na
With counter flow regeneration the most highly regenerated portion of the ion exchange bed is at the
unit outlet so leakage is significantly better in service operation!
Co – Flow Regeneration
The regeneration of the resin involves the following
main steps with co-flow regeneration
Backwash
Bed Settle
Establish motive water
Regenerant Injection
Slow / Displacement Rinse
Fast Rinse
Co-Flow Regeneration
7
1
Feed Water
Valve Identifiers
5
1. Inlet
2. Oulet
3. Drain
4. Regen / Slow
Rinse Inlet
4
6
5. WWI
6. WWO
Regen
7. Vent (manual)
Effluent
3
2
Treated Water
Plant Operation / Treated Water Quality
SAC / Degasser / SBA / Mixed Bed Treated water Quality
Cation TWQ:
Anion TWQ:
MB TWQ:
pH 2 – 3
pH > 7
AT ALL TIMES!!!!!
Conductivity Increase
(R water x 1.5 to 2)
Conductivity low
pH 7+
(Depending Sodium leakage
exit cation)
Conductivity 0.056 - 0.1
us/cm
Reactive Silica low
Na < 0.01 mg/l
Co-flow Regen (Typ.)
Co-flow Regen (Typ.)
Silica < 10 - 20 ug/l
0.5-2.0 mg/l Na
0.05 – 0.3 mg/l SiO2
Counter flow Regen
(Typ.)
Counter flow Regen (Typ.)
Trace Na / No hardness
0.02-0.5 mg/l Na
SAC
5 mg/l CO2
Degasser
Typically 7.3 - 9
0.025 – 0.1 mg/l SiO2
SBA
Mixed Bed
Minimum Level of Instrumentation for Cation – Anion – Polishing M Bed
(Cation – Anion with co-flow regeneration)
Pump
Raw
Water
Flow
Pressure
Cation
Pressure
Anion
Pressure
Conductivity
Silica (Optional depending on clients Treated Water
specification)
Treated
Water
Minimum Level of Instrumentation for Cation – Anion – Polishing M Bed
(Cation – Anion with co-flow regeneration)
Flow
Flow
Pressure
Pressure
Pump
Cation
Anion
LS
Pressure
Pressure
LS
Raw
Water
Tank
Degasser Tower
Conductivity
Silica (Optional depending on clients Treated
Water specification)
Pump
Treated
Water
Tank