The Basics of Demineralisation by Ion Exchange

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The Basics of Demineralisation by Ion Exchange

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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.

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Raw Water Supply

• Water in the UK can come from many sources
• Four principle types of supply are widely
  encountered
• 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)

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Raw Water Supply

• Ions present in all natural waters
• Cations Anions
• Sodium (Na) Bicarbonate (HCO3) / Carbon
  Dioxide (CO2)
• Calcium (Ca) Sulphate (SO4)
• Magnesium (Mg) Chloride (Cl)
• Potassium (K) Nitrate (NO3)
• Iron (Fe) Silica (SiO2)
• In addition dissolved organics can be
  present which can be important on some sites with
  regard to resin selection.

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Typical IEx Plant Designs

• To achieve high water quality the majority of
  plants
• employed in the UK fit into the following
  categories
• Cation Anion (Main subject for todays
  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.

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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.

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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).

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• Cation Unit Representation in service and after
  co-flow regeneration
• In Service OperationResin
• Resin - SO3H Na ? Resin - SO3Na
  H
• 2Resin - SO3H Ca ? 2Resin
  SO3Ca 2H
• Order of Selectivity Fe gt Ca gt Mg gt K
  gt 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.

Raw Water
Calcium
Magnesium
Sodium
H (unused)
In Service
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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.

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• Anion Unit Representation in service and after
  co-flow regeneration
• In Service OperationResin
• Resin Amine OH Cl ? Resin
  Amine Cl OH
• 2Resin Amine OH SO4 ? 2Resin
  Amine SO4 2OH
• Order of selectivity SO4 gt NO3 gt Cl
  gt Bicarbonate / CO2 gt 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.

Raw Water
Sulphate
Nitrate
Chloride
Bicarbonate / CO2
Silica
OH- (unused)
In Service
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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.
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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
• Higher capacity
• Better Rinse
• Lower pressure drop
• Higher breaking weight
• Are more suitable to some specialist engineering
  designs (e.g. Packed beds)

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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)

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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)

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• Co-flow vs Counter Flow Regeneration (Cation
  Representation)

Counter flow (Example showing upflow regen.)
Co-flow
Service flow
Co-flow Regeneration
Counter Flow Regeneration
After regen
After regen
Ca Mg Mg Na Na Na Na Na
Ca Ca Ca Mg Mg 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!
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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

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Co-Flow Regeneration
7
1
Feed Water

• Valve Identifiers
• Inlet
• Oulet
• Drain
• Regen / Slow Rinse Inlet
• WWI
• WWO
• Vent (manual)

5
4
6
Regen
3
Effluent
2
Treated Water
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Plant Operation / Treated Water Quality

• SAC / Degasser / SBA / Mixed Bed Treated water
  Quality

Cation TWQ
Anion TWQ
MB TWQ AT ALL TIMES!!!!!
pH 2 3 Conductivity Increase (R water x 1.5 to
2) Trace Na / No hardness Co-flow Regen
(Typ.) 0.5-2.0 mg/l Na Counter flow Regen
(Typ.) 0.02-0.5 mg/l Na
pH gt 7 Typically 7.3 - 9 Conductivity low
(Depending Sodium leakage exit cation) Reactive
Silica low Co-flow Regen (Typ.) 0.05 0.3 mg/l
SiO2 Counter flow Regen (Typ.) 0.025 0.1 mg/l
SiO2
pH 7 Conductivity 0.056 - 0.1 us/cm Na lt 0.01
mg/l Silica lt 10 - 20 ug/l
5 mg/l CO2
SAC
Degasser
SBA
Mixed Bed
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Minimum Level of Instrumentation for Cation
Anion Polishing M Bed(Cation Anion with
co-flow regeneration)
Pump
Raw Water

Flow
Pressure

Cation
Anion
Pressure
Pressure
Silica (Optional depending on clients Treated
Water specification)
Conductivity
Treated Water
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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
Degasser Tower
LS
Silica (Optional depending on clients Treated
Water specification)
Conductivity
Raw Water Tank
Pump
Treated Water Tank