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 localwater plc or the clients own supply extracted on site from a river /borehole.

The drinking water use is sterile and includes all many of thenatural 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 affectingperformance 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. Ground Waters – Pumped from boreholes or wells, these supplies have ahigh 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.

2. Surface Waters – Upland sources low in dissolved solids but with a highproportion of dissolved organics.

3. Surface Waters – Lower levels with moderate dissolved solids and with moderate to high organics.

4. Mix of surface and ground waters of variable quality – (river supplements)

Raw Water Supply

Ions present in all natural waters:

Cations AnionsSodium (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.

Typical IEx Plant Designs

To achieve high water quality the majority of plants

employed in the UK fit into the following categories:

1. Cation – Anion (Main subject for today’s presentation)

2. Cation – Anion – Polishing Cation

3. Cation – Anion – Mixed Bed

4. 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

In Service OperationResin

Resin - SO3H + Na Resin - SO3Na + H

2Resin - SO3H + Ca 2Resin – SO3Ca + 2H

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.

Calcium

Magnesium

Sodium

H+ (unused)

In Service

Raw Water

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

In Service OperationResin

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.

Sulphate

Nitrate

Bicarbonate / CO2

OH- (unused)

In Service

Raw Water

Chloride

Silica

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 plantsthere 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 bicarbonatepresent in the water mechanically and cheaply. When the Ca / Mg associated with the bicarbonate passesthrough 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)

Co-flow Counter flow (Example showing upflow regen.)Service flow

Ca

Mg Mg

Na Na

Na Na Na

After regen:

Co-flow Regeneration

After regen:

Counter Flow Regeneration

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!

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

4

5

Feed Water

Treated Water

3 Effluent

1

2

6

Regen

7

Valve Identifiers

1. Inlet

2. Oulet

3. Drain

4. Regen / Slow Rinse Inlet

5. WWI

6. WWO

7. Vent (manual)

Plant Operation / Treated Water QualitySAC / Degasser / SBA / Mixed Bed Treated water Quality

SAC Degasser SBA Mixed Bed

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 5 mg/l CO2

pH > 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 < 0.01 mg/l

Silica < 10 - 20 ug/l

Cation TWQ: Anion TWQ: MB TWQ:

AT ALL TIMES!!!!!

Minimum Level of Instrumentation for Cation – Anion – Polishing M Bed(Cation – Anion with co-flow regeneration)

Silica (Optional depending on clients Treated Water specification)

Conductivity

Raw Water

Treated Water

Cation Anion

Pressure

Pressure Pressure

Flow

Pump

Minimum Level of Instrumentation for Cation – Anion – Polishing M Bed(Cation – Anion with co-flow regeneration)

Pump

Silica (Optional depending on clients Treated Water specification)

Conductivity

Raw Water

Tank

Treated Water

Tank

Cation

Degasser Tower

Anion

Pressure

Pressure

Pressure

FlowFlow

LS

Pump

LSPressure