What is a Demineralisation Plant?

A Demineralisation Plant consists of two pressure vessels containing cation and anion exchange resins. Various types of ion exchange resins can be used for both the cation and the anion process, depending on the type of impurities in the water and what the final water is used for.

Typically, the cation resin operates in the hydrogen cycle. The cations in the water (i.e. calcium, magnesium and sodium) pass through the cation exchange resin where they are chemically exchanged for hydrogen ions.

The water then passes through the anion exchange resin where the anions (i.e. chloride, sulphate, nitrate and bicarbonate) are chemically exchanged for hydroxide ions.

The final water from this process consists essentially of hydrogen ions and hydroxide ions, which is the chemical composition of pure water.

The simplified demineralisation plant consists of:

Composite resin vessels with charge of strong cation and anion resin; control-panel encompassing a conductivity measurement and alarms, etc; acid and caustic injection facility from bulk, semi-bulk or carboy containers.

What is demineralised water used for?

The high-purity water from a demineralised plant is typically used for high pressure boiler feed, wash water for computer chip manufacture, pharmaceutical process water, micro-electronics and any process where high-purity water is a requirement.

Why are there numerous types of resins used in demineralisation plants?

The type of resins employed and selected depends on numerous factors:

• Treated water quality required
• Input water quality
• Presence of organic foulants
• Water temperatures
• Flow through plant required
• Regeneration method - either counter-current or co-current regeneration

There is a vast range of resins to select from, e.g. purafine resins, enhanced capacity resins, gel polystyrene resins, potable clear gel resins, resins for water containing organic matter, resins to achieve low silica levels.

What is co-current flow regeneration?

The resin contained in the pressure vessel has about 50% free space above the resin (known as resin-free board). This free space allows backwashing, removal of any entrained solids, re-classification of the resin bed and it relieves bed compaction.

Water and acid/caustic regeneration is carried out in a down-flow direction.

What is counter-current flow regeneration?

With counter-flow regeneration, the regenerant acid and caustic passes in the opposite direction to the service flow water.

With counter-flow regeneration, the regenerant passes through the resin near to the outlet of the unit and, hence, counter-current flow regeneration has lower leakage to service than the co-current method.

How do you regenerate a demineralisation plant?

demineralisation plants regenerate automatically according to conductivity.

The regenerant for the cation column is 28% hydrochloric acid and 30% caustic soda for the anion column.

What is mixed-bed demineralisation?

Polishing mixed beds come after the cation and anion standard vessels and, as the name implies, they are there to polish the water. The bed is an intimate mix of anion and cation resins. These resins are not regenerated at a pre-set time according to conductivity and are not operated to exhaustion but are regenerated on time and volume.

What is the quality of the treated water from a demineralisation plant?

The quality of the water depends on the type of scheme used:

Cation-Anion-Polishing Mixed Bed

With this configuration, this type of plant should produce water of the following quality:

- Conductivity 0.1 µS/cm at 25°C
- Sodium 0.01 mg/l
- Reactive silica 0.02 mg/l

Cation-Anion (Counter-Current Regeneration)

With typical counter-flow regeneration, the average water quality is:

- Conductivity 0.5 to 1.0 µS/cm at 25°C
- Sodium 0.05 to 0.1 mg/l
- Reactive silica 0.025 mg/l

Cation-Anion (Co-Current Regeneration)

With typical co-current regeneration, the outlet quality will depend on the regenerant applied, resin employed and raw water quality.

- Conductivity 5 to 30 µS/cm at 25 C
- Sodium 0.5 to 3 mg/l
- Silica 0.1 to 0.3 mg/l

How do I size a demineralisation plant?

For the sizing of a demineralisation plant, a good in-depth water analysis is normally required which gives the breakdown of total anions and total cations and any potential organic foulants. The final required quality water specification is required, as well as flow rate and water used per day.

For help on sizing, please contact us.