FAQ on Demineralization Plants

Demineralization plant is employed for removal of minerals or dissolved salts from the water. Salts on dissolving dissociate into electrically charged particles called ions: for example common salt will be split into sodium ion (a positively charged ion or cation) and chloride (a negatively charged ion or an anion). If such a solution is brought into contact with a suitable ion exchange material (called resin), some ions from the solution are taken up by the resin and an equivalent number are transferred from the resin to the solution. Ion exchange is thus a reversible interchange of ions between a liquid and a solid. A simple Demineralization Plant consists of two beds of chemically treated resin beads operating in series. The first column-cation exchanger-converts the dissolved solids in the raw water to the equivalent acids; these acids are removed as the water passes through the second column-anion exchanger. The final product from this process consists essentially of pure water. When exhausted, the cation exchange resin is regenerated with acid and the anion exchange resin with alkali.

In essence the DM plant comprises of 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.

The high-purity water from a demineralization plant is typically used as feed water for high pressure boilers in many industries; as wash water in computer chip manufacture and other micro-electronics manufacturing processes, as pharmaceutical process water, and any process where high-purity water is a requirement. DM water is used as process water in the manufacture of chemicals and fertilizers, food products such as soft drinks, automobilesfor rinsing of parts, textiles, etc.

The regeneration is usually carried out in three steps. Firstly, the ion exchange column is backwashed with an upflow of water. The pressure vessel has about 50% free space above the resin bed (known as free board). This free space allows removal of any entrained solids, and re-classification of the resin bed by backwashing. Backwashing also relieves bed compaction. Secondly, a predetermined amount of acid or alkali is injected into the column in a downward direction (same direction as the service flow or co-current) to displace sodium/calcium/magnesium in the cation exchanger and chlorides/sulphates/alkalinity in the anion exchanger taken up during the service cycle. Lastly, the column is rinsed to remove excess regenerant. The entire operation takes about 3 hours for a two-bed DM plant.

With counter-flow regeneration, the regenerant acid or caustic passes in the direction opposite to the flow of water during the service cycle. With counter-flow regeneration, the fresh regenerant enters at the bottom of the resin bed and passes in an upward direction (opposite to the downflow direction during service cycle or counter-current). Hence, bottom layer of the resin bed is always in highly regenerated condition. This means lower leakage or slip of ions during the service cycle producing better quality of treated water than the co-current method.

The mixed bed is a single column of cation resin and anion resin mixed together. Water passing through the column comes into contact with these materials and is subjected to almost infinite number of demineralizing stages. Thus demineralized water of extreme purity is produced.

As with two-bed demineralizers, mixed bed units are regenerated with acid and alkali: but the ion exchange resins must be separated before this can be done. Bed separation is accomplished by backwashing: this carries the lighter anion resin to the top of the bed and the heavier cation resin sinks to the bottom. Two completely separated layers are thus formed, into which the acid and alkali solutions and rinse water are introduced through specially designed distributors. After regeneration, the two resins are mixed with compressed air.

Normally mixed bed unit treats water from the two-bed DM plant that is already of high purity and their ionic load is low. They can consequently be operated at high flow rates, and are of relatively smaller size.

For the sizing of a demineralization 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 water quality specification, as well as flow rate and water used per day is required.

The alkalinity or bicarbonates and carbonates present in raw water appear as carbonic acid or dissolved carbon dioxide at the outlet of cation exchanger. Weak base anion resin such as does not remove weak acids such as carbon dioxide or silica. The demineralized water is therefore passed through a degassing tower for removal of carbon dioxide or CO₂.The tower, made of rubber-lined steel is filled with packing rings through which the demineralized water percolates. Low pressure air introduced at the bottom of the tower scrubs out CO₂, and the degassed water collects in a sump beneath the tower.

All DM plants are provided with conductivity indicators that have two basic elements: a conductivity cell with electrodes of special design between which demineralized water flows and a sensitive milliammeter for measuring the current passing between the electrodes. This current is proportional to conductivity of the water.