Demineralization is a process which uses specific ion exchange resins for the removal of dissolved mineral ions present in water in form of cations such as sodium , magnesium , iron , calcium , copper etc and anions such as chlorides , sulphates, nitrates , carbonates , bicarbonates etc.
Since majority of these impurities in water are in form of dissolved salts hence De-mineralization plants produce water of high purity standards which is almost devoid of any dissolved minerals and quite similar to distilled water.
Demineralization reduces scale formation, deposition and corrosion on tubes thereby increasing life of pipes and tubes in plants and also prevents deposition of minerals in turbine blades in power plants.
Ion exchange is the process in which the impurity ions present in water are replaced by ions released by the resin. The impurity ions thus get attached to the resin beads. With due passage of time , the resin beads get saturated with impurity ions and hence after sometime the need arises to re-generate or refresh the resin beads periodically.
There are basically 2 types of ion exchange resins :
1. Cation exchange resins release H+ ions or other positively charged ions in water for exchange with impurity cations present in water. These impurity cations are usually in form of metals such as sodium , magnesium , calcium , copper , iron etc.
2. Anion exchange resins release OH- or any other negatively charged ions in water for exchange with impurity anions like sulphates , chlorides , carbonates , bi-carbonates, nitrates etc.
Reservoir - is used for storage of water which is to be used for de - mineralization.
Clarifier - these are settling tanks which use mechanical means for continous removal of solida by sedimentation. A clarifier is generally used to remove solids particulates or suspended solids from the liquid. The concentrated impurities discharged from the bottom of the tank are known as sludge while the particles that float on the surface of liquid are called scum.
Before the water enters the clarifier - coagulation and flocculation reagents such as polyelectrolytes and ferric sulphate are added to water. These reagents cause the finely suspended particles to clump together to form larger denser particles which settle down as sludge more quickly , stably , easily and effeciently thereby helping in conservation of energy.
These are vertical or horizontal pressure tanks containing multiple layers of coarse and fine sand. The suspended contaminants in water are captured in the media bed and filtered water passes through the discharge manifold. The captured contaminants from the media bed are removed through the process of backwash. After the backwash mode , the Sand Filter is rinsed and then put back in Service mode.
Ultrafiltration membranes have pore size ranging from .01 to .1 micron and are typically used to remove oil , grease and colloidal particles from water. These membranes are preferred widely since they are highly tolerant to high temperature , pH ( acid and alkalinity ) , chlorine and are easily backwashable thereby avoiding the issue of membrane fouling.
The 2 bed de-ionization consistes of two vessels - one containing a cation exchange resin in form of H+ or other positively charged ion and other containing the anion resin in form of hydroxyl ion OH- or other negatively charged ion.
In order to keep the water electrically balanced one hydrogen ion H+ is exchanged for every monovalent cation and similarly for every divalent cation like Ca2+ , Mg2+, two divalent cations are exchanged.The same principle applies when considering anion exchange for the other vessel.
The de-cationised water then flows through the anion column where all the negatively charged ions are exchanged with hydroxyl ions which combine with hydrogen ions to form water.
Degasser is an integral part of De-mineralization plant where it is placed between cation and anion exchangers and remove CO2 which is generated by the breaking up of carbonic acid at outlet of cation exchanger.
After cation exchange , the carbonates and bi-carbonates are convererted into carbonic acid or CO2. Carbon dioxide gas is soluble in water but also escapes in air like it does in coke. A degasser removes the CO2 thereby reducing the ionic load on the strong base anion resin thus reducing the consumption of caustic soda.
HCO3 - H2O + CO2
Degassers are usually made up of acid proof materials ( wood & rubber lined steel ) as it has to handle acidic water of cation exchangers.
Typically in a degasser , air is blown at the bottom which rises counter current to the downward trickling water.The spray pipes or trays divide water into droplets or thin films exposing new surfaces to gas phase. Trays also agitate the water by splashing water thus allowing dissolved gases to leave water readily. Agitation also gets rid of the basic tendency of water to retain gas bubbles through surface tension or viscosity.
The last traces of salinity and silica can be removed here by mixed bed polishing where highly re-generated strong acid cation and strong base anion resins are mixed in a single pressure vessel. When water passes through the mixed bed it comes in contact with millions of cation and anion exchangers resulting in high quality of effluent water.
At the time of re-generation , the entire bed is backwashed. This expands the resin bed and allow the heavy cation resin to sink at bottom whereas the lighter anion resin rises to the top. The cation resin is regenerated through acid application ( sulphuric or hydrochloric acid ) whereas the anion resin is regenerated through caustic application mainly through caustic soda or Sodium Hydroxide ( NaOH).
Although the water quality obtained from mixed bed de - ionizer is appreciably higher than that produced by a 2 bed plant, the mixed bed plants are more sensitive to impurities in water supply and involve complicated re-generation process.
Hence mixed bed de-ionizers are normally used to polish the water to higher levels of purity after it has been initially treated by either a 2 bed de-ionizer or a Reverse Osmosis unit.