It is quite difficult to find clear definitions and standards for distilled, demineralized and deionized water. Probably the easiest way to familiarize in the topic of producing (ultra) pure water is to start with the oldest and best-know method: distilling. Distilled water is water that has been boiled in an apparatus called a "still" and then recondensed in a cooling unit ("condenser") to return the water to the liquid state. Distilling is used to purify water. Dissolved contaminants like salts are left behind in the boiling pot as the water vapor rises away. It might not work if the contaminants are volatile so that they also boil and re-condense, such as having some dissolved alcohol. Very elegant stills can selectively condense (liquefy) water from other volatile substances, but most distillation processes allow carry-over of at least some volatile substances, and a very little of the non-volatile material that was carried into the water vapor stream as bubbles burst at the surface of the boiling water. Maximum purity from such stills is usually 1.0 MΩ.cm) dissolving into the distillate the pH is generally 4.5-5.0. Additionally, you have to be careful not to re-contaminate the water after distilling it.
Demineralization: Any process used to remove minerals from water, however, commonly the term is restricted to ion exchange processes.
Ultra pure water: Highly-treated water of high resistivity and no organics; usually used in the semiconductor and pharmaceutical industries.
Deionization entails removal of electrically charged (ionized) dissolved substances by binding them to positively or negatively charged sites on a resin as the water passes through a column packed with this resin. This process is called ion exchange and can be used in different ways to produce deionized water of various qualities.
Strong acid cation + Strong base anion resin systems.
These systems consist of two vessels - one containing a cation-exchange resin in the hydrogen (H+) form and the other containing an anion resin in the hydroxyl (OH-) form (see picture below). Water flows through the cation column, whereupon all the cations are exchanged for hydrogen ions. The decationised water then flows through the anion column. This time, all the negatively charged ions are exchanged for hydroxide ions which then combine with the hydrogen ions to form water (H2O).
These systems remove all ions, including silica. In the majority of cases it is advisable to reduce the flux of ions passed to the anion exchanger by installing a CO2 removal unit between the ion exchange vessels. This reduces the CO2 content to a few mg/l and brings about a reduction of the following strong base anion resin volume and in the regeneration reagent requirements.
In general the strong acid cation and strong base anion resin system is the simplest arrangement and a deionized water that may be used in a wide variety of applications can be obtained with it.
Strong acid cation + weak base anion + Strong base anion resin systems.
This combination is a variation of the previous one. It provides the same quality of deionized water, while offering economic advantages when treating water which contains high loads of strong anions (chlorides and sulphates). The subtitle shows that the system is equipped with an extra weak base anion exchanger before the final strong base anion exchanger. The optional CO2 removal unit may be installed either after the cation exchanger, or between the two anion exchangers (see picture below). The regeneration of the anion exchangers takes place with caustic soda (NaOH) solution first passing through the strong base resin and then through the weak base resin. This method requires less caustic soda than the method described before because the remaining regeneration solution after the strong base anion exchanger is usually sufficient to regenerate the weak base resin completely. Moreover, when raw water contains a high proportion of organic matter, the weak base resin protects the strong base resin.
In mixed-bed deionizers the cation-exchange and anion-exchange resins are intimately mixed and contained in a single pressure vessel. The two resins are mixed by agitation with compressed air, so that the hole bed can be regard as an infinite number of anion and cation exchangers in series (mixed bed resin).
To carry out regeneration, the two resins are separated hydraulically during the loosening phase. As the anion resin is lighter than the cation resin it rises to the top, while the cation resin falls to the bottom. After the separation step the regeneration is carried out with caustic soda and a strong acid. Any excess regenerant is removed by rinsing each bed separately.
The advantages of mixed bed systems are as follows: - the water obtained is of very high purity and its quality remains constant throughout the cycle, - pH is almost neutral, - rinse water requirements are very low. The disadvantages of mixed bed systems are a lower exchange capacity and a more complicated operating procedure because of separation and remixing steps which have to be carried out.