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ZERO-ION Demineralizers

Twin Bed Demineralizer (DM) Plant

Technology and Process Description

Raw water is passed through a MacroFilter for the reduction of excess turbidity, suspended solids, bad colour, odour, residual chlorine and volatile organic compounds. This water is subsequently sent into a two bed DM plant.

The process of demineralization in which the ions present in water are removed is accomplished in stages, deploying ion-exchange resins. Twin-Bed Demineralizers are used to remove ionized impurities from the water. Deionization or demineralization is the process of removing up to 95% of the dissolved mineral salts from the raw water by way of ion exchange technology. A DM plant comprises two separate vessels internally connected in series. The first vessel is filled with strongly acidic cation (SAC) exchange resins and the second one is filled with strongly basic anion (SBA) exchange resins. Two distinctly different types of resins are utilized for this process.

When water containing cations (positively charged ions such as sodium(Na+), calcium(Ca2+), magnesium (Mg2+), potassium(K+) etc.) passes through the cation resin bed, the cation resin exchanges the positive ions for hydrogen ions (H+). To keep the water electrically balanced, for every monovalent cation (e.g. Na+), one hydrogen ion is exchanged and for every divalent cation (eg., Ca2+ or Mg2+), two hydrogen ions are exchanged.

Chemical reactions (cation exchange)
RH2 + Ca(HCO3)2 -----> R Ca + 2H2CO3
RH2 + MgSO4 -----> R Mg + H2SO4
RH + NaCl -----> R Na + HCl
RH + KNO3 -----> RK + HNO3

Similarly, when the cation-free water flows through the anion resin bed, the anion resin exchanges the negative ions (chlorides(Cl-), sulphates (SO42-), carbonates (CO32-), Bicarbonates (HCO3-), Nitrate (NO3-)) for hydroxide ions (OH-).

Chemical reactions (anion exchange)
R(OH)2 + H2CO3 -----> RCO3 + 2 H2O
R(OH)2 + H2SO4 -----> RSO4 + 2 H2O
ROH + HCl -----> RCl + H2O
ROH + HNO3 -----> RNO3 + H2O

The hydrogen ions from the cation bed combine with the hydroxyl ions from the anion bed to form high purity water.

Weak Acid Cation and Weak Base Anion are also used in DM plants to handle water containing high amounts of alkalinity, chlorides and sulphates.

Mixed Bed Demineralizer (MBD) plants

MBD plant intimately combines strongly acidic cation and strongly basic anion resins within a single pressure vessel. MBD plants are also called as polishing units and are used for applications that require a higher level of water quality. Thorough mixture of cation and anion exchangers in a single column makes an MB plant very effective as post-treatment to either a twin-bed DM plant or RO Plant. These units are used downstream of the DM unit to achieve very high purity levels in the treated water. Standard units are mounted on self-supporting, fully assembled skids with necessary piping / valves and include the necessary chemical regeneration systems, piping and valves.

Mixed bed resins are regenerated with acid and alkali but the resins must be separated before this can be done. Bed separation is done by backwashing. The backwashing carries the lighter anion resin to the top and the cation resin sinks to the bottom.

Where ultra pure water is required two mixed beds are provided. The first mixed bed is known as primary mixed bed and the second as secondary mixed bed.

All DM plants are available in Manual, Semi-Automatic and Fully-Automatic versions. Fully automatic units also include Programmable Logic Controllers (PLC) based control systems. These systems can be further upgraded for a direct link to DCS.