EXTERNAL TREATMENT METHOD-----WATER TECHNOLOGY---CY2111 183101 –ENGINEERING CHEMISTRY –I Lecture Notes

EXTERNAL TREATMENT METHOD:

.Ion exchange method  ( Demineralisation)

Working:

1. Here all the cations and anions are completely removed .  It uses twp cylinders pf cation exchange cylinder and anion exchange cylinder  filled with resins.
2. Resins are linear, insoluble, cross linked , organic polyers. There are 2 types.

i)                    Cation exchange resins – RH₂   (e.g)  Sulphonated coals , RSO₃H

ii)                  Anion exchange resins . R’(OH)₂ (e.g)Urea-formaldehyde resin,Amines R-NH₂

3. The water is fed into cylinder –I  where all the cations are replaced by RH₂  

Resins.

             RH₂  + CaCl₂  à  R Ca  +  2 HCl

4. The cation free water is fed to cylinder II, where all the anions are replaced.

R’(OH)₂  +  2 HCl  à  R’Cl₂ + 2H₂O

5. So, the resultant water is free from all types of ions.

Regeneration:

            On prolonged use, as  all the resins are exhausted, there will be no H⁺ orOH ^– ions to exchange the unwanted ions. So, they have to be regenerated.

Cation resins are regenerated by HCl and anion resins by NaOH.

R Ca  +  2 HCl  à  RH₂  + CaCl₂

R’Cl₂ + 2NaOH  à    R’(OH)₂  +  2NaCl

Advantages of Ion exchange method:

i)                    Very good for use in high pressure boilers also.

ii)                  It can treat highly acidic or alkaline water.

iii)                We can get pure water as hardness as low of 2 ppm.

Drawbacks of Ionexchange method:

i)                    Expensive

ii)                  Fe, Mn cannot be removed as they form complexes with resins

iii)                Cannot be used for turbid water as they clog the resins.

Diagram: