Ion exchange or deionization or demineralization process:
Principle: A reversible exchange of ions takes place between the stationary ion exchange phase and the external liquid mobile phase.
Ion exchange resins are insoluble, cross linked, porous, high molecular weight, organic polymers and the functional groups attached to the chains are responsible for the ion exchange properties.
Ion exchange resins are generally copolymers of Styrene- divinyl benzene
Ion exchange resins are of two types
Cation exchange resins
Anion exchange resins
Cation exchange resins (RH+):
These are capable of exchanging H+ ions with the cations.
It is mainly styrene divinyl benzene copolymer
They have acidic functional groups like -SO3H,
-COOH etc
Anion exchange resin (ROH-)
These are capable of exchanging OHions with the anions.
It is nothing but a copolymer of styrene divinyl benzene
They contain basic functional groups like quartenary ammonium ion.
On treatment with dil NaOH they have capability to exchange its OHions with anions in the water
Process:
The hard water is passed first through cation exchange column
The cation exchanger removes all the cations like Ca+2, Mg+2 from it
and equivalent amount of H+ ions are released from the column to water.
2RH+ + Ca2+ --> R2Ca + 2H+
2RH + + Mg2+ --> R2Mg + 2H+
The hard water is now passed through anion exchange column,
which removes all the anions like SO4 -2, Cl- from it
and equivalent amount of OHions are released from the column to water.
R'OH- + Cl- --> R'Cl + OH-
2R'OH- + SO42- --> R'2SO4 + OH-
H+ and OHions are released from the cation exchange and anion exchange columns respectively get combined to produce water molecule.
H+ + OH- --> H2O
Water coming out from the exchanger is free from cations and anions and is known as deionized or deminerealized water.
Regeneration of exhausted bed:
When the beds are getting exhausted (capacity to exchange H+ & OHions are lost) then they are regenerated
The exhausted cationic exchanger is regenerated by washing with acids like HCl, H2SO4 etc
R2Ca2+ + 2H+→ 2RH+ + Ca2+
The exhausted anion exchanger is regenerated by washing with bases like NaOH
R'2SO42- +2OH - → 2R'OH- + SO42-
The columns are washed with deionized water and washings are passed to sink or drain. Now they are ready for softening process.
Advantages
Produce water of very low hardness (2ppm )
The process can be used to soften highly acidic or highly alkaline waters.
Water produced by this process water can be used in high pressure boilers
Disadvantages
High capital cost and chemicals & equipment are costly
If water contains turbidity efficiency of the process decreases.
Tags
waterchemistry