Softening of Water

Removal of Calcium, magnesium, iron salts etc. to feed soft water into boilers

Three processes

Cold and Hot Lime soda process #

Only required amount of lime and soda are to be added to remove temporary and permanent hardness

• Lime: Ca(OH)₂: 74g/mol
• Soda Na₂CO₃: 106g/mol

Temporary #

1. Ca(HCO₃)₂: 1 lime
2. Mg(HCO₃)₂: 2 limes

Permanent #

1. CaSO₄: 1 soda
2. MgSO₄: 1 soda + 1 lime

(A product of these reactions is CaCO₃↓)

Cold Lime soda process #

1. Calculated amount of chemicals are added + accelerators + coagulators
2. Contents in the tank are vigorously stirred
3. Water is softened
4. Soft water rises upwards and heavy sludges settle down
5. Softened water passes through a filtering media Calcium hardness=40ppm, Magnesium hardness = 0

Hot Lime soda process #

Here the chemicals along with water are heated near about the boiling point of water by exhaust steam. Process

1. Reaction tank: complete mixing of the ingredients takes place
2. Conical sedimentation vessel: Sludge settles down
3. Sand filter: Sludge is completely removed. Advantages:
4. Reaction is faster, sludge settles rapidly.
5. No coagulant needed.
6. Precipitation reaction becomes almost complete
7. Dissolved gasses are removed
8. Viscosity of water is lowered and hence is better filtered.
9. Residual hardness low compared to cold water.

Advantages and disadvantages of Lime Soda process #

Advantages #

1. Economical
2. Improves corrosion resistance of water
3. Mineral content ↓
4. pH↑⇒pathogenic bacteria↓

Disadvantages #

1. Huge amount of sludge is removed
2. Residual hardness⇒Water isn’t suitable for high pressure boiler

Zeolite process #

Zeolite: hydrated sodium alumino silicate (Na₂OAl₂O₃⋅xSiO₂⋅yH₂O) Commono zeolite=natrolith: x,y=3,2

Permutit=Artificial zeolite used for softening.

• Porous and glassy
• Greater softening capacity than green sand
• China clay + feldspar + soda ash

Method

• Na₂Ze+Ca(HCO₃)₂=2NaHCO₃+CaZe
• Na₂Ze+CaSO₄=Na₂SO₄+CaZe

Water softened using this can be used for laundry purposes

Regeneration #

CaZe+2NaCl=Na₂Ze+CaCl₂

Advantages #

1. Hardness as low as 10ppm
2. Equipment used is small, easy to handle
3. Less time for softening
4. No sludge formation⇒clean
5. Easy to regenerate
6. Flexibility: Any hardness can be removed without adjusting the process.

Disadvantages #

1. Unfiltered water cannot be used.
2. Water containing acid cannot be used since it can destroy the zeolite.
3. Causes corrosion:
   1. Method releases lots of NaHCO₃ ⇒{heat} Liberates CO₂ ⇒ Causes corrosion
   2. Therefore, not suitable for boilers.

Ion-exchange/demineralization process #

Ion exchange resins: Organic polymers which are crosslinked having microporous structure and the functional groups are attached to the chains which are responsible for ion exchange properties

1. Cation exchange resin (RH⁺)
   1. Exchange their H⁺ with Ca²⁺, Mg²⁺
2. Anion exchange resin (ROH⁻)
   1. Exchange their OH⁻ with Cl⁻, SO₄⁻²

Method

1. Passed through cation exchange resin
   1. 2RH⁺+Ca⁺²→R₂Ca⁺²+2H⁺
2. Passed through anion exchange column
   1. ROH⁻+Cl⁻→RCl⁻+OH⁻
3. H⁺ and OH⁻ combine to form water Thus the water coming out from the two exchangers is ion free and called deionized/demineralized water.

Regeneration #

• R₂Ca⁺²+2H⁺→2RH⁺+Ca²
• R₂SO₄⁻²+2OH⁻→ROH⁻+SO₄⁻²

Mixed bed deionizer: A single cylinder containing both the mixture of cation exchanged and strongly basic anion exchanger. Water comes in contact with the two for a large # of times and hardness is reduced to 1ppm.

Advantages #

1. Easy regeneration
2. Both acidic and alkaline water can be softened
3. Residual hardness is very low⇒can be used in boilers

Disadvantages #

1. Equipment and process is costly
2. Turbid water cannot be used with filteration.