water conditioning and water hardness.ppt

water conditioning and water hardness

1. Softening or Conditioning Methods of Hard Water
It is mandatory to soften water to make it free from hardness producing
substances, suspended impurities and dissolved gases, etc.
Softening or conditioning of water - the process of removing hardness
producing
salts from water.
Two methods – External treatment and Internal treatment
External Treatment – the removal of hardness producing salts from the
water before feeding it into the boiler.
•Lime-soda process
•Zeolite or Permutit process
•Demineralisation or Iron exchange process

2. Distillation

3. External Treatment – the removal of hardness producing salts from the water
Before feeding it into the boiler.
Lime-soda process
Zeolite or Permutit process
Demineralisation or Iron exchange process

4. Water softening processes
1. Lime Soda Process: The lime soda process involves the chemical conversion of
all the soluble hardness causing salts by the addition of soda (Na
2
CO
3
) and lime
[Ca(OH)
2] into insoluble precipitates which could easily be removed by settling and
filtration.
 It removes temporary hardness, permanent Mg hardness, dissolved Fe, Al salts,
dissolved CO
2
and H
2
S gases and free mineral acids present in water.

7. Zeolite (or) Permutit Process
Hydrated sodium aluminosilicate – Na
2
O.Al
2
O
3
.xSiO
2
.yH
2
O; x = 2 – 10, y = 2 – 6
Natrolite – Na
2
O.Al
2
O
3
.3SiO
2
.2H
2
O
Natural zeolites are green sand and are usually non-porous.
Permutit – artificial zeolite, porous.
Sodium zeolite – Na
2
Ze which exchange Na+ ions with the hardness producing
ions like Ca
2+
, Mg
2+
in water.

8. Process

9. Ca(HCO
3
)
2
+ Na
2
Ze CaZe + 2NaHCO
3
Mg(HCO
3
)
2
+ Na
2
Ze MgZe + 2NaHCO
3
CaSO
4
+ Na
2
Ze CaZe + Na
2
SO
4
MgSO
4
+ Na
2
Ze MgZe + Na
2
SO
4
CaCl
2
+ Na
2
Ze CaZe + 2NaCl
MgCl
2
+ Na
2
Ze MgZe + 2NaCl
The sodium salts formed in the above reactions remain dissolved in the softened
water and do not impart any hardness.

10. Regeneration
Sodium zeolite gets exhausted due to its conversion into Ca and Mg zeolites.
Regeneration is done by percolating 10% brine solution through the exhausted
zeolite. The Ca and Mg zeolites are converted back into sodium zeolite.
Na
2
Ze + CaCl
2
CaZe + 2NaCl
Na
2
Ze + MgCl
2MgZe + 2NaCl

11. Advantages of the process
Water obtained by this process will have a residual hardness between
7 and 15 ppm
The method is cheap, because the regenerated zeolite can be used again.
This process does not produce any sludge and hence a clean process.
The equipment is compact and occupies less space.

12. Disadvantages of the process
The process exchanges only calcium and magnesium ions with sodium ions and
hence the softened water contain more sodium and also more of dissolved salts.
It does not remove the acidic ions such as bicarbonate and carbonate and remain
as sodium salts contributing to the alkalinity and causes boiler corrosion.
The water containing turbidity and suspended impurities cannot be treated by this
method because turbidity clogs the pores of the zeolite bed.
The process cannot tolerate acidity or alkalinity as the zeolite disintegrates.
The process is not very efficient for treating water containing large quantities of
Fe
2+
and Mn
2+
ions as these ions convert sodium zeolite into their respective zeolites
which are difficult to be regenerated.

13. Demineralization or Ion Exchange Process
This process removes almost all the ions present in water.
Soft water does not contain hardness producing Ca
2+
and Mg
2+
ions but it may
contain other ions like Na
+
, K
+
, Cl
-
, SO
4
2-
.
Every soft water is not demineralized water whereas every demineralized water
is soft water.
Ion exchangers are resins with a long chain, cross-linked, insoluble organic
polymers with a microporous structure. The functional groups attached to the
chains are responsible for the ion exchanging properties.

14. The following two types of resins are used for demineralization process:
Cation exchange resins and Anion exchange resins
Cation exchange resins – possess acidic group such as –COOH or –SO3H groups.
Cations in hard water are exchanged with H
+
ions of this resins. This resin may be
represented as RH
2
.
examples: sulphonated coal, sulphonated polystyrene
Anion exchange resins – possess basic groups such as OH- or NH2- group. Anions
in hard water are exchanged with –OH ions of this resins. It may be represented as
R’(OH)
2
.
examples – cross-linked quaternary ammonium salts, urea-formaldehyde resin.

15. SO
3
H
SO
3
H
SO
3
H
SO
3
H
SO
3
H
SO
3
H
CHCH
2
CHCH
2
CHCH
2
SO
3
H
CH
CHCH
2
CHCH
2
CHCH
2
SO
3
H
CHCH
2
CHCH
2
CHCH
2
CH
A strongly acidic sulphonated
polystyrene cation exchange resin

16. SO
3
H
CH
2
NR
3
OH
_
+
_
+
HOR
3
NH
2
C
_
+
HOR
3
NH
2
C
CH
2
NR
3
OH
_
+
_
+
HOR
3
NH
2
C
CHCH
2
CHCH
2
CHCH
2CH
CHCH
2
CHCH
2
CHCH
2
CHCH
2
CHCH
2
CHCH
2
CH
A strongly basic quaternary ammonium
anion exchange resin

18. Process
Demineralization or Ion Exchange Process

19. Hard water is first passed through the cation exchange resin. The cations like
Na
+
, K
+
, Ca
2+
, Mg
2+
, etc. in hard water get exchanged with H
+
ions of the resin.
The water coming out from this column is acidic in nature.
RH
2
+ CaCl
2 RCa + 2HCl
RH
2
+ MgSO
4
RMg + H
2
SO
4
RH + NaCl RNa + HCl

20. The water is then passed through the anion exchange resin which exchanges the
anions like Cl
-
, SO
4
2-
, HCO
3
-
with OH
-
ions of the resin.
The water coming out from this column is completely free from both cations and
anions. H
+
and OH
-
ions combine to produce water molecule.
The water coming out from the second column is neutral and is free from all ions.
It is known as deionised or demineralised water.
R'(OH)
2
+ 2HCl R'Cl
2
+ 2H
2
O
R'(OH)
2
+ H
2
SO
4 R'SO
4
+ 2H
2
O
H
+
+OH
_
H
2
O

21. Regeneration of resins
The exhausted cation exchange resin is regenerated by percolating a dilute HCl
solution through it.
The exhausted anion exchange resin is regenerated by percolating dilute NaOH
solution through it.
RH + NaClRNa + HCl
RH
2
+ CaCl
2
RCa + 2HCl
R'Cl
2
+ 2NaOH R'(OH)
2
+ 2NaCl

22. Advantages
The hardness of water can be reduced to about 2 ppm and hence it is suitable for
use in high pressure boilers.
Highly acidic or highly alkaline water can be softened by using this process.
Limitations
The resin used in the process are quite expensive.
If water contains turbidity, the efficiency of the process is reduced.
Water containing Fe and Mn cannot be treated because they form stable product
with the resins.

23. Differences between zeolite and demineralization processes
S. No. Zeolite Process Demineralization process
1.Only cations are exchanged.Both cations and anions are
exchanged.
2.Since acidic water decomposes
the zeolite it cannot be treated.
Acidic water can be treaated.
3.Treated water contains more
dissolved salts which causes
priming, foaming and caustic
embrittlement in boilers
Water treated by this process
contains no dissolved salts and
no priming or foaming is
caused.
4.Disadvantages
Water with Fe, Mn and
turbidity cannot be treated.
Disadvantages
Water with Fe, Mn and
turbidity cannot be treated.