Understanding Water Hardness and Impurities in Water Technology

 
WATER
 
TECHNOLOGY
 
1
 
Course: 
B.Tech
Subject: Engineering
 
Chemistry
Unit:
4
 
Sources 
of
 
Water
 
A) Surface
 
Waters
Rain 
Water 
- Pure but contaminated with
 
gases
River 
Water 
- High dissolved salts
 
moderate
organics
Lake 
Water 
- Const. composition but high
 
organics
Sea
 Water
 
-
 
High 
salinity, 
pathogens,
 
organics
 
B) 
Underground
 
Waters
Spring/Well 
Water 
- Crystal clear but
 
high
dissolved
salts and high purity from
 
organics
 
2
 
Ionic and
 
dissolved
 
Cationic
Calcium
M
a
g
n
es
i
um
 
Nonionic and undissolved
Turbidity, 
silt, mud, 
dirt
 
and
other suspended
 
matter
 
Anionic
B
i
car
b
o
n
a
t
e
Carbonate
Hydroxide
 
S
u
lfa
t
e
 
Color,
 
Plankton
Organic
 
matter,
 
Gases
CO
2
H
2
S
NH
3
CH
4
O
2
 
Chloride
 
Colloidal 
silica,
 
Sodium
Potassium
A
m
m
o
n
ium
Iron
Manganese
 
Nitrate
P
h
osp
h
a
t
e
 
M
i
c
r
o
o
r
g
a
n
i
s
m
s,
Bacteria
 
MAJOR IMPURITIES OF
 
WATER
 
Alkalini
t
y
 
3
 
H
ARD
 
W
ATER
 
Hardness 
refers to the
presence of calcium and
magnesium 
ions in
 
water
(and 
sometimes
 
iron).
Ions 
come 
from
 
dissolved
rock the 
water has passed
through.
Affects 
properties of
 
tap
water
 
1
 
H
ARD
 
W
ATER
 
Minerals in hard
 
water
interact with
 
soap.
Interferes 
with 
soap’s
ability to
 
lather.
 
2
 
S
OFT
 
W
ATER
 
Water 
with very low concentrations of
 
minerals.
Soap lathers easily and is 
sometimes 
difficult to 
rinse
off.
 
3
 
H
ARDNESS OF
 
W
ATER
 
7
 
Hardness 
in 
Water 
is characteristic that prevents the
 
‘lathering
of 
soap’ 
thus 
water which 
does not produce lather 
with soap
solution 
readily, 
but 
forms 
a white curd 
is 
called hard
 
water.
 
Type 
of
 
Hardness
 
Temporary 
or Carbonate
 
Hardness
Permanent 
Hardness or non-carbonate
 
Hardness.
 
8
 
Temporary
 
Hardness
 
Temporary 
Hardness 
is caused by the presence of 
dissolved 
bicarbonate
 
of
calcium, 
magnesium 
and other heavy metals and the carbonate of
 
iron.
It 
is mostly 
destroyed by 
more 
boiling of 
water, 
when bicarbonates are 
decomposed
yielding insoluble
 
carbonates.
 
Ca(HCO
3
)
2
 
Heat
 
CaCO
3
 
+
 H
2
O
 
+
 CO
2
Calcium
 
bicarbonate
 
Calcium
 
Carbonate
 
Mg(HCO
3
)
2
 
Heat
 
Mg(OH)
2
 
+
 2CO
2
Magnesium
 
Bicarbonate
 
Magnesium
 
hydroxide
 
 
Calcium/Magnesium Carbonates thus 
formed 
being 
almost 
insoluble,
 
are
deposited 
as 
a 
scale 
at the bottom of 
vessel, 
while carbon dioxide escapes
 
out.
 
P
ERMANENT
 
H
ARDNESS
 
9
 
Non 
Carbonate Hardness is 
due 
to the presence of 
chlorides, sulfates
 
of
calcium, Magnesium, 
iron 
and other heavy
 
metals
 
2C
17
H
35
COONa 
+
 
CaCl
2
 
2C
17
H
35
COONa 
+
 
MgSO
4
 
Sodium
 
stearate
(sodium
 
soap)
 
H
a
rdn
e
ss
 
(C
17
H
35
COO)
2
Ca 
+
 
2NaCl
Calcium
 
stearate
(Insoluble)
 
Sodium
 
stearate
(sodium
 
soap)
 
H
a
rdn
e
ss
 
(C
17
H
35
COO)
2
Mg 
+
 
2Na
2
SO
4
Magnesium
 
stearate
(Insoluble)
 
10
 
Draw 
backs (or) 
Disadvantages 
of Hard
 
Water
Domestic
 
Use
Industrial
 
Use
 
1.
Washing
2.
Bathing
3.
Drinking
4.
Cooking
 
1.
Textile
 
Industry
2.
Sugar
 
Industry
3.
Dyeing
 
Industry
4.
Paper
 
Industry
5.
Pharmaceutical
 
Industry
6.
In 
Steam 
generation in
 
Boilers
 
The 
sticky precipitate 
adheres on 
the
fabric/cloth 
and 
gives spots 
and 
streaks. 
Fe
salts 
stain the
 
cloths.
 
Produces sticky scum 
on the 
bath 
tub
 
and
the body
Bad 
to the 
digestive 
system 
and calcium
oxalate formation is possible 
in 
urinary
tracts
Requires 
more 
fuel and time. Certains food
don’t 
cook soft and also 
gives 
unpleasant
taste
 
11
Boiler troubles 
due 
to Hard
 
Water
1. Scale and
 
Sludge
2. Caustic
 
embitterment
3. 
Priming 
and
 
Foaming
4. Boiler
 
corrosion
 
Boiler
 
wall
 
Slimy 
loose precipitate
 
called
sludge suspended in
 
water
1.
 
Sludge
water
 
Sludge 
is 
a soft, loose and 
slimy 
precipitate 
formed 
within the 
boiler. 
It can be easily
scrapped 
off 
with a 
wire
 
brush.
It is 
formed 
at 
comparatively 
colder 
portions 
of the boiler and collects in areas of
 
the
system, 
where the flow 
rate 
is slow 
or at
 
bends.
It 
is 
formed by 
substances 
which have greater solubility's in hot 
water 
than in cold
water, 
e.g. 
MgCO
3
, 
MgCl
2
, CaCl
2
, 
MgSO
4
 
etc.,
 
Remedy: 
Sludges can be removed 
using 
wire brush or 
mild
 
acid
 
4
 
D
ISADVANTAGE 
O
F 
SLUDGE
 
FORMATION
:
 
-poor conductor of
 
heat
-disturbs the working of
 
boiler.
- If along with 
scales 
: both get deposited as
 
scales.
 
Preventation 
of Sludge
 
formation
:
-
Well 
softened
 
water
-
Drawing 
off 
a portion of concentrated
 
water.
 
12
 
13
1.
 
Scale
 
wat
e
r
 
Hard adherent 
coating 
on
 
inner
walls of
 
boiler
 
Scales are hard substances which sticks very 
firmly 
to the
 
inner
surfaces of the boiler
 
wall.
Scales are 
difficult 
to 
remove 
even 
with 
the help of a 
hammer
 
and
chisel.
Examples: CaSO
4
, CaCO
3
,
 
Mg(OH)
2
 
Boi
l
er
 
6
wall
 
5
 
14
Reasons for formation of
 
scale
1. Presence of Ca(HCO
3
)
2 
in low pressure
 
boilers
 
Ca(HCO
3
)
2
 
CaCO
3
 
+ 
H
2
O 
+
 
CO
2
Calcium
 
bicarbonate
 
Calcium Carbonate
 
(scale)
Low 
pressure boilers 
but in high
 
pressure
boilers 
it 
is 
soluble by 
forming
 
Ca(OH)
2
2. Presence of 
CaSO
4 
in high pressure
 
boilers
T
o
C
 
Solubility of
 
CaSO
4
15
 
3200
 
ppm
230
 
15
 
ppm
320
 
27
 
ppm
Cold
 
water
 
soluble
Super
 
heated
 
water
 
Insoluble
 
(scale)
3. Presence of 
MgCl
2 
in high 
temperature
 
boilers
 
MgCl
2 
+ 2
 
H
2
O
Magnesium
 
chloride
 
Mg
 
(OH)
2
 
+
 
2HCl
scale
4. Presence of
 
SiO
2
It 
forms 
insoluble 
hard adherent
CaSiO
3 
and 
MgSiO
3 
as
 
scales
 
Mg(OH)
2 
can also be generated by thermally decomposing
 
Mg(HCO
3
)
2
 
15
Disadvantages of scale
 
formation
 
1.
Fuel 
wastage 
– scales have 
low thermal
 
conductivity
2.
Degradation of boiler material and increases of 
risk 
of
 
accident
3.
Reduces the 
efficiency 
of the boiler 
and- 
deposit on the valves and
 
condensers
4.
The boiler 
may 
explode – if crack occurs in
 
scale
Remedies: Removal of
 
scale
 
1.
Using 
scrapper, 
wire brush
 
often
2.
By 
thermal 
shock- heating and cooling 
suddenly 
with cold
 
water
3.
Using 
chemicals – 5-10% 
HCl 
and by adding
 EDTA
 
16
Prevention 
of scale
 
formation
 
Scale formation 
can be prevented by two
 
methods
1.
Internal conditioning or Internal
 
Treatment
2.
External conditioning or External 
treatment- 
will be discussed
 
later
1. 
Internal 
conditioning 
methods 
- of boiler water to prevent 
scale
 
formation
 
1.
Phosphate conditioning – addition of phosphate
 
compound
2.
Carbonate 
conditioning 
– addition of carbonate
 
compound
3.
Calgon conditioning – addition of sodium hexa 
meta
 
phosphate
4.
Colloidal conditioning – spreading of 
organic 
compounds like
tannin, agar gel
5.
Sodium Aluminate – removes oil and
 
silica
6.
Electrical
 
Conditioning
7.
Radioactive
 
Conditioning
8.
Complexometric method – using
 
EDTA
 
17
1. Phosphate
 
conditioning
Scale formation 
can be prevented by adding sodium phosphate to the
boiler water which reacts with the hardness producing ions and
forms easily 
removable phosphate 
salts 
of respective
 
ions
 
3CaCl
2 
(Boiler water) +
 
2
 
Na
3
PO
4
 
Ca
3
(PO
4
)
2  
+ 6
 
NaCl
 
18
NaH
2
PO
4 
(acidic in nature)
 
,
Na
2
HPO
4 
(weakly alkaline
in
 
nature),
Na
3
PO
4
 
(Alkaline in
 
nature)
Calcium 
can not be precipitated below a pH = 9.5, hence the 
selection
 
of
phosphate has to be based on the pH of the boiler feed
 
water.
Selection 
of Phosphate
 
compound
2. Carbonate
 
conditioning
 
CaSO
4 
(Boiler water)
 
+
 
Na
2
CO
3
 
CaCO
3
 
+
 
Na
2
SO
4
Caution: Excess 
Na
2
CO
3 
can result in caustic
 
embrittlement
 
19
3. Calgon
 
conditioning
 
2CaSO4 (Boiler water) +
 
[Na4P6O18]2-
C
a
l
c
ium
sulfate
 
[Ca2P6O18]2- +
 
2Na2SO4
Soluble 
complex
 
ion
of 
calcium 
- can be
removed
 
easily
 
Na2
[
Na4
(
P
O
3
)
6
 
2
N
a+
 
+
 
[Na4P6O18]2-
Calgon –
sodium hexa
meta
phosphate
Calgon tablets are used in the cleaning of washing 
machine
 
drums
 
4.Colloidal
 
conditioning:
 
In low pressure 
boilers- 
scale : adding org. substances like kerosine,
agar-agar,
 
tannin,etc.
-
 
Yield non-sticky & loose
 
deposits
 
5
. 
Treatment
 
withNaAlO
2
:
 
NaAlO
2 
+
 
2H2O
MgCl
2 
+
 
2NaOH
 
NaOH + Al(OH)
3
(gelatinous
 
ppt)
Mg(OH)
2 
+
 
2NaCl
 
20
 
6.Electrical
 
conditioning:
Sealed glass bulbs-mercury: battery
 
Water 
boils- mercury 
bulb - 
emit 
electrical
 
discharged
 
7. 
Radioactive conditioning:
Tablets 
- 
radioactive
 
salts
 
8. 
Complexometric
 
method:
-
1.5% 
alk. 
Soln of
 
EDTA
-
EDTA 
: 
cation 
& form stable & 
soluble
 
complex.
 
21
 
Other
 
treatment:
1.
Prevent : iron
 
oxide
2.
Protects boiler unit from corrosion by 
wet
 
steam.
3.
Reduces the carry over of oxides with
 
steam
 
22
 
C
AUSTIC
 
EMBRITTLEMENT
:
 
Is a 
type 
of boiler
 
corrosion.
 
Caused : highly alkaline
 
Water.
 
Boiler water – 
becomes
 
‘caustic’
Water 
evaporates – 
caustic soda
 concentration
 
increase.
which 
attacks 
surrounding area of boiler
 
machine.
Iron converted in to sodium
 
ferroate.
Causes 
embrittlement 
of boiler parts, strssed parts( bends, 
joints,
 
etc.)
Iron : in dilute NaOH – cathodic
 
side
Iron : in concentrated NaOH – anodic
 
side.
 
23
Na
2
CO
3 
+ 
H
2
O 
→ 2 
NaOH 
+
 
CO
2
 
24
 
Preventation 
of caustic
 
Embrittlements:
Sodium phosphate – sodium
 
carbonates
Adding lignin : preventing 
infilteration 
of caustic soda
 
solution
Sodium sulphate
: 
Na
2
SO
4
 
concentration
NaOH 
concentration
Kept 1:1, 2:1 &
 
3:1,
Pressure : 10,20 & above
 
20.
IV. 
Boiler
 
corrosion
 
Degradation or 
destruction 
of boiler 
materials 
(Fe) 
due 
to the 
chemical 
or
electrochemical attack 
of dissolved gases or 
salts 
is 
called 
boiler
 
corrosion
Boiler corrosion is of three
 
types
 
1.
Corrosion 
due 
to dissolved
 
O
2
2.
Corrosion due to dissolved
 
CO
2
3.
Corrosion 
due 
to 
acids formed 
by dissolved
 
salts
1. Corrosion 
due 
to dissolved oxygen
 
(DO)
 
2 Fe
 
+ 2 
H
2
O 
+
 
O
2
 
2
 
2
 
Fe(OH)
2
2
 
[Fe
2
O
3
.2H
2
O]
Rust
 
4 
Fe(OH)
2
 
+
 
O
Ferrous
hyd
r
o
x
i
d
e
Removal 
of Dissolved Oxygen
 
(DO)
 
The dissolved oxygen present in the boiler feed water can be 
removed 
by
 
the
addition of sodium sulphite or hydrazine and the reactions can be written
 
as
 
below
 
2
 
Na
2
SO
3
 
+
 
O
2
 
2
 
Na
2
SO
4
 
N
2
H
4 
+
 
O
2
S
o
dium
sulphi
t
e
DO
Sodium
 
sulphate
Hydrazine
Nitrogen
1. By the addition of
 
chemicals
 
N
a
2
S
 
+
 
2O
2
N
2
 
+
 
2H
2
O
 
N
a
2
S
O
4
2. Corrosion 
due 
to dissolved
 
CO
2
 
Presence of bicarbonate 
salts 
of 
either magnesium 
or 
calcium 
also causes the
 
release
of 
CO
2 
inside the boiler apart from the dissolved
 
CO
2
Mg(HCO
3
)
2
 
MgCO
3 
+ 
H
2
O 
+
 
CO
2
 
CO
2
 
+
 
H
2
O
 
H
2
CO
3 
(causes slow
 
corrosion)
3. Corrosion due to dissolved
 
salts
 
MgCl
2   
+
 
2
 H
2
O
 
Mg(OH)
2  
+
 
2HCl
 
Fe
 
+
 
2 HCl
 
FeCl
2
 
+
 
H
2
 
FeCl
2
 
+
 
2
 H
2
O
 
Fe(OH)
2  
+
 
2HCl
 
Prevention
 
:
(1) Corrosion can be prevented by adding 
alkali 
to neutralize
 
acidity
& 
anti-oxidant 
to remove
 
oxygen
(2) By keeping pH value 8 to
 
9.
(3) Oxygen is removed from boiler feed-water by adding
 
Na2SO3.
(4) Oxygen can 
also 
removed by treating it with hydrazine
 
hydrate
NH2–NH2
NH2-NH2 + 
O2 
→ 2N2 +
 
2H2O
 
28
III. 
Priming 
and
 foaming
 
Foa
m
ing
 
Pri
m
ing
 
Normal
 
bubble
 
Carry over
 
bubble
Foaming
It 
is 
the 
production of 
continuous foam
or hard 
bubblers 
in 
boilers. Foaming is
due to the 
presence 
of 
substance like
oil in boiling
 
water.
Priming
It 
is the process in which some
particles in water are carried along
with 
the 
steam. 
The 
resulting
process 
is called as 
wet 
steam 
or
carry 
over. 
The 
process of formation
of wet 
steam in boilers 
is 
called 
as
priming.
 
7
 
Causes of
 
Priming
(1) the presence of 
large 
amount 
of dissolved
 
solids
(2) high 
steam
 
velocities.
(3) sudden
 
boiling
(4) improper boiler
 
design
(5) sudden increase in 
steam-production
 
rate.
 
Disadvantages of
 
Priming
(1) Dissolved 
salt 
in boiler water are carried 
out 
by the wet 
steam 
to
 
turbine
blades - which reduces their
 
efficiency.
(2) Dissolved 
salts 
may 
enter the parts of other 
machinery 
may 
decrease
 
the
life 
of the
 
machinery.
(3) Actual height of the water 
column 
cannot be judge 
properly,
 
Thereby
 
making 
the 
maintenance 
of the boiler pressure 
becomes
 
difficult.
 
30
 
Prevention 
of
 
Priming
(1) By improving boiler
 
design.
(2) By 
fitting mechanical steam
 
purifiers.
(3) By 
maintaining 
low water 
level 
in
 
boilers
(4) By using soft
 
water.
(5) By 
decreasing 
the amount of dissolved
 
salts.
 
( B) FOAMING
 
:
It is the production of foam or bubbles in boiler which do not break
 
easily.
Causes of Foaming
 
:
It is due to the presence of oily substances in
 
water.
(1) Low level of water in
 
boiler.
(2) The presence of dissolved salts in
 
water.
(3) Sudden increase in steam production
 rate.
 
31
 
Disadvantages of foaming
 
:
(1) Actual height of the water 
column 
cannot be
 
judge.
(2) Dissolved 
salts 
in water carried by the wet 
steam may
 
damage
turbine blads or 
machinery
 
parts.
(3) 
Boiler 
pressure cannot be
 
maintained.
 
Prevention 
of Foaming
 
:
(1) By the addition of anti-foaming 
chemicals like castor 
oil,
 
Gallic
acid, 
tennic 
acid
 
etc.
(2) removing oil from boiler water by adding compounds
 
like
sodium
 
aluminate.
 
32
II . Softening of water/ External 
treatment 
of water –
 
External
Conditioning of
 
water
Softening of hard water can be done by the
following
 methods
1.
Lime 
soda
 
process
2.
Zeolite
 
methods
3.
Ion exchange resin
 
method
4.
Mixed bed deionizer
 
method
 
1. 
Lime 
soda
 
process
It 
is 
a process 
in which 
Lime 
(Ca(OH)
2
) 
and 
soda (Na
2
CO
3
) 
are added to
the 
hard water to convert the 
soluble 
calcium and magnesium
 
salts
 
to
insoluble compounds 
by a 
chemical reaction. The 
CaCO
3 
and 
Mg(OH)
2 
so
precipitated 
are filtered 
off 
and removed
 
easily.
It is further divided in to two
 
types
1.
Cold 
lime 
soda
 
process
2.
Hot 
lime 
soda
 
process
1. Cold 
lime 
soda
 
process
 
In 
this process 
a 
calculated quantity of 
Ca(OH)
2 
(lime) and 
Na
2
CO
3 
(soda)
are 
mixed with 
water 
at room temperature and added to the hard 
water. 
The
following reactions takes place depending on the nature of
 
hardness
 
If it is permanent hardness and 
due 
to 
calcium
 
salt
Ca
2+
 
+
 
Na
2
CO
3
 
CaCO
3
 
+ 
2Na
+
 
(soda)
slimy 
suspended
 
precipitate
 
If it is due to Magnesium
 
salt
Mg
2+
 
+
 
Ca(OH)
2
 
Mg(OH)
2
 
+ 
Ca
2+
 
(lime)
slimy 
suspended precipitate
Ca
2+
 
+
 
Na
2
CO
3
 
CaCO
3
 
+ 
2Na
+
 
(soda)
slimy 
suspended
 
precipitate
Chemical
 
reactions
Step
 
1
 
35
 
If it is 
Temporary 
hardness and 
due 
to 
calcium
 
salt
Ca(HCO
3
)
2
 
+
 
Ca(OH)
2
 
2CaCO
3
 
+
 
2H
2
O
slimy 
suspended
 
precipitate
 
If it is 
due 
to Magnesium
 
salt
Mg(HCO
3
)
2 
+
 
2Ca(OH)
2
 
2CaCO
3
 
+ 
Mg(OH)
2 
 
+
 
2H
2
O
slimy 
suspended
 
precipitates
Chemical 
reactions
 
contd..
 
The 
precipitates 
CaCO
3 
and 
Mg(OH)
2 
are very fine and forms sludge like
precipitates 
in 
the 
boiler water and are 
difficult to 
remove because it 
does
not 
settle easily making it 
difficult 
to filter 
and 
the 
removal 
process. Finally
reduces the 
efficiency 
of the
 
boiler.
Therefore, it 
is 
essential to 
add 
small 
amount of coagulant (such 
as 
Alum,
Aluminium sulfate, sodium aluminate etc) which hydrolyses to flocculent
precipitate 
of 
Al(OH)
3 
which entraps the fine
 
precipitates.
Step
 
2
 
36
 
8
When 
coagulants are added flocculation takes place followed by the 
formation
 
of
flocculants.
 
NaAlO
2 
 
+
 
2H
2
O
 
NaOH 
+
 
Al(OH)
3
Coagulant
Flocculent-
Gelatinous precipitate
which 
entraps the fine
precipitates of CaCO
3
and
 
Mg(OH)
2
 
Al
2
(SO
4
)
3
 
+ 3
 
Ca(HCO
3
)
2
 
2Al(OH)
3
 
+ 
CaSO
4 
+
 
CO
2
 
Alu
m
in
i
um
sulfate
 
Hard
 
water
sample
Flocculent-
Gelatinous precipitate
which entraps the fine
precipitates of CaCO
3
and
 
Mg(OH)
2
The 
Al(OH)
3 
formed 
by 
the addition of 
coagulants initiates the process 
of 
flocculation 
and
entraps the fine 
precipitates 
and 
becomes 
heavy. 
The 
heavier flocs 
then 
settles 
at 
the bottom
and filtered 
off
 
easily.
 
Method
 
:
 
Raw water & calculated
 
quantities
 
of
 
chemicals: from the top
in to the inner vertical circular
 
chamber.
There is a vigorous stirring & continuous 
mixing 
: softening
of water take
 
place.
Softened water 
comes 
into the outer 
co-axial 
chamber, 
it rises
upwards.
Heavy sludge settles down & softened water reaches
 
up.
Softened water : passes through a filtering 
media 
to
 
ensure
complete removal of
 
sludge.
Filtered soft water finally flows 
out 
continuously through the
outlet at the
 
top.
Sludge settling at 
the 
bottom 
of the 
outer chamber is
 
drawn
off
 
occasionally.
 
38
 
(ii) 
Hot 
lime-soda
 
process:
 
Treating 
water with 
softening 
chemicals 
at a 
temp. 
of 80 to
 
150˚c.
Process : operated 
close 
to the boiling
 
point.
 
consists three
 
parts:
 
(a)
reaction tank: in which raw 
water, 
chemicals 
& 
steam 
are
 
mixed,
(b)
Conical 
sedimentation 
vessel : sludge 
settles
 
down.
(c)
Sand 
filter 
: 
complete 
removal of sludge from softened
 
water.
 
39
 
40
 
9
 
Advantages 
of 
hot 
lime-soda
 
process:
The 
precipitation 
reaction 
is almost
 
complete,
Reaction takes place
 
faster,
Sludge settles down
 
rapidly;
No 
coagulant 
is
 
needed,
Dissolved 
gases 
(which 
may 
cause corrosion)
 
are
removed,
Viscosity 
of 
soft 
water is 
lower, 
hence filtered
 
easily,
Residual 
hardness is low compared 
to cold lime-soda
process.
 
41
 
Advantages of 
Lime 
– soda
 
process
:
1.Economical
2
.
Pr
o
ce
s
s
 
incre
s
es
 
pH
 
val
u
e of
 
t
h
e
 
treated
 
wate
r
,
 
thereby
 
corrosion
of 
the 
distribution pipes is
 
reduced.
3.Mineral 
content 
of water is
 
reduced
4.pH of water raises thus reducing content of pathogenic
 
bacteria
5. iron & Mn:
 
removed.
 
Disadvantages of 
Lime 
– soda
 
process
:
1.Huge amount of sludge is formed and its disposal is
 
difficult
2.Due to residual hardness, water is 
not 
suitable 
for 
high pressure
boilers
 
42
 
2. 
Z
EOLITE OR PERMUTIT
 
PROCESS
:
 
Zeolite - hydrated sodium alumino silicate, capable of
exchanging reversibly its sodium ions 
for
 
hardness-
producing ions in
 
water.
The general chemical structure of
 
Zeolite:
Na
2
O.Al
2
O
3
.
x
SiO
2
.
y
H
2
O
(
x 
= 2-10 and 
y 
=
 
2-6)
 
43
Micro pores 
of
 
Zeolite
 
10
Porous 
structure of
 
Zeolite
 
Two
 
types:
 
(i)
Nat
u
ral
 
Ze
o
l
i
te:
 
no
n
-p
o
rous.
e.g.,
 
Natrolite
(ii)
Synthetic 
zeolite: porous & possess gel
 
structure.
 
They are prepared by heating together china 
clay, 
feldspar and
soda
 
ash.
 
Zeolites possess higher exchange capacity 
per 
unit weight than
natural
 
zeolite.
 
44
 
Process:
 
Hard 
water 
is percolated at a 
specified 
rate through a bed of
 
zeolite,
kept in a
 
cylinder.
The hardness-causing ions are retained by the zeolite 
as 
CaZe
 
and
MgZe; while the outgoing water contains sodium
 
salts.
 
Na
2
Ze 
+ 
Ca(HCO
3
)
2
Na
2
Ze 
+
 
Mg(HCO
3
)
2
 
CaZe + 
2NaHCO
3
MgZe +
 
2NaHCO
3
 
CaZe
 
+
 
2NaCl
 
Na
2
Ze 
+ 
CaCl
2
Na
2
Ze 
+
 
MgCl
2
 
MgZe
 
+
 
2NaCl
 
45
To 
remove temporary
 
hardness
To 
remove 
permanent
 
hardness
 
46
 
11
 
R
EGENERATION 
OF
 
Z
EOLITE
:
 
At
 
this stage, the supply of hard water is stopped 
and 
exhausted zeolite is
reclaimed by treating 
the 
bed with concentrated NaCl
 
solution.
 
CaZe 
or
 
MgZe
(exhausted
 
zeolite)
 
+
 
2NaCl
(Brine)
 
Na
2
Ze
 
+
 
CaCl
2
(Reclaimed 
zeolite)
 
(washings)
 
47
 
L
IMITATIONS 
OF 
Z
EOLITE
 
PROCESS
:
 
1.
If the water is 
turbid 
---- then the 
turbidity 
causing
 
particles
clogs the pores of the Zeolite and making it
 
inactive
2.
The ions such as 
Mn
2+ 
and 
Fe
2+ 
forms 
stable complex Zeolite
which can 
not 
be regenerated that easily as both 
metal 
ions
bind 
strongly and irreversibly to the zeolite
 
structure.
3.
Any acid present in water (acidic water) should be neutralized
with soda before admitting the water to the plant, since acid
will 
hydrolyze 
SiO
2 
forming silicic
 
acid
 
48
 
A
D
V
A
N
T
A
G
E
S
:
 
Residual hardness of water is about 10 
ppm
 
only
Equipment is 
small and 
easy to
 
handle
Time 
required for softening of water is
 
small
No sludge 
formation 
and the process is
 
clean
Zeolite can be regenerated easily using brine
 
solution
Any 
type 
of hardness can be 
removed 
without any 
modifications 
to the
 
process.
DISADVANTAGES
 
:
1.
 
Soft water contains 
more 
sodium salts than in lime 
soda
 
process
 
3
 
2.
 
It replaces only Ca
2+ 
and 
Mg
2+ 
with 
Na
+ 
but leaves all the other ions like 
HCO 
-
 
and
 
CO 
2-  
in the softened water (then it 
may
 
form
 
NaHCO
 
and 
Na
 
CO
 
which releases
 
CO
 
2
 
3
 
3
 
2
 
3
when the 
water 
is 
boiled and causes
 
corrosion)
 
3.
 
It also causes caustic 
embitterment 
when sodium carbonate hydrolyses to give
 
NaOH
49
 
I
ON 
E
XCHANGE
 
P
ROCESS
:
 
Ion exchange resins are insoluble, cross 
linked, 
long chain 
organic
polymers with a microporous structure, and the functional groups
attached to the chain is responsible for the 
“ion-exchange”
 
properties.
 
Acidic functional groups (-COOH, -SO3H, etc.) exchange H+ with other
cations.
 
Basic functional groups
 
(-NH2=NH
 
etc.)
 
exchange 
OH- 
with other
anions.
 
50
 
Classification of
 
Resins
 
A. 
Cation-exchange
 
Resins(RH+)
 
:
 
Strongly acidic
(SO
3
-
H
+
) 
and weakly acidic 
(COO
-
H
+
) 
cation
 
exchange
resins
 
51
 
12
 
2.  Anion Exchange resin
 
(ROH
-
)
 
 
Strongly basic
 
(R
4
N
+
OH
-
)
and weakly basic 
(RNH
2
+
OH
-
) 
anion exchange
 
resins
 
52
13
 
53
14
Process or Ion-exchange 
mechanism 
involved in water
 
softening
 
2 
RH
+  
+ 
Ca
2+
 
(hard
 
water)
 
R
2
Ca
2+
 
+ 2
 
H
+
2 
RH
+  
+ 
Mg
2+ 
(hard
 
water)
 
R
2
Mg
2+  
+ 2
 
H
+
 
2 
ROH
-  
+ 
SO
4
2-
 
(hard
 
water)
 
R
2
SO
4
2+
 
+ 2
 
OH
-
2 
ROH
-
 
+
 
Cl
-
 
(hard
 
water)
 
R
2
Cl
-
 
+ 2
 
OH
-
 
H
+
 
+ O
H
-
 
H
2
O
Reactions occurring at Cation exchange
 
resin
Reactions occurring at Anion exchange
 
resin
At the end of the
 
process
 
5
 
5
Regeneration 
of 
ion exchange
 
resins
R
2
Ca
2+   
+ 
2H
+
 
(dil. HCl
 
(or)
 
H
2
SO
4
)
 
2 RH
+ 
+ 
Ca
2+ 
(CaCl
2
,
 
washings)
 
R
2
SO
4
2-   
+ 
2OH
-
 
(dil.
 
NaOH)
 
2 
ROH
- 
+ 
SO
4
2- 
(Na
2
SO
4
,
 
washings)
Advantages
1.
The 
process can be used 
to soften 
highly 
acidic or alkaline
 
waters
2.
It 
produces 
water 
of very 
low hardness 
of 
1-2ppm. So 
the 
treated
waters 
by 
this 
method can be used in high pressure
 
boilers
 
D
i
sa
d
va
n
ta
g
es
 
1.
The 
setup is costly and 
it 
uses 
costly
 
chemicals
2.
The 
water 
should not be turbid and 
the 
turbidity level should not be
 
more
than
 
10ppm
Regeneration 
of 
Cation 
exchange
 
resin
Regeneration 
of 
Anion 
exchange
 
resin
 
56
IV. 
Softening of water by 
Mixed 
Bed
 
deioniser
Description and process of 
mixed 
bed
 
deionizer
 
1.
It 
is 
a single cylindrical 
chamber 
containing a 
mixture 
of anion and cation exchange
resins
 
bed
2.
When 
the hard water is 
passed 
through this bed 
slowly 
the cations and anioins of
 
the
hard water 
comes 
in to contact with the 
two 
kind of 
resins many number 
of
 
times
3.
Hence, it 
is 
equivalent to 
passing 
the hard water 
many number 
of 
times 
through a
series of cation and anion exchange
 
resins.
4.
The 
soft water 
from this method contains less than 1ppm of 
dissolved salts 
and 
hence
more 
suitable for
 
boilers
 
deionizer
 
c
 
c 
Mixed
 
bed
 
c
 
 
                 
c
 
a
 
a
 
c
 
a
 
a
 
c
 
a
a
 
Anion
 
exchange
resin
Demineralised
water
 
Cation
 
exchange
resin
 
Hard
 
water
 
Mixed
 
resin
bed
 
15
 
57
 
Regeneration of 
mixed 
bed
 
deionizer
 
1.
 
When 
the bed 
(resins) 
are exhausted or cease to 
soften 
the 
water, 
the 
mixed 
bed 
is 
back
washed 
by forcing the water from the bottom in the upward
 
direction
 
2.
Then the light weight anion exchanger 
move 
to the top and 
forms 
a upper 
layer 
above
 
the
heavier cation
 
exchanger
3.
Then the anion exchanger is regenerated by 
passing 
caustic 
soda 
solution 
(NaOH) 
from
 
the
top and then rinsed 
with 
pure
 
water
4.
The lower cation exchanger bed 
is 
then 
washed with 
dil.H
2
SO
4 
solution and then
 
rinsed.
5.
The 
two 
beds are then 
mixed 
again by forcing 
compressed 
air to 
mix 
both and the 
resins
 
are
now ready for
 
use
 
deionizer
 
c
 
c
 
Mixed bed
 
c
 
c
 
a
 
a
 
a
 
c
 
a
 
a
 
c
 
a
 
Mixed
 
bed
 
c
 
Exhausted
 
c
 
c
 
c
 
a
 
a
 
a
 
c
 
a
 
a
 
c
 
a
Ba
c
k
w
a
s
h
w
a
ter
 
c 
c c 
c c
 
c
 
aa 
a a a
 
a
Back
 
washed
Co
m
pre
ss
ed
air
 
Reg
e
n
e
rat
e
d
Mixed bed
deionizer
 
c
 
c
 
c
 
a
 
c
 
a
a
 
c
 
a 
a 
c
 
a
 
Low
 
density
resin
 
High
den
s
i
ty
resin
 
NaOH
 
16
 
D
ESALINATION 
OF BRACKISH
 
WATER
 
Process of 
removing 
common 
salt 
from
 
water.
Brackish water: water containing dissolved salts with
 
a
peculiar salty
 
taste
Sea 
water : 3.5% salts.
1.Electrodialysis,
 
2.Reverse
 
osmosis.
 
58
 
E
LECTRO
 
DIALYSIS
 
Principle:
Electrodialysis 
is an 
electrochemical process 
whereby
electrically 
charged 
particles, 
ions, 
are transported 
from a
raw 
solution 
(retentate, diluate) into 
a 
more concentrated
solution 
(permeate, concentrate) through ion-selective
membranes 
by applying an electric
 
field.
 
59
 
60
 
17
 
T
HEORY 
OF
 
E
LECTRODIALYSIS
Electro dialysis chamber comprises 
of sheet 
like 
barriers 
made 
out 
of 
high-capacity,
highly 
cross-linked 
ion exchange resins that 
allow 
passage of ions 
but not 
of
 
water.
 
There are two 
types 
: (a) 
Cation 
exchange and (b) Anion exchange
 
membranes
 
Cation 
exchange 
membranes 
consists of an insoluble 
matrix 
and 
mobile
 
cation
reside in the pore space that allows the pass through of only
 
cations.
 
Anion exchange 
membranes 
consists of an insoluble 
matrix 
and 
mobile
 
anion
reside in the pore space that allows the pass through of only
 
anions.
 
Cation- and 
Anion- 
exchange 
membranes 
are 
installed alternatively 
in the
 
tank.
 
By 
impressing electricity 
on the electrodes, the positive anode 
attracts 
negative
 
ions
in solution, while the negative cathode 
attracts 
positive ions in the
 
solution.
61
 
Na+
 
:
 
negative
 
pole,
Cl- : positive
 
pole.
Concentration of brine decreases : central
 
compartment.
It increases: two side
 
compartment.
Desalinated brine :
 
removed
Concentrated brine : replaced by fresh
 
water.
Electrodialysis cell : consists of a 
large 
no. of rigid plastic
membrane.
Saline water is passed & electric field is
 
applied.
Positive 
charges 
: repel positive ions & 
permit 
negative
 
ion.
Negative
 
charges.
Water: 
deprived of its
 
salts,
Salt concentration : increases in adjacent
 
compartment.
 
62
 
Advantage:
1.
Most compact
 unit,
2.
Economical,
3.
If 
electricity is 
easily available, it 
is 
best
 
suited
 
63
 
R
EVERSE
 
OSMOSIS
:
 
When 
two 
solutions of unequal concentration are separated 
by a
semi-permeable membrane, 
flow 
of solvent takes 
place 
from dilute
to concentration side, 
due 
to increase 
in 
osmostic pressure, which 
is
termed as
 
osmosis.
 
However, 
when a 
hydrostatic pressure in excess of osmotic pressure
is applied 
on 
the concentrated side, the solvent flow is reversed 
from
concentrated side to dilute side, across 
the 
membrane. This principle
is termed as 
reverse
 
osmosis.
 
64
 
65
 
Reverse
 
Osmosis
 
18
 
The semi-permeable 
membrane 
(in reverse osmosis) is
selective in 
not 
permitting the passage of dissolved solute
particles such as molecules, ions, etc.) It permits only the flow
of water molecules (solvent) from the concentrated to dilute
side.
 
Cellulose acetate, polyamide, etc., are used as
 
membrane
 
Reverse osmosis process requires only mechanical force
 
to
generate the required 
hydrostatic
 
pressure.
 
66
 
P
OTABLE
 
WATER
 
Water 
: safe to
 
drink,
Fit 
for 
human consumption, 
should satisfy the 
following
requirements:
 
1.
Clear &
 
odourless,
2.
Pleasant in
 
taste,
3.
Perfectly
 
cool,
4.
Turbidity 
should not exceed
 
10ppm,
5.
Free 
from objectionable 
dissolved
 
gases
6.
Objectionable 
minerals: 
lead,arsenic, 
Mn,
 
Cr
7.
 
pH:8.0
8.
Reasonably
 
soft,
9.
Free 
from 
disease-producing
 
microorganisms.
 
67
 
B
REAK
-
POINT
 
CHLORINATION
 
Involves in addition of 
sufficient 
amt. 
of chlorine
 
to
oxidise 
:organic 
matter, 
reducing
 
substances.
Dosage of applied chlorine to 
water rich 
in
 
organic
compound or 
ammonia 
is gradually
 
increased.
Four
 
stages:
The addition of chlorine at the dip or break
 
:
’ breakpoint’
 
chlorination.
This 
indicates the 
point at which free 
residual
chlorine begins to
 
appear.
All 
tastes, 
odour 
disappear at 
break 
point : water
 
free
from bad 
tastes 
and
 
odours.
 
68
 
Persistent & 
powerful 
disinfection possessed by
available free chlorine, any type of pathogenic
 
organisms
present :
 
destroyed.
Advantages:
1.
Oxidises 
completely 
organic 
compound, ammonia
 
&
other reducing
 
compound.
2.
Removes
 
colour
3.
Removes 
disease producing
 
bacteria
4.
Removes 
odour &
 
taste.
 
69
 
D
E
-
CHLORINATION
 
Over-chlorination 
after the break point : produces unpleasant 
taste
 
&
odour
Removed 
by 
filtering 
the 
over-chlorinated 
water through a bed
 
of
molecular
 
carbon.
Activated C : added directly & after a short reaction period
 
:
removed by
 
filteration.
SO
2 
/ sodium sulphite / sodium
 
thiosulphate.
 
SO
2 
+ Cl
2 
+
 
2H
2
O
Na
2
SO
3 
+ Cl
2 
+
 
H
2
O
 
H
2
SO
4 
+ 2HCl
Na
2
SO
4 
+
 
2HCl
 
70
 
R
EFERENC
E
:
 
Image 
1:
 
http://postimg.org/image/9dskyjsvj/
Image 
2:
 
http://postimg.org/image/3kawku281/
Image 
3:
 
http://postimg.org/image/ritblpo01/
Image 
4: 
http://www.slideshare.net
Image
 
5:
http://www.slideshare.net
Image 
6:
 
http://postimg.org/image/rhjdsam69/
Image 
7:
 
http://postimg.org/image/d0w4dpwox/
Image 
8:
 
http://postimg.org/image/ficgyq5z5/
Image 
9: 
http://postimg.org/image/c7o3yug1t/
Image 
10:
 
http://postimg.org/image/a82wtowch/
 
71
 
Image 
11:
 
http://postimg.org/image/sc5xebu0x/
 
Image 
12:
 
http://postimg.org/image/51wcwe6yp/
Image 
13:
 
http://postimg.org/image/dnzmgziyp/
Image 
14:
 
http://www.slideshare.net
Image
 
15:
http://www.slideshare.net
Image
 
16:
http://www.slideshare.net
Image 17:
http://postimg.org/image/p1m5s6thd/
Image 
18:http://post
im
g.or
g
/image/mqetl9cbr/
 
72
Slide Note
Embed
Share

Water technology in the field of Engineering Chemistry explores sources of water, major impurities, hardness of water, and distinctions between hard and soft water. The content delves into the impact of mineral concentrations on soap lathering, categorizes hardness into temporary and permanent forms, and highlights the characteristics of different water sources. It also discusses the effects of dissolved substances like calcium, magnesium, and heavy metals on water quality.


Uploaded on Jul 29, 2024 | 1 Views


Download Presentation

Please find below an Image/Link to download the presentation.

The content on the website is provided AS IS for your information and personal use only. It may not be sold, licensed, or shared on other websites without obtaining consent from the author. Download presentation by click this link. If you encounter any issues during the download, it is possible that the publisher has removed the file from their server.

E N D

Presentation Transcript


  1. WATER TECHNOLOGY Course: B.Tech Subject: Engineering Chemistry Unit:4 1

  2. Sources of Water A) SurfaceWaters Rain Water - Pure but contaminated with gases River Water - High dissolved salts moderate organics Lake Water - Const. composition but high organics Sea Water - High salinity, pathogens, organics B) UndergroundWaters Spring/Well Water - Crystal clear but high dissolved salts and high purity from organics 2

  3. MAJOR IMPURITIES OF WATER Ionic anddissolved Cationic Calcium Magnesium Anionic Bicarbonate Carbonate Hydroxide Nonionic and undissolved Turbidity, silt, mud, dirt and other suspendedmatter Gases CO2 H2S NH3 CH4 O2 Alkalinity Sodium Potassium Ammonium Iron Manganese Color,Plankton Organicmatter, Sulfate ChlorideColloidal silica, Nitrate Phosphate Microorganisms, Bacteria 3

  4. HARD WATER Hardness refers to the presence of calcium and magnesium ions in water (and sometimes iron). Ions come from dissolved rock the water has passed through. Affects properties of tap water 1

  5. HARD WATER Minerals in hard water interact with soap. Interferes with soap s ability to lather. 2

  6. SOFTWATER Water with very low concentrations of minerals. Soap lathers easily and is sometimes difficult to rinse off. 3

  7. HARDNESS OF WATER Hardness in Water is characteristic that prevents the lathering of soap thus water which does not produce lather with soap solution readily, but forms a white curd is called hard water. Type of Hardness Temporary or Carbonate Hardness Permanent Hardness or non-carbonate Hardness. 7

  8. Temporary Hardness Temporary Hardness is caused by the presence of dissolved bicarbonateof calcium, magnesium and other heavy metals and the carbonate of iron. It is mostly destroyed by more boiling of water, when bicarbonates are decomposed yielding insoluble carbonates. Ca(HCO3)2 Calcium bicarbonate Heat CaCO3 + H2O + CO2 Calcium Carbonate Mg(HCO3)2 Magnesium Bicarbonate Heat Mg(OH)2 + 2CO2 Magnesium hydroxide Calcium/Magnesium Carbonates thus formed being almost insoluble, are deposited as a scale at the bottom of vessel, while carbon dioxide escapes out. 8

  9. PERMANENT HARDNESS Non Carbonate Hardness is due to the presence of chlorides, sulfatesof calcium, Magnesium, iron and other heavymetals (C17H35COO)2Ca + 2NaCl Calcium stearate 2C17H35COONa + CaCl2 Sodiumstearate Hardness (sodium soap) (Insoluble) 2C17H35COONa + MgSO4 (C17H35COO)2Mg + 2Na2SO4 Magnesium stearate Sodiumstearate Hardness (Insoluble) (sodium soap) 9

  10. Draw backs (or) Disadvantages of Hard Water Domestic Use Industrial Use 1. Washing 1. Textile Industry 2. Sugar Industry 2. Bathing 3. Dyeing Industry 3. Drinking 4. Paper Industry 4. Cooking 5. Pharmaceutical Industry The fabric/cloth and gives spots and streaks. Fe salts stain the cloths. sticky precipitate adheres on the 6. In Steam generation in Boilers Produces sticky scum on the bath tub and the body Bad to the digestive system and calcium oxalate formation is possible in urinary tracts Requires more fuel and time. Certains food don t cook soft and also gives unpleasant taste 10

  11. Boiler troubles due to Hard Water 1. Sludge 1. Scale and Sludge Slimy loose precipitatecalled sludge suspended inwater 2. Caustic embitterment 3. Priming and Foaming water 4. Boiler corrosion 4 Boilerwall Sludge is a soft, loose and slimy precipitate formed within the boiler. It can be easily scrapped off with a wire brush. It is formed at comparatively colder portions of the boiler and collects in areas of the system, where the flow rate is slow or at bends. It is formed by substances which have greater solubility's in hot water than in cold water, e.g. MgCO3, MgCl2, CaCl2, MgSO4etc., 11 Remedy: Sludges can be removed using wire brush or mild acid

  12. DISADVANTAGE OF SLUDGE FORMATION: -poor conductor of heat -disturbs the working ofboiler. - If along with scales : both get deposited asscales. Preventation of Sludge formation: - Well softened water - Drawing off a portion of concentratedwater. 12

  13. 1. Scale Hard adherent coating on inner walls of boiler water Boiler wall 6 5 Scales are hard substances which sticks very firmly to theinner surfaces of the boiler wall. Scales are difficult to remove even with the help of a hammer and chisel. 13 Examples: CaSO4, CaCO3, Mg(OH)2

  14. Reasons for formation of scale 1. Presence of Ca(HCO3)2 in low pressureboilers Low pressure boilers but in high pressure boilers it is soluble by forming Ca(OH)2 Ca(HCO3)2 Calcium bicarbonate CaCO3 + H2O + CO2 Calcium Carbonate (scale) 2. Presence of CaSO4 in high pressureboilers ToC Solubility of CaSO4 4. Presence of SiO2 15 3200 ppm 230 15ppm It forms insoluble hard adherent CaSiO3 and MgSiO3 asscales 320 27ppm Cold water soluble Super heated water Insoluble (scale) 3. Presence of MgCl2 in high temperatureboilers MgCl2 + 2H2O Magnesium chloride Mg (OH)2 + 2HCl scale Mg(OH)2 can also be generated by thermally decomposing Mg(HCO3)2 14

  15. Disadvantages of scale formation 1. Fuel wastage scales have low thermal conductivity 2. Degradation of boiler material and increases of risk of accident 3. Reduces the efficiency of the boiler and- deposit on the valves andcondensers 4. The boiler may explode if crack occurs in scale Remedies: Removal of scale 1. Using scrapper, wire brush often 2. By thermal shock- heating and cooling suddenly with cold water 3. Using chemicals 5-10% HCl and by adding EDTA 15

  16. Prevention of scale formation Scale formation can be prevented by two methods 1. Internal conditioning or InternalTreatment 2. External conditioning or External treatment- will be discussedlater 1. Internal conditioning methods - of boiler water to prevent scaleformation 1. Phosphate conditioning addition of phosphate compound 2. Carbonate conditioning addition of carbonate compound 3. Calgon conditioning addition of sodium hexa meta phosphate 4. Colloidal conditioning spreading of organic compounds like tannin, agar gel 5. Sodium Aluminate removes oil and silica 6. Electrical Conditioning 7. Radioactive Conditioning 16 8. Complexometric method using EDTA

  17. 1. Phosphate conditioning Scale formation can be prevented by adding sodium phosphate to the boiler water which reacts with the hardness producing ions and forms easily removable phosphate salts of respectiveions 3CaCl2 (Boiler water) + 2 Na3PO4 Ca3(PO4)2 + 6NaCl 17

  18. Selection of Phosphate compound Calcium can not be precipitated below a pH = 9.5, hence the selectionof phosphate has to be based on the pH of the boiler feedwater. NaH2PO4 (acidic in nature), Na2HPO4 (weakly alkaline in nature), Na3PO4(Alkaline in nature) 2. Carbonate conditioning CaSO4 (Boiler water) + Na2CO3 CaCO3+ Na2SO4 Caution: Excess Na2CO3 can result in causticembrittlement 18

  19. 3. Calgon conditioning Na2[Na4(PO3)6 2Na+ + [Na4P6O18]2- Calgon sodium hexa meta phosphate [Ca2P6O18]2- +2Na2SO4 2CaSO4 (Boiler water) + [Na4P6O18]2- Calcium sulfate Soluble complexion of calcium - can be removed easily Calgon tablets are used in the cleaning of washing machinedrums 19

  20. 4.Colloidal conditioning: In low pressure boilers- scale : adding org. substances like kerosine, agar-agar, tannin,etc. - Yield non-sticky & loose deposits 5. Treatment withNaAlO2: NaAlO2 +2H2O MgCl2 +2NaOH NaOH + Al(OH)3(gelatinousppt) Mg(OH)2 +2NaCl 20

  21. 6.Electrical conditioning: Sealed glass bulbs-mercury: battery Water boils- mercury bulb - emit electrical discharged 7. Radioactive conditioning: Tablets - radioactive salts 8. Complexometric method: - 1.5% alk. Soln of EDTA - EDTA : cation & form stable & solublecomplex. 21

  22. Other treatment: Prevent : iron oxide Protects boiler unit from corrosion by wetsteam. Reduces the carry over of oxides with steam 1. 2. 3. 22

  23. CAUSTIC EMBRITTLEMENT: Is a type of boilercorrosion. Caused : highly alkalineWater. Na2CO3 + H2O 2 NaOH +CO2 Boiler water becomes caustic Water evaporates caustic soda concentrationincrease. which attacks surrounding area of boilermachine. Iron converted in to sodiumferroate. Causes embrittlement of boiler parts, strssed parts( bends, joints,etc.) Iron : in dilute NaOH cathodicside Iron : in concentrated NaOH anodicside. 23

  24. Preventation of caustic Embrittlements: Sodium phosphate sodiumcarbonates Adding lignin : preventing infilteration of caustic sodasolution Sodium sulphate: Na2SO4concentration NaOH concentration Kept 1:1, 2:1 & 3:1, Pressure : 10,20 & above20. 24

  25. IV. Boiler corrosion Degradation or destruction of boiler materials (Fe) due to the chemical or electrochemical attack of dissolved gases or salts is called boilercorrosion Boiler corrosion is of threetypes 1. Corrosion due to dissolvedO2 2. Corrosion due to dissolvedCO2 3. Corrosion due to acids formed by dissolvedsalts 1. Corrosion due to dissolved oxygen(DO) 2 Fe + 2 H2O +O2 2 Fe(OH)2 4 Fe(OH)2 + O Ferrous hydroxide 2 [Fe2O3.2H2O] 2 Rust

  26. Removal of Dissolved Oxygen (DO) 1. By the addition of chemicals The dissolved oxygen present in the boiler feed water can be removed bythe addition of sodium sulphite or hydrazine and the reactions can be writtenas below 2 Na2SO3 + O2 2 Na2SO4 Sodium sulphite Na2S + 2O2 Na2SO4 Sodium sulphate DO N2+ 2H2O N2H4 +O2 Nitrogen Hydrazine

  27. 2. Corrosion due to dissolvedCO2 Presence of bicarbonate salts of either magnesium or calcium also causes therelease of CO2 inside the boiler apart from the dissolvedCO2 Mg(HCO3)2 MgCO3 + H2O +CO2 CO2+ H2O H2CO3 (causes slowcorrosion) 3. Corrosion due to dissolvedsalts MgCl2 + 2 H2O Mg(OH)2 + 2HCl Fe + 2 HCl FeCl2+ H2 FeCl2+ 2 H2O Fe(OH)2 + 2HCl

  28. Prevention : (1) Corrosion can be prevented by adding alkali to neutralizeacidity & anti-oxidant to remove oxygen (2) By keeping pH value 8 to 9. (3) Oxygen is removed from boiler feed-water by addingNa2SO3. (4) Oxygen can also removed by treating it with hydrazinehydrate NH2 NH2 NH2-NH2 + O2 2N2 + 2H2O 28

  29. III. Priming and foaming Foaming It is the production of continuous foam or hard bubblers in boilers. Foaming is due to the presence of substance like oil in boiling water. Priming Foaming Normalbubble It is the process in which some particles in water are carried along with the steam. process is called as wet steam or carry over. The process of formation of wet steam in boilers is called as priming. The resulting Priming Carry overbubble 7

  30. Causes of Priming (1) the presence of large amount of dissolvedsolids (2) high steamvelocities. (3) sudden boiling (4) improper boiler design (5) sudden increase in steam-productionrate. Disadvantages of Priming (1) Dissolved salt in boiler water are carried out by the wet steam toturbine blades - which reduces theirefficiency. (2) Dissolved salts may enter the parts of other machinery may decreasethe life of the machinery. (3) Actual height of the water column cannot be judge properly,Thereby making the maintenance of the boiler pressure becomesdifficult. 30

  31. Prevention of Priming (1) By improving boiler design. (2) By fitting mechanical steam purifiers. (3) By maintaining low water level in boilers (4) By using soft water. (5) By decreasing the amount of dissolved salts. ( B) FOAMING : It is the production of foam or bubbles in boiler which do not break easily. Causes of Foaming : It is due to the presence of oily substances in water. (1) Low level of water in boiler. (2) The presence of dissolved salts in water. (3) Sudden increase in steam production rate. 31

  32. Disadvantages of foaming: (1) Actual height of the water column cannot bejudge. (2) Dissolved salts in water carried by the wet steam maydamage turbine blads or machineryparts. (3) Boiler pressure cannot bemaintained. Prevention of Foaming : (1) By the addition of anti-foaming chemicals like castor oil,Gallic acid, tennic acid etc. (2) removing oil from boiler water by adding compoundslike sodium aluminate. 32

  33. II . Softening of water/ External treatment of water External Conditioning of water Softening of hard water can be done by the following methods 1. Lime soda process 2. Zeolite methods 3. Ion exchange resin method 4. Mixed bed deionizermethod 1. Lime soda process It is a process in which Lime (Ca(OH)2) and soda (Na2CO3) are added to the hard water to convert the soluble calcium and magnesium salts to insoluble compounds by a chemical reaction. The CaCO3and Mg(OH)2so precipitated are filtered off and removedeasily. It is further divided in to twotypes 1. Cold lime soda process 2. Hot lime soda process

  34. 1. Cold lime soda process Step 1 In this process a calculated quantity of Ca(OH)2(lime) and Na2CO3(soda) are mixed with water at room temperature and added to the hard water. The following reactions takes place depending on the nature ofhardness Chemical reactions If it is permanent hardness and due to calciumsalt Ca2++ Na2CO3 CaCO3 + 2Na+(soda) slimy suspended precipitate If it is due to Magnesiumsalt Mg2++ Ca(OH)2 Mg(OH)2 + Ca2+(lime) slimy suspended precipitate Ca2++ Na2CO3 CaCO3 + 2Na+(soda) slimy suspended precipitate

  35. Chemical reactions contd.. If it is Temporary hardness and due to calciumsalt Ca(HCO3)2+ Ca(OH)2 2CaCO3 + 2H2O slimy suspended precipitate If it is due to Magnesiumsalt Mg(HCO3)2 +2Ca(OH)2 2CaCO3 + Mg(OH)2 +2H2O slimy suspended precipitates Step 2 The precipitates CaCO3and Mg(OH)2are very fine and forms sludge like precipitates in the boiler water and are difficult to remove because it does not settle easily making it difficult to filter and the removal process. Finally reduces the efficiency of theboiler. Therefore, it is essential to add small amount of coagulant (such as Alum, Aluminium sulfate, sodium aluminate etc) which hydrolyses to flocculent precipitate ofAl(OH)3which entraps the fineprecipitates. 35

  36. 36 8

  37. When coagulants are added flocculation takes place followed by the formationof flocculants. Flocculent- Gelatinous precipitate which entraps the fine precipitates of CaCO3 and Mg(OH)2 NaAlO2 + 2H2O NaOH +Al(OH)3 Coagulant Al2(SO4)3 + 3 Ca(HCO3)2 Hard water sample 2Al(OH)3 + CaSO4 +CO2 Aluminium sulfate Flocculent- Gelatinous precipitate which entraps the fine precipitates of CaCO3 and Mg(OH)2 The Al(OH)3formed by the addition of coagulants initiates the process of flocculation and entraps the fine precipitates and becomes heavy. The heavier flocs then settles at the bottom and filtered off easily.

  38. Method : Raw water & calculated quantities of chemicals: from the top in to the inner vertical circular chamber. There is a vigorous stirring & continuous mixing : softening of water take place. Softened water comes into the outer co-axial chamber, it rises upwards. Heavy sludge settles down & softened water reaches up. Softened water : passes through a filtering media to ensure complete removal of sludge. Filtered soft water finally flows out continuously through the outlet at the top. Sludge settling at the bottom of the outer chamber is drawn off occasionally. 38

  39. (ii) Hot lime-sodaprocess: Treating water with softening chemicals at a temp. of 80 to150 c. Process : operated close to the boilingpoint. consists three parts: (a)reaction tank: in which raw water, chemicals & steam aremixed, (b)Conical sedimentation vessel : sludge settlesdown. (c)Sand filter : complete removal of sludge from softenedwater. 39

  40. 40 9

  41. Advantages of hot lime-soda process: The precipitation reaction is almost complete, Reaction takes place faster, Sludge settles down rapidly; No coagulant is needed, Dissolved gases (which may cause corrosion) are removed, Viscosity of soft water is lower, hence filtered easily, Residual hardness is low compared to cold lime-soda process. 41

  42. Advantages of Lime soda process: 1.Economical 2.Process increses pH value of the treated water, thereby corrosion of the distribution pipes is reduced. 3.Mineral content of water is reduced 4.pH of water raises thus reducing content of pathogenic bacteria 5. iron & Mn: removed. Disadvantages of Lime soda process: 1.Huge amount of sludge is formed and its disposal is difficult 2.Due to residual hardness, water is not suitable for high pressure boilers 42

  43. 2. ZEOLITE OR PERMUTIT PROCESS: Zeolite - hydrated sodium alumino silicate, capable of exchanging reversibly its sodium ions for hardness- producing ions in water. The general chemical structure of Zeolite: Na2O.Al2O3.xSiO2.yH2O (x = 2-10 and y = 2-6) Micro pores of Zeolite 43 10 Porous structure of Zeolite

  44. Twotypes: (i) Natural Zeolite: non-porous. e.g., Natrolite (ii) Synthetic zeolite: porous & possess gel structure. They are prepared by heating together china clay, feldspar and soda ash. Zeolites possess higher exchange capacity per unit weight than natural zeolite. 44

  45. Process: Hard water is percolated at a specified rate through a bed of zeolite, kept in a cylinder. The hardness-causing ions are retained by the zeolite as CaZeand MgZe; while the outgoing water contains sodium salts. To remove temporary hardness Na2Ze + Ca(HCO3)2 Na2Ze + Mg(HCO3)2 CaZe + 2NaHCO3 MgZe + 2NaHCO3 To remove permanent hardness Na2Ze + CaCl2 Na2Ze + MgCl2 CaZe + 2NaCl MgZe + 2NaCl 45

  46. 4611

  47. REGENERATION OF ZEOLITE: At this stage, the supply of hard water is stopped and exhausted zeolite is reclaimed by treating the bed with concentrated NaCl solution. CaZe or MgZe (exhausted zeolite) + 2NaCl (Brine) Na2Ze + CaCl2 (Reclaimed zeolite) (washings) 47

  48. LIMITATIONS OF ZEOLITE PROCESS: 1. If the water is turbid ---- then the turbidity causing particles clogs the pores of the Zeolite and making it inactive 2. The ions such as Mn2+ and Fe2+ forms stable complex Zeolite which can not be regenerated that easily as both metal ions bind strongly and irreversibly to the zeolite structure. 3. Any acid present in water (acidic water) should be neutralized with soda before admitting the water to the plant, since acid will hydrolyze SiO2 forming silicicacid 48

  49. ADVANTAGES: Residual hardness of water is about 10 ppmonly Equipment is small and easy to handle Time required for softening of water issmall No sludge formation and the process isclean Zeolite can be regenerated easily using brinesolution Any type of hardness can be removed without any modifications to the process. DISADVANTAGES : 1. Soft water contains more sodium salts than in lime soda process 2. It replaces only Ca2+ and Mg2+ with Na+ but leaves all the other ions like HCO -and CO 2-in the softened water (then it may form NaHCO and Na CO which releasesCO2 3 when the water is boiled and causes corrosion) 3 3 2 3 3. It also causes caustic embitterment when sodium carbonate hydrolyses to give NaOH 49

  50. ION EXCHANGE PROCESS: Ion exchange resins are insoluble, cross linked, long chain organic polymers with a microporous structure, and the functional groups attached to the chain is responsible for the ion-exchange properties. Acidic functional groups (-COOH, -SO3H, etc.) exchange H+ with other cations. Basic functional groups (-NH2=NH etc.) exchange OH- with other anions. 50

Related


More Related Content

giItT1WQy@!-/#giItT1WQy@!-/#giItT1WQy@!-/#giItT1WQy@!-/#giItT1WQy@!-/#