WATER

1Q. Briefly explain temporary and permanent hardness of water.

Ans. Temporary Hardness:

• Caused by the presence of dissolved bicarbonates of calcium (Ca(HCO₃)₂) and

magnesium (Mg(HCO₃)₂).
• Can be removed by boiling the water, which decomposes the bicarbonates into
insoluble carbonates that precipitate out.
• Also known as carbonate hardness.

Permanent Hardness:

• Caused by the presence of dissolved chlorides and sulfates of calcium (CaCl₂,

CaSO₄) and magnesium (MgCl₂, MgSO₄).
• Cannot be removed by simple boiling.
• Requires specific chemical treatments like adding washing soda, using ion exchange
resins, or the Calgon process to remove the dissolved salts.
• Also known as non-carbonate hardness.

2Q. Explain the terms Priming and foaming

Ans. Priming:

• Priming is the carryover of water droplets along with steam from a boiler. This
results in "wet steam" containing liquid water particles.
• Causes:
o High water level in the boiler.
o Sudden or rapid boiling of water.
o Presence of dissolved solids and impurities in the boiler water.
o Improper boiler design or overloading.
• Effects:
o Reduces the efficiency of the steam.
o Can cause damage to turbines and other downstream equipment due to water
impingement.
o Leads to scale formation and deposits in superheaters and pipelines.
• Prevention:
o Maintaining the correct water level in the boiler.
o Avoiding sudden increases in steam demand.
o Ensuring proper boiler design with adequate steam separation space.

Foaming:

• Foaming is the formation of a stable layer of bubbles or froth on the surface of the
boiler water. These bubbles do not break easily.
• Causes:
o High concentration of dissolved solids in the boiler water.
o Presence of oily or greasy substances, soaps, or certain organic matter in the
water, which reduce surface tension.
 o High alkalinity of the boiler water.
o Suspended impurities in the water.
• Effects:
o Interferes with the accurate reading of the water level gauge, leading to
potential safety hazards.
o Reduces heat transfer efficiency.
o Can lead to the carryover of impurities and dissolved salts into the steam.
• Prevention:
o Maintaining low concentrations of dissolved and suspended solids through
regular blowdown.
o Using oil separators and ensuring oil does not contaminate the feedwater.
o Controlling the alkalinity of the boiler water.
o Adding anti-foaming agents (defoamers) to break the surface tension of the
bubbles.

3Q. What is caustic embrittlement? Explain its causes and prevention.

Ans. Caustic embrittlement is a type of corrosion that occurs in boilers, particularly in

riveted or stressed areas of mild steel, due to the buildup of concentrated alkaline (caustic)
substances, primarily sodium hydroxide (NaOH). It leads to the metal becoming brittle and
prone to cracking, potentially causing catastrophic boiler failures. It is also known as caustic
cracking or a form of stress corrosion cracking.

Causes of Caustic Embrittlement:

1. Presence of Concentrated Hydroxide: In high-pressure boilers, sodium carbonate

(Na₂CO₃) used for water softening can decompose to form sodium hydroxide
(NaOH):

Na₂CO₃ + H₂O → 2NaOH + CO₂

As water evaporates to form steam, the concentration of NaOH in the remaining

boiler water increases.

2. Localized Concentration in Stressed Areas: Alkaline water enters minute cracks

and crevices, particularly around rivets, bends, joints, and other highly stressed areas,
through capillary action. The water then evaporates, leaving behind a highly
concentrated solution of NaOH within these confined spaces.
3. Susceptible Material: Mild steel, commonly used in boiler construction, is
susceptible to attack by concentrated alkali at elevated temperatures.
4. Tensile Stress: The presence of tensile stress in the boiler material, often residual
stress from fabrication (like riveting or welding) or operational stress due to pressure,
accelerates the cracking process. The concentrated NaOH attacks the grain boundaries
of the stressed steel.
5. Formation of Sodium Ferrate: The concentrated NaOH reacts with the iron in the
steel to form sodium ferrate (Na₂FeO₂), which is soluble in water. This corrosive
action weakens the metal structure, leading to embrittlement and cracking.
6. Fe + 2NaOH → Na₂FeO₂ + H₂
Prevention of Caustic Embrittlement:

1. Using Sodium Phosphate as Softening Agent: Instead of sodium carbonate, using

sodium phosphate (Na₃PO₄) for water softening is preferred. It doesn't readily
decompose to form high concentrations of NaOH.
2. Maintaining Proper pH Control: Carefully monitoring and controlling the pH of the
boiler water within a safe range can help prevent excessive alkalinity.
3. Adding Inhibitors: Substances like tannin, lignin, or sodium sulfate (Na₂SO₄) can be
added to the boiler water. These substances can block the hairline cracks, preventing
the infiltration and concentration of NaOH in stressed areas. Sodium sulfate should be
maintained in a specific ratio with the NaOH in the boiler water to inhibit caustic
embrittlement. A common guideline is to maintain a Na₂SO₄ to NaOH ratio of 2:1 or
3:1 in low-pressure boilers and higher ratios in high-pressure boilers.
4. Stress Relieving: Reducing residual stresses in boiler components through proper
fabrication techniques and post-weld heat treatment can minimize the susceptibility to
cracking.
5. Avoiding High Localized Heat Fluxes: Preventing excessive localized heating can
reduce the rate of evaporation and concentration of caustic substances in specific
areas.

4Q. Explain the EDTA method to estimate the total hardness of water.

Ans. The EDTA (Ethylenediaminetetraacetic acid) method is a widely used titrimetric

method to determine the total hardness of water, which is primarily due to calcium
(Ca²⁺) and magnesium (Mg²⁺) ions. The method is based on the formation of
stable chelate complexes between EDTA and metal ions at a suitable pH.

Principle

• EDTA forms a 1:1 stable, colorless complex with Ca²⁺ and Mg²⁺ ions in water.
• The titration is performed at pH ≈ 10 (using NH₄Cl-NH₄OH buffer) to ensure all
metal ions react with EDTA.
• Eriochrome Black T (EBT) is used as an indicator, which forms a wine-red
complex with Ca²⁺ and Mg²⁺ ions.
• When EDTA is added, it displaces the indicator from the metal-EDTA complex,
turning the solution from wine-red to pure blue at the endpoint.

Reactions Involved

1. With metal ions (Ca²⁺/Mg²⁺):

M2++EBT→M-EBT (Wine-red)M2++EBT→M-EBT (Wine-red)

 2. With EDTA:
M-EBT+EDTA→M-EDTA+EBT (Blue)

Procedure

Materials Required:
• Standard EDTA solution (~0.01 M)
• Buffer solution (NH₄Cl + NH₄OH, pH ≈ 10)
• Eriochrome Black T (EBT) indicator
• Water sample (50 or 100 mL)
• Burette, conical flask, pipette

Steps:
1. Take 50 mL of the water sample in a conical flask.
2. Add 1-2 mL of buffer solution (to maintain pH ≈ 10).
3. Add a few drops of EBT indicator (solution turns wine-red due to Ca²⁺/Mg²⁺-EBT
complex).
4. Titrate with standard EDTA solution until the color changes from wine-red to
blue (endpoint).
5. Note the volume of EDTA used (V₁ mL).
6. Repeat the titration for concordant values.

Calculation

Total Hardness (as CaCO₃ in mg/L):

Total Hardness=V1×M×1000×100Volume of water sample (mL)Total Hardn
ess=Volume of water sample (mL)V1×M×1000×100
• V₁ = Volume of EDTA used (mL)
• M = Molarity of EDTA solution
• 100 = Molecular weight of CaCO₃ (g/mol)
5Q. Explain - (water softening by internal treatment)

(1)carbonate and Phosphate conditioning.

(2) Calgon Conditioning:

Ans. (1) Carbonate Conditioning:

• Principle: This method is primarily used in low-pressure boilers. It involves adding

sodium carbonate (Na₂CO₃) directly to the boiler water. The carbonate ions (CO₃²⁻)
react with the calcium sulfate (CaSO₄), a common scale-forming impurity, to
precipitate it as calcium carbonate (CaCO₃), and form soluble sodium sulfate
(Na₂SO₄).
• CaSO₄ (aq) + Na₂CO₃ (aq) → CaCO₃ (s) ↓ + Na₂SO₄ (aq)

Calcium carbonate forms a soft, loose sludge rather than a hard, adherent scale,
which is easier to remove through periodic blowdown of the boiler.

• Advantages:
o Simple and relatively inexpensive.
o Converts hard scale-forming calcium sulfate into a softer sludge.
• Limitations:
o Not suitable for high-pressure boilers: At higher temperatures and
pressures, sodium carbonate can hydrolyze to form sodium hydroxide
(NaOH):
o Na₂CO₃ + H₂O → 2NaOH + CO₂

The increased concentration of NaOH can lead to a serious problem called caustic
embrittlement, where the boiler metal becomes brittle and cracks, especially in stressed
areas. * Calcium carbonate scale can still form, especially from the decomposition of calcium
bicarbonate. * May not be very effective for removing magnesium hardness.

(2) Phosphate Conditioning:

• Principle: This method is commonly used in high-pressure boilers to prevent the

formation of calcium and magnesium scales. It involves adding sodium phosphates
(such as trisodium phosphate (Na₃PO₄), disodium hydrogen phosphate (Na₂HPO₄), or
sodium dihydrogen phosphate (NaH₂PO₄)) to the boiler water. The phosphate ions
(PO₄³⁻) react with calcium and magnesium ions to form insoluble calcium phosphate
[Ca₃(PO₄)₂] and magnesium phosphate [Mg₃(PO₄)₂] sludges.
• 3Ca²⁺ (aq) + 2PO₄³⁻ (aq) → Ca₃(PO₄)₂ (s) ↓
• 3Mg²⁺ (aq) + 2PO₄³⁻ (aq) → Mg₃(PO₄)₂ (s) ↓

These phosphate sludges are soft and non-adherent and can be easily removed by
blowdown.

• Advantages:
o Effective in preventing both calcium and magnesium sulfate and carbonate
scales.
 o The type of phosphate used can be adjusted based on the alkalinity of the
boiler water to achieve optimal results.
o Helps in maintaining a suitable pH range in the boiler water, which can aid in
corrosion control.
o Can help prevent caustic embrittlement if properly controlled, as it can react
with any free hydroxide ions. For instance, disodium phosphate can react with
excess caustic to form trisodium phosphate.
• Limitations:
o Requires careful monitoring and control of phosphate dosage and boiler water
pH.
o If alkalinity is not properly maintained, magnesium may precipitate as
magnesium hydroxide or even adherent magnesium phosphate.
o Can lead to the formation of phosphate sludge, requiring regular and adequate
blowdown.
o At very high temperatures, phosphate can sometimes undergo "hide-out,"
depositing on heat transfer surfaces and potentially leading to corrosion under
the deposit.

(3) Calgon conditioning: is an internal boiler water treatment method used to prevent scale
formation, particularly calcium sulfate scale. It involves adding sodium hexametaphosphate
(Na₆P₆O₁), commercially known as Calgon, to the boiler water.

Mechanism:

Calgon works by a process called sequestration. The polyphosphate ions in Calgon react
with the calcium and magnesium ions (which cause hardness) to form stable, soluble
complex ions. This prevents these hardness-causing ions from reacting with sulfate,
carbonate, or silicate ions present in the water to form insoluble scales on the boiler surfaces.

For example, with calcium sulfate:

2 CaSO₄ (aq) + Na₂[Na₄(PO₃)₆] (aq) → Na₂[Ca₂(PO₃)₆] (aq) + 2 Na₂SO₄ (aq)

The Na₂[Ca₂(PO₃)₆] complex is soluble and thus prevents the precipitation of hard calcium
sulfate scale.

Advantages:

• Effective in preventing scale formation, especially calcium sulfate.

• The formed complex is soluble, so there is no sludge formation issue.
• Can help in gradually removing existing scales.

Limitations:

• At high temperatures, Calgon can hydrolyze to form orthophosphates, which might

lead to calcium phosphate sludge if phosphate conditioning is not properly managed.
• Less effective against silica scales.
• Can be more expensive than other conditioning methods.
6Q. Why phosphate conditioning is preferred over carbonate conditioning?
Ans. • Suitable for High-Pressure Boilers: Carbonate conditioning risks caustic
embrittlement at high temperatures and pressures. Phosphate conditioning is safer in these
conditions.
• Effective Against Both Ca and Mg: Phosphate precipitates both calcium and magnesium
as soft sludges. Carbonate primarily targets calcium sulfate.
• Better Scale Prevention: Phosphate generally forms less adherent and more easily
removable sludges compared to potential CaCO₃ scale in carbonate conditioning.
• pH Buffering: Phosphate systems can help buffer the boiler water pH, contributing to
corrosion control.
• Reduced Caustic Embrittlement Risk (with control): While high alkalinity is still a
concern, controlled phosphate treatment doesn't inherently generate high caustic
concentrations like carbonate hydrolysis.

OR
Why calogon conditioning better than phosphate conditioning ?
Ans. • Scale Dispersion: Calgon doesn't just precipitate hardness; it forms soluble
complexes with calcium and magnesium ions, effectively dispersing them and preventing
them from forming hard, adherent scales. This is particularly useful in systems where sludge
removal might be less efficient.
• Iron and Manganese Sequestration: Calgon can also help prevent the deposition of iron
and manganese oxides, which can cause fouling and corrosion in boiler systems. Phosphates
are less effective at this.
• Lower Sludge Formation: Since Calgon primarily disperses hardness rather than
precipitating it, the volume of sludge produced is significantly lower compared to phosphate
conditioning. This can be advantageous in smaller boilers where frequent blowdown might be
impractical.
• Prevention of After-Precipitation: In systems where softened water still contains trace
amounts of hardness, Calgon can prevent "after-precipitation" of calcium carbonate in the
boiler due to temperature increases.
7Q. Explain the following:- (water softening by external treatment)

(1))Lime soda process (2)Zeolite Softening Process of Water (3)Ion-exchange Resin

Ans. (1) Lime-Soda Process:

• Principle: This method removes both temporary (carbonate) and permanent (non-
carbonate) hardness by adding lime (calcium hydroxide, Ca(OH)₂) and soda ash
(sodium carbonate, Na₂CO₃) to the water.
• Reactions:
o Removal of Temporary Hardness (Calcium Bicarbonate):
o Ca(HCO₃)₂ (aq) + Ca(OH)₂ (s) → 2CaCO₃ (s) ↓ + 2H₂O (l)
o Removal of Temporary Hardness (Magnesium Bicarbonate):
o Mg(HCO₃)₂ (aq) + Ca(OH)₂ (s) → Mg(OH)₂ (s) ↓ + CaCO₃ (s) ↓ + 2H₂O
(l)
o Removal of Permanent Hardness (Calcium Sulfate/Chloride):
o CaSO₄ (aq) + Na₂CO₃ (aq) → CaCO₃ (s) ↓ + Na₂SO₄ (aq)
o CaCl₂ (aq) + Na₂CO₃ (aq) → CaCO₃ (s) ↓ + 2NaCl (aq)
o Removal of Permanent Hardness (Magnesium Sulfate/Chloride):
o MgSO₄ (aq) + Ca(OH)₂ (s) → Mg(OH)₂ (s) ↓ + CaSO₄ (aq)
o MgCl₂ (aq) + Ca(OH)₂ (s) → Mg(OH)₂ (s) ↓ + CaCl₂ (aq)

The formed CaSO₄ and CaCl₂ are then removed by soda ash as shown above.

• Process: The calculated amounts of lime and soda ash are added to the hard water in
a reaction tank. The precipitates of CaCO₃ and Mg(OH)₂ settle down and are removed
by sedimentation and filtration. The treated water is then collected.
• Advantages:
o Cost-effective for treating large volumes of hard water.
o Can remove both temporary and permanent hardness.
o Can also reduce the levels of iron, manganese, and some microorganisms.
• Disadvantages:
o Produces a large volume of sludge, requiring disposal.
o Requires careful calculation of chemical dosages based on water analysis.
o Treated water may have high alkalinity, requiring further treatment in some
cases.

(2) Zeolite Softening Process of Water:

• Principle: This is an ion exchange process where hard water is passed through a bed
of zeolite (natural or synthetic hydrated sodium aluminum silicate, represented as
Na₂Ze). The calcium and magnesium ions in the hard water are exchanged for sodium
ions present in the zeolite.
• Reactions:
• Ca²⁺ (aq) + Na₂Ze (s) → CaZe (s) + 2Na⁺ (aq)
• Mg²⁺ (aq) + Na₂Ze (s) → MgZe (s) + 2Na⁺ (aq)

The zeolite retains the calcium and magnesium ions, and the outgoing water is
softened (rich in sodium ions).
 • Process: Hard water flows through a packed bed of zeolite granules. When the zeolite
becomes saturated with calcium and magnesium ions (it can no longer release sodium
ions), it is regenerated by passing a concentrated brine solution (NaCl) through the
bed. The high concentration of sodium ions reverses the exchange process, displacing
the calcium and magnesium ions and restoring the zeolite to its sodium form. The
wastewater containing CaCl₂ and MgCl₂ is then flushed out.
• CaZe (s) + 2NaCl (aq) → Na₂Ze (s) + CaCl₂ (aq)
• MgZe (s) + 2NaCl (aq) → Na₂Ze (s) + MgCl₂ (aq)

• Advantages:
o Efficiently removes both temporary and permanent hardness.
o Relatively simple to operate and automate.
o Requires less space compared to the lime-soda process.
o Treated water has very low hardness.
• Disadvantages:
o The zeolite can be clogged by suspended solids; therefore, pretreatment may
be needed for turbid water.
o Not suitable for highly acidic or alkaline waters, which can damage the
zeolite.
o Water softened by this process has a higher sodium content, which may be
undesirable for some applications or individuals on sodium-restricted diets.
o Can be affected by the presence of iron and manganese ions.

(3) Ion-exchange Resin:

• Principle: Similar to the zeolite process, this method uses synthetic organic polymers
called ion-exchange resins that contain functional groups capable of exchanging ions
with the surrounding solution. For water softening, cation exchange resins are used,
typically containing sulfonic acid groups (-SO₃H) that are initially loaded with
sodium ions (Na⁺).
• Mechanism: When hard water passes through a bed of cation exchange resin in the
sodium form (R-Na), the divalent calcium and magnesium ions have a higher affinity
for the resin sites and displace the sodium ions.
• 2 R-Na (s) + Ca²⁺ (aq) → R₂-Ca (s) + 2 Na⁺ (aq)
• 2 R-Na (s) + Mg²⁺ (aq) → R₂-Mg (s) + 2 Na⁺ (aq)

The water exiting the resin bed is softened as it now contains more sodium ions and
fewer calcium and magnesium ions.

• Regeneration: Once the resin is exhausted (saturated with calcium and magnesium),
it is regenerated by passing a concentrated brine solution (NaCl) through the bed. The
high concentration of Na⁺ ions forces the calcium and magnesium ions off the resin
and reloads it with sodium ions, restoring its softening capacity.
• R₂-Ca (s) + 2 NaCl (aq) → 2 R-Na (s) + CaCl₂ (aq)
• R₂-Mg (s) + 2 NaCl (aq) → 2 R-Na (s) + MgCl₂ (aq)
 • Types of Resins: Besides cation exchange resins for softening (typically strong acid
cation resins), there are also anion exchange resins used for other water treatment
purposes like demineralization (removing anions like sulfate, chloride, nitrate).
• Advantages (similar to Zeolite):
o Highly effective in removing hardness.
o Can achieve almost complete removal of calcium and magnesium.
o Relatively compact systems.
o Amenable to automation.
• Disadvantages (similar to Zeolite):
o Susceptible to fouling by suspended solids and some metal ions, requiring pre-
filtration.
o Increases the sodium content of the treated water.
o Can be damaged by oxidizing agents like chlorine.
o Requires periodic regeneration with brine solution.

8Q. Explain the following:-

(i) Reverse Osmosis (ii) Electro dialysis

Ans. (i) Reverse Osmosis (RO)

• Principle: Reverse osmosis is a water purification process that forces water through a
semi-permeable membrane from a region of higher solute concentration to a region of
lower solute concentration by applying pressure greater than the osmotic pressure.
This membrane allows water molecules to pass through but blocks most dissolved
salts, minerals, organic matter, and microorganisms. Essentially, it reverses the
natural flow of osmosis.
• Process:
1. Pressurization: The water to be purified is subjected to high pressure.
2. Membrane Filtration: This pressurized water is then forced against a semi-
permeable membrane.
3. Separation: Water molecules pass through the tiny pores of the membrane,
while most dissolved and suspended impurities are retained on the high-
pressure side.
4. Permeate and Concentrate: The purified water that passes through the
membrane is called the permeate (or product water). The concentrated solution
containing the rejected impurities is called the concentrate, reject, or brine,
which is typically discarded or sent for further treatment.
• Applications: Desalination of seawater and brackish water, purification of drinking
water, industrial water treatment, food and beverage industry, pharmaceutical
applications.
• Advantages: Highly effective in removing a wide range of contaminants, relatively
simple to operate, can be used for various water sources.
• Disadvantages: Requires significant pressure and energy, produces a waste stream
(concentrate), membranes can be fouled and require periodic cleaning or replacement,
removes beneficial minerals along with contaminants (remineralization may be
needed for drinking water).
(ii) Electrodialysis (ED)

• Principle: Electrodialysis is a membrane-based water treatment process that uses an

electric field to transport dissolved ions (salts and minerals) through ion-selective
membranes, separating them from the water.
• Process:
1. Electrodialysis Cell: The process takes place in an electrodialysis cell, which
consists of a stack of alternating cation-exchange membranes (CEMs) and
anion-exchange membranes (AEMs) placed between two electrodes (anode
and cathode).
2. Ion Migration: When a direct current (DC) voltage is applied across the
electrodes, positively charged ions (cations) are attracted to the cathode
(negative electrode), and negatively charged ions (anions) are attracted to the
anode (positive electrode).
3. Selective Membranes: The CEMs allow only positively charged ions to pass
through, while AEMs allow only negatively charged ions to pass through.
4. Separation: As ions migrate towards the electrodes, the alternating
arrangement of membranes creates compartments where ions are either
depleted (diluate stream - purified water) or concentrated (concentrate/brine
stream).
5. Continuous Flow: Feed water is continuously pumped through the
compartments, resulting in a continuous flow of purified water and a
concentrated brine solution.
• Applications: Desalination of brackish water, demineralization of process water,
recovery of salts from industrial wastewater, concentration of salt solutions, food
processing.
• Advantages: Lower energy consumption compared to RO for brackish water with
moderate salinity, higher water recovery rates in some applications, less susceptible to
fouling by non-ionic substances, can selectively remove ions.
• Disadvantages: Less effective for highly saline water (seawater desalination is often
more energy-efficient with RO), does not remove non-ionic contaminants or
suspended solids effectively (pretreatment may be needed), membrane performance
can be affected by certain organic compounds and scaling.
9Q. What do you understand by softening of water? Elaborate the functions of lime and soda
in hot lime-soda process.

Ans. Water softening is the process of removing or reducing the concentration of hardness-
causing salts, primarily calcium (Ca²⁺) and magnesium (Mg²⁺) ions, from water. Hardness in
water is generally categorized into:

• Temporary hardness, caused by bicarbonates of calcium and magnesium.

• Permanent hardness, caused by chlorides, sulfates, and nitrates of calcium and
magnesium.

Soft water is preferred in industries and homes because hard water can lead to:

• Scale formation in boilers and pipes.

• Wastage of soap due to scum formation.
• Reduced efficiency in heat exchangers and other systems.

Hot Lime-Soda Process:

The hot lime-soda process is a chemical method used to soften water by adding lime
(Ca(OH)₂) and soda ash (Na₂CO₃) at elevated temperatures. Heating improves the reaction
rate and helps in better precipitation of hardness-causing salts.

Functions of Lime and Soda:

1. Lime [Ca(OH)₂]:

• Removes temporary hardness by reacting with bicarbonates:

Ca(HCO3)2+Ca(OH)2→2CaCO3↓+2H2OCa(HCO₃)₂ + Ca(OH)₂ → 2CaCO₃↓ +

2H₂OCa(HCO3)2+Ca(OH)2→2CaCO3↓+2H2O
Mg(HCO3)2+2Ca(OH)2→Mg(OH)2↓+2CaCO3↓+2H2OMg(HCO₃)₂ + 2Ca(OH)₂ →
Mg(OH)₂↓ + 2CaCO₃↓ + 2H₂OMg(HCO3)2+2Ca(OH)2→Mg(OH)2↓+2CaCO3
↓+2H2O

• Removes dissolved CO₂ and free acids:

CO2+Ca(OH)2→CaCO3↓+H2OCO₂ + Ca(OH)₂ → CaCO₃↓ + H₂OCO2+Ca(OH)2

→CaCO3↓+H2O HCl+Ca(OH)2→CaCl2+H2OHCl + Ca(OH)₂ → CaCl₂ +
H₂OHCl+Ca(OH)2→CaCl2+H2O

• Removes some permanent hardness due to magnesium salts:

MgCl2+Ca(OH)2→Mg(OH)2↓+CaCl2MgCl₂ + Ca(OH)₂ → Mg(OH)₂↓ +

CaCl₂MgCl2+Ca(OH)2→Mg(OH)2↓+CaCl2
2. Soda Ash [Na₂CO₃]:

• Removes permanent hardness caused by calcium salts:

CaCl2+Na2CO3→CaCO3↓+2NaClCaCl₂ + Na₂CO₃ → CaCO₃↓ + 2NaClCaCl2+Na2

CO3→CaCO3↓+2NaCl CaSO4+Na2CO3→CaCO3↓+Na2SO4CaSO₄ + Na₂CO₃ →
CaCO₃↓ + Na₂SO₄CaSO4+Na2CO3→CaCO3↓+Na2SO4

Advantages of Hot Lime-Soda Process:

• Faster reaction rates due to heat.

• Better precipitation and settling of sludge.
• Reduces dissolved gases (like CO₂ and O₂).
• Economical for treating large volumes of water.

10Q. What are scales and sludges and why are they formed in boilers.What are their
disadvantages and how can their formation be prevented?

Ans. In boiler systems, scales and sludges are unwanted deposits that form as a result of the
use of hard water. These deposits reduce efficiency, damage equipment, and increase
maintenance costs.

1. Scales:

Scales are hard, adherent deposits that form on the inner walls of boiler tubes and other heat-
exchange surfaces.

Causes of Scale Formation:

Scales are mainly formed by:

• Precipitation of calcium and magnesium salts (e.g., CaSO₄, CaCO₃, Mg(OH)₂) due
to heating.
• These salts have inverse solubility, meaning they become less soluble as temperature
increases.

2. Sludges:

Sludges are soft, loose, and non-adherent deposits that settle at the bottom of the boiler. They
are usually formed by:

• Salts like MgCl₂, MgSO₄, CaCl₂, NaCl, and other impurities.

 • These salts remain soluble at higher temperatures but precipitate slowly as the boiler
water cools.

Disadvantages of Scales and Sludges:

Scales:

• Poor heat transfer: Scales act as thermal insulators, leading to wastage of fuel.
• Overheating and bursting: Localized overheating may cause tube failures or even
explosions.
• Reduced efficiency: Lower heat transfer rate decreases boiler performance.
• Corrosion: Some scales may trap water underneath and promote corrosion.

Sludges:

• Choking of pipes and valves: Sludge can block boiler components and reduce flow.
• Reduced efficiency: Sludge buildup may interfere with heat transfer.
• Foaming and priming: Sludges can lead to water carryover with steam.

Prevention of Scale and Sludge Formation:

1. Water Softening:
o Use external treatment methods like lime-soda process, zeolite softening, or
ion-exchange to remove hardness before feeding water to the boiler.
2. Internal Treatment:
o Add chemicals (e.g., phosphates, tannins, EDTA) that convert scale-forming
salts into sludge, which can be removed by blowdown.
3. Blowdown Operation:
o Periodically remove part of the water containing dissolved solids and sludge
from the boiler.
4. Proper Boiler Maintenance:
o Regular cleaning and inspection.
o Monitoring and adjusting water chemistry.

11Q. What are ion exchange resins? how are they used for softening of water, explain with
reactions and diagram? How can they be regenerated after getting exhausted ?

Ans. Ion Exchange Resins and Water Softening

1. What are Ion Exchange Resins?

Ion exchange resins are insoluble, porous, polymer-based materials that can exchange
specific ions within them with ions in a solution that passes through them. These resins
contain functional groups that can hold and release ions.
 • Cation exchange resins exchange positive ions (e.g., Na⁺, H⁺).
• Anion exchange resins exchange negative ions (e.g., Cl⁻, OH⁻).

They are commonly made of styrene-divinylbenzene copolymers with attached functional

groups such as –SO₃H (sulfonic acid) for cation exchange and –NR₃⁺OH⁻ (quaternary
ammonium hydroxide) for anion exchange.

2. Use of Ion Exchange Resins in Water Softening

Water softening means removal of hardness-causing ions, mainly:

• Calcium (Ca²⁺)
• Magnesium (Mg²⁺)

Hard water is passed through a cation exchange resin in sodium (Na⁺) form or hydrogen
(H⁺) form, which exchanges Ca²⁺ and Mg²⁺ ions with Na⁺ or H⁺.

Reaction with Sodium form (Na⁺):

Let the resin be denoted as R–Na.

Ca²⁺ + 2R–Na → R₂–Ca + 2Na⁺

Mg²⁺ + 2R–Na → R₂–Mg + 2Na⁺

This removes the hardness ions from water.

(Optional) Deionization (complete removal of all ions):

• Cation exchanger (R–H):

Ca²⁺ + 2R–H → R₂–Ca + 2H⁺
• Anion exchanger (R–OH):
Cl⁻ + R–OH → R–Cl + OH⁻

The H⁺ and OH⁻ combine to form H₂O, yielding deionized water.

3. Diagram: Ion Exchange Water Softening

4. Regeneration of Ion Exchange Resins

After prolonged use, the resin becomes exhausted (saturated with Ca²⁺ and Mg²⁺), and it
must be regenerated.

For Na⁺ form resins:

Use brine solution (NaCl) to replace Ca²⁺/Mg²⁺ with Na⁺:

R₂–Ca + 2Na⁺ → 2R–Na + Ca²⁺

R₂–Mg + 2Na⁺ → 2R–Na + Mg²⁺

Brine restores the resin back to the sodium form.

For H⁺ form resins:

Use dilute HCl or H₂SO₄:

R₂–Ca + 2H⁺ → 2R–H + Ca²⁺

For Anion Resins:

Regenerated with NaOH:

R–Cl + OH⁻ → R–OH + Cl⁻

NUMERICALS-
1Q. A water sample contains 248mg caso4 per litre.calculate the hardness in terms of caco3
equivalent
Ans.
2Q. A water sample is alkaline to methyl orange only. 100ml of the water sample require
30ml of N/50 H2SO4 for neutralization . Calculate type of alkalinity in ppm.
Ans.
3Q. Calculate amount of lime (90% pure) and soda (85% pure) required for softening of
10,000 liters of water containing following salts: CaCl₂ = 11.1 ppm, MgSO4 = 6.0 ppm,
CaCO3 = 10.0 ppm, MgCO3 = 8.4 ppm, SiO2 = 1.2 ppm
4Q. 1 g of CaCO3 is dissolved in dil. HCl and the solution diluted to one liter. 100 mL of this
solution required 90 mL of EDTA solution for titration using EBT as indicator, while 100 mL
of a water sample required 60 mL of EDTA. On the other hand, 100 mL of boiled water
sample required 30 mL of EDTA. Calculate temporary and permanent hardness of the water
sample.
Ans.