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BT-101 · Engineering Chemistry/Quick Revision Short Notes

Engineering Chemistry (BT-101) - Unit 2 Short Notes

How unit 2 is examined

This unit covers the troubles caused by hard water in boilers and the methods that soften water; boiler troubles and softening methods carry most marks, with a lime-soda numerical asked often.

Boiler troubles (Sludge & Scale, Priming & Foaming, Boiler Corrosion, Caustic Embrittlement)

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">High weight</span>

Definition. <mark>Boiler troubles are the problems, such as scale, sludge, priming, foaming, corrosion and caustic embrittlement, that arise when hard or impure water is used for steam generation.</mark>

Key points.

  1. Sludge is a soft, loose, slimy precipitate of salts such as $MgCO_3$, $MgCl_2$, $CaCl_2$ and $MgSO_4$ that are highly soluble in hot water; it collects at cooler, low-flow parts of the boiler and is removed by blow-down.
  2. Scale is a hard, adherent coating of sparingly soluble salts such as $CaSO_4$, $CaCO_3$, $Mg(OH)_2$ and calcium/magnesium silicate; it forms when the salts precipitate on the hot tube surface as water evaporates and their solubility falls, and it cannot be removed by blow-down.
  3. Bad effects of scale are wastage of fuel because scale is a poor heat conductor, overheating of tubes leading to weakening, cracks and boiler explosion, reduced boiler efficiency, choking of pipes, and deposits that cause uneven expansion.
  4. Priming is the carry-over of liquid water droplets with the steam, and foaming is the formation of a stable foam of bubbles on the water surface; both are caused by dissolved salts, oil and suspended impurities, and by high steam velocity, sudden boiling and a very high water level.
  5. Priming and foaming give wet steam, wastage of heat, water carried into turbines and engines with deposits on them, and unsteady water-level readings; they are cured by using softened water, avoiding sudden steam withdrawal, blow-down, and adding antifoaming agents such as castor oil.
  6. Boiler corrosion is the decay of boiler metal caused by dissolved oxygen, dissolved $CO_2$ and dissolved salts; oxygen forms rust ($4Fe+3O_2+xH_2O \rightarrow 2Fe_2O_3\cdot xH_2O$), $CO_2$ forms carbonic acid ($CO_2+H_2O \rightarrow H_2CO_3$), and $MgCl_2$ hydrolyses in hot water to give HCl, which attacks iron repeatedly ($MgCl_2+2H_2O \rightarrow Mg(OH)_2+2HCl$; $Fe+2HCl \rightarrow FeCl_2+H_2$).
  7. Corrosion is prevented by removing dissolved oxygen by adding sodium sulphite ($2Na_2SO_3+O_2 \rightarrow 2Na_2SO_4$) or hydrazine ($N_2H_4+O_2 \rightarrow N_2+2H_2O$), by mechanical deaeration under heat and vacuum, by removing $CO_2$ with ammonia or by deaeration, and by using softened water with blow-down.
  8. Caustic embrittlement is the intercrystalline cracking of boiler metal at stressed places such as rivets, bends and joints, caused by concentrated $NaOH$ in the boiler water.
  9. In caustic embrittlement, water softened by the lime-soda process contains residual $Na_2CO_3$, which hydrolyses at high pressure: $Na_2CO_3+H_2O \rightarrow 2NaOH+CO_2$. The $NaOH$ seeps into hair-line cracks and, as water evaporates, becomes very concentrated.
  10. A concentration cell forms in which the stressed iron at the crack (anode) dissolves and the unstressed iron around it (cathode) is protected, so the crack deepens and the metal fails.
Dilute NaOH | Iron at plane surface (cathode) | Concentrated NaOH | Stressed iron at bends, rivets (anode)
  1. Caustic embrittlement is prevented by using sodium phosphate instead of sodium carbonate for softening, by adding tannin or lignin, which block the hair-line cracks, and by adding sodium sulphate, which deposits in the cracks (keep the ratio $Na_2SO_4:NaOH$ about 1:1 to 3:1 as pressure rises).
Point Sludge Scale
Nature Soft, loose, slimy Hard, adherent, crystalline
Salts $MgCO_3$, $MgCl_2$, $MgSO_4$, $CaCl_2$ $CaSO_4$, $CaCO_3$, $Mg(OH)_2$, silicates
Where Cooler, low-flow parts Hot heating surface
Heat transfer Poor but tolerable Very poor
Removal Blow-down Scraping, chemicals, thermal shock
Risk Choking of pipes if excess Fuel wastage, explosion

Prevention of boiler troubles. External treatment (lime-soda, zeolite, ion exchange) removes hardness before the water enters the boiler; internal conditioning adds phosphate (Calgon or sodium phosphate) to make loose sludge, and colloidal conditioning with tannin or agar; blow-down removes sludge, and antifoaming agents control foaming.

Answer frame. Open with the definition of boiler troubles and name the four types; draw the concentration-cell sketch for embrittlement (iron at the plane surface as cathode, stressed iron as anode); then develop scale and sludge (with the table) first, then priming and foaming, then corrosion with equations, then embrittlement in the order cause, cell, prevention; close with the prevention steps of softening, blow-down and conditioning.

Pitfall: Do not say blow-down removes scale; it removes only loose sludge, while scale needs scraping or chemicals.

Asked: [7 marks] (Nov 2022) What is caustic embrittlement? How do you prevent it? Asked: [7 marks] (Dec 2023) Give a detailed account on: i) Caustic embrittlement ii) Boiler corrosion Asked: [7 marks] (Jun 2023, Dec 2024) Explain the formation and composition of scales in boilers. How are they different from sludges? What are bad effects? / Write a note on scales and sludges. Asked: [7 marks] (Dec 2024) What are boiler troubles? How are they caused? Suggest steps for minimizing the boiler troubles. Asked: [7 marks] (Jun 2025) Write short note on i) Sludge and Scale ii) Caustic Embrittlement iii) Boiler Corrosion

Softening methods (Lime-Soda, Zeolite and Ion Exchange Methods)

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">High weight</span>

Definition. <mark>Softening is the removal of the calcium and magnesium ions that cause hardness, by precipitation (lime-soda) or by exchanging them for ions that do not cause hardness (zeolite, ion exchange).</mark>

Lime-soda process.

  1. Lime $Ca(OH)_2$ and soda ash $Na_2CO_3$ convert soluble calcium and magnesium salts into insoluble $CaCO_3$ and $Mg(OH)_2$, which settle and are filtered off.
  2. Lime removes temporary hardness and $CO_2$, and converts magnesium to $Mg(OH)_2$; soda removes permanent hardness because it precipitates calcium and, after magnesium is converted, the calcium formed.

$$Ca(HCO_3)_2+Ca(OH)_2 \rightarrow 2CaCO_3\downarrow+2H_2O$$ $$Mg(HCO_3)_2+2Ca(OH)_2 \rightarrow 2CaCO_3\downarrow+Mg(OH)_2\downarrow+2H_2O$$ $$MgSO_4+Ca(OH)_2 \rightarrow Mg(OH)_2\downarrow+CaSO_4;\quad CaSO_4+Na_2CO_3 \rightarrow CaCO_3\downarrow+Na_2SO_4$$ $$CaCl_2+Na_2CO_3 \rightarrow CaCO_3\downarrow+2NaCl$$

  1. Cold process works at room temperature, is slow and needs a coagulant such as alum or sodium aluminate; it leaves residual hardness of about 50 to 60 ppm.
  2. Hot process works at 80 to 100 C with steam, so reaction is fast, the precipitate settles and filters quickly without a coagulant, dissolved $CO_2$ and $O_2$ are expelled, and residual hardness falls to about 15 to 30 ppm.

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  1. Advantages are that it is economical, reduces iron and manganese, raises the pH of water and so lowers corrosion, and the sludge settles readily; disadvantages are the disposal of large sludge and residual hardness of 15 to 60 ppm, so it is unsuitable for high-pressure boilers.

Zeolite (permutit) process.

  1. Zeolite is hydrated sodium aluminosilicate $Na_2Al_2Si_2O_8\cdot xH_2O$ (written $Na_2Z$), which exchanges its sodium ions for $Ca^{2+}$ and $Mg^{2+}$ as hard water passes through the bed.

$$Na_2Z+Ca(HCO_3)_2 \rightarrow CaZ+2NaHCO_3$$ $$Na_2Z+MgSO_4 \rightarrow MgZ+Na_2SO_4$$

  1. When the bed is exhausted (all sodium replaced by calcium and magnesium), it is regenerated by passing 10% NaCl brine: $CaZ+2NaCl \rightarrow Na_2Z+CaCl_2$; the washings of $CaCl_2$ and $MgCl_2$ are drained.

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  1. Advantages are that it gives water of about 10 ppm hardness, needs no sludge disposal, is compact, and the bed is regenerated easily; limitations are that turbid or coloured water clogs the bed, $Fe^{2+}$ and $Mn^{2+}$ must be removed first, acidic water damages it, and it leaves sodium salts (and alkalinity) in the water.

Ion-exchange (demineralisation) process.

  1. Synthetic cross-linked organic resins remove all dissolved ions: the cation exchange resin ($RH_2$, sulphonic acid type) exchanges $H^+$ for cations, and the anion exchange resin ($R'(OH)_2$, amine type) exchanges $OH^-$ for anions.

$$RH_2+Ca^{2+} \rightarrow RCa+2H^+ \qquad R'(OH)_2+2Cl^- \rightarrow R'Cl_2+2OH^-$$

  1. Hard water first passes the cation column, and the $H^+$ released makes the water acidic; it then passes the anion column, and the $H^+$ and $OH^-$ combine to give pure water ($H^+ + OH^- \rightarrow H_2O$); a degasifier removes dissolved $CO_2$ between the columns.
  2. The exhausted cation resin is regenerated with dil. HCl or dil. $H_2SO_4$ ($RCa+2HCl \rightarrow RH_2+CaCl_2$), and the exhausted anion resin with dil. NaOH ($R'Cl_2+2NaOH \rightarrow R'(OH)_2+2NaCl$).

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  1. Advantages are that it removes all ions, gives water of about 1 to 2 ppm hardness suitable for high-pressure boilers, and softens acidic and alkaline water alike; disadvantages are high cost and that turbid or high-iron water fouls the resin.
Point Lime-soda Zeolite Ion exchange
Principle Precipitation Exchange of $Na^+$ Exchange of $H^+$ and $OH^-$
Residual hardness 15 to 60 ppm about 10 ppm 1 to 2 ppm
Sludge Yes No No
Regenerant None 10% NaCl Acid and alkali

Answer frame. For lime-soda, open with the principle, give the four reactions, then cold against hot, and close with advantages and disadvantages. For hot lime-soda, draw the plant labelled as in the caption and stress temperature 80 to 100 C and residual hardness 15 to 30 ppm. For zeolite, open with the definition of zeolite, draw the softener, give the two reactions, regeneration with 10% NaCl, and close with advantages and limitations. For ion exchange, open with the principle of demineralisation, draw the two columns with the degasifier, give the resin reactions, regeneration with acid and alkali, and close with advantages.

Pitfall: A zeolite softener does not remove alkalinity or dissolved salts as such; it only replaces Ca and Mg by Na, so the water stays salty, whereas ion exchange gives demineralised water.

Asked: [14 marks] (Nov 2022) Describe: i) Zeolite process ii) Ion-exchange process with neat diagram Asked: [7 marks] (Jun 2022, Dec 2023) Discuss the lime soda process for softening of hard water. How is water softened by the lime-soda process? What are its advantages? Asked: [7 marks] (Jun 2022, Jun 2023, Dec 2024) Describe the ion exchange process used for softening of water with neat diagram. / Write a note on ion-exchange process. Asked: [7 marks] (Dec 2023) Discuss the zeolite process for softening of water. Asked: [7 marks] (Jun 2023) Explain the hot lime-soda process. What are its advantages? Asked: [7 marks] (Jun 2025) Describe ion exchange process used for softening of water with neat diagram. A zeolite softener was 90% exhausted by removing the hardness completely when 10,000 L of hard water passed through it. The exhausted bed required 200 L of 3% NaCl solution for regeneration. Calculate the hardness of the water sample.

Related numerical problems

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Definition. <mark>Lime-soda numericals convert every impurity to its $CaCO_3$ equivalent, then find lime for temporary hardness and magnesium salts, and soda for permanent hardness.</mark>

Formula. $$CaCO_3\text{ equivalent} = \frac{\text{mass of salt}\times 100}{\text{molar mass of salt}}$$ $$\text{Lime} = \frac{74}{100}\big[Ca(HCO_3)_2 + 2\,Mg(HCO_3)_2 + \text{Mg permanent salts}\big]\times\frac{V}{\%\text{purity}}\ \text{(CaCO}_3\text{ eq.)}$$ $$\text{Soda} = \frac{106}{100}\big[\text{Ca permanent salts} + \text{Mg permanent salts}\big]\times\frac{V}{\%\text{purity}}$$

Key points.

  1. Molar masses used are $CaCl_2$ 111, $Mg(HCO_3)_2$ 146, $CaSO_4$ 136, $Ca(NO_3)_2$ 164, $MgSO_4$ 120, $Ca(HCO_3)_2$ 162; $NaCl$ needs no lime or soda and is ignored.
  2. $Mg(HCO_3)_2$ counts twice in lime, because it needs two moles of lime; magnesium sulphate needs lime once and then soda once.
  3. Multiply by 120/100 for 20% excess and divide by the purity, then by $10^6$ to convert mg to kg for the volume.

Example (Jun 2023, Jun 2025). Given: $CaCl_2$ 222, $Mg(HCO_3)_2$ 73, $CaSO_4$ 6.8, $NaCl$ 58.5, $Ca(NO_3)_2$ 162, $MgSO_4$ 73 mg/L; lime 74% pure, soda 90% pure, 20% excess; 10,000 L.

Salt mg/L $CaCO_3$ eq. (mg/L) Needs
$CaCl_2$ 222 200 soda
$Mg(HCO_3)_2$ 73 50 lime x 2
$CaSO_4$ 6.8 5 soda
$Ca(NO_3)_2$ 162 98.78 soda
$MgSO_4$ 73 60.83 lime and soda

Lime = $\frac{74}{100}(2\times 50+60.83)\times\frac{100}{74}\times\frac{120}{100} = 160.83\times1.2 = 193.0$ mg/L, so for 10,000 L it is $193.0\times10^4$ mg.

Soda = $\frac{106}{100}(200+5+98.78+60.83)\times\frac{100}{90}\times\frac{120}{100} = 1.06\times364.61\times1.111\times1.2 = 515.3$ mg/L.

Lime required = 1.93 kg; soda required = 5.15 kg.

Second version (20,000 L; $Ca(HCO_3)_2$ 40, $Mg(HCO_3)_2$ 36.5, $CaSO_4$ 34, $MgSO_4$ 30, $CaCl_2$ 27.75 ppm). The $CaCO_3$ equivalents are 24.69, 25, 25, 25 and 25 ppm. Lime = $0.74\times(24.69+2\times25+25)\times 20000/10^6 = 0.74\times99.69\times0.02$ = 1.475 kg; soda = $1.06\times(25+25+25)\times0.02$ = 1.59 kg. Lime = 1.48 kg; soda = 1.59 kg.

Zeolite regeneration numerical (Jun 2025). Given: 200 L of 3% NaCl; 90% exhausted; 10,000 L water. NaCl mass = $200\times0.03 = 6$ kg = 6000 g. $CaCO_3$ eq. = $6000\times\frac{50}{58.5} = 5128.2$ g. Hardness removed = $0.9\times5128.2 = 4615.4$ g. Hardness = $\frac{4615.4\times10^3\text{ mg}}{10000\text{ L}}$ = 461.5 mg/L (ppm).

Answer frame. Open with the lime-soda principle; tabulate salts with $CaCO_3$ equivalents; mark which need lime and which soda; substitute in the formulas with purity and excess; close with the final answer in kg.

Pitfall: Do not include NaCl in either requirement, and do not forget the factor 2 for $Mg(HCO_3)_2$ in lime.

Asked: [7 marks] (Jun 2023, Jun 2025) A water sample contains per litre $CaCl_2$ 222 mg, $Mg(HCO_3)_2$ 73 mg, $CaSO_4$ 6.8 mg, $NaCl$ 58.5 mg, $Ca(NO_3)_2$ 162 mg and $MgSO_4$ 73 mg. Lime is 74% pure, soda 90% pure, 20% excess. Calculate the lime and soda for 10,000 L. / What is the lime soda process? Calculate lime and soda for 20,000 L with $Ca(HCO_3)_2$ 40, $Mg(HCO_3)_2$ 36.5, $CaSO_4$ 34, $MgSO_4$ 30, $CaCl_2$ 27.75 ppm.

Last-minute revision

  • Boiler troubles: scale, sludge, priming and foaming, corrosion, caustic embrittlement.
  • Sludge is soft and loose ($MgCO_3$, $MgCl_2$); scale is hard and adherent ($CaSO_4$, $CaCO_3$, silicates).
  • Corrosion causes: dissolved $O_2$, $CO_2$ and $MgCl_2$ hydrolysis; remove $O_2$ with $Na_2SO_3$ or hydrazine.
  • Caustic embrittlement: $Na_2CO_3+H_2O \rightarrow 2NaOH+CO_2$; stressed iron is the anode; prevent with phosphate, tannin, lignin, sulphate.
  • Lime removes temporary hardness; soda removes permanent hardness.
  • Hot lime-soda: 80 to 100 C, residual hardness 15 to 30 ppm, no coagulant.
  • Zeolite is $Na_2Z$, regenerated with 10% NaCl, residual hardness about 10 ppm.
  • Ion exchange: $RH_2$ cation resin, $R'(OH)_2$ anion resin, regenerated with dil. HCl and dil. NaOH, gives 1 to 2 ppm.
  • $CaCO_3$ eq. = mass $\times$ 100 / molar mass; lime factor 74/100; soda factor 106/100; $Mg(HCO_3)_2$ counts twice in lime.
  • Jun 2023 numerical answers: lime 1.93 kg, soda 5.15 kg.

Memory hooks

  • Scale is Stuck, sludge is Soft.
  • "Lime for Lime-temporary, Soda for Solid permanent."
  • Zeolite regenerates with Salt (brine); ion exchange with Acid then Alkali, cation first.
  • Embrittlement: the Stressed iron is the Sacrificed anode.
  • Cation before anion, then degasifier in between.

Coverage checklist

  • Boiler troubles (Sludge & Scale, Priming & Foaming, Boiler Corrosion, Caustic Embrittlement): Nov 2022 caustic embrittlement, Dec 2023 embrittlement and corrosion, Jun 2023/Dec 2024 scale and sludge, Dec 2024 boiler troubles, Jun 2025 short note.
  • Softening methods (Lime-Soda, Zeolite and Ion Exchange Methods): Nov 2022 zeolite and ion exchange, lime-soda (Jun 2022, Dec 2023), ion exchange (Jun 2022, Jun 2023, Dec 2024, Jun 2025), zeolite (Dec 2023), hot lime-soda (Jun 2023), zeolite regeneration numerical (Jun 2025).
  • related numerical problems: lime-soda requirement for 10,000 L (Jun 2023, Jun 2025) and 20,000 L.
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