Engineering Chemistry · June 2024

Hard water versus soft water

(i) Blow-down process

Blow-down is the process of removing a portion of the concentrated water from the bottom of the boiler through a blow-down valve, and replacing it with fresh softened feed water. As water evaporates, the concentration of dissolved salts and sludge rises; blow-down keeps this concentration within safe limits.

• Purpose: prevents sludge accumulation, scale formation, priming, foaming and caustic embrittlement.
• Types: intermittent blow-down (opened at intervals, removes sludge from the bottom) and continuous blow-down (a small continuous stream, keeps dissolved solids constant).
• Frequency depends on the hardness of feed water: harder water needs more frequent blow-down.
• Limitation: it involves loss of hot water and therefore of heat; it is a supplement to, not a substitute for, proper softening.

In a boiler, water evaporates continuously and the concentration of dissolved salts keeps rising. When the solution becomes saturated, the salts separate out as a precipitate on the inner walls. If the precipitate is loose and slimy it is called sludge; if it forms a hard, adherent coating it is called scale.

Sludge

Sludge is a soft, loose, slimy precipitate formed in the comparatively colder parts of the boiler and where the flow is slow (bends, bottom of the boiler). It is formed by salts whose solubility is greater in hot water than in cold, such as MgCO₃, MgCl₂, MgSO₄ and CaCl₂.

Disadvantages: sludge is a poor conductor of heat, so fuel is wasted; excess sludge settles in pipes and chokes them, disturbing circulation; when formed along with scale it gets entrapped in the scale and both are deposited together.

Prevention: use well-softened water and carry out frequent blow-down.

Scale

Scale is a hard deposit firmly sticking to the inner surface of the boiler, very difficult to remove even with a hammer and chisel. Causes of scale formation:

1. Decomposition of calcium bicarbonate (low-pressure boilers): Ca(HCO₃)₂ → CaCO₃↓ + H₂O + CO₂. CaCO₃ scale is soft and is the main scale in low-pressure boilers.
2. Deposition of calcium sulphate (high-pressure boilers): solubility of CaSO₄ decreases with rise in temperature, so it separates out as a hard, adherent scale on the hot portions.
3. Hydrolysis of magnesium salts: MgCl₂ + 2H₂O → Mg(OH)₂↓ + 2HCl. Mg(OH)₂ forms a soft scale.
4. Presence of silica: SiO₂ forms calcium and magnesium silicates (CaSiO₃, MgSiO₃), which give a very hard, glassy scale that is very difficult to remove.

Bad effects of scale

• Wastage of fuel: scale is a poor conductor of heat, so the metal must be heated more to transfer the same heat. Approximate fuel wastage: 0.325 mm scale → 10%, 0.625 mm → 15%, 1.25 mm → 50%, 2.5 mm → 80%, 12 mm → 150%.
• Lowering of boiler safety: the metal is overheated, becomes soft and weak, and may fail under high steam pressure.
• Decrease in efficiency: scale deposits in valves and condensers and partially chokes them.
• Danger of explosion: scale expands differently from the metal; when it cracks, water suddenly comes in contact with the overheated metal, a large amount of steam forms at once and high pressure may burst the boiler.

Difference between sludge and scale

Prevention and removal of scale

• External treatment: soften feed water by lime-soda, zeolite or ion-exchange process before use.
• Internal treatment (conditioning):
  • Colloidal conditioning (low-pressure boilers): kerosene, tannin or agar-agar coat the particles so they stay as loose sludge.
  • Phosphate conditioning (high-pressure boilers): 3CaCl₂ + 2Na₃PO₄ → Ca₃(PO₄)₂↓ + 6NaCl (soft sludge).
  • Carbonate conditioning (low-pressure boilers): CaSO₄ + Na₂CO₃ → CaCO₃↓ + Na₂SO₄.
  • Calgon conditioning: sodium hexametaphosphate forms a soluble complex: Na₂[Na₄(PO₃)₆] + 2CaSO₄ → Na₂[Ca₂(PO₃)₆] + 2Na₂SO₄.
  • Sodium aluminate: gives gelatinous Al(OH)₃ and Mg(OH)₂ that entrap fine particles as sludge.
• Removal: scraping with wire brush (soft scale), thermal shocks (heating and sudden cooling), chemical washing (5–10% HCl for CaCO₃; EDTA for CaSO₄), and frequent blow-down.

The ion-exchange (demineralisation) process removes all cations and anions from water using ion-exchange resins. These are insoluble, cross-linked long-chain organic polymers (for example styrene–divinylbenzene copolymer) with a microporous structure, carrying functional groups that can exchange ions.

• Cation exchange resins (RH₂): contain acidic groups –SO₃H or –COOH; they exchange H⁺ for cations.
• Anion exchange resins (R′(OH)₂): contain basic groups such as quaternary ammonium –NR₃⁺OH⁻ or amino groups; they exchange OH⁻ for anions.

Process

Hard water is passed first through the cation-exchange column, which removes Ca²⁺, Mg²⁺, Na⁺ etc. and releases an equivalent amount of H⁺:

The acidic water then passes through the anion-exchange column, which removes Cl⁻, SO₄²⁻, HCO₃⁻ and releases OH⁻:

The water coming out is free from all ions and is called deionised or demineralised water. Dissolved CO₂ may be removed in a degasifier.

Regeneration

When the resins are exhausted, the cation exchanger is regenerated with dilute HCl (or H₂SO₄) and the anion exchanger with dilute NaOH. The washings are drained off.

Advantages

• Produces water of very low hardness (about 0–2 ppm) — suitable for high-pressure boilers.
• Removes all ions, including silica (with strongly basic resin), so total dissolved solids become very low.
• Works for highly acidic or alkaline water.

Disadvantages

• Equipment and resins are costly; regenerating chemicals (acid, alkali) are expensive.
• Turbid water reduces efficiency, since suspended matter clogs the resin; water must be filtered first.

A lubricant is a substance introduced between two moving or sliding surfaces to reduce friction between them. The process of reducing friction in this way is called lubrication. The lubricant keeps the surfaces apart, so the frictional resistance is replaced by the much smaller internal resistance of the lubricant layer.

Functions of a lubricant

• Reduces friction, wear and tear of the moving parts.
• Reduces heat generation and acts as a coolant by carrying heat away.
• Reduces loss of power, so efficiency of the machine increases.
• Acts as a seal (e.g., between piston and cylinder) and prevents entry of dust and moisture.
• Protects the surfaces against corrosion and reduces noise.

Classification of lubricants

Lubricants are classified on the basis of their physical state:

1. Liquid lubricants (lubricating oils)

• Animal and vegetable oils — e.g., castor oil, olive oil, palm oil, rapeseed oil, lard oil, whale oil. They have good oiliness but oxidise easily, form gums and are costly.
• Mineral (petroleum) oils — hydrocarbons (C₁₂–C₅₀) obtained by fractional distillation of petroleum. Cheap, abundant and stable; their oiliness is low, so additives are used.
• Blended (compounded) oils — mineral oil with additives: oiliness agents (vegetable oils, fatty acids), extreme-pressure additives (chlorinated, sulphurised or phosphorus compounds), antioxidants, pour-point depressants, viscosity-index improvers, detergents and anti-foam agents.
• Synthetic lubricants — silicones, polyglycols, diesters, fluorocarbons; used at extreme temperatures (jet engines, aircraft).

2. Semi-solid lubricants (greases)

Grease is a lubricating oil thickened with a metallic soap (Ca, Na, Li, Al soaps). Examples: calcium-based (cup) grease, soda-based grease, lithium-based grease, axle grease; petroleum jelly (vaseline) is also semi-solid. Greases are used where the load is heavy and speed is low, where oil cannot stay in place (vertical shafts, open gears), for intermittent operation, and where the bearing must be sealed against dust and moisture.

3. Solid lubricants

Graphite, molybdenum disulphide (MoS₂), talc, mica, soapstone, boron nitride and PTFE (Teflon). They are used at very high temperature or load, where liquid lubricants would burn or squeeze out, and where contamination by oil is not acceptable (food and textile machinery). Graphite has a layered hexagonal structure with weak van der Waals forces between layers, so the layers slide over each other easily. It is used as dry powder or dispersed in oil (oildag) or water (aquadag). MoS₂ has a sandwich-like layer structure and is stable up to about 800 °C.

4. Synthetic and emulsion lubricants

Oil-in-water and water-in-oil emulsions are used as cutting fluids, where both cooling and lubrication are required.

(i) Viscosity index

Viscosity is the internal resistance of a liquid to flow. The viscosity of a lubricating oil decreases as temperature rises. Viscosity index (VI) is a number that expresses the rate of change of viscosity with temperature. A high VI means the viscosity changes only slightly with temperature; a low VI means it changes a lot.

On the Dean and Davis scale, a Pennsylvanian (paraffinic) oil, whose viscosity changes little, is given VI = 100, and a Gulf Coast (naphthenic) oil, whose viscosity changes greatly, is given VI = 0. The test oil and both standard oils have the same viscosity at 210 °F. Their viscosities at 100 °F are compared:

Viscosity is measured with a Redwood or Saybolt viscometer (time of flow in seconds).

Significance

• A machine part may work over a wide range of temperature (cold start in winter to very hot running). A high-VI oil remains thick enough at high temperature to keep the film intact, and thin enough at low temperature to flow and lubricate at start-up.
• Paraffinic oils have high VI; naphthenic and aromatic oils have low VI. VI is raised by adding VI improvers such as polyisobutylene and polymethacrylates.
• Multigrade engine oils (e.g., 10W-40) are high-VI oils made with VI improvers.

Free-radical polymerisation is a chain-growth polymerisation of vinyl monomers (CH₂=CHX) initiated by free radicals produced from initiators such as benzoyl peroxide, azobisisobutyronitrile (AIBN) or H₂O₂, by heat or light. It takes place in three steps.

1. Chain initiation

The primary radical adds to the double bond of the monomer and forms a new, larger free radical.

2. Chain propagation

The growing radical adds monomer units one after another very rapidly, and the chain grows.

3. Chain termination

(a) By combination (coupling): two growing radicals join to form one dead polymer molecule.

(b) By disproportionation: one radical abstracts a hydrogen atom from the other, giving one saturated and one unsaturated chain end.

Chain transfer to monomer, solvent or polymer may also stop growth of one chain while starting another (this causes branching, as in LDPE).

Example — polyethylene: n CH₂=CH₂ → [–CH₂–CH₂–]_n (at high pressure with traces of O₂ or peroxide). Other examples: PVC (X = Cl), polystyrene (X = C₆H₅), PMMA.

Biodegradable polymers are polymers that are broken down by micro-organisms (bacteria, fungi, algae) in the environment, by hydrolysis and enzymatic oxidation, into harmless products such as CO₂, water, methane and biomass, within a reasonable period. They usually contain hydrolysable linkages (ester, amide) in the main chain. They help reduce the plastic-waste problem caused by non-biodegradable polymers like polythene and PVC.

Examples

• PHBV (poly-β-hydroxybutyrate-co-β-hydroxyvalerate): copolymer of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid, produced by bacteria.
• Nylon-2-nylon-6: alternating polyamide of glycine (H₂N–CH₂–COOH) and ε-aminocaproic acid (H₂N–(CH₂)₅–COOH).
• PLA (polylactic acid) from lactic acid (corn starch, sugarcane); PGA (polyglycolic acid); PCL (polycaprolactone); starch- and cellulose-based plastics.

Example in detail: PHBV

PHBV is obtained by copolymerisation of 3-hydroxybutanoic acid and 3-hydroxypentanoic acid; it is actually produced by bacteria (e.g., Alcaligenes eutrophus) as an energy reserve:

Its properties vary with the ratio of the two acids: butanoic acid units give stiffness, pentanoic acid units give flexibility. It is degraded by soil bacteria into CO₂ and water. Uses: speciality packaging, orthopaedic devices and controlled release of drugs.

Applications of biodegradable polymers

1. Medical and pharmaceutical

• Absorbable surgical sutures (PGA, PLA, Dexon, Vicryl): dissolve in the body after the wound heals, so no second operation is needed to remove stitches.
• Controlled drug delivery: drugs enclosed in PLA/PLGA capsules or microspheres are released slowly as the polymer degrades.
• Orthopaedic implants: pins, screws, plates for bone fixation that dissolve as the bone heals.
• Tissue engineering scaffolds, wound dressings, dental and nerve-guide devices.

2. Packaging

• Carry bags, food containers, films and wraps, disposable cups, plates and cutlery.
• Bottles and egg cartons; loose-fill packing material made from starch.

3. Agriculture

• Mulch films that degrade in the soil after the crop season.
• Controlled release of fertilisers, pesticides and herbicides; seed coatings; biodegradable plant pots and nursery bags.

4. Consumer goods and others

• Disposable diapers, sanitary products, cosmetic containers, tea bags.
• PLA filaments for 3-D printing; textile fibres; fishing nets and lines that do not persist in water.
• Biodegradable electronic components and temporary structures.

Advantages: reduce plastic pollution and landfill load, made from renewable resources, non-toxic degradation products. Limitations: higher cost, lower strength and heat resistance, and they need suitable composting conditions to degrade quickly.

Water-line corrosion

Water-line corrosion is a type of differential-aeration (concentration-cell) corrosion seen in steel water tanks, ships and partially immersed structures. The metal surface just above the water line is in contact with water that is highly oxygenated (open to air), while the metal just below the water line is in contact with poorly oxygenated water. The less aerated part becomes the anode and corrodes, while the well-aerated part becomes the cathode. So a band of corrosion forms just below the water line.

In ships it is aggravated by marine plants and animals attached to the hull near the water line. Prevention: special anti-fouling paints, corrosion inhibitors, cathodic protection, and use of corrosion-resistant materials.

Oxidation corrosion is a type of dry corrosion brought about by the direct action of oxygen on metals, at low or high temperature, in the absence of moisture. Alkali and alkaline-earth metals are oxidised even at low temperature; most other metals (except Ag, Au, Pt) are oxidised at high temperature.

Mechanism

The metal is first oxidised at the surface to metal ions, and the electrons released reduce oxygen to oxide ions; the two combine to form a thin film of metal oxide:

Once the film covers the surface, further oxidation can continue only if metal ions diffuse outward through the film, or oxide ions diffuse inward. Since metal ions are smaller, outward diffusion of Mⁿ⁺ is usually faster. Whether corrosion continues depends on the nature of the oxide film:

• Stable film: a fine-grained, tightly adhering, non-porous film (on Al, Cr, Pb, Sn, Cu) acts as a protective barrier and stops further corrosion.
• Unstable film: the oxide decomposes back to metal and oxygen (on Ag, Au, Pt), so no net oxidation occurs: 2Ag₂O → 4Ag + O₂.
• Volatile film: the oxide evaporates as soon as it forms (MoO₃ on molybdenum), exposing fresh metal, so corrosion is rapid and continuous.
• Porous film: the film has pores or cracks through which oxygen reaches the metal, so corrosion continues (alkali and alkaline-earth metals such as Li, Na, K, Mg).

Pilling–Bedworth rule

The protective nature of the oxide film is judged by the specific volume ratio = volume of oxide formed ÷ volume of metal consumed. If the ratio is less than 1, the oxide cannot cover the surface completely and the film is porous and non-protective (e.g., Li, Na, K, Mg). If the ratio is greater than 1 (but not too large), the film is compact, non-porous and protective (e.g., Al, Cr, Ni). A very large ratio (W: 3.6) causes stress and cracking of the film.

Spectroscopy is the branch of science that deals with the study of interaction of electromagnetic radiation with matter. When radiation falls on a molecule, it can absorb energy only in definite quanta, ΔE = hν = hc/λ, which raise it from a lower to a higher energy level (electronic, vibrational or rotational). A plot of absorption (or emission) against wavelength or frequency is called a spectrum.

The periodic table is a tabular arrangement of elements in order of increasing atomic number, such that elements with similar properties fall in the same vertical column (group). According to the modern periodic law (Moseley), the physical and chemical properties of elements are a periodic function of their atomic numbers. The long form of the periodic table has 18 groups and 7 periods, and is divided into s-, p-, d- and f-blocks according to the subshell receiving the last electron. Periodicity of properties arises from the regular recurrence of similar outer electronic configurations.

All four are answered below; write any two in the exam.

(i) Phase diagram of single component system (water)

The water system is a one-component system (C = 1). Phase rule gives F = 1 − P + 2 = 3 − P. Since F cannot be negative, at most 3 phases (ice, water, vapour) can coexist, and the maximum F is 2, so the system is represented on a pressure–temperature diagram.

Curves (two phases in equilibrium, P = 2, F = 1, univariant)

• OA — vaporisation curve: water ⇌ vapour. It shows the vapour pressure of liquid water at different temperatures. It ends at A, the critical point (374 °C, 218 atm), beyond which liquid and vapour become indistinguishable.
• OB — sublimation curve: ice ⇌ vapour. It shows the vapour pressure of ice at different temperatures and extends down to absolute zero.
• OC — fusion (melting) curve: ice ⇌ water. It slopes towards the pressure axis (negative slope), showing that the melting point of ice decreases with increase of pressure. This is because ice occupies more volume than water; by Le Chatelier's principle, pressure favours melting.
• OA′ — metastable curve (dashed): water can be cooled below its freezing point without forming ice (supercooled water). OA′ is the vapour-pressure curve of supercooled water ⇌ vapour, a continuation of OA. It lies above OB, i.e., supercooled water has a higher vapour pressure than ice at the same temperature, so it is metastable; slight disturbance or adding an ice crystal converts it to ice.

Areas (one phase, P = 1, F = 2, bivariant)

Area AOC — water; area BOC — ice; area below AOB — vapour. Within an area, both T and P can be changed independently without changing the number of phases.

Triple point O (P = 3, F = 0, non-variant)

The three curves meet at O, where ice, water and vapour coexist in equilibrium, at 0.0098 °C (≈ 0.01 °C) and 4.58 mm Hg. If temperature or pressure is changed even slightly, one of the phases disappears.