Caustic embrittlement is the cracking of boiler metal caused by
Answer: A
Free caustic concentrates in the crevices at riveted or stressed joints and attacks the iron along its grain boundaries, giving intercrystalline cracks with no loss of thickness. Adding tannin, lignin or phosphate prevents it.
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Boiler corrosion by dissolved oxygen is prevented chemically by adding
Answer: B
Correct answer: sodium sulphite or hydrazine
Both are oxygen scavengers. Sodium sulphite is oxidised to the sulphate, and hydrazine to nitrogen and water - which is why hydrazine is preferred at high pressure, since it adds no dissolved solids. Chemical dosing follows mechanical deaeration, which removes the bulk of the oxygen; the chemical deals with the last traces.
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Dissolved carbon dioxide corrodes a boiler and its condensate lines by forming
Answer: C
Correct answer: carbonic acid
Carbon dioxide dissolves to form carbonic acid, which lowers the pH and attacks steel. The problem is worst in condensate lines, because the gas leaves with the steam and redissolves as the steam condenses, so the returning condensate is acidic. Neutralising or filming amines are dosed into the steam to deal with it.
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In the lime-soda process, lime is used to remove
Answer: D
Correct answer: temporary hardness and magnesium salts
Lime supplies hydroxide, which converts bicarbonate to carbonate so that calcium precipitates as the carbonate, and which precipitates magnesium as its hydroxide. It also deals with dissolved carbon dioxide. What it cannot remove is the permanent calcium hardness, since precipitating calcium sulphate needs a carbonate source rather than a hydroxide one.
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In the lime-soda process, soda ash is used to remove
Answer: A
Correct answer: permanent calcium hardness
Soda ash supplies carbonate ions, which convert the soluble calcium sulphate and chloride into insoluble calcium carbonate. That is precisely the part lime cannot handle, which is why the two reagents are used together - lime for the temporary hardness and the magnesium, soda ash for the permanent calcium hardness.
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The hot lime-soda process is preferred to the cold because at high temperature the reactions are
Answer: B
Correct answer: faster and the precipitate settles more readily
At elevated temperature the reactions go to completion more quickly, the precipitates form larger and denser particles that settle rapidly, and dissolved gases are driven off. The result is a smaller plant for a given throughput and a softer product water. The heat is usually available as exhaust steam, which is what makes the process economic.
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The zeolite or permutit process softens water by exchanging
Answer: C
Correct answer: sodium ions for calcium and magnesium ions
The zeolite holds exchangeable sodium ions, and as hard water passes through, the calcium and magnesium in it displace that sodium and are held on the bed. The water leaves with its hardness ions replaced by sodium, which does not form scale. When the bed is exhausted it is regenerated with brine, which drives the exchange back the other way.
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An exhausted zeolite bed is regenerated using
Answer: D
The high sodium concentration drives the exchange backwards, displacing the accumulated calcium and magnesium as their chlorides, which are washed to waste. Acid and alkali are used to regenerate ion exchange resins, not zeolite.
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The zeolite process cannot be used on water that is
Answer: A
Suspended matter blocks the bed and acid attacks the alumino-silicate structure irreversibly, so both must be dealt with before the water reaches it. Its inability to remove anions or to reduce the dissolved solids is a separate limitation.
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Water softened by the zeolite process still contains
Answer: B
Correct answer: dissolved sodium salts
The zeolite process exchanges rather than removes: every calcium or magnesium ion taken out is replaced by two sodium ions, so the total dissolved solids are essentially unchanged and the water leaves carrying sodium sulphates, chlorides and bicarbonates. That is acceptable for a low pressure boiler but not for a high pressure one, where total dissolved solids matter.
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