Gaseous Pollutant Control Questions and Answers

Q: Which change would NOT improve a carbon bed's working capacity?

Correct answer: Raising the bed temperatureAdsorption is exothermic, so raising the temperature shifts the equilibrium against it and the working capacity falls. The other three all help: cooling improves capacity directly, drying removes the water that competes for micropore sites, and removing competing organics leaves more of the surface available for the species of interest. Heat is used deliberately in the other direction - to drive the adsorbate off during regeneration.

Q: An engineer proposes thermal incineration for a very large stream of dilute odorous air. This is

Correct answer: Unwise: the fuel bill would be prohibitiveThe incinerator must heat the whole stream, almost all of which is air carrying no pollutant and no calorific value, so the fuel cost scales with the volume while the benefit scales with the small pollutant mass. Adding a catalyst lowers the temperature required but does not change the fundamental arithmetic for a stream this dilute. Biofiltration, which works at ambient temperature, is the appropriate answer - and the compounds being biodegradable is what makes it viable, not what would rule it out.

Q: Which of the following would NOT reduce ammonia slip?

Correct answer: Raising the ammonia injection ratioRaising the injection ratio is what causes slip: the extra reagent beyond what the mixing can bring into contact with nitrogen oxide passes straight through. The other three all reduce it - better mixing lets more of the reagent find its reactant, more catalyst volume gives the reaction longer to complete, and operating within the correct temperature window ensures the reaction proceeds at all rather than the ammonia surviving unreacted.

Q: Which of these is LEAST relevant to choosing a gaseous control method?

Correct answer: The colour of the gas streamColour tells you almost nothing about how to treat a gas. The choice turns on concentration, which sets the method family; on solubility, which decides whether absorption is practical; on whether the material has recovery value, which may justify condensation or adsorption over destruction; and on flow rate, temperature, humidity and the presence of catalyst poisons. A visible plume may be what prompts the investigation, but it does not select the technology.

Q: Which statement about the three T's of incineration is TRUE?

Correct answer: Failing any one gives partial oxidationTemperature, time and turbulence are jointly necessary: the gas must be hot enough for the reaction to proceed, held long enough for it to finish, and mixed well enough that every parcel of pollutant meets oxygen. Deficiency in any one leaves partial oxidation products - carbon monoxide, aldehydes, and from chlorinated feedstocks dioxins and furans. Turbulence matters in thermal units as much as anywhere, and residence time cannot be dispensed with at any temperature.

91.

Which statement about adsorption is FALSE?

A.High humidity reduces the working capacity
B.Steam is commonly used for regeneration
C.Beds are operated to full saturation before switching
D.Breakthrough occurs when the mass transfer zone reaches the outlet

Answer: C

Correct answer: Beds are operated to full saturation before switching

This is false. The bed is switched at breakthrough, when the mass transfer zone reaches the outlet and the effluent concentration starts to rise. Running on to saturation would mean emitting pollutant at a steadily rising concentration in order to use the last of the capacity, which defeats the purpose of the bed. The other three statements are all correct.

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92.

Which change would NOT improve a carbon bed's working capacity?

A.Cooling the incoming stream
B.Drying the incoming stream
C.Removing competing organic species
D.Raising the bed temperature

Answer: D

Correct answer: Raising the bed temperature

Adsorption is exothermic, so raising the temperature shifts the equilibrium against it and the working capacity falls. The other three all help: cooling improves capacity directly, drying removes the water that competes for micropore sites, and removing competing organics leaves more of the surface available for the species of interest. Heat is used deliberately in the other direction - to drive the adsorbate off during regeneration.

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93.

Which of these is NOT true of selective non-catalytic reduction?

A.It achieves eighty to ninety per cent removal
B.It requires no catalyst
C.It operates in a narrow furnace temperature window
D.Its capital cost is far below the catalytic route

Answer: A

Correct answer: It achieves eighty to ninety per cent removal

That figure belongs to the catalytic route. Selective non-catalytic reduction achieves only 30 to 50 per cent, and its case rests entirely on the other three statements, which are true: no catalyst to buy, house or replace, hence a far lower capital cost, at the price of depending on a narrow furnace temperature window of roughly 850 to 1100 degrees.

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94.

An engineer proposes thermal incineration for a very large stream of dilute odorous air. This is

A.Unwise only if the compounds are biodegradable
B.Unwise: the fuel bill would be prohibitive
C.Sound
D.Sound if a catalyst is added

Answer: B

Correct answer: Unwise: the fuel bill would be prohibitive

The incinerator must heat the whole stream, almost all of which is air carrying no pollutant and no calorific value, so the fuel cost scales with the volume while the benefit scales with the small pollutant mass. Adding a catalyst lowers the temperature required but does not change the fundamental arithmetic for a stream this dilute. Biofiltration, which works at ambient temperature, is the appropriate answer - and the compounds being biodegradable is what makes it viable, not what would rule it out.

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95.

Which statement about wet limestone desulphurisation is TRUE?

A.It needs no reheat of the treated gas
B.It is the cheapest route to install
C.Forced oxidation of the sulphite gives a saleable gypsum
D.It produces a dry solid product

Answer: C

Correct answer: Forced oxidation of the sulphite gives a saleable gypsum

Air blown into the reaction tank oxidises the calcium sulphite to calcium sulphate dihydrate, which dewaters well and sells to the plasterboard and cement industries - turning a disposal cost into revenue. The other three statements belong to other routes or are simply false: wet scrubbing leaves the gas saturated and usually needing reheat, it is the most expensive route to install, and its product is a wet slurry rather than a dry solid.

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96.

Which of the following would NOT reduce ammonia slip?

A.Improving the mixing of reagent into the gas
B.Adding catalyst volume
C.Operating within the correct temperature window
D.Raising the ammonia injection ratio

Answer: D

Correct answer: Raising the ammonia injection ratio

Raising the injection ratio is what causes slip: the extra reagent beyond what the mixing can bring into contact with nitrogen oxide passes straight through. The other three all reduce it - better mixing lets more of the reagent find its reactant, more catalyst volume gives the reaction longer to complete, and operating within the correct temperature window ensures the reaction proceeds at all rather than the ammonia surviving unreacted.

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97.

Which statement about packed and plate towers is FALSE?

A.Packed towers handle solids in the gas better than plate towers
B.Packed towers give a lower pressure drop
C.Packed towers are easily built in corrosion-resistant materials
D.Plate towers tolerate a wider turndown

Answer: A

Correct answer: Packed towers handle solids in the gas better than plate towers

This is the false statement, and it reverses the usual comparison. Packing plugs and channels when solids are present, and cleaning it means removing and washing the packing. Plate towers, with open downcomers and large passages, tolerate solids far better and can be cleaned in place. The other three are true: packed towers give lower pressure drop, are readily built in plastics and ceramics for corrosive duty, and are less tolerant of turndown than plates.

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98.

Which of these is LEAST relevant to choosing a gaseous control method?

A.Whether the material has recovery value
B.The colour of the gas stream
C.The pollutant's concentration
D.The pollutant's solubility

Answer: B

Correct answer: The colour of the gas stream

Colour tells you almost nothing about how to treat a gas. The choice turns on concentration, which sets the method family; on solubility, which decides whether absorption is practical; on whether the material has recovery value, which may justify condensation or adsorption over destruction; and on flow rate, temperature, humidity and the presence of catalyst poisons. A visible plume may be what prompts the investigation, but it does not select the technology.

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99.

Which statement about the three T's of incineration is TRUE?

A.Turbulence matters only in catalytic units
B.Residence time may be ignored above 800 degrees
C.Failing any one gives partial oxidation
D.Temperature alone is sufficient

Answer: C

Correct answer: Failing any one gives partial oxidation

Temperature, time and turbulence are jointly necessary: the gas must be hot enough for the reaction to proceed, held long enough for it to finish, and mixed well enough that every parcel of pollutant meets oxygen. Deficiency in any one leaves partial oxidation products - carbon monoxide, aldehydes, and from chlorinated feedstocks dioxins and furans. Turbulence matters in thermal units as much as anywhere, and residence time cannot be dispensed with at any temperature.

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100.

Which of the following best explains why chemical absorption suits a sparingly soluble gas?

A.The reagent raises the gas's solubility
B.The reaction lowers the tower's pressure drop
C.Chemical absorption needs no liquid
D.Reacting it keeps the driving force high

Answer: D

A physical absorber would approach equilibrium and stall, whereas a reagent that consumes the dissolved gas keeps its concentration near zero. The cost is a consumed reagent and a pollutant that cannot be recovered unchanged.

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