Gaseous Pollutant Control Questions and Answers

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.