Gaseous Pollutant Control Questions and Answers

Answer: C

Correct answer: Cool and dry the gas beforehand

The table shows capacity falling with both temperature and humidity, so a warm humid stream is doubly disadvantaged. Cooling raises the capacity directly and, by condensing out moisture, removes the water that would otherwise compete for the adsorption sites - though cooling alone raises the relative humidity, so a condensing cooler or a subsequent reheat is used. Heating would lower the relative humidity but reduce capacity through the temperature effect.

Answer: D

Correct answer: Its far lower capital cost, and no catalyst

The non-catalytic route needs only injection lances into the furnace: no reactor vessel, no catalyst to buy, house, monitor and replace every few years, and no additional pressure drop for the fans to overcome. Where the emission limit can be met with 30 to 50 per cent removal, that saving decides the matter. Its operating temperature is far higher, not lower, and its reagent consumption is greater rather than less.

Answer: A

Transfer units measure how hard the separation is and the height of a transfer unit measures how well the packing performs. Improving the packing shortens the tower without changing the number of units the duty demands.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason states the trade exactly. Between breakthrough and saturation the bed still has capacity, but every hour of that capacity is bought by emitting pollutant at a rising concentration. Since the whole purpose of the bed is to prevent that emission, it is switched at breakthrough, and the design capacity used is the working capacity up to that point rather than the equilibrium capacity.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the mechanism. The fuel needed is what it takes to raise the whole gas stream to the operating temperature, so halving that temperature roughly halves the heat duty. Where fuel dominates the operating cost - a continuous stream of moderate concentration - catalytic incineration wins, provided the stream is free of the halogens, sulphur and metals that would poison the catalyst.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the cause. The gas leaves a wet absorber at 50 to 60 degrees and saturated, so it has little buoyancy and will condense acid in the ductwork and stack. Reheating it by 20 or 30 degrees restores plume rise and protects the downstream metal, at the cost of fuel or of a gas-to-gas heat exchanger. Avoiding this is one of the arguments for the semi-dry routes.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason explains the mechanism. Mixing between reagent and flue gas is never perfect, so pushing removal higher requires injecting more ammonia than stoichiometry demands; the fraction that finds no nitrogen oxide passes through as slip. Removal and slip therefore rise together, and the practical limit on removal is usually set by the slip the downstream equipment and the permit can tolerate.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason is the decisive argument. The fuel required by an incinerator scales with the volume of gas to be heated, while the benefit scales with the mass of pollutant destroyed, so a very dilute stream is the worst possible case for combustion. A biofilter treats the same stream at ambient temperature with almost no energy input, needing only that the compounds be biodegradable and the bed kept moist.

Answer: C

The driving force is maintained the whole way up the tower, which the reason explains, and a physical absorber would saturate long before. The cost is a consumed reagent and a pollutant that cannot be recovered unchanged.

Answer: D

The assertion is true and the reason is true, but the reason is about the choice of packing rather than about flooding. Flooding is avoided because liquid cannot drain against the rising gas and the tower ceases to operate.