Gaseous Pollutant Control Questions and Answers

Answer: C

Correct answer: 20 seconds

The empty bed residence time is the bed volume divided by the volumetric gas flow:

It is called empty bed because the volume used is the whole bed, not the void fraction, which makes it a simple and comparable design parameter. Twenty to sixty seconds is typical for odour control; readily degraded compounds need less, and recalcitrant ones more.

Answer: D

Correct answer: 1.58 t/day

What matters is the residual, not the removal:

Eleven percentage points of removal separate the two routes, but the residual emission differs by nearly a factor of four - which is the general point that as removal approaches 100 per cent, each further point costs disproportionately and matters disproportionately.

Answer: A

Multiplying gives \( 216 \times 3 = 648 \) kilograms of steam. The steam cost and the condensate handling are the principal running costs of a carbon adsorption system, and they set the value below which solvent recovery does not pay.

Answer: B

The ratio is \( 1073/313 = 3.43 \). Sizing an oxidiser chamber on the cold flow would make it less than a third of the volume actually needed, and the residence time would fall well short of what the reaction requires.

Answer: C

Correct answer: Both Statement I and Statement II are correct

Both are true and the second explains the first. The rate of absorption depends on the difference between the actual and equilibrium concentrations at the interface. In physical absorption the dissolved gas accumulates and that difference shrinks; in chemical absorption a reagent consumes it as fast as it dissolves, holding the free concentration near zero and the driving force near its maximum. This is why sulphur dioxide is scrubbed with an alkaline slurry rather than with water.

Answer: D

The second is correct and it explains why beds are switched early: continuing past breakthrough buys the last of the capacity at the price of an emission. Working capacity is therefore well below the equilibrium figure.

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both are true and the second gives the reason. A catalyst lowers the activation energy so that complete oxidation proceeds at 300 to 400 degrees instead of the 700 to 900 a thermal unit requires. Far less fuel is needed to bring the stream to temperature, and the smaller temperature rise also means less thermal nitrogen oxide is formed. The offsets are the cost of the catalyst and its vulnerability to poisoning by halogens, sulphur and heavy metals.

Answer: B

Correct answer: Both Statement I and Statement II are correct

Both are true and the second describes the mechanism. Limestone slurry absorbs the sulphur dioxide to form calcium sulphite, which is a troublesome sludge; blowing air into the reaction tank oxidises it to calcium sulphate dihydrate, that is gypsum, which dewaters readily and can be sold to the plasterboard and cement industries. Turning a disposal cost into a product is a large part of the economic case for the wet route.

Answer: C

Correct answer: Statement I is incorrect but Statement II is correct

Statement II is correct: the non-catalytic route injects ammonia or urea directly into the furnace and depends on a temperature window of roughly 850 to 1100 degrees, below which the reaction is too slow and above which the reagent is simply oxidised to more nitrogen oxide. Statement I inverts the comparison. It achieves only 30 to 50 per cent removal against the catalytic route’s 80 to 90 - its attraction is its low capital cost, not its efficiency.

Answer: D

Correct answer: Both Statement I and Statement II are correct

Both are true and the second explains the first. Activated carbon is hydrophobic but not perfectly so, and above about 50 per cent relative humidity water begins to condense in the finer micropores, occupying the sites the organic vapour would use. The working capacity can fall by half in a saturated stream, which is why humid gases are cooled and dried, or heated to lower the relative humidity, before the bed.