Gaseous Pollutant Control Questions and Answers

Answer: A

Correct answer: Ammonia slip has risen and formed salts

Pushing the molar ratio above what the mixing can use leaves unreacted ammonia in the gas. Downstream, where the gas cools, it combines with sulphur trioxide to form ammonium sulphate and bisulphate, which are sticky solids that deposit on the air heater surfaces, foul them and raise the draught loss. This is the practical limit on how far removal can be pushed, and the reason ammonia slip is itself a monitored and limited quantity.

Answer: B

The absence of a by-product market removes wet limestone's main economic advantage, and the spray dryer avoids an effluent stream altogether. Dry sorbent injection would be cheaper still but would use far more reagent for less removal.

Answer: C

Liquid can no longer drain against the rising gas and accumulates in the packing, and both symptoms follow from that. Reducing the gas or liquid rate restores operation, and the design margin exists to keep the tower clear of the condition.

Answer: D

Correct answer: breakthrough

Breakthrough is the point at which the mass transfer zone reaches the outlet of the bed and the effluent concentration starts to rise. It comes before saturation, sometimes well before, so the working capacity used in design is always less than the equilibrium capacity. The bed is switched at breakthrough, since running past it means emitting the pollutant to buy the remaining capacity.

Answer: A

Correct answer: slip

Ammonia slip is the unreacted reagent leaving with the flue gas. It rises as the injection ratio is pushed up for higher removal, and it matters for three reasons: ammonia is itself a pollutant, it forms ammonium salts that foul downstream heat transfer surfaces, and it contaminates the fly ash, which may then be unsaleable to the cement industry. Permits typically limit it to a few parts per million.

Answer: B

Correct answer: flooding

Flooding is the condition in which the upward gas flow exerts enough drag on the descending liquid to prevent it draining. Liquid accumulates, the pressure drop rises almost vertically and liquid is carried out of the top. Channelling, weeping and entrainment are distinct troubles: channelling is maldistribution through the packing, weeping is liquid falling through the holes of a tray, and entrainment is droplet carryover.

Answer: C

Correct answer: Henry’s

Henry’s law states that at equilibrium the partial pressure of a gas above a liquid is proportional to its concentration in the liquid, \( p = Hx \). It fixes the equilibrium line on which every absorption calculation rests, and a small Henry constant means a readily soluble gas needing a small tower. Raoult’s law applies to the solvent rather than a dilute solute, Dalton’s law concerns partial pressures in a gas mixture, and Fick’s law describes diffusion.

Answer: D

Sufficient residence time lets the reaction complete once temperature and mixing have started it. Falling short on any one of the three produces partial oxidation products that may be more harmful than the original stream.

Answer: A

Correct answer: Spray dryer absorption

The plant is small, so the high capital cost of a wet limestone system is hard to justify; it has no gypsum market, so the wet route’s chief economic advantage disappears; and the limit is modest, so 80 to 90 per cent removal suffices. Spray dryer absorption meets all three: moderate cost, a dry solid product handled with the fly ash, and no liquid effluent or reheat. Dry sorbent injection would probably fall short of the limit.

Answer: B

Correct answer: Heating the whole volume costs more than it yields

An incinerator must raise the entire gas stream to the oxidation temperature, and in a dilute stream almost all of that gas is air carrying no calorific value at all. The fuel bill is set by the volume, while the benefit is set by the pollutant mass, so the economics collapse as the concentration falls. The pollutant would oxidise perfectly well; it is simply not worth what it costs to heat the air around it.