Gaseous Pollutant Control Questions and Answers

Answer: D

Correct answer: Only three

Absorption, adsorption and incineration are correctly described. The fourth is wrong: condensation removes a vapour by cooling it below its dew point so that it becomes a liquid - a purely physical change of phase involving no reagent and no chemical reaction. That is precisely why it recovers the material in usable form.

Answer: A

Correct answer: Only three

The first three pairs are correct. The fourth is wrong: wet limestone scrubbing leaves the gas cooled and saturated with water vapour, so it commonly does require reheat before the stack, both to restore buoyancy for plume dispersion and to prevent acid condensation in the ductwork. Avoiding that reheat is one of the advantages claimed for the semi-dry and dry routes.

Answer: B

Correct answer: Only three

The first three are correct. The fourth contradicts the name: non-catalytic reduction uses no catalyst at all, which is exactly what distinguishes it - it injects the reagent into the furnace and relies on a narrow high-temperature window to drive the reaction. Its low capital cost follows from having no catalyst bed to buy, house and eventually replace.

Answer: C

Correct answer: Only three

The first three are well matched. The fourth is wrong: a water-saturated stream is the worst case for activated carbon, since water competes for the adsorption sites and can halve the working capacity. Such a stream must be cooled and dried, or heated to lower its relative humidity, before it reaches the bed - or another method chosen altogether.

Answer: D

Correct answer: Only three

Breakthrough, ammonia slip and flooding are correctly defined. The fourth confuses two quantities: the height of a transfer unit is the height that accomplishes one transfer unit, and the total packed height is that multiplied by the number of transfer units required, \( Z = HTU \times NTU \). The first is set by the packing and the flow rates, the second by the separation demanded.

Answer: A

Incineration oxidises the pollutant to carbon dioxide and water, so nothing remains to be handled afterwards. The other three transfer it to a liquid, a solid or a condensate that must then be disposed of or recovered.

Answer: B

Selective catalytic reduction reacts the oxides of nitrogen with ammonia over a catalyst. The other three all capture sulphur dioxide with an alkaline reagent, differing only in whether the reagent is wet, semi-dry or dry.

Answer: C

Adsorption capacity rises as the temperature falls and as competing molecules are removed, so cooling and drying the stream helps directly. The other three each occupy or weaken the sites the target molecule would otherwise use.

Answer: D

Correct answer: Breakthrough curve

The breakthrough curve is the plot of outlet concentration against time for a fixed bed, and it belongs entirely to adsorption - it is what determines when the bed must be switched or regenerated. Height of a transfer unit, flooding velocity and packing type are all absorption tower quantities, describing a continuous countercurrent contactor rather than a bed that loads up and must be taken out of service.

Answer: A

Flooding belongs to a packed absorption tower where liquid cannot drain against the gas flow, and a dry catalyst bed has no liquid at all. The other three are the standard mechanisms by which catalyst activity is lost.