Gaseous Pollutant Control Questions and Answers

Answer: C

Correct answer: Simplicity, at poorer removal and sorbent use

Dry sorbent injection blows powdered lime or sodium bicarbonate straight into the duct, and the reacted solid is collected with the fly ash. There is no absorber vessel, no slurry handling and no liquid effluent, so it is by far the cheapest route to install. The price is a removal of only 50 to 70 per cent and a large stoichiometric excess of reagent, since the gas-solid contact is brief and imperfect. Saleable gypsum and the highest removal both belong to wet limestone scrubbing.

Answer: D

The catalyst allows the reaction to proceed at around three hundred to four hundred degrees with high selectivity. Ammonia slip and catalyst poisoning are the two operating concerns, and the catalyst itself is a substantial part of the cost.

Answer: A

Ammonia or urea is injected into a narrow temperature window of about eight hundred and fifty to eleven hundred degrees. Removal is typically thirty to fifty per cent, against eighty or ninety for the catalytic route, but capital cost is far lower.

Answer: B

Injecting more reagent than the reaction consumes raises removal but sends the excess up the stack, where it forms ammonium salts and fouls downstream equipment. The trade between removal and slip is the central control problem of both reduction processes.

Answer: C

Correct answer: Passing it through a moist microbial bed

A biofilter passes the gas slowly through a bed of compost, peat or wood chips carrying a moist biofilm. The pollutant partitions into the water layer and is metabolised by the microorganisms to carbon dioxide, water and biomass. It works only on biodegradable compounds at low concentration, needs the bed kept moist and near neutral pH, and requires a long residence time - but it consumes almost no energy, which is what makes it right for large dilute odorous streams.

Answer: D

Packing gives a large interfacial area with a modest pressure drop and is easily built in corrosion-resistant materials. Plate towers handle solids and large turndown better, since a packed bed fouls and channels when solids are present.

Answer: A

Multiplying it by the number of transfer units gives the packed height required, which separates the equipment's efficiency from the separation's difficulty. A more efficient packing shortens the tower without changing the number of units needed.

Answer: B

Correct answer: Upward gas prevents the liquid draining

As the gas velocity rises, the drag it exerts on the descending liquid increases until the liquid can no longer drain freely. It accumulates in the packing, the void space fills, the pressure drop rises very steeply, and liquid is carried out of the top of the tower. Towers are therefore designed at 60 to 70 per cent of the flooding velocity, leaving margin for the load variations of normal operation.

Answer: C

Steam raises the bed temperature and lowers the partial pressure of the adsorbate, driving it off so it can be condensed and recovered. Recovering a valuable solvent in this way often pays for the installation on its own.

Answer: D

Reacting the dissolved gas keeps its concentration in the liquid near zero and so maintains the driving force through the whole tower. The penalty is that the reagent is consumed and the pollutant is no longer recoverable in its original form.