Catalysis and Catalytic Reactors Questions and Answers

Answer: C

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

Both are true and the reason gives the mechanism. Catalysis happens on the surface, so what matters is exposed metal atoms rather than metal mass. Dispersing platinum as crystallites a few nanometres across on a high area support puts a large fraction of the atoms at the surface, where a lump of the same metal would expose a vanishing fraction - which is why a vehicle catalyst uses grams rather than kilograms.

Answer: D

Correct answer: regeneration by burning off the deposit

Regeneration is the cure rather than the disease. The three genuine modes are poisoning, in which an impurity binds the active sites irreversibly; coking, in which carbonaceous material buries them; and sintering, in which the crystallites coalesce at temperature and the exposed area falls. Coking is often reversible, poisoning usually is not, and sintering never is.

Answer: A

Only some kinds of deactivation are reversible, and sintering in particular is not. Knowing which is which decides whether a catalyst charge can be recovered or must be replaced. Sintering is the one that cannot be undone.

Answer: B

The metal crystallites coalesce and lose surface area, and no ordinary treatment restores the dispersion. This is why the maximum operating temperature is a hard limit rather than a guideline. Crystallite growth destroys the surface area.

Answer: C

Correct answer: burning it off in dilute oxygen

Coke is burned off in air heavily diluted with steam or nitrogen, and the dilution is the point: the combustion is strongly exothermic, and undiluted air would produce a temperature excursion that sinters the metal and destroys the support - converting a reversible deactivation into a permanent one. The burn is therefore controlled by limiting the oxygen supplied rather than the rate of reaction.

Answer: D

Both are true and the reason gives the hazard: the operation trades one deactivation mechanism for a worse and irreversible one if the temperature is allowed to run. Control of the burn is essential. Dilution keeps the peak temperature down.

Answer: A

Correct answer: the catalyst must be circulated continuously

Fluidisation makes the bed behave as a liquid, so catalyst can be drawn off, regenerated and returned continuously - which is exactly what a fluid catalytic cracker needs, its catalyst coking within seconds. The bed also gives excellent heat transfer and a near-uniform temperature. What it gives up is plug flow and mechanical simplicity, and it grinds its own catalyst by attrition.

Answer: B

The first two are correct and are the reasons the arrangement is used. The third is wrong, since gas bypassing in bubbles makes the contacting much poorer than in a fixed bed. Attrition of the particles is the other cost.

Answer: C

Gas bubbling through the bed gives poor contacting rather than plug flow. The other three are the genuine advantages that justify the arrangement. Contacting is where it loses out. Bubbles carry gas past the solid unreacted.

Answer: D

The fine particles have a high effectiveness factor and the liquid gives good heat transfer. Separating the catalyst from the product afterwards is the main practical difficulty. Three phase reactions are its usual home.