Non-isothermal Reactor Design 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. Between the ignition and extinction points three steady states coexist, and which one the reactor occupies depends on where it came from - so raising the feed temperature ignites at one value and lowering it extinguishes at a lower one. The reactor therefore remembers its history, which is what a hysteresis loop means.

Answer: D

Correct answer: True

The middle steady state of the three is unstable in the sense that any small departure grows: above it generation exceeds removal and the reactor heats further, below it the reverse. But it is often the most productive point, and a controller measuring temperature and adjusting the coolant can hold the reactor there indefinitely. Unstable means it will not stay there on its own, not that it cannot be operated.

Answer: A

Correct answer: heat generation exceeds heat removal

Generation rises exponentially with temperature through the Arrhenius term while removal rises only linearly with the temperature difference to the coolant. Once generation is the steeper of the two, any excess raises the temperature, which raises the generation further, and the feedback accelerates. That is why runaway is sudden rather than gradual once it begins.

Answer: B

Correct answer: increasing the reactor volume for the same duty

Heat is generated in the volume and removed through the surface, so a larger vessel has less area per unit volume and is harder to cool - which is why a reaction perfectly safe in a flask can run away in a plant. The other three all help: gradual feeding limits how much can react at once, dilution raises the heat capacity and lowers the adiabatic rise, and more area per unit volume improves removal directly.

Answer: C

The first two are correct and are standard measures. The third is inverted, since the heat transfer area per unit volume falls as the vessel grows and cooling therefore becomes harder. Scale-up works against thermal safety.

Answer: D

Correct answer: high at the start and falling

Two effects pull in opposite directions. Rate rises with temperature, and for an exothermic reversible reaction the equilibrium conversion falls with it. Early on, far from equilibrium, rate governs and the temperature should be high; later, as equilibrium is approached, the ceiling governs and the temperature should fall to lift it. The locus of the optimum is a declining curve, which multi-bed reactors with interstage cooling approximate in steps.

Answer: A

Both are true and the reason gives the design, and sulphuric acid converters and ammonia synthesis reactors are both built on exactly this principle. The profile falls as conversion rises. Each bed runs adiabatically between coolers.

Answer: B

An endothermic reaction absorbs heat, and none of the other three could then be true. The three together are the signature of an exothermic reaction. Sign of the heat of reaction settles it. Endothermic behaviour is the opposite in each respect.

Answer: C

Correct answer: True

An endothermic reaction absorbs heat, so it cools its own surroundings and slows itself down. The feedback is negative rather than positive, which makes runaway impossible in principle - the reaction is self-limiting. The corresponding difficulty is the opposite one: keeping enough heat going in to sustain a useful rate.

Answer: D

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

Both are true and the reason gives the cause. Because the reaction consumes heat, the temperature and therefore the rate fall unless heat is supplied at least as fast as the reaction takes it. The design problem becomes one of heat transfer rather than of kinetics, which is why steam reforming and ethane cracking are done in fired tubes of small diameter - the geometry is chosen to get heat in.