Reactors in Series and Parallel Questions and Answers

Answer: C

Correct answer: True

Splitting the feed equally between two identical reactors gives each half the flow and half the volume, so the space time in each is the same as in the single large reactor - and since a plug flow reactor’s conversion depends only on the space time, each branch achieves exactly what the single reactor would, and the recombined stream matches it. The result holds for CSTRs in parallel too, and it fails if the split is unequal.

Answer: D

There is no mixing between them, so the fluid simply continues its journey through the second. Interstage cooling or feed addition would break the equivalence. Nothing mixes between the two vessels. The fluid simply continues on its way.

Answer: A

Both are true and the reason gives the reason for the difference, and interstage cooling is used deliberately for exothermic reversible reactions to restore a favourable equilibrium. The temperature history is what changes.

Answer: B

Correct answer: A-2, B-4, C-1, D-3

Series and parallel behave quite differently for the two ideals. Plug flow reactors in series are simply one longer reactor, and in parallel with equal space times they are equivalent to one larger one - so a plug flow system is insensitive to the arrangement. Stirred tanks are not: in parallel they merely make one larger tank, while in series they approach plug flow as the number rises, which is where all the benefit lies.

Answer: C

Two parallel plug flow reactors behave exactly as one of the combined volume, so nothing is gained. The other three all genuinely improve the conversion for the same total volume. Parallel plug flow gains nothing at all.

Answer: D

Each tank leaves half, so one eighth survives all three. Conversions multiply as fractions remaining rather than adding, which is the point the question makes. One eighth survives all three stages. Adding conversions gives an impossible answer.

Answer: A

It is the fractions remaining that multiply, so three stages each removing half leave one eighth rather than removing everything. Adding conversions would give a hundred and fifty per cent, which is plainly impossible.

Answer: B

Correct answer: 2 - 4 - 1 - 3

A single stirred tank is worst, since all of it sits at the exit concentration. Two in series improve on it, four improve further, and plug flow is the limit the cascade approaches but never reaches. The order is the same ranking as the tanks-in-series item, read from the poorest arrangement to the best.

Answer: C

The recycle mixes exit material back into the feed, which is exactly what a stirred tank does continuously. A very large recycle ratio makes the two indistinguishable in behaviour. Recycle spans the whole range of mixing.

Answer: D

The first two are correct and describe the standard construction. The third is wrong, since the method works for any rate curve and is particularly useful where no analytical solution exists. Measured rate data can be used directly.