4 replies to this topic

[rana680]

Recently in an interview i was asked this question:

If there is a black box with inlet and outlet streams and you have to tell what type of reactor is inside whether an ideal PFR or an Ideal CSTR then what would you tell ?

My answers went on going till every answer i gave was considered as wrong. I gave the following answers:

1. CSTR will have a mixer but nothing as such for a PFR. Wrong !!!!!!!!!

2. Calculate theoretical conversions for the given volume for both PFR and CSTR. A PFR gives higher conversion per unit volume and this way i will decide which one is it.
Above answer was considered to be wrong.

3. I will use a lot of temperature detectors and place them in various positions along the reactor. Since the reactors are ideal therefore the CSTR will show same temperature for all of the detectors.
Wrong again.

Now they said that I am not able to understand them. They said you have to tell by just looking at that black box.

Can anyone answer it ?

[kkala]

"tell by just looking at that black box" may mean that the key to right answer is some condition, quantity, note, etc, written for some input or output stream. It seems they have written something forcing for one reactor type out of the two. Can you give the exact picture of the black box with anything around it? It might be something like observation puzzle. So try to interprete streams and get information.
For instance, one can initially assume all streams as liquids, with insignificant volume change during reaction. Can you justify it from available data? If so, check if there is a lateral stream out of the box. In case that a concentration of a substance in lateral stream is same as in stream out, CSTR is indicated; otherwise it must be PFR.
Same thought can be with temperatures, on the condition that the reaction is not isothermic (check).
Slurry in streams in or out indicate CSTR, but this is not certain. Probably the guess is based on simpler indications.
Can you ask them to provide the solution? It would be interesting.

[rana680]

No picture was shown. It was an oral question and whatever I wrote in the question is the only thing that was told to me. I cannot ask them the answer now.

[kkala]

So "tell by just looking at that black box" may mean "tell by just thinking of that black box"? The question seems not clear to my understanding. Perhaps they had something in mind that we cannot get.

[EyMiller245]

With so little information given, it would be impossible to tell. The fact is, with reactions, you can almost always think of some obscure combination of situations that can give any outcome you want. The degrees of freedom for reaction problems are immense.

My best guess for this would they wanted to see your thought process or it was one of those riddle type questions.

As an example: Ideal CSTRs are well mixed and there for an energy balance would reveal that there is an agitator putting energy into the system. Of course if you dont know anything of the reaction you could easily counter that by saying the reaction was just endothermic enough to nullify this.

Even if they gave you a "right" answer, you could probably just prove them wrong using this same process

Or, just be like "a CSTR looks like a tub and a PFR looks like tubes"

Thats just my take on it at least.