cold spot in a multitubular reactor

[reactorshell]

have anyone come across the phenomenon of a 'cold spot' within a enothermic multitubular non-isothermal reactor. The idea is that the temperatures within the reactor drops to a very low temperature in the front section of the reactor, somewhat like the reversible of the hot spot of an exothermic reactor.

The question is: what could have caused such a phenomenon? I would presume that the heat transfer resistance would probably be much higher on the tube side. However, would it be possible that the shell-side, where a heating medium is flowing, does not have a high heat transfer coefficient as we may have suspected?

 

[joerd]

I would agree with you that it looks like the "reversed hot spot", so the cause would also be similar, such as improper catalyst loading. It also depends on the heating medium. If this is steam, the heat transfer coefficient on the shell side would indeed be very high. However, hot oil could have a low heat transfer coefficient, especially if there is preferential flow in the shell side, and you have "dead corners" with no or little flow. Regards,

Joerd

 

[Montemayor]

First and foremost, there is no such term as "enothermic". Agreeably, you didn't spell check but it could be either eXothermic or enDothermic that you meant to write. So, to clear things up correctly, do you mean endothermic?

If the reaction is an endotherm (as I believe), right off, various things could have happened:

1) Your catalyst is dead or dying up front; this could be because of poisoning or fouling originating in your feed stock; a question here is: how do you know the temperature is cold "up front"? do you have temperature sensors at the front of the monitored tube(s) as well as "in the back"? I've never seen this done - but it is possible.
2) Since it is logical to assume that you don't have a temperature probe in 100% of all the catalyst tubes, it
follows then to ask what tubes do you monitor and where are they located. Some tubes near the outside reactor wall are flow-starved sometimes and they transfer less heat.

Are you depending on the reactor's heat input to preheat the reactants or do you pre-heat externally within close proximity of the desired reaction temperature and then, subsequently feed the individual reactants into the reaction tubes for almost immediate reaction? Details on how you are carrying out this endotherm would help to get down to specifics on what can be happening.

What is the temperature that you call a "cold spot"? Is it colder than your heating fluid? I doubt it, but I have to ask because you haven't identified it. The affected tubes and their locations should be identified also. Also, by saying "non-isothermal reactor" do you mean "adiabatic reactor"?

 

[reactorshell]

joerd,

It's interesting how you mentioned the possibility of dead zones within the shell side of the reactor, because I do have exactly the same suspicions. However, I have been cracking on head on how I can actually prove this. Do you have any ideas on this? Anyone else?

Montemayor,

i must admit i wrote the posting in a hurry and therefore there's alot of typos. But I guess it should be obvious that I'm refering to an ENDOTHERMIC, NON-ADIABATIC, NON-ISOTHERMAL reactor. Anyway, the answers to your questions..

1)There are three tubes containing a thermowell each, which holds ten thermocouple points spaced evenly along the length of the tubes. THe temperature readings are sent to the DCS so that we can monitor the conditions within the reactor.

I do not discount the possibility of fouling or catalyst poisoning. However, the fact that the temperature drops to a very low temperature (>30 C) when it should not have been the case seems to suggest that the catalyst activity could be in fact too high for the heating medium to supply the necessary amount of heat.

When the temperature drops to a low level, there is a possibility of condensation of the heavy ends within the pores of the catalyst. This could lead to polymerization of the species and therefore severe plugging of the tubes could be observed in the reactors. This is just a theory from us. We have no idea how we can prove this though.

2) Unfortunately, we have only three thermowells within a reactor which has 2000+ tubes. The tubes with the thermowells are placed at 120 degrees from each other at a distance of 0.5*Dshell from the centre of the reactor shell.

We did have some doubts as to how representative these temperature measurements could be. Guess we just have to do with what we have for now.

The reaction is endothermic. The reaction mixture is preheated externally and sent into the reactor for reaction. I wouldn't say reaction is immediate in the front section of the tubes. However, reaction is fast enough for us to notice the significant drop in temperature within the front end of the reactor. As the rate of reaction slows down, the temperature at teh back of the reactor slowly picks up again, as we would expect because of the heating medium flowing on the shell side.

I really appreciate the two of you taking time to discuss about these. Thanks alot.

Comments anyone else?