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Hot Spots In Adiabatic Reactor


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#1 apathy

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Posted 25 January 2008 - 01:13 AM

Do the hot spots always occur in adiabatic reactor?
Is there any way of calculating and even reducing the hot spots temperature?
Would having a shorter (shallow) catalyst bed help with the temperature distribution?

Thanks!


Edit: would distributing feed by using multitubular reactor help reducing this phenomenon?

#2 Austro

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Posted 25 January 2008 - 01:40 AM

For an exothermic reaction and adiabatic rxn, there will always be hot spots as far as the temperature increasing as you go farther in the length of the PBR/PFR. Not sure about there always being "hot spots" as far as radially. No hot spots should occur in an adiabatic CSTR since it is stirred. A trickle bed will definitely have hot spots.

One can easily multiply the conversion (i.e. X) of the limiting reagent times the molarity of the limiting reagent (M) times the Heat of rxn (H) divided by the heat capacity of the fluid (Cp) in Energy.

I.e. Change in T = X*M*H/Cp Cp is in energy per volume per temperature. Of course divide the equation by density if you have Cp on a per mass basis.. you should know how to convert this.

This equation will give you the temperature increase at any point in the reactor provided you know where a certain conversion occurs. (You should be able to know this if you know the rate law)

For an adiabatic bed I dont think there is any way to reduce hot spot temperature. However, you could dilute the reactant or mix it with a high Cp fluid (like water) to lower the concentration and increase the Cp, both decreasing the temperature increase. (i.e. M decreases, Cp increases).

Edit: If they are adiabatic no, because there is no heat transfer. If they are exposed to cool air/water/etc., it will lower the temperature in the reactor because there will be more area for heat transfer.

#3 apathy

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Posted 26 January 2008 - 04:47 PM

Ah I see.. I just thought that with small diameter and height, the catalyst (packing) inside the reactor would have pretty much equal temperature (pseudo-isothermal)

Thanks for the answer! smile.gif

Although that raises another question, why is adiabatic reactor chosen for some processes compared to isothermal reactor, other than economical reason?

#4 Austro

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Posted 26 January 2008 - 06:49 PM

QUOTE (apathy @ Jan 26 2008, 05:47 PM) <{POST_SNAPBACK}>
Ah I see.. I just thought that with small diameter and height, the catalyst (packing) inside the reactor would have pretty much equal temperature (pseudo-isothermal)

Thanks for the answer! smile.gif

Although that raises another question, why is adiabatic reactor chosen for some processes compared to isothermal reactor, other than economical reason?


The reason you would want smaller reactors is because, like I said earlier, if you have a bunch in parallell, you can have the same amount of production but it will be much easier to heat or cool them since you will have a higher surface area. Also, since they are smaller, there wont be as much of a difference in temperature radially in the reactor if transfer heat to/from them.

Other than economical? We are engineers, ultimately everything we do is done because we think it is an economically optimum.

Anyway, one would use an adiabatic reactor if selectivity/temperature was not a problem. This way, the more the reactants heat up, the higher the conversion, so allowing the temperature to increase would be good. However, it can't get so hot that you rupture your reactor of course.

As I understand, most of what is done in industry is having reactors in series where some conversion is reached before it is cooled/heated and then fed to another adiabatic reactor and so on until the desired conversion is obtained.

#5 djack77494

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Posted 28 January 2008 - 02:19 PM

Hot spots are avoided by ensuring good (equal) fluid distribution across the reactor. If the fluid is perfectly distributed, then there are no radial gradients of temperature. There is an axial gradient from the feed temperature to the product temperature, but it would be very well behaved for a well designed reactor.

You can obtain better flow distribution if your catalyst bed is deeper. Unfortunately, this comes at the expense of a higher pressure drop. You can view this as happening if you accept the fact that the individual catalyst particles will not be perfectly distributed. In a shallow catalyst bed, the pressure drop (driving force) is small and insufficient to overcome catalyst maldistribution. In a deeper bed, as the fluid passes by an area of denser than average catalyst, it is effectively mixed with fluid that has just passed through an area of less dense than average catalyst. THere's still some local maldistribution, but the reactor is more effective at averaging together hot and cold spots to bring all closer to the average.
Doug




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