Agitator and heating times 1

[bragamor]

Hi all,

Is it better to stop using the agitator when heating a reactor?

Thanks for your help on this one

 

[Latexman]

No.

Good luck,
Latexman

 

[bragamor]

I forgot to mention that we have an external hot oil heat exchanger to heat the reactor.

 

[Latexman]

It's still no.

Good luck,
Latexman

 

[bragamor]

Thanks Latexman, can you tell me a bit more about why the answer is no (any article, book). According to one of the heat transfer books I am reading, agitators reduce de delta T, therefore the use of agitators increase the heating time...

 

[Latexman]

You are welcome. It's just my opinion based on your extremely concise question and many years of experience. My answer is very general - no agitation yields poor heat transfer coefficient.

If you would add more pertinent details to the description of the equipment/problem, I can elaborate. As it stands, we don't know very much about your RX. Is the RX contents pumped through the hot oil heat exchanger and then returned to the RX, or does it have hot oil circulating through the hot oil heat exchanger and then through a jacket on the RX?

Good luck,
Latexman

 

[bragamor]

You are quite right, I did not give you much information.

Our reactor is a vertical one (capacity 180.000 pounds aprox), the product is pumped from the RX to the external heat exchanger, and back to the reactor. At the shell side of the external heat exchanger we have the hot oil that is constantly circulating at about 530F and 600gpm.

Hope this information helps some more.

Thanks again for your hellp

 

[IRstuff]

Generally speaking, temperature gradients are less than desirable for liquid processes, since either desired reactions occur in a narrow range of temperatures, or undesirable reactions occur outside of a narrow range of temperatures. Therefore, it's typically better that your liquid is all at the same, uniform temperature.

So, yes, the delta T across the fluid is low, but that means that it's all at the same temperature, which is usually the desire.

A classic example of non-uniform heating is a microwave oven, where you can get boiling on the surface and still have frozen food in the center. Not many processes are happy with that type of temperature distribution.

TTFN

FAQ731-376

 

[Latexman]

With the agitator off, the hot, returning liquid may not mix in with the RX contents and may "short circuit" to the outlet nozzle to the hot oil heat exchanger. As such, the T of that stream will be higher, thus reducing the delta T of the exchanger and lowering heat transfer. A well mixed RX will give the lowest T going to the hot oil heat exchanger and the highest delta T and heat transfer.

Good luck,
Latexman

 

[CastMetal]

to Latexman, while I agree that the heat transfer is greatest with a uniform Rx temperature. Depending upon heat loses involved in the reactor and many of the other issues you brought up. Pre-heating the tank to near the desired temperature could be quicker without agitation. Again depending on tank design i could see scenarios where heat loss is greatly increased due to agitation if the hot fluid return was in a central or more well insulated portion of the tank.

I think it may be quick to dismiss all situations as needing agitation throughout the heating process.

 

[Latexman]

I see your point. With hot oil at 530 F, I assumed the RX is well insulated. If it is, heat loss would be very very low. If it isn't, that's mind boggling to me. bragamor, is it?

Good luck,
Latexman

 

[IRstuff]

If you think you're going to be losing more heat than you gain from the heaters, there's already a serious problem. Nonetheless, having your liquid at different temperatures, possibly drastically different, just doesn't sound like a desirable situation.

TTFN

FAQ731-376

 

[CastMetal]

The comment wasn't supposed to be the ideal situation just an observation that some scenarios may justify not agitating the liquid

 

[IRstuff]

Let's agree to disagree. Chemical processing as a discipline includes a strong component of economics. Running a liquid heating process with suboptimal insulation is basically throwing money out the door.

TTFN

FAQ731-376

 

[CastMetal]

agreed

 

[bragamor]

Yes the rx is perfectly insulated, Latexman