Reactor heating

[lamo]

Good day gentlemen,

I'm a bit baffled by the selection of temperature difference in the design of heating of continuous process reactor.
The reactor is a 10 m3 pressure vessel with an agitator and heating coil. The reactor will be continuously fed with two substances of 170 C and 20 C. The mixture inside the reactor will be heated to 220 C by 240 C thermal oil coils and jacket and will flow out through the bottom.
If no agitator was present I could use heat exchanger logic -> entering mixture is of 80 C, leaving 220 C; entering thermal oil is of 240 C, leaving 200 -> use the resulting LMTD of 55 C as temperature difference or gradient. Use the LMTD for sizing the coils.

But because of the agitator the flow won't be from top to bottom. And the operating principle should be - the whole reactor heats up to 220 C and then the temp is maintained. So neither counter-current nor co-current heat exchange can be assumed.

Has anybody have experience with such designs? How would you select the LMTD for coil sizing?

Thank you for your 2 cents

 

[Latexman]

Don't forget any heat of reaction!

Even though a continuous reactor, I'd look at the start-up and shutdown procedure and do at least three heat and material balances, including steady state. You don't want to wait forever to heat up on start-up or cool down on shutdown.



Good luck,
Latexman

Engineers helping Engineers

 

[Compositepro]

If no agitator were present warm fluid would rise in the reactor and not exit out the bottom.

 

[georgeverghese]

Check the agitator literature to see what flow profile will be set up in the reactor (the baffles may have some influence on this too), then choose the appropriate LMTD. Else take the conservative approach and assume the entire reactor volume is at a constant temp of 220degC - in this case, the hot oil exit temp would have to be >220degC.

 

[lamo]

Thank you for your input.

Compositepro - even though warm fluid would want to rise, the bottom is always open, and to travel to it the liquid must go through heated zones.

I'm thinking of going with, as georgeverghese stated, conservative approach. It's just that circulating 50 m3/h of thermal oil with a deltaT of 15 C seems wasteful, when there's only 15 m3/h of product going in/out of the reactor.

 

[georgeverghese]

With the agitator running, there may not much of a temp profile for the contents - it is probably all well mixed throughout the entire volume.