High Temperature Heat Transfer Fluid
High Temperature Heat Transfer Fluid
(OP)
Hello,
I am trying to design a Fluidized bed reactor where i have to supply large amounts of heat to the solid bed at high temperatures : 600-800 C.
A large part of the heat has to be supplied indirectly, through immersed in bed heat exchanger tubes, and will be supplied either by combustion of hydrogen or of coal.
My question is in what conditions steam can be used for heat exchange at these temperatures? Does it imply a special choice of materials for the exchangers? Thank you very much as i am new to the job.
Vlatko
I am trying to design a Fluidized bed reactor where i have to supply large amounts of heat to the solid bed at high temperatures : 600-800 C.
A large part of the heat has to be supplied indirectly, through immersed in bed heat exchanger tubes, and will be supplied either by combustion of hydrogen or of coal.
My question is in what conditions steam can be used for heat exchange at these temperatures? Does it imply a special choice of materials for the exchangers? Thank you very much as i am new to the job.
Vlatko





RE: High Temperature Heat Transfer Fluid
RE: High Temperature Heat Transfer Fluid
RE: High Temperature Heat Transfer Fluid
Upon burning hydrogen with air, nitrogen-diluted superheated steam would be generated, right ? Are you trying to substitute superheated steam for combustion gases ?
RE: High Temperature Heat Transfer Fluid
RE: High Temperature Heat Transfer Fluid
RE: High Temperature Heat Transfer Fluid
thanks for the very quick replies.
To answer 25362's question, yes i was considering burning hydrogen and using the resulting gases to supply the heat, but was wondering about the material constraints on the heat exchangers.
Hydrogen burning not being ideal for this application, i was also looking for anotther heat transfer medium at these temperatures.
What are the advantages of molten salts over superheated steam for example?? Just the specific volume? or something else..
Does anyone know of any other high-temperature heat transfer fluids? Thanks very much
Vlatko
RE: High Temperature Heat Transfer Fluid
The normal heat transfer salts (nitrates/Nitrites) generally have a max operating temperature of approximately 600°C.
The neutral salts will go much higher. I have never seen them used for any thing other than for heat treating.
Here is the salt we built our process, 450°C, on. We have made a couple of modifications to the composition.
U-TEC-TIC Salt
http://www.hummelcroton.com/utt/utt.html
We have used these salts in the bench scale and pilot plants with good results. Park has "Neutral Salts and Tool Steel Salts (Chlorides) that operate much at higher temperatures. We have use them but have had corrosion problems.
Partherm 290 and others
http://www.heatbath.com/Products-link-page.htm
Here is another HTS that we have used for process cooling.
HITEC
http://www.coastalchem.com/PDFs/HITECSALT/HITEC%
20Heat%20Transfer%20Salt.pdf
http://www.coastalchem.com/index.html
I wasn't directly involved with this project but years ago we did have a shallow fluidized bed, once through, that took it's heat, around 1300°C, from a NOX incinerator. It operated for a couple of years.
RE: High Temperature Heat Transfer Fluid
As I recall, the US DOE had proposed such a process in order to simulate the activity of the pebble bed reactor- they wanted to heat up high pressure helium in the bed tubes that are placed in the parallel fluid bed.
RE: High Temperature Heat Transfer Fluid
Thanks for the replies. Unfortunately i can't allow any pollution.
I have read the numerous threads concerning the uselessness of superheated steam as a heat transfer fluid.
However, maybe in the case i am working on it may prove to be the right answer because:
I have 2 reactors directly using steam at around 800C. Another reactor generates heat at 350C.
Furthermore,for the reactor i wish to heat indirectly the heat transfer coefficient of the fluid bed to the immersed exchangers is expected to be low (app. 200 W/m2/K) similar, from what i gather, to the coeff. of superheated steam.
Do you think these reasons could justify the use of superheated steam in heat transfer? If yes, does it require any special materials??
Thanks,
Vlatko
RE: High Temperature Heat Transfer Fluid
Perhaps the following threads may shed some light on the metallurgy for high temperatures:
thread725-37130
thread367-58995
thread330-62473
thread330-55502
As for htc the steam-side coefficient appears correct, what about the process side ?
RE: High Temperature Heat Transfer Fluid
On the process side, the fluidized bed-to-wall htc is expected to be 200-400 W/m2/K so it should be quite similar. I did the calculation and it apears that to achieve the reaction in the time i need, it requires 1 m2 of exchanger / 0.185 m3 of fluidized bed reactor in the worst case.. in the best it's approx 1 m2 / 1.4 m3.
Since i have little practical experience i don't quite realize if it is a lot or not.. What do you think?
Thanks a lot,
Vlatko
RE: High Temperature Heat Transfer Fluid
Probably a good reading material with plenty of references would be Ullmann's Encyclopedia of Industrial Chemistry Vol. B4 (VCH).
RE: High Temperature Heat Transfer Fluid
If the process side fluid is a hydrocarbon, heat fluxes may be greater than the endothermicity of the reaction and coke may form on the tube walls. Experts also say that wall-originated Fe can catalyse coke formation. Coke would then act as a thermal blanket.
If the coke barrier is porous enough, coke precursors could still diffuse to form new coke at a self-accelerating rate. Since no details were given on the process in question, this is pure speculation.
RE: High Temperature Heat Transfer Fluid
The advantages are: high heat transfer coefficient from sodium to tube wall, steady temperature so no overheating problems etcm high heating capacity.
The combustion gas can be used as heating medium, but the disadvantage is that the heat transfer coefficient is low. Large amount of transfer surface has to be used.
RE: High Temperature Heat Transfer Fluid