Reactor Tube 1

[thryll]

I am currently experiencing a problem with my Laboratory reactor tube. It has been breaking more often recently. It is made of an extruded alumina approx 2.25 inch OD 2.0 ID. The temperatures the tube is experiencing is 1200-1400C. Are there any types of coatings which could provide further strength? I was perhaps thinking of another material but supplies are limited in this temp range any type of assistance is greatly appreciated.
Thanks

 

[donmackenzie]

thryll,
Are you operating at pressures significantly above atmospheric? If not, strength per se is probably not an issue.

My first guess would be that thermal stresses are causing your tube to crack. I would suggest taking steps to minimize rates of temperature change in the tube. Be sure to ramp your temperature up and down slowly, vary gas flows through the reactor gradually, and so on, to avoid this problem. Or have you been doing this already?

I'm not aware of any materials you could use to strengthen the material at the temperatures in question, though as I said, strength should not be an issue unless you're operating at high pressures.

Let us know what you figure out.

 

[thryll]

I have been ramping up and down slowly (60 C per hour) the trouble comes into play where I am injecting certain gasses. One of my nozzles is in @ 1100 C at a 90 degree angle and the gas which I am injecting i believe is at a lower temperature and this is causing the stress. I cannot decrease the flow and this position is critical.
Perhaps someone may help me find out what the temperature is of the gas at this postition ?
I have the flow(3Lpm) the gas 1 Lpm=Ar carrier (H2O) and the other two is Ar the temperature is 150 C 20 inches later it is 1100C. I believe the gas is probably only around 800 C(I have no Idea how to calculate this number I am just guessing) and the temperature is stressing the tube. Your thoughts on strengthening ?

 

[kenvlach]

thryll,
I am sure you are right about the ‘cold’ inlet gas causing thermal stress in the alumina tube, both in the wall surrounding the inlet and where the inlet gas impinges on the far wall. Don’t believe there are any suitable coatings; anything that would adhere to the alumina would react and cause spalling on every cool-down. Really no way to strengthen the alumina; therefore, try to reduce the stress.
I have 3 alternatives:
1) Use a mullite tube. Believe better thermal shock resistance than for alumina.
Possibly, MgO-or yttria-stabilized zirconia tube might work, due to higher toughness.
2) Insert a diffuser shield in front of the nozzle. This might be difficult, you would have to support it from one end with a rigid ceramic rod w/o it banging around when flow is switched on or off. Alternatively, tie a diffuser shield over the nozzle with a Pt wire which would run up through the gas inlet tube.
3) Maybe best. Flow the gas inside a ceramic tube (inside the alumina tube) from either end of the furnace tube to the injection point. This will give it approximately the same temperature, and if you do it from the exit end, you benefit from counter-current heat transfer (less cooling needed at the exit fittings). For better mixing at the injection point, use a ceramic tube closed one end with diffuser exit holes drilled near the closed end using a 1 mm diamond-coated bit.

 

[hacksaw]

Alumina is hard to beat in terms of its non-reactivity and gas tightness.

A diffuser of some kind will work, you can also place a section of a slightly smaller Alumina tube in the zone where the gas is impinging on the outer tube.

Of course anything that diffuses the gas flow and prevents localized cooling will help.

In the case I'm familiar with an internally purged probe, the heated gas in the annulus was used to preheat the purge gas traveling down the center.

 

[thryll]

I was looking at mullite but the fracture toughness wasn't as high. I thought the tube was breaking due to some other force. I am having trouble finding a supplier for YsZ. On my materials property guide MgO states its maximum usage temp is 500 C. Is that a misprint? Should it be 1500 C? Can you recomend any suppliers I currently only have one which is reliable.
Thanks

 

[kenvlach]

MgO (periclase) has an mp of 2852^oC and is used for 'basic' firebrick, so it must have a higher use T than 500^oC. However, in my earlier post, I was referring to MgO-stabilized zirconia (along with YSZ). CaO-stabilized zirconia is also available.

Re partially-stabilized zirconia: “Even though it has a very high coefficient ot thermal expansion (~12 x 10^-6 per ^oC) and only moderate strength, it is extremely thermal shock resistant due to the high fracture toughness.” p. 142 in Modern Ceramic Engineering, D. W. Richerson, Marcel Dekker (1982).
Note: stabilized zirconia is used in oxygen sensors in automobile exhaust manifolds, so must be pretty thermal shock resistant.

When I was in grad school, we got zirconia tubes (for solid oxide electrolyte) from a Japanese supplier. Can’t recall the name. Suggest you try Ceramics Bulletin, published by the American Ceramics Society, which has an annual supplier issue. Try

http://www.ceramics.org/

or

http://www.ceramicbulletin.org

Note: their server seems to be down today, but I presume they have an online listing of advertisers.

 

[kenvlach]

You can search for zirconia suppliers at the Thomas Register site:

http://www10.thomasregister.com/ss/...g=zirconia&wherefrom=&pdm=&json=n&indexpage=n

Searching for zirconia yields 65 companies, most with online catalogs.

 

[hacksaw]

There are some really high grade ceramics to choose from but why make life difficult.

Just use a glass frit diffuser if you injection point is too hot for glass then fabricate one out of ceramic high temperature stainless.

 

[BRT549]

ABB Raymond in Naperville, Illinois had some high temperature calciners in service at their test center when I was there. Many different tube materials were available. They have materials that would go over 2200 C. They seem to change hands frequently (I think they went from being Bartlett-Snow Pacific to Combustion Engineering to ABB), but they may still be there at (708) 971-2500. Try Dave Kolecke at ext. 354.