Wet Scrubber Design 4

[sampi]

Hi 2 all, I have to design a wet scrubber for an effluent gas stream containing a very fine dust. This stream is abtained by burning SiH4 with O2 (it is a spontaneous combustion) to form SiO2 and H2O. The plan is do design a scrubber to remove the SiO2. I just started my career as chem eng and is still a rookie. do someone out there maybe have some guidelines 4 me?

Thank you in advance
Sampi

 

[Homayun]

Is SiO2 a gas? Do you know what liquid is needed to scrub the gas with? If yes,do you have access to a simulation software like Aspen or Provision or Hysys? If yes, make a simulation of your design case. Specifiy the degree that you want to remove SiO2 from your stream and then specify the equipment.

 

[sampi]

to Homayun

thanks for the reply! SiO2 is a solid, very fine powder that forms as combustion product. looks almost like a spiders web - saw it once at a leakage in the pipesystem.

SiH4(g) + O2(g) --> SiO2(s) + H2O(g, due to high flame temp)

The scrubbing liquid is H2O. What I have in mind (due to plant setup and safety) is a ejector-venturi scrubber at the "end" of a combustion chamber. I don't know if this sort of combination "design" has ever been attempted. basic design equations, whether empirical or derived from first principles of scrubbers will probably help. any other suggestions?

I dont have acces to aspen, by I can try to get...

 

[cub3bead]

You might find this web page of use.

http://www.geocities.com/CapeCanaveral/cockpit/8882/silica.html

If you end up with the solids as christobalite rather than amorphous silica you have a suspected carcinogen to deal with.

Venturi type scrubbers have been used in flue gas desulfurization systems. But they impose a greater pressure drop than spray tower designs. For large scrubbing volumes the spray towers win out but I suspect you are dealing with a realitvely small volume.

 

[TD2K]

If vapor side pressure drop is a problem, you can also consider a wet gas scrubber that uses eductors to contact the liquid and vapor streams. We are installing on an FCCU regenerator stream. Your pumping costs go up but you significantly decrease the gas pressure requirements.

You need to provide information such as the flow rates, pressure, temperatures, particle sizes, required emission limits, etc to get much more than general suggestions.

 

[siretb]

Silica particles can be very fine. I have had one case where the diameter was 50 nanometers.
That's ways too small to be efficiently collected by a wet scrubber. So unless you are certain that your particle size distribution is large enough, be very very carefull.

In the micron range, I would consider high energy venturis, or some proprietary designs. I still doubt that will be enough in your case.
Above the micron, the empty towers (with spraying nozzles will do something).
If you really need to remove much of very fine silica, why not consider a bag house filter, with precoating, or a WESP (Wet Electrostatic Precipitator).

Do you have the particle size distribution?

 

[sampi]

What I know is that the particles sizes of the SiO2 is between 0.1 an 0.001 micron! What happens is during combustion, for some reason, the SiO2 form spider-web-like structures and floats in the air. (I think the get charged and them cling to one another). Will a wet srcubber be able to remove the SiO2 in this form?

The other thing I am struggling to get hold of is the industrial emission standards that should state how much of what substances at what size can be blown off into the atmosphere - this will give an indication of at what efficiencies the equipment needs to be operated.

I have even thought about a Dry ESP. I will burn about 200g/hr siO2 at times in the vent line or more when an "incident" occurs.

The other problem is money, because the company didn't budget for "vent cleaning", so I need to come up with a effective solution that not too pricy but effecient enough to keep the environmentalists off our back. And bviously health and safety IS my concern.

 

[siretb]

I have not seen , so far, a spider web like structure. I do not say you don't have it. I think a wet scubber would be inadequate.
That said, it's quite easy and probably not that expensive to set up a small pilot, if you want to try.
As for a dry ESP, I also think it will be too limited. What removal efficiency do you want to acheive?

Why not a plain filter? What's your gas flowrate and what's the temperature?

 

[sampi]

Flame Temp ~ 1600oC (2900oF)

Gas Temperature will probably be around room temperature due to mixing in the pipes with other gases, but the absolute theoretical max. temperature, at the exit of the compressors, is 150oC (300oF). With the MAX. flow rate
200g SiH4/h. It will probably be ~100g/h under normal operating conditions.

I am still waiting for feedback from the department of environmental affairs to know what the allowable emission standards are. This will determine the efficiency needed, that I asumme will be quite high maybe 80-90+%.

I just a test by burning pure silane (SiH4) out of the bottle. My observation was that about 60% of the SiO2 forms as that spider-web-like structures and the rest is in the form of "smoke" (atomized).

Its quite easy to remove the "webs" (you can grab some them with your hand - so a assume a scrubber will maybe do the trick), so maybe by removing them I will allready be withing emission specs.

The total production (max) of SiO2 is 375g/h or 0.1g/s, so if ~60% can be easily reamoved, that leaves you with about 150g/h or 0.04g/s of particles ranging probably between
0.1 and 0.01 microns.