tank truck venting and flame arrestors

[StoneCold]

I am working on revamping a tank truck venting system. We periodically load waste solvents in to a waste solvent tanker. The vent on the tanker goes to two carbon drums in series. There have been some instances where the carbon drums have heated up enough to burn the paint off the bottoms. The vents are currently just open lines going to the carbon drums. There are no check valves or purge gases so the tanker breathe though the carbon drums. We are thinking of adding flame arrestors right before the carbon drums. We do not have a flare system. What I am wondering is what are other people doing? Is there a code on this type of venting? Any help would be appreciated.

 

[Flareman]

I don't know whether there is a code covering your specific activites but the US Coast Guard does regulate a similar activity when it comes to ship tanker activities.

Normally, such venting is associated with loading and unloading and sometimes with cleaning/washing. In most of these cases atmospheric venting is frowned upon and the vapors are collected into a disposal system such as a flare or incinerator (or in your case the carbon tanks).

The normal action would be to include a "detonation arrestor" (not a flame arrestor) at each end of the line between the sender and receiver vessels. "Detonation arrestors" are really good flame arrestors which stop the flame even after it is accelerating down the line in a flash back state whereas a flame arrestor is only suitable directly at the vent. That's where the Coast Guard comes in. Most suitable detonation arrestors carry the CG approval so ask about that when you go to the supplier.

An internet search for detonation arrestors will give you lots of suppliers.


David

 

[StoneCold]

Thanks for the tip Flareman

 

[TrevorP]

Is it possible to back vent the solvent gases into the feed tank? This may help to reduce emmissions overall.

 

[chicopee]

Your reference about carbon tanks- are these tanks with activated charcoal filters? and how do they get so hot to melt paint?I f these are activated charcoal filters,are they reheated to reguvinate the filters?

 

[StoneCold]

TrevorP
No it is not possible to vent back to the waste solvent or waste supply tanks. Usually the waste material is from a reactor or a drum so that is not an option.

Chicopee
The carbon drums are literally portable activated carbon drums for scrubbing vapor phase emmisions. They get heated by adsorbing the VOC's. We currently have no dillution system in place so the VOC content of the vapor stream is between 15 and 100% v/v which creates a lot of heat when it is adsorbed. You could regenerate the units but we don't have the facilities to do that.

Thanks for your interest TrevorP and Chicopee

 

[TrevorP]

I don't know of any code on this, but I do know that "hotspots" and thermal runaway can lead to fires in carbon beds. This usually depends on the VOC's involved and the concentration. Since I don't know what they are in your case, I can't comment further. However, it seems if you have some history of observing local hotspots, it is certainly a possibility. From your description, it seems that a carbon bed fire could ignite the vapours in the tanker. It looks like you need to take some precautions to prevent this.

I concur that a detonation arrestor between the tanker and the carbon bed is highly recommended. You might also want to consider dilution air to prevent overheating, and/or a temperature trip. You should also consider whether overheating can occur due to decomposition, reaction between solvents or atmospheric oxidation, and take appropriate steps to prevent or reduce this.

Some good references on this are:

http://www.osha-slc.gov/dts/hib/hib_data/hib19970730.html

; and "Prevent Thermal Runaways in Carbon Beds", Hofelich, LaBarge and Drott, Chemical Engineering, January 2000. (

http://www.che.com

).

They make sobering reading about what can happen if it goes wrong!

 

[Flareman]

BradStone
TrevorP's posts and the handy link remind me that you will need temperature sensors on the element of any detonation arrestors you use, to monitor for a continuing reaction in the line.


David

 

[skearse]

In response to TrevorP's posting, you may want to be careful introducing dilution air into the mixture. The only reason that you may have avoided a fire (so far) is by having the vapor concentration above the UEL for the solvents. I would consider using an inert, such as nitrogen, if possible, instead of air.

 

[chicopee]

Due to high temperatures generated during adsorption, I suggest the installation of an automatic dry chemical fire extinguishing system to protect equipment in the event of a fire. Such protection is often installed on expensive equipment and normally such request would be made by insurance companies.
From my previous question of 4/25/03, I was not aware of the exothermic reaction that develops with the concentrations of VOC that you stated. I have done a fair amount of research on activated charcoal filters and it seems that exothermic reaction was never a bone of contention probably because the concentrations that I was interested to clean up were in the 5 to 100 ppm.

 

[TrevorP]

Unfortunately, I cannot take sole credit for the dilution air idea - it is on OSHA's website ! (See link above). Yes, it may be right that a fire has not taken effect due to being above the UEL with the current set-up, though, unless the tank is N2 blanketted before loading and maintained throughout, this cannot be relied upon. The idea of the air is that there is enough of a heat sink in the air to remove the heat of adsorption of the VOC's by the activated Carbon. This effectively removes one of the elements of the fire triangle, namely source of ignition. Putting enough air through to keep the system below the LEL adds an extra degree of safety, although I agree N2 could be used as an option. It depends on which is cheaper, and what is available.

 

[chicopee]

Remember--Dilution is not the solution it is pollution.

 

[TrevorP]

Chicopee, I was not suggesting removing the CA facility - you said yourself that they are pretty good at removing most VOCs, even at ppm levels. I do not think a bit of extra air added for cooling its cooling effect, will dramatically adversely affect environmental performance. Provided, of course, that the drums are not saturated.

 

[gacguy]

Bradstone
At this high concentration I'm surprised that the temperature hasn't exceeded the ignition point of the organic on the carbon.The heat of adsorption is approximately twice the heat of vaporization, so in your case you have a considerable amount of heat released. Is there a long time between tanker fillings? If so, and due to diurnal breathing, the carbon would have the opportunity to cool down between fillings?
An accepted way to prevent a temperature increase due to adsorption of organics is to prewet the carbon with water. If you fill the adsorber with water, let it stand for 30-45 minutes and then drain it you will reduce the risk of excessive temperature rise. At the concentrations you indicate, there should not be a large decrease in the capacity of the carbon for the VOC's.
I agree that a detonation arrestor is the way to protect against a flame propagating back into the tanker. The one drawback is that the tanker allowable pressure must be sufficient to overcome the pressure drop in the detonator. Otherwise you would have to install a blower

 

[StoneCold]

Thanks Guys for all your input. We are going to put flame arrestors, not detonation arrestors right at the carbon drums. We are also adding a nitrogen dillution point upsteam of the carbon scrubbers to help reduce the temperature. We have considered soaking the carbon with water but since the drums are online for about two months I am sure that they would dry out and that method of heat removal would be negated. It would also increase the corrosion of the drums and we currently reuse the drums.

Thanks

 

[gacguy]

Bradstone
One point about wetting. The water is eventually displaced by the organic and then you have the organic as a heat sink.

If you look at any literature from vendors of flame arrestors you will see that they are always at the end of the line, not between the vessel and the source of the flame.