# Scrubber set-up (what chemicals needed/retention time)



## Acid_Bath76 (Aug 7, 2011)

I've been looking through a lot of postings the last couple days. I want to start increasing my batch sizes, but I need to do this the right way. Fume hoods have been beaten to death on here, and I think there's more than enough info on how to build a good one. 4metals has an amazing posting on waste disposal,and I'm slowly starting to cover all my bases. Almost there! I'm a small scale refiner/hobbyist, but my longevity (including that of my hobby) depends on a proper set-up. 
The fume hood has to have enough power to get all the fumes out of the hood and into the scrubber. The scrubber has to have enough power to suck the fumes through the scrubber coming from the hood. I imagine the scrubber CFM needs to be greater than the hood CFM in order to draw the fumes through whatever media/chemicals the gasses will be exposed to. What types of chemicals/media are used in the scrubber? How long should these exhaust fumes be retained in the scrubber? If I missed a posting, let me know. No need to waste your time, but if this isn't something already covered.... chime in. Thanks!!


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## goldsilverpro (Aug 7, 2011)

For scrubbing NOx in a typical vertical scrubber utilizing only NaOH solution in a spray system at the top, the figure I have always been told is that the fume retention time in the scrubber should be at least 8-10 seconds. This is achieved by balancing the scrubber volume, the cfm rating of the blower, and the opening size of the air inlet(s). As a safety measure, the size is often doubled.


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## NoIdea (Aug 8, 2011)

One of the best scrubbers I’ve seen to date is one comprising of a 6inch ID PVC pipe filled with cheap garden hose, that stuff you use for garden watering system, cut into 1 inch piece at a 45 degree angle. The stripping media (Alkali for acid) is fed through the top and off gas is fed through the bottom. The high surface area of the stripping medium exposed to the gas is enormous. Regulation of the stripping medium feed rate is vital, to much and you will stall the off gas feed, blowing nasty’s back at you. 

This is the design I intend to build for by fume cupboard. Simple set up utilizing bits and pieces around the garage and based around commercially design systems. I’m a very visual learner, so those like me here it is, for those that aren’t I’m sorry.


A. Flow control valve
B. Stripping medium pump
C. From fume cupboard
D. Stripping column
E. Stripper medium input stripper
F. Sucker, blower backwards
G. Cleanly stripped gases
H. Stripping medium (mild alkali)

No need for filters and any goodies lost in the gas flow will be trapped in the stripping solution.

Retention time is governed by height and diameter of the column, flow rate of both mediums, and packing size. Too big reduces total surface area though increase flow rate(less friction), too small and gas flow will struggle. Sorry I should be able to do the maths, have done in the past, been a while, I find it easier using trial and error.


Deano


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## Acid_Bath76 (Aug 8, 2011)

Perfect! Thanks for the drawing. The visual probably saved me a couple hours!


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## 4metals (Aug 8, 2011)

An 8 to 10 second retention time makes for an awfully big internal volume or an awfully low flow rate! The retention time is a function of the volume and the surface area of the packing combined, that's why the Mas Pac and other popular packings list the sq/ft surface area per cu/ft packing.


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## Barren Realms 007 (Aug 8, 2011)

4metals said:


> An 8 to 10 second retention time makes for an awfully big internal volume or an awfully low flow rate! The retention time is a function of the volume and the surface area of the packing combined, that's why the Mas Pac and other popular packings list the sq/ft surface area per cu/ft packing.



The retention time I feel is a mute point. Your combination of air flow and surface surface area of your media have to be able to cause turbulance or break up the molecules of air to come in contact with your scrubbing solution. Your dirty air you are moving thru the scrubber will need to flow across a spread out media surface. It is the action of the air coming in contact with the scrubbing fluid flowing across the surface area of the media breaking up or mixing the air that will actually do the scrubbing. The air you are trying to scrub or clean just coming in contact with droplets of scrubbing solution is not going to do that great of a job.


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## Acid_Bath76 (Aug 8, 2011)

Barren Realms 007 said:


> 4metals said:
> 
> 
> > An 8 to 10 second retention time makes for an awfully big internal volume or an awfully low flow rate! The retention time is a function of the volume and the surface area of the packing combined, that's why the Mas Pac and other popular packings list the sq/ft surface area per cu/ft packing.
> ...



Well, for at least eight to ten seconds right? Can anyone expound on "retention is a function of the volume and the surface area of the packing combined", or maybe provide some type of formula to base sound construction plans from? I imagine a lot of this will depend on the amount of exhaust coming from the hood. The more fumes, the larger the surface area will have to be in order to retain the gas for sufficient time to expose it to the media for at least eight to ten second. 
My goal is to construct something larger than what I'm currently requiring. As I increase batch size, I can decrease the suction to increase dwell time. Regardless, before I start drafting anything up, I need to get this "function" squared away. Thanks guys! If I can find a solution online, I'll post it here.

so far, I've found this site... at a glance it seems pretty good. 
http://www.engineeringtoolbox.com/scrubbers-air-washers-d_139.html


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## 4metals (Aug 8, 2011)

The trouble with scrubbing all of the air passing out of your hood is at 100 cu/ft per sq/ft of hood opening, even a modest sized hood with a 2 x 2 opening needs 400 cfm. Anything large enough to house some production will have to be at least 4 feet wide by 3 feet high. Even with a swing down door closing the top half you're talking 600 cfm. At 600 cfm the air moves at 10 cu/ft/sec through the hood. For 8 seconds of retention we need 80 cubic feet or a packed area of 600 gallons in size. 

That is why reactions should be carried out in closed reaction vessels where the entire fume is sucked through a scrubber at a much slower rate. Now your scrubbers internal packing volume does not need to be as large. 

The scrubber in this thread http://goldrefiningforum.com/phpBB3/viewtopic.php?f=40&t=9115
sucks into the scrubber at a modest 10 CFM providing a 30 second retention time in the packing. This has proven sufficient for reactions up to 300 ounces of karat gold using the right chemistry for scrubbing.


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## Acid_Bath76 (Aug 8, 2011)

This is just genius. It just amazes me everything I'm learning tangent to refining. Thanks for the help guys. I think I've got enough to go on. I'll post pics as this progresses.


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## goldsilverpro (Aug 9, 2011)

One place I worked had a scrubber about 6' in dia x 15' tall - 424ft3 (3180 gallons equivalent = big!). It was a typical setup with a NaOH solution reservoir at the bottom and this solution was pumped to spray nozzles at the top. Above the reservoir was a perforated shelf that contained a layer of packing (saddles) about a foot, or more, thick (in Hong Kong, we used reject plastic flowers from a manufacturer). The fumes entered between the NaOH reservoir and the packing and the scrubbed air exited through the blower mounted at the top. Very similar to what is pictured in the link given by Acid_Bath76, except, there was no water overflow. There was a drum of 50% caustic soda sitting on the ground next to the scrubber and a pH probe constantly measured the pH of the NaOH solution in the scrubber. When the pH dropped to a certain level, a small pump automatically added 50% caustic from the drum until the pH was raised to the proper level.
http://www.engineeringtoolbox.com/scrubbers-air-washers-d_139.html

The variable blower was 2600 cfm max. At full speed, at a 10 second retention time, this would require a 2600/6 (because 10 sec. is 1/6 of a minute) = 433 ft3 scrubber volume (pretty close to the actual scrubber volume given above). They had several open fume hoods and NOx was generated in each one. Theoretically, the scrubber should have handled the fumes when the blower was on max, but it didn't. They had to turn it down to about 1800 to prevent a plume of very visible red fumes from exiting from the top - that's the way they adjusted it - visually. So, in this case, it required about a 2600/1800 x 10 = 14.4 second retention time to scrub the NOx.

Another place I worked used 12, 4-liter beakers for dissolving karat gold. On top of each beaker was a loose fitting plastic lid with a 1" plastic flex hose coming off the top. There was a hole that could be opened and closed on the lids to add acids. All the hoses were connected to an exhaust manifold. They also had a 50 gal Pfaudler kettle, with a lid and about a 3" dia exhaust pipe connected to the manifold. They had no open fume hoods where NOx was generated. The scrubber had a 275 cfm blower and was about 4' dia x 12' high (150 ft3) and was built the same as the one above. The scrubber volume was about 3 times what was theoretically needed - for both overkill and to allow for a little expansion.

The NOx is a lot more difficult (and slower) to scrub than many other types of acid fumes. The reason is that the NO produced by the dissolving reaction must be first oxidized to NO2 before it will scrub. This requires time and an oxidizer (oxygen, e.g.). Any unoxidized NO will go through the system and, when it's exhausted and it hits the air, it will be oxidized to NO2 = red fumes. Some people use a 2 stage scrubber, with H2O2 in the first stage and NaOH in the second. The peroxide oxidizes the NO to NO2 and this is scrubbed fairly easily by the NaOH. In this 2 stage system, a smaller total scrubber volume is needed.

As 4metals intimated, the way around all this is to use closed systems rather than wide open fume hoods. That way, a much, much smaller blower can be used and this would result in a much, much smaller scrubber being required. Less air opening (in) = less cfm needed (out) = less scrubber volume needed.

It's all math, folks!

Please correct me if any of my assumptions are wrong. One thing I've learned is that these systems are anything but intuitive. I've seen a lot of money wasted by intelligent people building exhaust and scrubber systems, intuitively, without doing the proper homework, that flat-out didn't work. I must admit that I was involved in a couple of these in the old days.

Chris


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## HAuCl4 (Aug 9, 2011)

big johnson recycles NOx and turns to nitric for less waste:
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=8619&p=80788&hilit=big+johnson#p80788


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## goldsilverpro (Aug 9, 2011)

HAuCl4 said:


> big johnson recycles NOx and turns to nitric for less waste:
> http://goldrefiningforum.com/phpBB3/viewtopic.php?f=49&t=8619&p=80788&hilit=big+johnson#p80788



That's another story, though, isn't it? This thread is about scrubbing.


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## HAuCl4 (Aug 9, 2011)

It seems to me it's more or less the same story: Dealing with NOx. :shock: :lol: 8)


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## Acid_Bath76 (Aug 9, 2011)

Seems to me, that for the smaller refiners/hobbyists, the closed system is the way to go. Less costly to set up, and not nearly as much equipment needed. Something I was thinking about... I don't know how stringent inspectors are when they come and check your site out, but I can imagine they might give you a look if you don't have a commercial grade fume hood when refining. Even if you had a really cool looking closed system that you built yourself, but isn't mass produced. When I think of refining/chemistry work, a large fume hood and all the pipes and bubblers come to mind. I imagine the suits think the same way to a degree. Am I wrong? Do they just check exhaust content and leave it at that? Is this closed circuit system a recognized way of operation, or is it merely a "stop gap" for the smaller guy? I don't mean to get off topic, and maybe this question is best suited for a different area. Anyhow, I digress... closed system... fantastic. I'll stop now.


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## HAuCl4 (Aug 9, 2011)

You could perhaps use another oxidizer instead of nitric acid, and then you would not have any NOx to scrub, and much less waste to process.


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## goldsilverpro (Aug 13, 2011)

Here's a closed system I've recently been thinking about but have never implemented. It would use 5 gal buckets in a hot water bath instead of those bulky huge round bottom glass flasks. I'm basically a bucket guy.

I could first build a long fume hood from 3/4" plywood. There would be an exhaust blower (s) at the top, whose size would be based on 100 cfm for each square foot of hood opening. I would use just a standard steel squirrel cage blower(s), since there would be a very small amount of fumes entering it. I would put the motor on top of the hood, to protect it from any fumes, and use a belt to drive the blower. 

Sitting on a low bench inside the hood, would be an open top water tank about 10" deep. The width and length would depend on how many buckets one wanted to run at one time. The water would be about 6" deep and would be heated by circulating it with a small pump through a hot water heater. The thermocouple would be placed in the water tank. I know this hot water heater thing will work because I've done it. I once used this method to heat, utilizing heating coils underneath the SS tank, a 375 gal solution of 10% caustic soda to about 140F. I had a tumbler in it and used it to strip x-ray film. If I remember right, it took about an hour or two to get it to this temp. It was originally a milk storage tank and the coils were originally used for cooling.

The plastic bucket lids would each have a bulkhead adapter on it with a hose fitting and a plastic valve. There are a variety of types of bucket lids available. I think they even make a clamp type like you see on some drums. The flexible hoses from the lids would all be connected to to a manifold (4" plastic pipe?) mounted inside and near the top of the fume hood. A small plastic blower at the end of the scrubber would move the fumes from the manifold to a small 2 stage (H2O2/NaOH) scrubber. There would have to be an opening somewhere to provide make-up air. I'm thinking that a second bulkhead adapter, with a valve, could be mounted on the bucket lids to provide and adjust this make-up air. There could also be a open/close hole in the lids to add acids. It would sure be nice to have transparent lids, though, so you could see what is happening.

Although there are many small scrubber designs possible, some of which have been discussed on this forum, I'm thinking that a couple (or, more) of 55 gal plastic drums with clamp lids could be adapted with a spray system, bulkhead adapters, hoses, etc.

Like I said, this is only a thought experiment at present, and there are some flaws that I think could be worked out. Just thinking out loud.


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## janecho (Oct 25, 2013)

4metals said:


> An 8 to 10 second retention time makes for an awfully big internal volume or an awfully low flow rate! The retention time is a function of the volume and the surface area of the packing combined, that's why the Mas Pac and other popular packings list the sq/ft surface area per cu/ft packing.




I am new in designing the scrubber system. I need to treated the fumes produce from gold recovery system. Please kindly advise on my scrubber design.

From the comment stated above, air flow through the hood opening need to design at 100CFM/ft2. Base on this information, I have calculate the fumes produce in my system are 1500CFM. 

a) How can I design my scrubber column base on 1500CFM? 
Base on the retention time 8-10s, assuming column diameter=1.5m, surface area,A= 1.767m2 = 19sq ft. So, with retention time 10s, my scrubber height will be 4m. Is my calculation correct? Besides, the 4mHt calculated is consider as packing height or column height?

b) Can I treated the fumes by just using NaOH scrubber column? If I only use NaOH scrubber column, will I need bigger column? Is the calculation different? 

c) packing :- 
Geometic surface area: 44 sq ft/ cu. ft
Packing factor: 14/ft
If I am using the packing as above, will the packing height become lower? How can I calculate the packing height in order to treat 1500CFM of NO2 fumes by assuming column diameter 1.5m?

Please kindly help on above question. 

Thanks.


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## Lou (Oct 25, 2013)

You should look at BioNox solver or else NaOH-sodium sulfide based scrubbing chemistry.


1500 CFM at what concentration?

Have you considered using something other than aqua regia for the gold dissolution process?


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## 4metals (Oct 27, 2013)

The surface area of the packing material provides the surface for the chemical reaction to take place but the amount of time the fume spends in the scrubbers packed section is what is used to determine the scrubbers size. When sizing a scrubber I use 8 seconds as the target retention time for scrubbing. The surface area of the packing can effect the efficiency of the chemistry so I always determine the packed area and fill it with Maspac or other commercially available packing.

So if you are drawing 100 CFM you need a packed area of 13.333 cubic feet. If you divide the CFM of the blower by 7.5 you get the cubic feet of packed area you will need to have the 8 second fume retention time. 

1500 CFM will require a packed column of 200 cubic feet. In your example with a 1.5 meter diameter packed tower is about 18.6 cubic feet per foot of height. So a 10.75 foot packed section will handle the 1500 CFM of NOx. I would suggest adding 2 packed sections of equal height with a second spray head to assure complete reaction. I like to redistribute the fume every 1 to 1.5 column diameters of packing. 

This is a pretty big scrubber, I've done bigger but for pretty big refiners. You can reduce the size considerably by using sealed reactors. What type of refining are you doing?


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## butcher (Oct 27, 2013)

For heavy fume like digesting metals in nitric acid, I like to use a small scrubber system, if you have a lot of fume going through the hood mixing with all of that air flow through the hood you will need a much larger scrubber to scrub the fumes mixed with all of that airflow, after it leaves the hoods exhaust system.

Actually sometimes these fumes can be recycled, NO2 gas can be made into dilute nitric acid.

A smaller scrubber system right off of the reaction vessel can scrub the fumes, or prescrub the fumes before they go through the hood.


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## janecho (Oct 29, 2013)

Dear All, 

Thanks for the reply. 

I am using nitric acid for copper leaching. 
2 NO3- + 4H+ + Cu :arrow: 2NO2 + H20 + Cu2+

Lou, I am not sure what is the concentration of the fumes. I am using 62% of nitric acid for the leaching process. How to calculate the concentration NO2 will be produce? 

4metal, thanks for the calculation. I just confuse that 200ft3 is referring to packing volume or the scrubber column volume? If different type of packing being use, can help on reduce the packing height? By using the retention time of 8s, the packing height is calculate base on the empty surface area of the scrubber column. 1500CFM/7.5 =200 cu.ft. Dia= 1.5m, A = 19sq ft. So, the height = 3.2m. Since 3.2mHt is calculate base on the surface area without packing, It is means that NO2 need 3.2mHt to transfer along the scrubber when the column is without packing? If i am using the packing element as mention previously, the surface area will be increase, as well as the retention time. Therefore, the packing height will be much more lower that what have calculate? Am i correct? I am still confusing about it. 

Can i calculate the actual packing height needed base on the packing factor or packing surface area of the packing element I use? 

Thanks. 

Your help is much appreciated


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## butcher (Oct 29, 2013)

In concentrated nitric acid NO2 (nitrogen dioxide is formed) (red brown gas), this gas escaping is wasted nitric that could do more work if it was still in solution.
Cu + 4HNO3 --> Cu(NO3)2 + 2NO2(g) + 3H2O

(If a medium concentrated acid is used both NO and NO2 gas can be formed with the reaction. 

In dilute nitric acid (formula for 3M HNO3), NO gas forms (nitrogen oxide gas) from the reaction.
this wastes less nitric acid and can put more copper into solution for the amount of acid used.
3Cu + 8HNO3 --> 3Cu(NO3)2 + 2NO(g) + 4H2O

NO in air (or with O2) forms NO2 gas:
2NO + O2 --> NO2
NO gas will not dissolve in water easily and can be hard to scrub, but if mixed with oxygen can form NO2 which is fairly easy to dissolve in water.

NO2 gas dissolved in water forms a dilute nitric acid
3NO2 + H2O --> 2HNO3 + NO

Hydrogen peroxide in the water can help to convert NO to NO2 in solution helping this reaction of dissolving the NOx gas into solution.

Hydrogen peroxide in the original reaction of nitric acid and copper (with water involved) can also help to keep these gases in solution where more copper can be dissolved.

(Actually there are several NOx gases formed in these reactions but for simplicity in discussion we just mention these).

These gases can be routed from the reaction vessel (distilled off), and bubbled into a small amount of water with H2O2 added, in this first receiver (chilled) vessel nitric acid will form, from this vessel the gases that escape go to a second receiver to try and capture more NOx (that escaped through the first receiver) this gas bubbled again through water and H2O2, the remaining gases are bubbled through a caustic solution like KOH, NaOH where any un-reacted gas can form KNO3 Or NaNO3 (which can be used later to make HNO3), this last receiver scrubbing the NOx before going to the fume hood or a small spray type scrubber system, before mixing with the fume hoods air and going through the fume hood exhaust.


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## janecho (Oct 30, 2013)

4metals said:


> 1500 CFM will require a packed column of 200 cubic feet. In your example with a 1.5 meter diameter packed tower is about 18.6 cubic feet per foot of height. So a 10.75 foot packed section will handle the 1500 CFM of NOx. I would suggest adding 2 packed sections of equal height with a second spray head to assure complete reaction. I like to redistribute the fume every 1 to 1.5 column diameters of packing.
> 
> This is a pretty big scrubber, I've done bigger but for pretty big refiners. You can reduce the size considerably by using sealed reactors. What type of refining are you doing?



i) What you means by "adding 2 packed sections of equal height with a second spray head"? It is means that I need to have 2 unit of scrubber column of 1.5mDia x 3.3mHt in order to have complete reaction? 

ii) What do you means by 1 to 1.5 column diameter? Can you explain in detail? Can you show me with some example? 

Please kindly help.

Thanks.


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## 4metals (Oct 31, 2013)

1 to 1.5 column diameters in your case is 1.5 meters to 2.25 meters. So you should have a break because you need a packed section about 3.3 meters deep. The cubic foot area that is critical is the volume of the packing not the entire scrubber volume. We want the fume to be in the working area of the scrubber for 8 seconds and where the fume hits the chemistry is the working area. 

Fumes tend to "channel" or develop flow patterns through packing material the rule of thumb is that whatever your diameter is of the scrubber, that same height in packing will effectively transfer fumes with minimal channeling. Depending on the packing type, some recommend 1 times the diameter and some up to 1.5 If your packed bed is deeper than that, you run the risk of channeling. For the packed depth you need you will exceed the rule of thumb. To be on the safe side, I would break the packed beds in half and have a foot of open space. In the open space I would run a second liquid distribution section. A packing support, redistribution manifold will aid in fume redistribution. (Google scrubber internals) The liquid (Caustic) would be pumped into the packing from the top of the packing and in the split halfway up the packing where there is an air break. 

When a packed bed starts to channel, some areas will dry out from a lack of water flowing over them because the liquid flowing down can channel as easily as the air and fume flowing up. I have seen severe channeling where the caustic rich liquid (which has a high total dissolved solids) dries out and leaves a deposit of dry caustic which grows in the packing to the point the thing plugs solid. 

It is easier to build it right than to fix it so I always follow the engineering principles. They have a nasty habit of always failing on the coldest of winter nights when you take shortcuts.


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## the iron dwarf (Aug 21, 2014)

NoIdea said:


> One of the best scrubbers I’ve seen to date is one comprising of a 6inch ID PVC pipe filled with cheap garden hose, that stuff you use for garden watering system, cut into 1 inch piece at a 45 degree angle. The stripping media (Alkali for acid) is fed through the top and off gas is fed through the bottom. The high surface area of the stripping medium exposed to the gas is enormous. Regulation of the stripping medium feed rate is vital, to much and you will stall the off gas feed, blowing nasty’s back at you.
> 
> This is the design I intend to build for by fume cupboard. Simple set up utilizing bits and pieces around the garage and based around commercially design systems. I’m a very visual learner, so those like me here it is, for those that aren’t I’m sorry.
> 
> ...


the above looks easy enough to make
I see that for NO you need to add an oxidizer first so 2 columns may be needed, first with H2O2 and NaOH in the second or is it possible to do it other ways, was thinking of using nitric to dissolve silver ( with some base metals probably mostly copper ) on a very small scale but the red gas is a problem
would I be making NO or NO2?
should I look to the 3 bottles I also seen in this thread?


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