Cheap home made scrubbing unit (theoretical design)

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Chris

Though i'm not 4matals, as far as i understand, it all has to do with providing as much surface area as possible in the path of the fumes.

Also, i think we should line ourselves up. Are we talking about closed system or open one?

What i refer to close system is, that the suction applies only to the reaction vessel forming small vacuum environment in it, thus the need to provide some fresh air to the scrubber inlet so any NO will oxidise to NO2 and not to chock the fan.
Though, i'm not sure this is even necessary as NO reacts with NaOH to form NaNO2 and later NaNO3.

An open system i consider a suction from a fume hood, where generated NO had the time to completly react with oxigen in the air to form NO2.
NO react instantly with air to form NO2.

Maybe my terminology is off...?
 
Samuel is correct,

The reaction to reduce the fume takes place on the thin well oxygenated layer of water which is running over the packing in the tower. Tower packings are rated by the actual surface area per cubic foot of packing. They give the chemistry in the scrubber more contact area where the chemistry of choice can interact with the fume which is winding its way through the packing.

Even the scrubber Chris referred to would only handle about 4000 CFM and that was pretty big. Without packing I doubt even that would work but the packing, properly wetted and with the flow redistributed to prevent channeling is the real workhorse.
 
Johnnycat535 said:
Could you add H2O2 to your solution to oxidize the NO better?
Click to expand...

Yes.
But i would consider this impractical. H2O2 is expensive and will quickly decompose with all the disturbance in the scrubber.


4metals
Do you see a need for oxygenating the scrubbing solution even if it contains NaOH/Lime solution?
 
I always knew about the large surface area needed in the packed column but never understood the mechanism. In the scrubbers I worked with, they used berl saddles or tellerettes (designed by Edward Teller, I think), both of which have maximum surface area in the space they occupy. Also, I think they reduce channeling. In Hong Kong, we used certain shaped rejects from a plastic flower manufacturer as packing. In my copy of Chemical Engineers' Handbook, there are about 25 pages on packed columns. The title of the section this is in is "Gas-Liquid Contacting", which basically says it all. They improve the contact rate of the gas and liquid phases, which are flowing counter-currently.

One place I worked had a 2600 CFM variable speed blower to exhaust the fume hoods, kettles, etc., with a scrubber sized to match that volume with 8 or 9 second dwell time. They adjusted the blower until red fumes no longer could be seen coming out of the stack. The result was a maximum of about 1800 CFM being possible.
 
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299x2 600$+ shipping is kind of hard for a packing option... any suggestion? or maybe a better deal somewhere?

can garden hose cut in 1/4inch be used or will the acid dissolve it?
 
Do you see a need for oxygenating the scrubbing solution even if it contains NaOH/Lime solution?
Click to expand...

Caustic scrubbers are regarded by some to be 50% efficient at best at total NOx reduction, the thing is it is very efficient at removing the nitrogen dioxide from the fume (NO2). To completely (at least 95% or better) destroy all of the nitrogen compounds peroxide seems to be the poison of choice. That is caustic and peroxide together.

Everyone thinks that when the red is gone its all good. The thing is there are still colorless nitrogen compounds remaining like nitrous oxide (N20) which is colorless but considered a major greenhouse gas. The cost of testing a stack gas in unbelievably high and the EPA rarely if ever forces the testing if the fume is clear, hence there are lots of caustic only scrubbers out there.
 
In about 1980, a guy I worked for ran a lot of karat scrap. He had 12 hotplates lined up with a 4 liter beaker in a Corning Ware dish on each one. On top of each beaker was a loose fitting (to provide some make-up air for the blower), lift-off flat clear plastic lid, about 1/2" thick, with a vinyl hose (about 3/4" - 1") attached. The other end of the hose was attached to an exhaust manifold above made of plastic pipe. He did it this way because he liked working in open beakers. About the only other thing that was vented was a 50 gallon Pfaudler kettle. It had a loose-fitting fiberglass lid that was about 2/3 hinged for adding or removing material or adding acid. The unhinged portion had about a 3" vent pipe mounted on it.

All of this was vented to a scrubber in an outside fenced area with a 275 CFM blower mounted at the top. The scrubber was a standard vertical caustic unit with a packed column and spray nozzles at the top. The scrubber was about 4' dia. X 12' high, which was about 4 times larger than what was theoretically needed for the 275 CFM blower. That way, he could add a larger blower and more beakers and/or kettles, if needed. Sitting next to the scrubber was a drum of 50% (6.25#/gal) caustic soda and there was a pH probe in the solution reservoir at the bottom of the scrubber. Some caustic was metered into the reservoir automatically with a peristaltic pump when the pH fell to about 9. An engineering company designed, built, and installed everything including the beaker lids, manifolds, etc. The total cost was about $25K. The refinery was located in a busy industrial park and, after the scrubber was installed, there were no complaints. Inside the building, there were no noticeable fumes or odors, even though there was no fume hood.

_______________________________

There are certain types of scrap that work best in 5 gallon buckets. The particular material I have in mind would react slowly and would generate some NOx, but not much. Also, the acids would be pre-mixed and no additions would be needed during the process. I've often thought about putting a heavy plastic or fiberglass rectangular trough, about 8" deep, in a fume hood. It would be large enough to hold 8 - 12 buckets, in 2 rows. The trough would contain water which would be heated by circulating the water through a pump on a hot water heater. The buckets would be lidded with a small adjustable hole in them for make-up air and vent tubes attached to a manifold above. I haven't calculated it but I don't think it would take a very large scrubber, or 2. There would be 2 separate blowers, a small one on the scrubber and a large one on the hood. The one on the hood would probably only be used when everything has finished, cooled, and when it would be time to manipulate things.
 
Keith Gills said:
yes. I get the principles kinda, but I have questions.
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First you have to think of what is the needs with respect to volume of scrubbed gas/fumes.
What kind of gas/fumes, then select a design that will cover those needs.
Then you can start by selecting sizes and so on.
Are you using Nitric in some volume, do you get it cheap or do you want to facilitate reclaiming some of it and so on.
 
k. I'm trying to learn but I want to scale up later. so...something small right now. probably a 12 cubic ft. fume hood, and I'm not interested i reclaiming until I learn and go bigger whart is the motor and the exhaust pipe for? I'm trying to do like a 5gal. bucket.
 
Keith Gills said:
k. I'm trying to learn but I want to scale up later. so...something small right now. probably a 12 cubic ft. fume hood, and I'm not interested i reclaiming until I learn and go bigger whart is the motor and the exhaust pipe for? I'm trying to do like a 5gal. bucket.
Click to expand...
Use proper English, k do not translate well and we have members relying on translators.
How much material are you processing at a time?
What kind of material?
Which process are you using?
Lets start with that.
 
sorry. I never even considered that there weren't native speakers.I will do my best to be considerate. Am I correct in thinking that the fume hood pushes gas into a vacuum that pushs air out of a pipe to bubble the gas through the water?
 
There are different systems.
Basic system is:
Push/drag the gases through a chamber where they are wetted at least 9 seconds with a liquid suited for your need.
 
Keith Gills said:
Am I correct in thinking that the fume hood pushes gas into a vacuum that pushs air out of a pipe to bubble the gas through the water?
Click to expand...
Not quite right - there are basically 2 different types of fume scrubbers

1) uses a "closed" fume control system wherein the fumes are drawn "directly" from the reaction vessel - through the scrubber by way of vacuum --- as shown in this thread -----------

https://goldrefiningforum.com/threads/building-a-chemical-fume-hood-with-a-plain-steel-blower.23107/
2) is a "free air flow" system where in the fumes are ether blown through the scrubber - or drawn through the scrubber - depending on if you have your blower/fan placed between the fume hood and the scrubber (drawing fumes from the hood & blowing through the scrubber --- or the fan/blower placed after both the fume hood and the scrubber & thereby pulling the fumes through both the hood & scrubber --- which is the type system proposed in this thread by samuel-a

This system requires a VERY LARGE amount of "air flow" through the system in order for it to work properly at scrubbing the fumes & that is because it is not only drawing fumes from the fume hood - but also drawing air though the lab/room the fume hood is in --- in other words - this system need to move a LARGE volume of air CFM (Cubic Feet per Minute) through the system

That means mean ------------

1) a fan/blower large enough to pull air from the room - through the hood - through the scrubber & out the exit ducting --- that means if you plan to do full blown reactions in your fume hood - without running your fumes through a "closed" vacuum scrubber (system #1) first --- you will likely need at least 8 inch ducting with an 8 inch fan/blower (or larger) to move the volume of air from the room/lab - through the hood - through the scrubber & out the exit ducting --- then ---------

2) the scrubbing column in this system NEEDS to be at least 3 - 4 times larger then the (8 inch or larger) system ducting so that the air speed slows down in the scrubbing column to provide retention time in the column to effect scrubbing of the fumes --- in other words - the scrubbing column would need to be 24 - 32 inches in diameter & at least 6 foot long (or longer) with your ducting going back to 8 inch after the scrubber

A 5 gallon bucket as you scrubbing column is not going to work in this system type set up if you intend to do full blown reactions directly in your hood

For the small hobby refiner - you are much better served to set up with a "closed" system vacuum scrubber (system #1) --- running this system in you may not even need a scrubber tied into your fume hood &/or your fume hood scrubber can at least be scaled down in size because you are scrubbing MOST if not all your fumes in the closed fume scrubber system

In other words - if you run a closed vacuum scrubber system - then your fume hood only needs to work at keeping fresh air moving through the lab & hood thereby removing "low level" fumes in the lab & hood - rather than trying to remove LARGE volumes of fumes created by full blown reactions

Kurt
 

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