how to build an acid resistant hood & scrubber

Building an acid resistant hood.

A good sturdy acid resistant hood is the backbone of any refining and or assaying lab. They do get expensive if you buy them from a scientific supply house. To build one yourself you need to use 3/4" plywood and build the equivalent of a doghouse with a 4 foot length and 30" depth. It should look the attachment when done.

Next google spray on bed liners on your computer. I've used Line X brand and Rhino and both have worked well. Find a local guy doing the work and take the hood to have the inside of the hood sprayed. Make sure he gets the corners and the bottom extra heavy. If you're flush with cash he can do the outside as well, for me I used good epoxy paint on the outside. If you live in an area where a lot of fiberglass work is done, that is also an option, both work well, it's all about price.

If you make the bottom like a "boat" with a lip in front it will also serve as a spill containment because something always spills, especially when you don't have containment.

Now all you need is legs or a table to put this on, keep in mind you will be processing in 5 gallon buckets so plan your height accordingly.

If you add a "U" channel vertically on both sides of the opening, you can add a sheet of 1/4" plexiglass as a sliding door. Make the channels long enough to contain the plastic sheet from the bottom in the closed position to the top in the open position. Place a stop so the door always stays 3 or 4 inches up from the bottom, and drill a pin so it can be pinned in the open position.

You can also have a coating, in a variety of plastics, "chopped" on. The last time I had a hood coated, I found a local guy with the equipment that made tornado shelters. He "chopped" on a coating of PVC fairly inexpensively. I also used the same PVC coating on wooden silver cells with great success.

I don't know exactly how these "choppers" work. I think it is molten plastic, chopped into small increments by a "chopper" gun and sprayed on. No solvents are involved. Maybe, bed liners are applied the same way.

Once, I had a 12' long hood made from bare plywood and I used it for about 5 years with no noticeable deterioration. I used two fair sized belt-driven squirrel cage blowers, in tandem, run off the same motor. The motor sat on top of the hood. I never, never incinerated in that hood and never had any problems.

Because of the incineration factor I have been gathering slate from old pool tables when I find it free. I have no difficulty machining it to size however for containment I would be interested in suggestions as to sealants for the bottom meeting the sides that will hold up over time.

4Metal, thank you so much. My main fear about refining is about toxic fumes. Now with this hood design of yours I would reconsider about quitting because the money from gold recovery is simply irresistible.
Would it be necessary to attach an exhaust fan to the duct in order to suck out the fume easily? Will these fumes be harmful to your neighbor and the environment I mean when it gets outside over the roof? I hope it just neutralized in the air otherwise it (the hood) is still not the solution that I want. Thank you again in advance.

Would it be necessary to attach an exhaust fan to the duct in order to suck out the fume easily?

Click to expand...

Yes. This particular hood has an opening of 3' x 4' = 12 sq.ft. When it's wide open, for excellent exhaust you'll need an exhaust fan of 100 cfm/sq.ft., or a total of 1200 cfm. You must also consider what the exhaust fan is made from if your design pulls the fumes through the fan. Steel will rust out quickly.

Will these fumes be harmful to your neighbor and the environment I mean when it gets outside over the roof?

Click to expand...

Yes. The fumes are heavy and, under calm weather conditions, they can tend to hug the ground. They will corrode metals - especially cars and metal buildings. Also, any usage of nitric acid will produce very visible red-brown fumes which can readily be seen by the authorities.

I hope it just neutralise in the air otherwise it (the hood) is still not the solution that I want.

Click to expand...

The fumes won't necessarily be neutralized but they will tend to disperse into the air. How fast depends on the weather.

A fume scrubber to handle 1200 cfm would be quite large, due to the great amount of air dilution of the fumes. If you want a wide open hood, you should consider building a multi-layer fume scrubber like the one described by 4metals in another thread. A sealed dissolving system, with little or no air dilution, would require a much smaller scrubbing system.

The hood described in the earlier post is only the hood to contain the fumes, it is part of a system which removes the fumes from the hood and passes them through a scrubber. The scrubber is a device made of a large diameter PVC pipe which is filled with tower packing.

Tower packing is basically obstructions to the air path which provide surface area for the chemicals we are pumping down through the scrubber to react with the fumes we are sucking up through the scrubber. They can get expensive if you purchase packing from a manufacturer so there are options to improvise. Wiffle balls work great, but they probably cost too much as well. Ladies hair curlers work too. Marbles work but that’s what chemists use in smaller columns, I’d venture a guess that a 12” diameter by 6 foot deep bed of marbles would be too heavy to be practical. One option is to cut slices of plastic soda bottles into rings about half to three quarters of an inch thick. Don’t use the ends because we do not want any pockets that will trap air or liquids. Anything you can think of to provide a corrosion resistant surface, that doesn’t trap liquids or gasses, that isn’t too heavy, that will allow the air to flow through the spaces will work.

We will be able to use a considerably smaller scrubber than required for a 1200 cfm opening of the hood I described earlier because scrubbers work when there is some contact time available. So if you’re trying to scrub 1200 cubic feet of air and you want it to stay in contact with the packing for a few seconds, you need a large scrubber. We will only be scrubbing the fume generated from an enclosed reaction so we can do a good job with a smaller sized scrubber.

The way we get to split the fume and do double duty with one corrosion resistant blower is by building a PVC manifold to exhaust the fume to the scrubber. Using a 3” PVC pipe, and 3 tee’s and one 90 degree elbow make a manifold for 4 equidistant inlets. Bush down the openings to inch and a half. Our reaction vessel is nothing more than a spackle bucket, our contained reaction vessel is a spackle bucket with a lid. In the lid we drill a 2” hole off center and we connect a section of flexible tubing from the hole in the lid to one of the holes in the manifold. The piping of the manifold is directed to the bottom of the scrubber, below the packing support and above the liquid level.

The blower we need is an 8” intake blower, usually either PVC or FRP (fiberglass reinforced plastic) Now from the blower we go into a reducing wye, it has to either be or be bushed down to 8” in the top, 6” out the bottom, and 4” out the wye. The throat of the wye has to be a full 8” so an 8” wye and a 8 by 6 reducer on one leg and a 8 by 4 reducer on the other leg will work. The 6 inch drop goes into the top of the hood and the 4” drop runs to the top of the scrubber. In the 6” line, just above the hood where you can reach it, add a blast gate damper. This will allow fine tuning between the 6” exhaust and the scrubber.

The reason I make a 4 port manifold is that this scrubber will handle 1 if not 2 ports working together. Cap off the unused ports. The damper on the 6” line is for adjusting the exhaust so the suction going into one of the manifold ports is just enough to suck the fume so it doesn’t overflow the container, and not hard enough to suck out all of the fume. The goal is to allow the fume to be in the scrubber for as long as possible and this is achieved by sucking in as little red fume as necessary.

The liquid in the scrubber should flow at a rare of 3 to 5 gpm from the pump up to the spray nozzles. We will use liquid caustic to keep the pH over 10. A corrosion resistant pump is needed here.

Some thoughts about spray nozzles.

The water and caustic that performs the scrubbing of the fume is delivered through a pipe to the top of the scrubber where it sprays on the packing, wetting the surfaces so they can interact with the NO_x. To assure that the most surface is wetted as possible a manifold should be made forming an “X” over the top of the packing. Into this manifold you can either insert plastic spray nozzles to disperse the liquid or my personal favorite is to drill 1/8th inch holes.

Whether you use nozzles or holes I can assure you they will clog eventually, either from caustic buildup or from generic floating crap clogging the holes. Because of this the scrubber has to be built in such a way as to facilitate periodic cleaning of the nozzles.

There are many ways to skin a cat but the two I have used work like this.

Option 1
Do not glue at least 1 coupling leading to the blower in the 4” PVC duct. Also do not glue the cap which serves as the top of the scrubber. This will allow you to remove the top of the scrubber as needed to clean the nozzles. Another benefit of this method is it allows you to witness the flow coming from the manifold you have made and judge if there should be more or less holes. If you use this method, enter the side of the scrubber with a PVC bulkhead fitting followed immediately by a PVC union, followed by a cross tee at the center and the necessary 3 legs and end caps. This allows you to open the union and take the manifold to the sink and clean it in hot water. The down side to this method is the crusty growth you will get as caustic drips find their way out of the unglued fittings. Not hard to clean if you keep up with it.

Option 2
Make the “X” of the manifold using 2 straight pipes install 2 bulkhead fittings which allow you to thread in a bushing and pass a pipe through into the scrubber. Mount the bulkheads at 90 degrees so the pipes cross, and mount one 1” higher than the other. Cap the pipes and drill the holes, add unions outside so you can disconnect to unthread and clean the nozzles. Connect the piping together and run it to the pump. Mark the outside of the assembly so you know which way the nozzles spray, this way when you re-assemble they are spraying at the packing not up into the stack. If you use spray nozzles instead of holes, make sure you have clearance to get through the bulkhead, because the nozzles project downward about 1”. This method is a little more difficult to construct.

One helpful precaution is to add an in-line strainer to trap the floating crud before it clogs the nozzles, remember to clean it regularly.

This scrubber, because of height considerations, does not have a mist eliminator. What that does is it allows the mist to condense on its surface and drip back into the scrubber. If you choose to use spray nozzles, the finer mist may require you to add a mist eliminator. If you are running your scrubber, and your kids come in and say “Gee Daddy it’s raining and the sun is out” you need a mist eliminator

4 Metals:
You didn't mention that to get the air to flow thru the scrubber, that the blast gate needs to be closed.

So far I like all that you and Harold have posted about this subject.

I would rather see all of the air flow to go thru the scrubber. Even if there is low toxic fumes or smoke leaving the hood there are some. By scrubbing all of the air and by adding paper air filter elements to the hood discharge into the ductwork you are improving the air quality emissions that are emitted from your lab.

This is just a personal choice that I think is beneficial.

**** B

The blast gate should not be closed completely. Based on the diameters of the legs of the wye an 8" blower should supply exhaust to both the scrubber and the hood. If your packing provides enough air space for flow. The damper is only in there to increase flow in the column if it is too low from the resistance. If you went out and spent big bucks on tower packing you probably wouldn't need the damper. If you close the damper the flow through the scrubber will be too high and thus ineffective.

The goal is just enough flow through the scrubber to capture the fumes coming off the top of the reaction, not strong enough to suck out all of the fume.

Those so-called high dollar saddles that are typically used for packing are a mathematical design to produce the maximum surface area per unit volume. They are also called Tellerettes (sp?). I think they were designed by Edward Teller for the Manhattan Project.

I can think of no better product to slow fumes and air down inside of the scrubber than aquarium bioballs used inside of fish tank filter towers. They come in various sizes, and I've used the one inch size. Their purpose is to create a huge surface (plastic) for liquid to flow over.

http://www.northcoastmarines.com/bioballs.htm

This link will give you an idea as to shape and price. Each manufacturer has their own design and they all work.

Look at the diagram of the Scrubber. Below the 3" pvc pipe is where the neutralizing solution is, the fumes enter the scubber from the 3" pipe into an air pocket above the solution and below a grating (the horizontal line of vertical lines). The bioballs or whatever you use sits on top of the grating and is represented by the squiggly lines in the diagram. The solution rains down and the fumes are pulled upwards being slowed in their upward path by the media and the adjusted speed of the blower.

I hope this description is helpful.

Emil

Emil,

You are right bioballs will work very well because they actually are designed as tower packing. They are not cheap either, if you're going to spend the money for bioballs you can buy scrubber packing from Maspac for about the same amount.

4metals,

I looked at the maspac items and they're very similar to the bioball. Same principle.

For those that want a link to the Maspac packing:

http://www.maahaul.com/maspac.asp

A link to one type of bioball is in my previous post.

4metals, I have not yet found a fully plastic inline fan at a reasonable price. If the scrubber cleans the discharge air of all fumes is a fan that won't corrode necessary.

My fume hood has an opening of 2.5 sq'. What size scrubber would you recommend so that the exhaust air is clean.

Does the Sodium Hydroxide neutralize Sulfuric Acid, Hydrochloric Acid, AP and AR? What is the ratio of SH to water, and what should the pH be maintained at?

I'm sorry to bother you with all these questions, I want to be sure I'm doing it correctly.

Thanks,
Emil

Emil,

Yes a corrosion resistant blower is necessary, if you go to US Plastic http://www.usplastic.com/catalog/default.asp on the left side go to Pipe, Duct & Blowers and you will find some blowers. Generally an exhaust should be 100 cubic feet of flow per square foot of opening. You should have 250 cfm. The caustic scrubber will handle all of the acids you mentioned and it is maintained at a pH of 10 or higher.

The packing will get hot enough to melt but you cannot incinerate or burn or roast anything in a hood served by a plastic chemical scrubber. The scrubber is for fumes of chemical reactions.

The only door you need is as described for servicing the nozzles as long as you don't burn anything in the hood.

The photo you posted is from a post I posted in a thread about building an assay / refining lab. I built that system as well and in the text I mentioned that the manifold does not have to be in a hood but there were some precautions which were described and you should go back and read them.

Titanium reactors withstand aqua regia and they don't break like glass can, the down side is there is no visibility.

Emil

I've used these before and they last, have replaceable parts and are the least expensive I've found. This is the part where everybody who has found a better deal jumps in.
http://www.usplastic.com/catalog/variant.asp?catalog_name=USPlastic&category_name=58&product_id=11551&varian

Emil,

I've never had to design a scrubber without an in line blower. Your point of the expense for a hobby interest is completely understandable. The resistance of the tower packing in the scrubber is overcome by a blower which can draw enough suction (usually measured in inches of water column). Possibly a combination of a less restrictive packing and a strong blower using the wye configuration to keep it out of the air path may work. I know it works for just a hood, it would require a separate blower for powering just the scrubber.

I'm sure if it's doable there are guys on this forum who can rig it to work.

4metals,

Thanks for your concerns. A Y system with an inline fan pushes the air through and out into open air. In the process it sucks the air from the Fume Hood up and out to open air. Could this be done through the scubber, if necessary using a larger sized variable speed fan reduced to 4" pipe?

A 4" pipe from the top of the hood down to the bottom of the scrubber, 4" out the top of the scrubber into the bottom of the y out the top of the y to open air. The fan attached to the remaining opening of the y sucking air from the room and not being corroded.

How well do you think it would work? what are the negatives?

Emil

Do you mean like this drawing? I would guess it can work, for a small operation you can downsize it for one, inch and a half inlet. The hood would have to be exhausted separately using a similar arrangement.

4metals,

If the scrubber is exhausting the fumes why would I need a second system?

Emil

When you open the bottles of acid and fumes leak out, when you open the top of the vessel that is reacting and fumes leak out, I guess you could call it insurance, it is a minor amount but you wouldn't want your face in it either.