Introduction: Nitrogen Gas Generator
This instructable will walk you through the basics of making your own nitrogen gas generator. Using pressure swing adsorption with carbon molecular sieve you can make an endless supply from the air without using any consumables. You can use this for filling your tires (nitrogen stays in tires much longer than oxygen, reducing the time you need to refill them), having a non-combustible gas or, in my case, to feed into a liquid nitrogen generator.
I won't go into any theory here. I'll just go over the basic construction. A unit of the size I will describe can go between $6000 to $8000. You can build this for a fraction of the price. If you are interested in this subject in more detail you can go to my nitrogen generator web tutorial. There you will find more pictures, animations and other information. There is also a video going over the nitrogen generator. You can also view my liquid nitrogen generator tutorial.
If you are ready to build this for your shop just click next.
Step 1: Basic Theory
This is a big project and will most likely need to stay in your shop or garage. The generator runs off of compressed air. It has two tower beds, each filled with CMS (carbon molecular sieve). Pneumatic valves control the airflow into one at a time. Under pressure, oxygen is preferentially trapped, while nitrogen passes through and out the bottom. The controller opens the next bed for filling, isolating it from the first one as it opens it to the atmosphere. During decompression, the first bed releases the oxygen back into the atmosphere, regenerating the CMS.
The picture above shows the CMS, which looks like chocolate sprinkles. They are approximately 0.5 x 2mm in size. Let's go over a basic materials list.
Step 2: Supplies
The unit I will describe will produce 98.5 - 99% pure nitrogen gas at a flow of 1 SCFM.
Air compressor: 6HP, 6-9 SCFM depending on the pressure.
Air compressor tank: 30 Gallons
Two 8" steel tubes x 33" tall
CMS-180, 200 or H: 20kg per tower. You can get this from molecular sieve dealers. Refer to web tutorial about this material.
Two pairs of 150 psi flanges (one flange blank/one 8" flange): you can get these at Lincoln supply or McMaster-Carr.
1/4 - 3/8" stainless steel (304 or 316) plumbing. I use 1/4" as this keeps the cost lower. You will need enough pieces to make the connections.
Prefilter: you need a 5um particle filter/water trap and a 0.01um coalescing filter. I use Wilkerson filters. Again, refer to my web tutorial for all the part numbers.
Versa pneumatic valves: you need two of these.
Check valves: you need two
200 psi pressure gauge
Rotometer or other flow control device
Pneumatic controller: schematics to follow
You will need access to a welder
You will need to build a cradle to hold the towers, which will weigh over 150lbs each. Mine has wheels on it so I can move it around. You're not going to be carrying this up and down your stairs, though.
Step 3: Basic Construction
Before making this nitrogen generator you should know a few things. The unit is at least 300lbs when complete. You will be dealing with pressures close to 150 psi. You will need access to welding equipment and possibly steel plate rollers.
The PSA consists of two tower beds, solenoid pneumatic valves and a controller. I sized my unit to deliver about 30 L/min of 98% or better purity N2. The bed is made from an 8" ID schedule 40 steel pipe at 33" long. The top and bottom has a welded low-pressure (150 psi) steel flange plate. They are an 8" 150# raised-face slip-on flange and a 8" 150# raised-face blind flange. A gasket helps seal this plate with a solid blank flange plate on top. A hole is drilled in the center to accept a 1/4" steel nipple, which you weld to the outer, removable plate. You can go to 3/8", but this did not seem necessary for the flows I'm using, and this increases the costs of the other hardware and pipes.
Each of the two towers holds 20kg of CMS-200 or CMS-H. You need to prevent these grains from emptying through the plumbing. You accomplish this with an insertible steel screen. You need to fashion a ring of steel that will just fit the inner diameter of the tower. Weld some steel screening onto this ring using whatever method works best for you. The screening needs to have holes about 1mm in size so the CMS grains do not pass through. This still ensures good airflow. As an added precaution you will use 10-12 MERV air-conditioning filter paper to trap dust from degrading the CMS.
Insert the bottom screen component. I welded two loops of steel on opposite sides so I could drop and retrieve the screen with two long poles. The ring holding the screen fits snuggly, so you will not be able to reach down with your hands and grap it out. Next, drop the filter paper and then fill the tower almost to the top with the CMS. Now, place another layer of the filter paper, followed by the screen. There should be now way for the grains to drop through the bottom or get discharged out from the top. Screw down the top plate.
As mentioned above, these towers are very heavy. I have mine sitting on a rack with large wheels on the bottom so I can roll it around. You will not be carrying this up and down flights of stairs, so pick a good resting place to keep it.
Above are pictures showing the bed with the screen. Again, this screening is duplicated on the bottom, except there are steel loops on the insertable ring to allow for extraction.
Step 4: Valves and Plumbing
Water vapor and microscopic particles can fowl up the CMS. One needs to make sure the fresh gas is clean. The system uses two filters: a pre-filter and a coalescing filter. The first traps water vapor and 5 um particles. The second filter particles down to 0.01 um. These are 1/4" port Wilkerson filters. The part numbers are F16-02-000 and M16-02-000, respectively.
The PSA has high pressure tubing coming off of the check-valve. The tubing from the two beds join together on a TEE. The output then goes to a high pressure valve that can shut off all flow. From here, the output goes to a Yokogawa rotometer, so one can control and measure the output flow.
Step 5: Pneumatic Valves
The bed is partially pressurized by the previously charged bed and from the pressurized reserve tank of air. When one bed receives fresh gas, the other's valves isolates it from the fresh gas and vents its tank to the atmosphere. The PSA system uses a three-way Versa valve. My system uses a normally-closed valve, so it needs power to open it to allow the fresh gas through to the bed. The model number for the 3-way, 1/4 brass, normally closed, 120vac Versa valve is VSG-3321-120. There may be an additional letter at the end.
Fresh gas enters from the top and flows into the bed when there is 120vac on the valve. When the power is off, the valve closes to the fresh gasflow and opens to the atmosphere. Exhaust gas flows from the bed out the hole on top to the left of the inflow port. Since there are two beds there are two valves. A controller handles powering and de-powering the valves.
The enriched gas leaves the bed at the bottom of the tank. Remembering that only one tank is under pressure, we need a means to prevent the pressurized, enriched gas from entering the other tank from the bottom. The system uses a check-valve that only lets gas flow out.
I got the check valves for a few dollars on Ebay. There is one under each bed.
Step 6: Arduino Pneumatic Valve Controller
A simple arduino-based controller manages two solid state relays. The arduino runs a cycling program and manages the gates on two triacs. These allow the controller to energize and de-energize the Versa valves with 120vac.
Above is the timing diagram and the controller schematic. The c-code is simple enough to write.
Step 7: Final Product
So, there you have it. You can now take regular air and pull out 99% pure nitrogen gas. If you want purer gas you will need a second stage, and unless you are a chemist making industrial grade compounds you won't need this.
I have other high-detail web tutorials.
You can read about my liquid nitrogen generator here.
You can read about how to make a 3 or 12kw induction heater here.
You can see one of many video of my induction heaters on Youtube. This is the 3kw unit here.