Selecting a Reliable and Efficient Nitrogen Generation System

As a refresher, there are two main types of nitrogen generators designed for use inside manufacturing facilities. 

 
Pressure Swing Adsorption (PSA) Nitrogen Generators

PSA nitrogen generators are fed compressed air which goes to two vessels alternately. One vessel fed by pre-treated air produces nitrogen, and the other undergoes regeneration by depressurization, purging the adsorbed gases, and then repressurization. This cyclic process ensures a continuous supply of nitrogen. The adsorbent materials selectively capture gases like oxygen, carbon dioxide and other trace gases within the compressed air stream while allowing nitrogen to pass through. Though testing of the outlet gas typically shows that the air composition is the same as the inlet minus the oxygen. The term inert nitrogen is sometimes used to refer to the nitrogen generator outlet gas because it has everything else the compressed air will contain. There are a few different types of adsorption media commonly used in PSA nitrogen generators. These include:

• Activated carbon is a highly porous material with a large surface area typically made from organic materials like coal, wood or coconut shells.
• The carbon molecular sieve is a type of activated carbon and can also be made from a plant resin. It’s a porous carbon material with a narrow range of pore sizes engineered to have selective adsorption properties, favoring the adsorption of certain gases over others.

PSA nitrogen generators are primarily used in industrial applications needing high-purity nitrogen, typically 95% to 99.9995% purity. It is worth noting here that the savings from making onsite nitrogen over liquid nitrogen increases as the level of purity required decreases. This contrasts with bulk liquid nitrogen that is industry standard 99.998% pure regardless of the application’s need.

PSA nitrogen generators are commonly used in industries such as food and beverage packaging, electronics manufacturing, pharmaceuticals, chemical processing and laser cutting.

  
Membrane Nitrogen Generators

Membrane nitrogen generators use selective permeation through semi-permeable membranes to separate nitrogen from other gases in the air. Compressed air is fed to the membrane which consists of a bundle of hollow polymer fibers. The membrane's fibers have a unique structure that is designed to allow oxygen to pass through (permeate) its walls and vent out. Nitrogen is less permeable and doesn’t pass through the walls. Nitrogen follows the path of the hollow tubes and flows out the end of the generator. Water vapor can be a big issue for membranes. Water and oil can both clog the fibers and stop the membrane from functioning.

Membrane Nitrogen Generator by Atlas Copco.

Membrane nitrogen generators are simple in design and require minimal maintenance. They are usually used in lower purity applications with purity levels ranging from 90% to 99.5%. Higher purities can be achieved, but the air-to-nitrogen ratio increases dramatically, and the output purity is not stable.

Membranes become more productive at higher temperatures, meaning they can produce more nitrogen with less compressed air. This decrease in the air-to-nitrogen ratio potentially lowers the capital investment cost for companies that work in high temperature areas since they don’t need as large an air compressor and air treatment equipment. Additionally, the energy cost for nitrogen generation will be less.

Membrane nitrogen generators are typically used in applications such as heat treatment, laser cutting, tire inflation and nitrogen blanketing in storage tanks.

Now that we know a little bit about nitrogen generators, what can you do to make them more reliable and energy efficient? We must look at the entire nitrogen system because the nitrogen generator is only one piece of equipment in a much larger system.

 
Things to Consider When Choosing a Nitrogen Generation System

Nitrogen Generators

Nitrogen is not toxic but will cause asphyxia. Therefore, never directly inhale the produced gas and avoid working in the immediate vicinity of flowing nitrogen.

The placement of nitrogen generation equipment should be carefully considered because the discharge air from a nitrogen generator will be rich in oxygen, which causes an increased fire hazard. The oxygen concentration in the vicinity of the nitrogen generator will normally not exceed 40% and will quickly go to normal concentration in the air a short distance from the generator, in a normally vented area. It’s also possible to pipe the waste gas outside the facility. If the generator is in the same room as it’s being used and isn’t being consumed by the process, the oxygen and nitrogen gases will mix together to get a net zero change in the room.

Air Compressor

After the nitrogen system equipment is purchased and installed, the only costs associated with onsite nitrogen generation are the energy and maintenance costs of the equipment. The air compressor will be the largest energy user in the system, therefore take some care picking out a properly sized and energy efficient air compressor.

Air compressor maintenance costs are also important, and care should be taken to find an air compressor that has high quality parts. If a compressor manufacturer uses low quality parts or has oil that must be replaced every 2,000 hours compared to others that have an 8,000-hour periodicity, the maintenance costs will significantly lengthen the return on investment.

The air compressor will need to be capable of 100% duty cycle, therefore choose an oil-free or oil-injected rotary screw air compressor that is either fixed speed or includes a variable speed drive (VSD). A VSD air compressor is recommended because the pull from the nitrogen generator can vary with temperature and time in the cycle. Having an air compressor that stays loaded reduces energy costs significantly.

The compressor should be placed in a well-ventilated room where the air is as cool and clean as possible, or duct in clean air from a suitable place outside. Placing the air compressor outside is not recommended unless weather and temperature protections are taken.

If using compressed air from an existing plant air compressor, it is recommended to have a receiver tank dedicated for a PSA nitrogen generator and a check valve placed in between the tank and the nitrogen generator to prevent large plant demands pulling air from the nitrogen generator.

Compressed Air Dryers

When picking out a dryer for a PSA nitrogen generator, be aware that heated desiccant dryers can have a high discharge temperature that needs to be taken into consideration. As the air temperature goes up, so does the air-to-nitrogen ratio. For example, the air-to-nitrogen ratio may be 3.6 to 1 at 86°F (30°C), but will go up to 4.2 to 1 at 140°F (60°C). Not taking this into account could lead to lower purity nitrogen than expected from the generator, under sizing the air compressor or an inline aftercooler may be needed to lower the temperature of the air going into the nitrogen generator. 

Adsorption dryers may use a purge for regeneration. The amount of purge air needs to be taken into consideration when sizing the air compressor to prevent not having enough compressed air for the nitrogen generator.

Compressed Air and Nitrogen Storage Tanks

For PSA generators, the dry compressed air supply tank should have a regulator downstream to provide steady pressure to the generator. You could also use a variable speed air compressor to get steady pressure. The pressure band of a fixed speed air compressor will cause air velocities to continuously change inside the nitrogen generator, which could degrade the adsorption material over time.

Having a properly sized wet compressed air tank in the system (a tank between the air compressor and the dryer) is recommended to help prevent short cycling of the air compressor and remove some moisture from the air stream and take some load off the dryer.

It’s important to size the dry compressed air receiver correctly because the air requested by the nitrogen generator is not constant during the production cycle. Every time the vessel is pressurized, the air consumption can be three to four times the average consumption for a few seconds.

It’s recommended to have a nitrogen storage tank after the nitrogen generator for handling varying demands. Whether you are feeding the nitrogen applications or supplying nitrogen to a booster, having enough nitrogen storage will help with the overall efficiency and reliability of the system. Also, regulate the nitrogen leaving the tank to the lowest pressure needed.

Having more nitrogen storage capacity may allow the purchasing of smaller nitrogen system equipment if the nitrogen demand is not continuous 24/7. For example, if the nitrogen system operates 16 hours a day, the other eight hours could be used to fill a large capacity storage system. It could mean reducing the horsepower of the air compressor significantly, as well as lowering energy costs.

It may be a good idea to create extra nitrogen as a backup or have an oversized or extra nitrogen tank. Having a 12 or 16 pack of nitrogen bottles pressurized to 4,100 psig will provide hours or potentially days’ worth of nitrogen in the event of a problem with the nitrogen system.

Compressed Air Filtration

The number one cause of customers breaking their nitrogen generator is improper filtration. Also, every nitrogen generator manufacturer will void the warranty if the nitrogen generator is destroyed by inadequate filtration.

Adsorption material is expensive and must be protected. Therefore, high-quality filters are required to ensure the efficiency and reliability of a nitrogen generator. These filters remove particulate, liquids and oil vapors from the air stream.

Most, if not all, nitrogen generator manufacturers require the use of an activated carbon bed with oil-flooded air compressors. The activated carbon adsorbs the compressor oil and protects the expensive CMS from degrading. This can be the difference between having to replace the CMS every two to four years to replacing it every 10 to 20 years.

Changing the filter elements periodically is crucial for continued protection of the nitrogen generator.

Compressed Air Piping

Pay attention to good engineering practices when installing piping. There may be flow meters or other types of sensors on the nitrogen generator that can be affected by erratic air flow. Having 20 pipe diameters of straight pipe before the flow meter is a good rule of thumb. This varies depending on what the piping obstruction is; 15 pipe diameters may be good after a sweeping 90-degree elbow, while 20 pipe diameters may not be enough if there is something more disruptive in the line.

The general rule for the size of a nitrogen generator’s inlet and outlet connections is to size them for a maximum pressure drop of 2 psi over 100 feet of pipe. This assumes no bends, but adding some elbows shouldn’t have much effect.

Some piping materials or designs may allow oxygen migration through the piping into the nitrogen system, so consider the piping style and material prior to installation.

Pressure Swing Adsorption (PSA) Nitrogen Generators

A good nitrogen generator will use high quality adsorption material that needs to be protected because it’s expensive to replace – typically 30 to 45% of the cost of a new generator.

Some nitrogen generator manufacturers take a more proactive approach than others to ensuring the reliability and efficiency of their products, therefore you should compare features offered by different manufacturers. 

Built-in sensors, monitoring systems and automated flush mechanisms can be used to ensure required purity is met. If nitrogen demand requires a minimum purity level, find a generator that guarantees the required purity by stopping nitrogen flow and flushing until purity is met. This requires nitrogen being provided from a backup storage system.

If inlet compressed air quality drops below safe operating conditions, the nitrogen generator should have a way of protecting itself by flushing out the bad air, but not all nitrogen generators have this feature. Backup nitrogen will be required with this feature, also.

During startups of the nitrogen generator when there is no pressure in the tank, there is a risk of overflowing the adsorption material and damaging it. Therefore, only a small amount of high purity nitrogen should be allowed to flow through the nozzle until adequate pressure is achieved. Once enough pressure is in the tank, the minimum pressure valve can be opened to allow full flow through the nitrogen generator. Not all nitrogen generators have this feature.

PSA systems typically are not designed for use outside and perform poorly in direct sunlight. This is because PSAs operate more efficiently at lower temperatures.

It’s rare, but there are applications that have a maximum purity requirement. This is usually for applications that limit the speed of a chemical reaction with oxygen. Make sure the nitrogen generator picked out can support this, if needed.

Custom pressure swing adsorption nitrogen generator fabrication project by Arkansas Industrial Machinery.

Holtec pressure swing adsorption nitrogen generator.

Mikropor pressure swing adsorption nitrogen generator.

Regular Maintenance

While the design and technology of a nitrogen generator plays a significant role in its efficiency and operation, following the manufacturer’s recommendations for maintenance schedules and procedures will prolong the lifespan and reliability of the generator.

Employee Training and Awareness

Providing training to personnel involved in operating, maintaining and troubleshooting the nitrogen system enhances their knowledge and skills, which could mean fewer problems and increased reliability.

By considering these factors and choosing a reputable manufacturer with a track record of building quality nitrogen generators, you can increase the chances of having a long-lasting dependable nitrogen generation system.

 
About the Author

Kevin Conley is the Applications Engineer for Arkansas Industrial Machinery (AIM) and the head of its Industrial Gas Division. He has been at AIM for 16 years and has a Master’s of Industrial Distribution Degree from Texas A&M. 
 

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