Fire suppression is an emerging nitrogen blanketing application, where the nitrogen is used to snuff out fires in case of emergency. Nitrogen fire suppression systems are commonly used in oil processing applications including offshore oil rigs, FPSO vessels and hot oil fired heater snuffing at processing plants.
Accelerant Blanketing is a blanketing process that provides safety when dealing with highly flammable accelerants used in auto air bag production. The airbag’s inflation system reacts sodium azide (NaN3) with potassium nitrate (KNO3) to produce nitrogen gas. Hot blasts of the nitrogen inflate the airbag. During production those flammable gases are blanketed by with nitrogen for safety.
When you need nitrogen, there are basically only two ways to obtain it: buy it in bulk tanks from a distributor or create it in an on-site nitrogen generator.
PSA Nitrogen Generation
Each PSA (Pressure Swing Adsorption) nitrogen generation system has two adsorber beds, or tanks, filled with a carbon molecular sieve (CMS). Compressed air enters at the bottom of the bed. Oxygen molecules are trapped in the porosities of the sieve, allowing nitrogen molecules to pass through and be stored. When the first tank is saturated with oxygen, the harmless waste gas mixture is purged while the second adsorber starts the process over again.
The standard PSA nitrogen generation system includes a feed air compressor, generator, high pressure nitrogen booster, the necessary inlet/discharge reservoirs and proper plumbing.
Membrane Nitrogen Generation
Air separation using Membrane Technology occurs when compressed air is forced into a vessel containing a hollow tube in the middle surrounded by a polymeric hollow fiber. These hollow fibers selectively permeate oxygen, water vapor, and other impurities out of its side walls, allowing Nitrogen to flow through its center and emerge as product.
The standard membrane nitrogen generation system includes a feed air compressor, air booster, generator, the necessary inlet/discharge reservoirs and proper plumbing.
Bulk Purchase Nitrogen
Bulk nitrogen is very expensive and is usually accompanied by contractual obligation. However, there are applications where purchasing bulk tanks of liquid nitrogen may be the best choice, depending on amount of nitrogen used, how fast it is used and what pressure it is consumed at.
To fill a customer’s liquid container, a bulk liquid tanker truck is filled at the cryogenic air separation plant and weighed. The weight of the truck determines the number of gallons of liquid in the truck. The customer is charged based on this figure.
Since the DOT has strict guidelines about the pressure allowed in the truck, it starts releasing N2 into the air as soon as it’s filled. Estimates vary between 3 and 5% of the truck’s volume is released during the deliveries. If the truck stops at another customer’s location before yours, an estimate is made as to how much goes to one customer and how much goes to the next and the next, etc. In fact, the best-case scenario is that you get about 90-95% of what you are charged for into their bulk tank. Then the tank evaporation continues in the storage tanks at your plant.
Bulk Nitrogen vs. Nitrogen Generation
Nitrogen Generator systems operate automatically and in direct response to demand, where bulk nitrogen tanks sit and await use. Since Nitrogen boils at 320 degrees (F) below zero, the N2 in a bulk liquid tank is always boiling. Most standard bulk tanks have a relief valve, much like a pressure cooker, which releases evaporated (boiled off) liquid N2 when the pressure builds to a certain point. The speed which the customer uses N2 has very little effect on the amount of boil-off, as it is constantly boiling.
PSA vs. Membrane
Joe Moretti of Industrial Solutions LLC, a PSA nitrogen generation manufacturer specializing in laser cutting applications, commented on the simple difference between using a nitrogen generator as opposed to buying N2 in bulk, “Ask yourself, why would we pay someone for the elements that are free for the taking? You don’t need to, you can own the equipment and generate your own N2, and do it for less then you are currently paying for it.”
Joe makes a great point, but which style of nitrogen generation do you need? It is all a matter of purities and pressures. If your process generally calls for lower pressures and a top purity of about 95% nitrogen, a membrane generator may be the best option. Membrane nitrogen generators are perfect for applications such as, fire suppression and some food processing application. However, when your application requires higher purities, such as 95% – 99.999%, then a PSA nitrogen generator is the best option. PSA nitrogen generation is a perfect fit in the metal laser cutting industry and anywhere else elevated purities and pressures are required at either intermittent or continuous duty.