Selection Onsite Nitrogen Generators

Selecting Purity Level Onsite Nitrogen Generators

The useful and various properties of nitrogen (N2) in industrial applications rank it as one of the most specified gases in industry. For the customers, nitrogen options exist in the choice of delivery system, compliance with clean air standards, safety and purity. In researching these choices, users can accurately select the optimum nitrogen supply required, often looking at a considerable coist savings. Selecting purity levels of 99.99% or higher in many industries and applications adds a variety of costs, both financial and efficiency, which may be needlessly incurred.

Commercially Supplied Nitrogen: The Process and the Costs from Air Separation Unit

Liquid air separation plants provide nitrogen generated by using cooled air to separate out the oxygen and nitrogen as they become liquid. Cryogenic distillation accounts for approximately >95% of the total nitrogen production. Generating nitrogen from ASU is energy-intensive because the process entails condensing ambient air into liquid air by cooling and compressing it in a refrigeration cycle that utilizes the Joule-Thompson effect.

After N₂ is separated from the air, additional energy is needed to purify it to requirements and fill the appropriate transport container. Since this process is performed continuously on a large scale, its power usage generates hundreds or thousands of tons of greenhouse gases every day.

Nitrogen produced through cryogenic process can attain a purity of 99.999% or higher. This process is completed at a higher cost for a purity level offering no added benefit for most applications. Purchased nitrogen from a commercial gas company is the most expensive option. Costs are slightly lower for liquid nitrogen in a bulk tank. In cylinders, the cost rises.

The cost of transport of nitrogen via delivery tankers from a ASU facility to  an end users plant is factored into the price. Delivery of nitrogen uses a lot of energy and significantly contributes to the amount of CO₂ generated in the process of delivering nitrogen to end users. Obviously, the amount of energy required to transport the nitrogen depends on the distance between the facility and the end user’s plant, but the environmental impact of trucking nitrogen is significant. For example, a tractor trailer travelling around 100,000 miles per year generates about 360,000 pounds of carbon dioxide during that time period.

On-site Nitrogen Generators: The Process and Benefits

On-site nitrogen generators are safer and easier to handle than high-pressure cylinders and offer speed of delivery advantages over liquid nitrogen evaporation from ISO tankers. On- demand gaseous nitrogen generation uses one of two alternative methods.

They include; (a) Pressure Swing Absorption (PSA) and (b) membrane system technologies. The choice of generator largely depends on the purity of nitrogen needed. In both cases, the level of O₂ can be controlled to just the required purity level

Applications that need nitrogen of 95 to 98 percent purity (5% to 2% oxygen), such as fire and explosion prevention, can use membrane generators.

Applications such as the blanketing of oxygen sensitive compounds, specialty chemicals and pharmaceutical processing need a high purity stream and require the use of PSA generators.

Membrane

Membrane generator uses the hollow fiber technology. Treated/purified compressed air used is directed through the bore of a membrane tube. The smaller and more soluble oxygen and water molecules pass through the wall of the polymeric membrane tube thus enriching the nitrogen in the air stream. Membranes produce nitrogen in a continuous process assuring constant downstream purity. Like the PSA system, the nitrogen-enriched gas stream passes through a final filter to ensure the delivery of pure, sterile nitrogen.

PSA system

PSA systems take compressed air and filter the air using high quality coalescing filters to remove 99.99% of 0.01 micron particles. The system then separates nitrogen from oxygen based on the preferential adsorption and desorption of oxygen, water vapor and other contaminants on a Carbon Molecular Sieve (CMS) bed. PSA nitrogen generators have two bed columns: one with fresh CMS for the current adsorption process and another that swings to low pressure to desorb oxygen from saturated CMS. One column provides nitrogen while the other column is being regenerated. These column beds are cycled back and forth to generate and regenerate the beds. The resulting high purity nitrogen then feeds a buffer tank to allow for a continuous high-pressure output. The high purity nitrogen stream passes through a final filter to ensure the delivery of pure, sterile nitrogen.

Continuous with lesson "Purity Specified in Industrial Applications"

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