NITROGEN GENERATION AND GAS SEPARATION: A BURGEONING INDUSTRY

November 29, 2017

N2 Molecule

The universe is permeated with elemental nitrogen. Nitrogen due to its unique chemical makeup is inert and this property makes it ideal for use in many industrial applications.

Life as we know it depends on nitrogen and oxygen. Our atmosphere we breathe is 78% Nitrogen, 21% Oxygen and 1% trace gases. Industry has developed thousands of applications where Nitrogen is a critical component to their processes. The critical issue is separating the Nitrogen from the Oxygen in the Air we breathe.

Nitrogen separation from air can be accomplished in several ways. Which application is used for the separation process depends on the volume of Nitrogen needed, the location of the application, and the purity of the Nitrogen needed for the application.

The Three Primary Technologies Utilized for Separating Nitrogen from Air are:

 

Cryogenic Distillation

One way to separate oxygen and nitrogen is to cool the Air to the point it turns into a liquid (-196 degrees Celsius) and use fractional distillation, which separates elements by their different boiling points.  This process is very effective for large tonnage requirements. Typical applications for this process are for large Steel Manufacturing Facilities, Refineries, and Glass Furnaces, and making liquid Nitrogen for the general industry.

Nitrogen made in this way is normally transported to customers as a super cooled liquid (-196C). Customers normally have onsite storage tanks that the transport trucks off load into.

One of the benefits of Nitrogen in this form (super cooled liquid -196C) is that it can be used to flash freeze food products. Additionally it is a good source of High Purity Nitrogen 99.999%. This has many industrial applications.

Non Cryogenic Nitrogen and Oxygen Generators

Two other types of technologies are employed for On Site Generation of Nitrogen. In the Non Cryogenic process the separation of the Gases is accomplished by adsorption on specialized media (PSA Technology) through a PSA nitrogen generator or utilizing Hollow Fibers (Membrane Technology) through a membrane nitrogen generator. Both processes operate at ambient temperatures and pressures up to 500 PSIG. These systems can be made to make Nitrogen from the smallest quantity to extremely large systems. They are ideally suited to individual company needs.

These technologies are what are normally selected for thousands of point of use applications. Just a sampling of the applications will give the reader a sense of the breadth of the market; – large commercial airplanes have on board Nitrogen Generators to fill the fuel tank head space with Nitrogen to prevent explosions, most packaged food has nitrogen injected into the bag to prevent spoilage, wine bottles have nitrogen injected into the bottle prior to corking to prevent the wine from going bad, laser cutting system use Nitrogen to shield the beam, Electronic Manufactures use Nitrogen in Chip Manufacturing,+ thousands more…

Which technology is right for which application depends on the volume of gas needed, at what purity and at what pressure it is needed.

Pressure Swing adsorption
Pressure Swing Adsorption (PSA) can produce Nitrogen up to 99.999% purity The PSA consists of two adsorption vessels that are filled with a specialized absorbent (CMS). The process works by passing the compressed dry air into one of the adsorption vessels. As the air passes through the vessel Oxygen is absorbed on to the CMS and the product Nitrogen pass out the top of the vessel. Once the CMS has become saturated with Oxygen this vessel is isolated and vented to atmosphere. At this point the other vessel is put into operation and the cycle repeats. The purity of the product is adjusted by controlling the residence time of the air inside of the adsorption vessel.

The PSA process is normally specifically designed to a customer’s flow rate and purity requirements.

Membrane Technology
The Membrane Technology can efficiently produce Nitrogen at purities of 95 – 99%. The Membrane technology works differently from the PSA in that the compressed air is passed through a bundle of hollow fibers (each the size of a hair but with a hole down the center), as the compressed air passes down the fiber length the Oxygen is preferentially absorbed through the fiber wall and vented to atmosphere. The Nitrogen exits the end of the fiber and is collected into the distribution piping.

The purity of the Nitrogen is controlled by varying the residence time of air in the membrane fiber. This is accomplished by adjusting a control valve on the discharge of the membrane. In some cases, the resultant oxygen from the oxygen-nitrogen separation membrane is the desired product.

Differences between a PSA and Membrane Nitrogen Generator

The PSA process is a reliable technology however it is a quite complex process with four pressure vessels (Supply Air Buffer Tank, Two adsorption Vessels filled with CMS, and a Nitrogen Buffer Tank) and Eight Switching Valves. The process involves switching between each absorber vessel every 40 seconds.

The primary advantage of the PSA technology is that it can produce Nitrogen to 99.999% purities. This would be equivalent to liquid Nitrogen purities at a fraction of the cost from an industrial gas supplier. The Generated Nitrogen is drawn off the Nitrogen Buffer tank and is ready for use by the customer.

The Membrane process in contrast is quite simple. The flow rate determines the number and size of the Membrane Modules to be used. The Membrane Modules are connected together and compressed air is introduced into one end of the Membrane Module and the Nitrogen Gas exits the other. The Generated Nitrogen is ready at this point for use by the customer.

PSA or Membrane Technology – Which one to use?

Each has its own unique advantages. You will need to consult with an expert in the field to determine which system will work best for your particular industry and facility. In general we would we would recommend the following:

For applications requiring 99 – 99.999% Purities we would normally recommend a PSA Nitrogen Generator.

For applications requiring 95-99% we would normally recommend a Membrane Nitrogen Generator.

The most critical item for all applications is to know what purity is required. As a general rule the higher the purity the more expensive the product gas will cost. The traditional gas companies provide Nitrogen with purities of 99.999%. This is because this is a byproduct of the cryogenic distillation process. Many customer pay an extremely high price for high purity Nitrogen that they do not need. Selecting the correct purity for your application will save you thousands of dollars per year on a typical application.

Benefits of On-Site Nitrogen Generation

Buying an on-site nitrogen generator has multiple benefits for your business, including financial ones. In fact, your company can save as much as 300 percent on nitrogen by generating it at your facility. You will eliminate many delivery costs and limit loss through evaporation. Even your site liability insurance should go down.

Your company will also benefit from the convenience of having a steady supply of nitrogen on hand. You won’t lose time waiting for suppliers to deliver the Nitrogen that they need.

Other Things to Consider

When you choose Nitrogen Generator supplier, choose a company with a long history in the business. The warranty and service agreements are vital. You will want a company with experts in your system design to work with you to train your employees and offer advice on system optimization.