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The Nitrogen Generation System is usually for customers

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PostPosted: 2017-06-13 05:45:55    Post subject: The Nitrogen Generation System is usually for customers Reply with quote

There are actually four main processes to generate nitrogen on site; the old traditional air separation process where large volumes of compressed air are pressurized to a high pressure (several hundred PSI) and quickly expanding to at or near ambient pressure, through several stages of recirculation, compression, and expansion to liquefy the various constituents of air such as nitrogen, oxygen, argon primarily. Other rare gases are also liquefied at the same time. Each of the gases liquefies at a given cryogenic temperature. The Nitrogen Generation System is usually for customers who need high purity and use greater than 20 tons of nitrogen per day and are typically leased from an industrial gas company. There are some companies around that design, build and sell turnkey on site cryogenic plants.

In some cases, using a catalyst in conjunction with lower purity nitrogen from a membrane or PSA is used to achieve extremely low oxygen levels in the nitrogen gas, called De-Oxo, but is used rarely and will have some level of hydrogen gas in the nitrogen product. It is used to keep the compressed air requirement lower than say a PSA at 99.9995%, but the system is more complicated.

For the purpose of this article, we will focus on the two non-cryogenic methods of producing nitrogen gas; Pressure Swing Adsorption (PSA) and Membrane. Both of these technologies use compressed air as a feed stock in the production of nitrogen.

Pressure Swing Adsorption (PSA) uses, in most cases, two vessels, packed with carbon molecular sieve (CMS). The CMS adsorbs Oxygen as compressed air flows upward through one of the vessels while the other vessel is depressurized and a small amount of the nitrogen output is flowing downward (counter flow) to drive off or desorbs the oxygen and moisture collected by the vessel when it was online removing oxygen from the air. The vessels alternate this adsorption and desorption process. There are several steps in the process; equalization, pressurization/depressurization, adsorption/purge, then back to equalization. This is a basic description of the process, but there are several things happening at various times throughout the time cycle which is generally a 2 minute cycle total. PSA generally takes up a little more floor space and is a little noisier than most membrane systems. It requires in most cases an air buffer tank, nitrogen buffer tank, and sometimes is sold without the air purification system that is critical to the protection of the CMS beds. PSA is generally offered for sale using a 125 PSI air compressor, common to most plant air systems. PSA technology is generally used where higher purities are needed, and in many cases if the system is very large in capacity. Over the last 10-12 years, there have been many advances in the production of both membrane and pressure swing adsorption (PSA) technologies. Flow improvements in carbon molecular sieve (CMS) used in PSA equipment has shown approximately a 60% increase in flow rates and air to nitrogen ratios have come down dramatically.

Membrane systems use hollow fibers of man-made polymers, of various lengths, diameters, materials, and efficiencies to use differential pressure in a process known as selective permeation to separate the gases from the compressed air stream. Sometimes millions of these hollow fibers are packed into a vessel and are referred to as membrane bundles, or just membranes. The membranes are generally installed in parallel with each other to provide the needed capacity. The device uses the semi-permeable fibers to allow faster gases to quickly permeate the walls of the fibers and released to atmosphere. Faster gases are mainly oxygen, carbon dioxide, hydrogen, and water vapor. There are no moving parts to the separation process, which is attractive from a maintenance and simplicity standpoint. The systems require other components which do have moving parts as well as other electrical controls to make the system functional. Most industrial systems also have an air circulation heater which adds to the kW consumption of the system. The systems generally take up a small footprint and in most cases, very quiet. If they are not quiet, the wrong membrane was used or the piping and components are severely undersized to keep costs and footprint at a minimum.

There are currently only five companies manufacturing membranes and each has its own design variations. Among these variations from the manufacturers are their air-to-nitrogen ratio (ANR) sometimes expressed as the inverse of ANR called recovery (percentage of nitrogen to the total air consumed. Other differences are the fiber materials, diameters, pressure drop characteristics, degree of sensitivity to oil vapor, physical size, pressure ranges, temperature ranges, production efficiency, energy efficiency, and range of model selection, overall quality and cost. Each manufacture has is unique advantages and disadvantages when compared to each other. They are not all the same, by any means and as this article will demonstrate the selection process will make a huge impact on, not only the capital cost, but more importantly the life cycle operating cost which is primarily the electricity to run the system.

Some membranes, with the emphasis on SOME, can reach purities approaching that of a PSA, but they cannot do so economically or reliably. These membranes are put in series, two or three stages, creating a very high pressure drop across the system and are extremely inefficient and cost much more than a PSA at the higher purities. To keep capital costs competitive, membrane systems are usually quoted using a 150-200+ PSI air compressor, because a membrane will produce more nitrogen flow at a given purity at higher pressures. Membrane technology is generally used when purities required are 99% or lower and some are not efficient at that purity. Other reasons for choosing a membrane generator are portability, flexibility in system design for special space considerations, and they are simpler to understand the process and maintenance is generally simpler.

Both technologies use essentially the same filtration that requires essentially the same level of maintenance. PSA units have several valves that will leak at some point over the years and require routine maintenance. These leaking valves will cause the purity to fall off abruptly, so there are some maintenance considerations not found in membrane systems. In either case, the heart of both technologies, the membrane fiber or the carbon molecular sieve (CMS) must be protected from water, particulate, oil and in almost all cases, oil vapor. Regular filter maintenance must be performed or the nitrogen purity will decline to the point where the system is not fit for its purpose, costing a large percentage of the initial cost of the nitrogen generator to bring the system back up to original specifications. The compressed air purification system should consist of tank(s), aftercooler, water separator, air dryer (selected carefully for the application), and multi-stage particulate and coalescing filters and carbon bed, all designed to meet the unique needs of the membrane brand or PSA system.
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