The nitrogen membrane component of a nitrogen generator provides a low-cost, highly efficient means of separating air into its component gases. Because this technology requires no moving parts and consumes relatively little energy, it is surprisingly economical to operate and maintain—the main expense is the energy required to provide a stream of compressed feed air. Each system contains gas pressure control valves and instruments, a coalescing filter and carbon filter (which removes particles and liquid vapors from the feed line), and the nitrogen membrane module.
The membrane module consists of bundles of hollow fiber, semipermeable membranes. Each fiber has a perfectly circular cross-section and a uniform core through its center. The wall thickness of each fiber is thus consistent, which contributes to the physical strength of each membrane. Because the fibers are so small (about the diameter of a human hair), a great many can be packed into a limited space, providing an extremely large membrane surface area that can produce a relatively high volume product stream.
The hollow fibers of a membrane nitrogen generator are assembled parallel to a central core tube, and the bundle is inserted into an outer case to form the air separation module. Compressed air is introduced into the center of the fibers at one end of the module and contacts the membrane as it flows down to fiber bores. Oxygen, water vapor and other "fast gases" pass through the outside of the fibers. The oxygen-rich gas stream then flows through the fiber bundle to the periphery of the case, where it is discharged as a by-product.
While all but a small fraction of the oxygen passes through the membrane material to the exterior of the hollow fibers, most of the nitrogen present in the feed air is contained within the hollow fiber membrane. Since water vapor passes through the membrane along with the oxygen, this nitrogen product is essentially moisture-free. The nitrogen stream emerges at a pressure roughly 20-30% below that of the feed air pressure.
Because the heart of the system—the membrane module—contains no moving parts, a nitrogen generator requires minimal maintenance. The only attention a system typically needs is a watchful eye on the inlet gas flow and an occasional filter change. Terra's compact nitrogen generators make these tasks easy with a built-in low-pressure alarm to alert users of gas line kinks or compressor malfunctions via an easy-to-see indicator light.
Each nitrogen generator system consists of a membrane module, controls and instrumentation, and coalescing filters and traps to remove oil and liquids, organic contaminants, and other particulates from the feed air. Installation of these small nitrogen generators merely requires piping compressed air to the system and piping the nitrogen to its destinations. Order Terra's oil-less clean air compressor separately.
|N2 Generator Size||Model||Example Applications|
|Small||No. 2700-12B||3-4 chamber desiccator or small 2-port glovebox|
|Medium||No. 2700-11B||Two 10-chamber desiccators or two 4-foot gloveboxes|
|Large||No. 2700-9B||Six 10-chamber desiccators or six 4-port gloveboxes|
|*Higher flow rates for a given purity can be achieved with high feed pressure and temperature. Contact Terra for information on high-pressure compressors and inline gas heaters.|
For more and more manufacturers, on site nitrogen generators with membrane separation have become a preferable alternative to conventional sources of purified nitrogen (whether in liquid customer stations or nitrogen cylinders).
High-volume suppliers of purified nitrogen typically rely on the cryogenic distillation process, which provides excellent purity levels (up to 99.99%) but is energy-intensive and hence inherently expensive. For occasional nitrogen requirements that demand ultra-high purity, these sources remain the most economical; but for many other applications—including most high-volume storage situations—non-cryogenic generation on-site makes better sense.
Portable nitrogen generator membrane systems can be sized to deliver as little as 100 SCFH efficiently and economically, and in most cases lead to significant savings, even when high purity levels are necessary.
For nitrogen-purged storage and process systems with automatic humidity controls (like Terra desiccators, glove boxes and automated stockers), this purity level may not even be necessary. Terra systems automatically regulate the nitrogen purge to maintain the most critical humidity levels typically required, which can generally be achieved with purity levels below the 99.99% you are currently paying for.
These nitrogen gas generators provide additional savings—and greater convenience—by eliminating the need to rent tanks and pay for delivery services. Of course, they also free you from price increases on the part of your nitrogen distributor. Once your system is installed, you pay only minimal electricity expenses: your gas costs remain completely predictable.
And your supply remains secure. You'll never need to worry about overbuying to compensate for tight supplies or delivery disruptions.
The purity of nitrogen, flow rate of the gas, and feed-air temperature and pressure all affect the performance of the N2 generator. By altering these variables, you can generate nitrogen gas with precisely the right purity and flow for your applications.
Nitrogen purity can be regulated (up to a maximum of 99.5%) by adjusting the input air pressure and temperature. In most cases, the greatest system efficiency is achieved at purities of 95-99%, levels adequate for most cleanroom applications.
Temperature: Feed air temperature also affects the performance of the system. The higher the feed air temperature, the higher the feed air flow rate required, assuming constant air pressure and nitrogen purity. As feed air temperature rises, the membrane permeability increases, requiring an increased feed air flow rate to maintain product flow. However, feed air temperature also affects the membrane material. High feed air temperature shortens the life of the membrane. When the feed air temperature increases, the feed air flow required rapidly escalates. Therefore, the system must be designed to provide required feed air flow rate of the maximum anticipated feed air temperature, or an air conditioner must be used to control the feed air temperature.
Moisture: Dew point is the temperature at which a given mixture of water vapor and gas is saturated. The dew point and trace contaminants of the nitrogen-enriched product gas are dependent on the water level and quality of the feed air. Operating in the purity range of 95-99.5% nitrogen, saturated feed air results in a product gas that contains less than 5 ppm water, depending on feed air conditions. The atmospheric dew point equivalent is -65°C (-85°F). Water level in the feed gas is dependent on temperature and pressure. Therefore, if the feed air pressure is reduced, the dew point of the product may increase. If the temperature is increased, the feed dew point increases and the product stream dew point increases. Changes in dew point are minimized by using a refrigerated air dryer to condition the feed air.
CO2: Carbon dioxide in the product stream is typically less than 0.01% operating at 99% nitrogen, 135 psig and 25°C, based on feed air containing 0.03% carbon dioxide.