- Membrane method
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The Membrane method or mixing by gas separation is a method of gas blending to produce nitrox, a breathing gas mostly used for scuba diving.
Nitrox systems use semi-permeable membranes to separate out a portion of the nitrogen in air. Nitrox mixtures, containing between 24% and 40% oxygen (O2), are then delivered to a high-pressure compressor to fill scuba cylinders or storage tanks, or to a low-pressure compressor for pumping to surface-supply divers. Although this compressor is described as the “nitrox compressor”, it is also used for air.
The membrane system requires a source of clean, pressurized, and heated air for separation. The two most common sources are an LP compressor (LP supply) or HP air storage tanks (HP supply). The air must be properly filtered to CGA Grade D or E air quality prior to entering the membrane system, so it will not damage or plug the membrane fibers.
Standard systems are rated for maximum supply pressures of 250 psi (17 bar) for LP supply and 5000 psi (345 bar) for HP supply. An input pressure regulator reduces these pressures to acceptable levels for the membrane. The air is then heated to a temperature that provides stability over a wide range of ambient conditions and is optimal for membrane permeation.
The heated air enters the membrane, which is made up of thousands of miniature hollow fibers. The walls of these fibers are semi-permeable and designed for different gases to move through them (or permeate) at different speeds. The resulting gas mixture is known as the permeate. As air flows through the hollow fibers, both oxygen and nitrogen permeate through the fiber walls. The oxygen permeates faster than the nitrogen, which produces permeate with an oxygen content greater than air. The gas that reaches the end of the hollow fibers without permeating is almost entirely nitrogen and is discharged. The resulting permeate contains approximately 44% O2 and is constant under all operating conditions.
The permeate is a concentrated mixture that must be diluted with additional air prior to entering the nitrox compressor. It exits the membrane at ambient to slightly negative pressure and travels into the mixing tube, where it mixes homogeneously with filtered outside air. The amount of dilution, and thus the final percentage of O2, is obtained by adjusting the Input Pressure Regulator. As pressure is increased, permeate flow increases, air flow decreases, and a higher percentage of O2 nitrox is produced. As pressure is decreased, permeate flow decreases, air flow increases, and a lower percentage of O2 nitrox is produced. This relationship between permeate flow and air flow exists because the total of these two flow rates will always equal the intake flow rate demanded by the nitrox compressor. The resulting nitrox mixture is analyzed for its approximate percentage of O2 before entering the nitrox compressor and again prior to usage to determine the precise percentage of O2.
Differences between the membrane method and oxygen blending
Membrane method Oxygen blending No O2 equipment cleaning Risk of leakage and explosion Uses inexpensive compressed air Requires expensive O2 Low, non critical maintenance High, critical and expensive maintenance Uses standard oil lubricated compressors Expensive oil free compressor required High volume output Low volume production Continuous on-site production Only 50% of O2 cylinder usable Lightweight and compact Requires heavy O2 cylinders Dive boats live-aboard and remote sites Requires facility for safe production Sources
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