Low-Pressure
Compressor Nitrox
FIGURE 6-5: PRESSURE SWING ABSORPTION SYSTEM
process should be called pressure swing adsorption with
a “d” because nitrogen only adheres to the surface of
the sieve material and is not sucked up by it.) The
adhering nitrogen can be easily removed, so the material
can be reused repeatedly. The amount of nitrogen that
will be extracted from the air is largely a function of the
pressure of the air as it is delivered to the system. This
pressure is only about three to six atmospheres. Two beds
of the nitrogen adsorbing molecular sieve operate in
Chapter 6 - Oxygen Precautions and Preparing Nitrox
parallel. The incoming, filtered air passes over each in
turn, alternately. As one bed is removing atmospheric
nitrogen, the other is depressurizing and releasing the
nitrogen that it adsorbed on the previous swing. Oxygen
up to 95% purity can be made using this system. (The
balance is argon, which is about one percent of the
incoming air and is not adsorbed by the molecular
sieve.) The gas emerges at low pressure and if nitrox
rather than oxygen is the end product, it must be compressed
to fill scuba cylinders. Usually, a PSA system
feeds directly into a continuous-flow system to make
oxygen enriched air.
MEMBRANE SEPARATION
Membrane separation is also a nitrogen removal
process. Instead of a molecular sieve that adsorbs nitrogen,
membrane separation uses a semi-permeable
“membrane” that allows oxygen to flow through it more
readily than nitrogen. Filtered, clean air is introduced
under moderate pressure (12 to 18 atmospheres) and
flows through the membrane. A valve on the output side
controls the flow-rate of gas across the membrane–the
higher the flow rate, the higher the oxygen fraction of the
product gas. The oxygen content can be precisely controlled,
and preset controls from the manufacturer limit
the maximum to 40% oxygen. The product nitrox is then
Oxygen Precautions and Preparing Nitrox
73
Purge Gas
(Nitrogen)
Adsorption Bed
"A"
Adsorption Bed
"B"
or 95% Oxygen
(For Blending)
Pressure/Flow
Control Valve
DNX
MEMBRANE
MODULE
EANx
( TO COMPRESSOR)
PURGE GAS
(NITROGEN)
GRADE E AIR
GRADE GRADE E E AIR
GRADE GRADE E E AIR
FIGURE 6-6: MEMBRANE SEPARATION UNIT