6
Chapter 6 Oxygen Precautions and Preparing Nitrox 73
Adsorption Bed
Low-Pressure
Compressor Nitrox
Adsorption Bed
way to obtain nitrox, and the mix accuracy available is
not of real necessity in nitrox diving.
"A"
"B"
Pressure Swing Absorption
This method removes nitrogen from air rather than
adding oxygen to it. Pressure swing absorption (PSA)
uses a material called a molecular sieve that selectively
adsorbs a thin layer of nitrogen onto its surface from
the air that passes over or through it (Figure 6-5).
(Technically, the 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 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 continuousflow
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
Purge Gas
(Nitrogen)
or 95% Oxygen
(for blending)
more readily than nitrogen (Figure 6-6). 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 compressed for use in scuba
cylinders.
The membrane separation system requires only
simple routine maintenance. If the air entering the
system is properly filtered and the unit properly
maintained, the membrane is long-lived. The system is
light-weight and portable. For these reasons, as well as
the fact that it does not require handling pure oxygen,
either as a source or a product, membrane separation for
Figure 6-5 Pressure swing absorption system.
Pressure/Flow
Control Valve
DNX
Membrane
Module
Purge Gas
(Nitrogen)
EANx
( to compressor)
GRADE E AIR
GRADE E AIR
GRADE E AIR
Figure 6-6 Membrane separation unit.