The purity of nitrogen, flow rate of the gas, and feed
air temperature and pressure all affect the performance
of the membrane separation system. By altering these
variables, you can deliver nitrogen 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. See Figure 1.
The flow rate of the enriched nitrogen product is a
function of the purity required. The higher the nitrogen
purity required, the lower the flow rate produced, assuming
constant feed air temperature and pressure. To produce
a higher purity nitrogen product, more oxygen must permeate
the membrane. You increase oxygen permeation by increasing
the pressure difference across the membrane. This transports
more gas, including more of the total oxygen content
of the feed air, through the membrane. With more oxygen
permeating the membrane, the oxygen level in the nitrogen
product gas is reduced and purity improved.
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. See Figure 2 for the relationship
between feed air temperature and flow rate.
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
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%
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 -30°C (-20°F) at 95% purity, and -50°C (-58°F) at 99.9% purity.
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.
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.