Chemical Engineering August 2021 - 19

Technology Profile
Production of Nitrogen from Air
By Intratec Solutions
itrogen (N2) makes up 78%
of ambient air (20.95%
of air is oxygen). N2 is an
inert (non-flammable) gas
used in industrial plants to prevent
undesirable chemical reactions
from occurring. Nitrogen is the
most common inert gas used in the
chemical process industries, due to
its high natural abundance and low
relative cost.
N
Major uses of N2 include several
in metallurgy: for heat treatments;
as an alloy constituent; for scavenging;
and as a protective gas; as
well as in heat treatment. N2 is also
used as an inert gas in shielding and
firefighting, in food storage and in
processing. Nitrogen can be used in
the manufacture of other products,
including ammonia.
Most N2 is produced onsite at industrial
facilities, for local consumption.
However, it may be stored and
transported as a gas or a liquid, at
cryogenic temperatures. High-pressure
cylinders or cylinder clusters
can be used for storage and shipping
of small quantities of nitrogen
gas. Liquid nitrogen can be transported
and shipped in specially
insulated storage vessels (small
quantities) or by road and rail tankers
(large quantities).
Production process
The process of recovering N2 from
air comprises three major sections:
(1) purification; (2) refrigeration; and
(3) rectification (Figure 1) .
Purification. Initially, the air is purified
from dust particles, carbon
dioxide and water. This is accomAmbient
air
PATHWAY A
Cryogenic air
separation + argon
recovery
Oxygen, argon
Ambient air
PATHWAY C
Adsorptive air
separation
Nitrogen
PATHWAY B
Cryogenic air
separation
Ambient air
Oxygen
PATHWAY D
Membrane air
separation
Ambient air
■ Raw material ■ Process ■ Main product
FIGURE 2. Cryogenic separation is used to produce gases from air, but other routes exist
plished by compressing the air,
passing it through a heat exchanger
(causing water condensation) and
then through an adsorbent bed of
molecular sieve for the removal of
water, carbon dioxide and traces of
other impurities.
Refrigeration. At this point, the purified
air is cooled down/liquefied, as
required for rectification steps downstream.
This step is conducted in a
plate-fin heat exchanger, using refrigeration
values contained in the products
(oxygen, nitrogen) and waste
(nitrogen) streams.
Rectification. The last step in the
process consists in submitting the
cooled/liquified air to a fractional distillation
carried out in two columns
for nitrogen-oxygen separation,
operating at different pressures, in
such a way that nitrogen is distilled
as a vapor and oxygen is obtained
as a liquid. In addition, while oxygen
and argon have close boiling points,
a third column is used exclusively
for the removal of that contaminant,
vented to the atmosphere.
The main product obtained in the
process under analysis here is gaseous,
high-purity N2 (the product gas
is greater than 99.7% N2), at 60 bars
absolute.
Production pathways
Millions of tons of N2 are isolated
from atmospheric
air
every
year,
through different separation processes
that are selected according
to the production scale and purity
required (Figure 2). The production of
large quantities of relatively pure nitrogen
is carried out, mostly, through
cryogenic distillation. On the other
hand, moderate volumes of low-purity
nitrogen are commonly produced
via pressure-swing adsorption and
membrane permeation.
In addition to nitrogen, oxygen (O2)
and argon (Ar) can also be generated
commercially in air-separation processes
using cryogenic distillation
and argon recovery units. Oxygen is
also produced by vacuum-swing adsorption
(VSA).
■
Edited by Scott Jenkins
Editor's note: Content for this column was originally developed by
Intratec Solutions LLC (Houston; www.intratec.us) and is edited by
Chemical Engineering. The analyses presented are based on publicly
available and non-confidential information. The content represents
the opinions of Intratec only. More information about the methodology
for preparing the analyses can be found, along with terms of use,
at www.intratec.us/che.
1. Air compressor
2. Air pre-treatment
3. Main heat exchanger
4. Oxygen columns
5. Argon column
6. Nitrogen compressor
7. Cooling tower
CW Cooling water
FIGURE 2. The diagram shows the process for producing nitrogen from ambient air
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