An air separation plant separates atmospheric air into its
primary components, typically nitrogen and oxygen, and sometimes also argon and
other rare inert gases.
The most common method for air separation is cryogenic
distillation. Cryogenic air separation units (ASUs) are built to provide
nitrogen or oxygen and often co-produce argon. Other methods such as Membrane,
pressure swing adsorption (PSA) and Vacuum Pressure Swing Adsorption (VPSA),
are commercially used to separate a single component from ordinary air. High
purity oxygen, nitrogen, and argon used for Semiconductor device fabrication
requires cryogenic distillation. Similarly, the only viable sources of the rare
gases neon, krypton, and xenon is the distillation of air using at least two distillation
columns.
Cryogenic liquification process
Distillation column in a cryogenic air separation plant
Pure gases can be separated from air by first cooling it
until it liquefies, then selectively distilling the components at their various
boiling temperatures. The process can produce high purity gases but is
energy-intensive. This process was pioneered by Dr. Carl von Linde in the early
20th century and is still used today to produce high purity gases.
The cryogenic separation process requires a very tight integration of heat
exchangers and separation columns to obtain a good efficiency and all the
energy for refrigeration is provided by the compression of the air at the inlet
of the unit.
To achieve the low distillation temperatures an air
separation unit requires a refrigeration cycle that operates by means of the
Joule–Thomson effect, and the cold equipment has to be kept within an insulated
enclosure (commonly called a "cold box"). The cooling of the gases
requires a large amount of energy to make this refrigeration cycle work and is
delivered by an air compressor. Modern ASUs use expansion turbines for cooling;
the output of the expander helps drive the air compressor, for improved
efficiency. The process consists of the following main steps:
Before compression
the air is pre-filtered of dust.
Air is compressed
where the final delivery pressure is determined by recoveries and the fluid
state (gas or liquid) of the products. Typical pressures range between 5 and 10
bar gauge. The air stream may also be compressed to different pressures to
enhance the efficiency of the ASU. During compression water is condensed out in
inter-stage coolers.
The process air is
generally passed through a molecular sieve bed, which removes any remaining
water vapour, as well as carbon dioxide, which would freeze and plug the
cryogenic equipment. Molecular sieves are often designed to remove any gaseous
hydrocarbons from the air, since these can be a problem in the subsequent air
distillation that could lead to explosions. The molecular sieves bed must be
regenerated. This is done by installing multiple units operating in alternating
mode and using the dry co-produced waste gas to desorb the water.
Process air is
passed through an integrated heat exchanger (usually a plate fin heat
exchanger) and cooled against product (and waste) cryogenic streams. Part of
the air liquefies to form a liquid that is enriched in oxygen. The remaining
gas is richer in nitrogen and is distilled to almost pure nitrogen (typically
< 1ppm) in a high pressure (HP) distillation column. The condenser of this
column requires refrigeration which is obtained from expanding the more oxygen
rich stream further across a valve or through an Expander, (a reverse compressor).
Alternatively the
condenser may be cooled by interchanging heat with a reboiler in a low pressure
(LP) distillation column (operating at 1.2-1.3 bar abs.) when the ASU is
producing pure oxygen. To minimize the compression cost the combined
condenser/reboiler of the HP/LP columns must operate with a temperature
difference of only 1-2 kelvin, requiring plate fin brazed aluminium heat
exchangers. Typical oxygen purities range in from 97.5% to 99.5% and influences
the maximum recovery of oxygen. The refrigeration required for producing liquid
products is obtained using the JT effect in an expander which feeds compressed
air directly to the low pressure column. Hence, a certain part of the air is
not to be separated and must leave the low pressure column as a waste stream
from its upper section.
Because the
boiling point of argon (87.3 K at standard conditions) lies between that of
oxygen (90.2 K) and nitrogen (77.4 K), argon builds up in the lower section of
the low pressure column. When argon is produced, a vapor side draw is taken
from the low pressure column where the argon concentration is highest. It is
sent to another column rectifying the argon to the desired purity from which
liquid is returned to the same location in the LP column. Use of modern structured
packings which have very low pressure drops enable argon purities of less than
1 ppm. Though argon is present in less to 1% of the incoming, the air argon
column requires a significant amount of energy due to the high reflux ratio
required (about 30) in the argon column. Cooling of the argon column can be
supplied from cold expanded rich liquid or by liquid nitrogen.
Finally the
products produced in gas form are warmed against the incoming air to ambient
temperatures. This requires a carefully crafted heat integration that must
allow for robustness against disturbances (due to switch over of the molecular
sieve beds). It may also require additional external refrigeration during
start-up.
The separated products are sometimes supplied by pipeline to
large industrial users near the production plant. Long distance transportation
of products is by shipping liquid product for large quantities or as dewar
flasks or gas cylinders for small quantities.
Non-cryogenic processes
Pressure swing adsorption provides separation of oxygen or
nitrogen from air without liquification. The process operates around ambient
temperature; a zeolite (molecular sponge) is exposed to high pressure air, then
the air is released and an adsorbed film of the desired gas is released. The
size of compressor is much reduced over a liquification plant, and portable
oxygen concentrators are made in this manner to provide oxygen-enriched air for
medical purposes. Vacuum swing adsorption is a similar process, but the product
gas is evolved from the zeolite at sub-atmospheric pressure.
Membrane technologies can provide alternate, lower-energy
approaches to air separation. For example, a number of approaches are being
explored for oxygen generation. Polymeric membranes operating at ambient or
warm temperatures, for example, may be able to produce oxygen-enriched air
(25-50% oxygen). Ceramic membranes can provide high-purity oxygen (90% or more)
but require higher temperatures (800-900 deg C) to operate. These ceramic
membranes include Ion Transport Membranes (ITM) and Oxygen Transport Membranes
(OTM). Air Products and Chemicals Inc and Praxair are developing flat ITM and
tubular OTM systems, respectively.
Applications
Large amounts of oxygen are required for coal gasification projects;
cryogenic plants producing 3000 tons/day are found in some projects. In
steelmaking oxygen is required for the basic oxygen steelmaking. Large amounts
of nitrogen with low oxygen impurities are used for inerting storage tanks of
ships and tanks for petroleum products, or for protecting edible oil products
from oxidation.
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