Clarification of Answer by
tox-ga
on
06 Dec 2002 11:15 PST
bulabear-ga,
Sorry for the misinterpretation of your question, I'll attempt to
clarify now.
The process of seperating air into its component elements has been
around for quite some time. Since 1895 actually, when it was
discovered by Dr. Carl von Linde.
Oxygen was first extracted from the atmosphere by a chemical
process. This was superseded over 80 years ago by the cryogenic (low
temperature) process involving the liquefaction and distillation of
air.
The cryogenic air separation process is still by far the most widely
used. However, non-cryogenic techniques first developed during the
1970s -- pressure swing adsorption (PSA), and membrane diffusion --
are becoming increasingly significant for smaller or less demanding
on-site applications.
There are several factors influencing the choice of separation
technology. The best supply option for each customer depends upon the
following:
Volume required - Cryogenic separation is economical for large tonnage
users.
Low temperature applications - Only cryogenic systems provide the
liquefied gases essential for low temperature applications such as
food freezing.
Purity required - Non-cryogenic systems are generally unable to
achieve high purities economically, but less pure products may be
adequate for some applications.
Continuity of supply - Fluctuating demand is best satisfied from
liquid storage tanks filled by road tanker or an on-site plant. If a
gas supply is an essential process requirement, perhaps for safety
reasons, a non-cryogenic system would usually need to be backed up
with liquid storage for emergency use.
Customer location - Some places are too remote for economical delivery
of liquid supplies by road tanker or may be out of reach altogether,
such as on board a ship.
The purpose of air seperation is to obtain the many gases required
to drive processes in industry, as well as for recreational purposes.
It is the most cost-effective and efficient way to produce many of the
common gases. For example, although oxygen COULD be produced through
the electrolysis of water, such a process would requre massive amounts
of energy. Extracting it from the air where it is very abundant is a
much more viable process. Some of the noble gases can actually only
be obtained in large quantities from the air. Although the represent
only a minute fraction of the composition of air as indicated by our
previous answer, the absolultely massive amounts of air available more
than makes up for this deficiency. For example the Hitachi plant
produces argon, oxygen and nitrogen in a very high purity form
(http://www.pi.hitachi.co.jp/sanpu/lowtemp/to-apn-e.html)
The air in the earth's atmosphere contains gasses that have many
industrial and scientific uses once they are separated to highly pure
substances. The air in the earth's atmosphere is composed of nitrogen
(78.08%), oxygen, carbon dioxide and trace amounts of noble gases.
Separation
Gases are separated using a variety of methods. The separation
method used is determined by a variety of factors, which include the
volume required, low temperature applications, customer location,
continuity of supply, and purity level. The separation processes are
broken down into two categories: cryogenic separation processes and
non cryogenic processes. Cryogenic is the keystone of cryogenic
processing since it is the source of many cryogenics fluids for use in
science and technology. In volume of production the separation of air
into its components at low temperatures is by far the most important
of the separation processes.(Sittig 1963) Non cryogenic processes
are methods used to separate air at ambient, or room temperature. Non
cryogenic methods include pressure swing absorption, membrane
diffusion, and vacuum swing absorbent.
(http://www.boc.com/gases/air/noncryo/noncryo.htm)
Noncryogenic Methods:
> Pressure swing absorption
Pressure swing absorption (PSA) is a non cryogenic method of
separating gases. PSA works because the molecules of each gas are
different sized. The air is gathered by a compressor and placed into
a tank, which contains molecular sieves containing various absorbents,
depending on which gas is being produced. Pressure pushes the air up
against the sieves, and the smallest molecules pass through, therefore
separating the desired gas. Pressure is released and the process
occurs over again in the second tank in order to increase the purity
level. Nitrogen produced by PSA are very similar to those produced by
membrane diffusion. They are low cost, many units are on site, and
they have a low purity level.
(http://www.boc.com/gases/air/noncryo/psa.htmEquipmentused)
> Vacuum swing absorption
Vacuum swing absorption (VSA) is a non cryogenic air separation
method. In VSA compressed air is placed into a tank that contains
molecular sieves. Molecular sieve are little balls that look like
ferterlizer. The sieves contain absorbents to gather the gasses that
are unwanted. Once the sieves are saturated with the unwanted gasses,
a vacuum pump turns on and sucks the sieves clear of the unwanted
gasses, allowing the sieve to be reused. There are two tanks used in
this process. One is being used while the other is regenerating so
that the product is continuously available. VSA has the ability to be
used on site, but it also produces a low purity gas.
(http://www.boc.com/gases/air/noncryo/vsa.htm)
> Membrane diffusion
Membrane diffusion is the simplest method by which gasses are
separated from another. The process works by pushing compressed air
into one end of a membrane, which contains many hollow fibers through
which the air flows. The fibers absorb the fast gasses of oxygen,
carbon dioxide and water vapor, allowing the slow gas of nitrogen to
separate. Membrane systems are often smaller onset systems, which are
more convenient for customer control. Membrane diffusion gasses are
significantly less expensive than gasses from other separation
processes because the purity is low level of about 95%. They are
also less expensive because there is not the need for monitoring,
ordering and storing.
(http://www.boc.com/gases/air/noncryo/membrane.htm )
BOC Membrane Systems onset Nitrogen Supply
"While "fast gases" such as oxygen, carbon dioxide, and water vapor
quickly permeate the membrane, most of the nitrogen flows along the
membrane fiber as a separate product stream."
(http://www.boc.com/gases/air/noncryo/memscheme2.htm)
Cryogenic Method:
"Is the study and use of materials at very low temperatures. The
upper limit of cryogenic temperatures has not been agreed on, but the
National Bureau Of Standards has suggested that the term cryogenics be
applied to all temperatures below -150° C (-238° F or 123° above
absolute zero on the Kelvin scale). Some scientists regard the normal
Boiling Point of oxygen (-183° C or -297° F), as the upper limit (see
Absolute Zero). Cryogenic temperatures are achieved either by the
rapid evaporation of volatile liquids or by the expansion of gases
confined initially at pressures of 150 to 200 atm. The expansion may
be simple, that is, through a valve to a region of lower pressure, or
it may occur in the cylinder of a reciprocating engine, with the gas
driving the piston of the engine. The second method is more efficient
but is also more difficult to apply."
(http://www.fwkc.com/encyclopedia/low/articles/c/c005002389f.html)
Cryogenic air separation uses pressure and temperature control in
order to separate gasses. The cryogenic air separation process uses
the boiling points of gasses as the main principle. When a gas
reaches its boiling point, it turns into a liquid state. The
difference in boiling points causes gasses to separate because each
gas will turn to a liquid at a different point. The temperatures at
which gasses turn to a liquid are very low. Nitrogen liquefies at
-320.4 F, argon at -302.6 F, and oxygen -297.3 F. Once each gas
reaches its boiling point, it begins to condense and separate.
Cryogenic separation is used mainly for medium to large scaled
production of nitrogen, oxygen, and argon. Cryogenics is normally
used to produce gasses in liquefied form for storage or
transportation, and a gaseous form for pipeline transportation to
large industrial users.
(http://www.praxair.com/Praxair.nsf/1b1a158c246dbf3e8525654300683b77/45da5d07b15e43c385256569005b7db9?OpenDocument)
Industrial Uses
"Among the many important industrial applications of cryogenics are
the large-scale production of oxygen and nitrogen from air. The
oxygen can be used in a variety of ways, for example, in rocket
engines, for cutting and welding torches, for supporting life in space
and deeps vehicles, and for blast furnace operations. The nitrogen
goes into the making of ammonia for fertilizers, and it is used to
prepare frozen foods by cooling them rapidly enough to prevent
destruction of cell tissues. It can also serve as a
refrigerant and for transporting frozen foods.
Cryogenics has also made possible the commercial transportation of
liquefied natural gas. Without cryogenics, nuclear research would
lack liquid hydrogen and helium for use in particle detectors and for
the powerful electromagnets needed in large particle accelerators.
Such magnets are also being used in nuclear fusion research. Infrared
devices, masers, and lasers can employ cryogenic temperatures as well.
Cryogenic surgery, or cryosurgery, is being used for the treatment
of Parkinson's disease, the technique being based on the selective
destruction of tissue by freezing it with a small cryogenic probe. A
similar technique has also been employed to destroy brain
tumors and to arrest cervical cancer.
(http://encarta.msn.com/index/conciseindex/30/0300C000.htm?z=1&pg=2&br=1#s5)
Delivery
Cryogenics can be delivered by semitankers, pipelines and or
containors.
Environment
The process of separating air has little effect on the natural
environment. The pollutants that are in the air when it goes through
the process are released back into the environment which they came.
The manufactures of this process are very concusses about the
environment. Cryogenics plants such as "Praxair products and
technologies are used by many industries to benefit the environment.
In the face of increasingly stringent regulations, our global
customers rely on our expertise to help them cut emissions and energy
consumption, while improving productivity.
(http://www.praxair.com/Praxair.nsf/X1/envi?OpenDocument)
Hazards
"The primary hazards of dealing with cryogenics are those
associated with the human body and the surroundings. The firsts
includes frostbite, respiratory, ailments, and chemical burns; the
second, phases changes and low temperature effects; and the third,
ignition and combustion reactions".
I hope that helps. Ask for clarification if it is still required.
-Tox-ga
,
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