Chemistry Reference
In-Depth Information
dryness between fillings. This can eventually lead to oxygen continuing to
condense and accumulate in the open Dewar with the end result being a
flammable or explosive level of oxygen.
After cryogenic distillation of air, the fractionated nitrogen and oxygen
have sufficient purity for most applications. The argon, with a boiling point
−
186
∘
C is in the oxygen fraction. The argon can be separated and the
oxygen purified by more elaborate fractionation. Other noble gases such as
neon, krypton and xenon can also be isolated by the distillation of air. The
primary source of helium is from natural gas wells.
Oxygen and nitrogen can also be separated from air by PSA or by mem-
branes. Depending on the requirements, a combination of these techniques
is used. Both stationary and portable home oxygen concentrators based upon
PSA are used for patients prescribed oxygen therapy [15, 16]. A combination
of membrane technology with PSA can also be used [17].
For membrane separation of nitrogen and oxygen, typically the oxygen
has a high permeation rate and the nitrogen a low permeation rate. Pressure
Swing Adsorption separation uses zeolites. In the separation of air, typically
the oxygen molecules are adsorbed under pressure in the channels of the
zeolite and the nitrogen passes through. Upon release of the pressure, the
oxygen is desorbed and more air can be introduced into the vessel containing
the zeolites. By varying (pressure swing) the pressure, oxygen is selectively
adsorbed and then desorbed, allowing it to be separated from air.
Oxygen is used in the production of steel and in the chemical industry
as an oxidant. Examples where oxygen is used as an oxidant include the
reaction with ethylene to form ethylene oxide, the conversion of methanol to
formaldehyde, and the oxidation of cumene to form, ultimately, phenol and
acetone. Often, such as in the reaction of cumene, the oxygen content of air
is sufficient and the oxygen does not have to be concentrated.
H
H
H
O
O
2
CC
H
Ethylene oxide
O
2
CH
3
OH
CH
2
O
Formaldehyde
O H
O
H
O
O
H
+
O
2
+
Phenol
Acetone
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