The air we breathe is a mixture, consisting of mostly nitrogen (78% by volume), oxygen (21% by volume) and Argon (about 1% by volume), with traces of CO2 (388 ppm and rising - see http://celebrating200years.noaa.gov/datasets/mauna/image3b.html), Neon (15 ppm), Krypton (1ppm), Xenon (88 ppb), methane (16 ppb), as well as varying amounts of water vapor (not in the above analysis) and dust/bacteria, other particulates, as well as varying amounts of ammonia, sulfur dioxide (SO2), sulfuric acid, and nitric oxides/nitric acid, depending on how close you are to a coal burning facility, a freshly fertilized field, a feed-lot waste lagoon, etc. A sign of life is the presence of oxygen, which cannot exist in the presence of ultraviolet light and oxidizable materials, such as carbon and cellulose. In contrast, nitrogen is relatively inert, while gases like neon, argon, krypton and xenon are not called inert for nothing.
Air separation is big business, and huge volumes of purified component are made in North America each year - about 20 million tons of pure N2, for example. It is usually used as a blanketing/inerting gas, but also sometimes as a reactant (especially to make ammonia). The major use for pure O2 (an extremely reactive substance) is steel making and chemicals manufacture.
In a typical air sep plant, the air must be filtered and then dried (usually via molecular sieves), and possibly scrubbed of any CO2. if relatively pure components are needed, purification can be done (to 95% N2/95% O2, for example) via special molecular sieves and pressure swing absorption. Compressed air is pushed pass special absorbents or through special membranes, where some degree of separation (N2 and O2, for example) happens.. For "mole sieves", relieving the pressure can produce a highly enriched stream of air components.
However, for really pure components, such as N2 for an ammonia plant, nothing beats cryogenic distillation. Components can be separated on the basis of boiling points (listed below at atmospheric pressure):
Neon..........-246 C = 24 K
N2............-196 C = 77 K
Ar............-186 C = 87 K
O2............-183 C = 90 K
Kr............-154 C = 120 K
Xe............-108 C = 165 K
CO2............-77 C = 196 K
Water..........100 C = 373.15 K (boiling point)
For those of you who are not hip to the Kelvin and Centigrade temperature scale, there is 1.8 F to one degree C or K. Kelvin starts at absolute zero and counts upwards. There is a similar arrangement when one uses degrees Rankine, where water freezes at 492 R = 32 F = 0 C.
Both water and CO2 can be a real pain, because they freeze up into solids at 0 C and -77 C respectively, and will plug up lines at inopportune times, with bad results. Undesirable to say the least....
As you know, when gases are compressed, they heat up, and when they expand, their temperatures drop. In cryogenic distillation plants, filtered air is compressed, dried, and scrubbed of any CO2, cooled via a heat exchanger and then expanded, which drops its temperature to the boiling point of air, as long as the air is initially compressed enough and then cooled in its compressed state sufficiently. This is then sent to a fractional distillation column ("the cold box"), where N2 with traces of Ne boils off the top of the column and this cold N2 gas is then warmed by contacting it with the compressed cooled air. To operate correctly, some quantity of liquid N2 has to be returned to the top of the fractionation column (the reflux), and the cooling for the condenser also needs to be obtained by this expansion process. The column is usually operated under pressure, which raises the boiling points listed above somewhat. If the N2 is collected as a liquid off of this column, it is sent to cryogenic, vacuum insulated tanks as a liquid for stroage. The less volatile Ar/O2 mix is then further separated to an Ar/O2 azeotrope and pure O2, which is then collected either as a liquid or warmed to a gas, and used for a coolant if it is warmed. It is stored in stainless steel cryogenic containers as a liquid (LOX = Liquid Oxygen). Pure Ar is made by reacting the Ar/O2 mix with H2 over a catalyst, followed by additional cooling/drying/compression.
In general, it takes about 1 MW to make about 2.2 tons/hr of air components. Less energy is needed if the components can be used as gases, as the energy used to liquify them can be employed to cool fresh mixtures of air while the pure components are vaporized. See here for a further description: http://encarta.msn.com/encyclopedia_1741500785/Air.html
Purities of at least "five nines" - 99.999% - are obtainable using these cryogenic distillations. A final note - purified O2 can be very dangerous, especially as a liquid. It is "relatively harmless" in normal air - except during a house fire, for example - because it is diluted with all that nitrogen.
The air we breathe is a mixture, consisting of mostly nitrogen (78% by volume), oxygen (21% by volume) and Argon (about 1% by volume), with traces of CO2 (388 ppm and rising - see http://celebrating200years.noaa.gov/datasets/mauna/image3b.html), Neon (15 ppm), Krypton (1ppm), Xenon (88 ppb), methane (16 ppb), as well as varying amounts of water vapor (not in the above analysis) and dust/bacteria, other particulates, as well as varying amounts of ammonia, sulfur dioxide (SO2), sulfuric acid, and nitric oxides/nitric acid, depending on how close you are to a coal burning facility, a freshly fertilized field, a feed-lot waste lagoon, etc. A sign of life is the presence of oxygen, which cannot exist in the presence of ultraviolet light and oxidizable materials, such as carbon and cellulose. In contrast, nitrogen is relatively inert, while gases like neon, argon, krypton and xenon are not called inert for nothing.
Air separation is big business, and huge volumes of purified component are made in North America each year - about 20 million tons of pure N2, for example. It is usually used as a blanketing/inerting gas, but also sometimes as a reactant (especially to make ammonia). The major use for pure O2 (an extremely reactive substance) is steel making and chemicals manufacture.
In a typical air sep plant, the air must be filtered and then dried (usually via molecular sieves), and possibly scrubbed of any CO2. if relatively pure components are needed, purification can be done (to 95% N2/95% O2, for example) via special molecular sieves and pressure swing absorption. Compressed air is pushed pass special absorbents or through special membranes, where some degree of separation (N2 and O2, for example) happens.. For "mole sieves", relieving the pressure can produce a highly enriched stream of air components.
However, for really pure components, such as N2 for an ammonia plant, nothing beats cryogenic distillation. Components can be separated on the basis of boiling points (listed below at atmospheric pressure):
Neon..........-246 C = 24 K
N2............-196 C = 77 K
Ar............-186 C = 87 K
O2............-183 C = 90 K
Kr............-154 C = 120 K
Xe............-108 C = 165 K
CO2............-77 C = 196 K
Water..........100 C = 373.15 K (boiling point)
For those of you who are not hip to the Kelvin and Centigrade temperature scale, there is 1.8 F to one degree C or K. Kelvin starts at absolute zero and counts upwards. There is a similar arrangement when one uses degrees Rankine, where water freezes at 492 R = 32 F = 0 C.
Both water and CO2 can be a real pain, because they freeze up into solids at 0 C and -77 C respectively, and will plug up lines at inopportune times, with bad results. Undesirable to say the least....
As you know, when gases are compressed, they heat up, and when they expand, their temperatures drop. In cryogenic distillation plants, filtered air is compressed, dried, and scrubbed of any CO2, cooled via a heat exchanger and then expanded, which drops its temperature to the boiling point of air, as long as the air is initially compressed enough and then cooled in its compressed state sufficiently. This is then sent to a fractional distillation column ("the cold box"), where N2 with traces of Ne boils off the top of the column and this cold N2 gas is then warmed by contacting it with the compressed cooled air. To operate correctly, some quantity of liquid N2 has to be returned to the top of the fractionation column (the reflux), and the cooling for the condenser also needs to be obtained by this expansion process. The column is usually operated under pressure, which raises the boiling points listed above somewhat. If the N2 is collected as a liquid off of this column, it is sent to cryogenic, vacuum insulated tanks as a liquid for stroage. The less volatile Ar/O2 mix is then further separated to an Ar/O2 azeotrope and pure O2, which is then collected either as a liquid or warmed to a gas, and used for a coolant if it is warmed. It is stored in stainless steel cryogenic containers as a liquid (LOX = Liquid Oxygen). Pure Ar is made by reacting the Ar/O2 mix with H2 over a catalyst, followed by additional cooling/drying/compression.
In general, it takes about 1 MW to make about 2.2 tons/hr of air components. Less energy is needed if the components can be used as gases, as the energy used to liquify them can be employed to cool fresh mixtures of air while the pure components are vaporized. See here for a further description:
http://encarta.msn.com/encyclopedia_1741500785/Air.html
Purities of at least "five nines" - 99.999% - are obtainable using these cryogenic distillations. A final note - purified O2 can be very dangerous, especially as a liquid. It is "relatively harmless" in normal air - except during a house fire, for example - because it is diluted with all that nitrogen.
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