Liquid-air fractionation

Rayleigh and Ramsay discovered argon by the fractional distillation of liquid air. Fractional distillation is the process of letting liquid air slowly warm up. As the air warms, different elements change from a liquid back to a gas. The portion of air that changes back to a gas at -302.55°F (-185.86°C) is argon. 

Finally, in addition to the liquid nitrogen and liquid oxygen - enriched streams from the pressure to the low-pressure column, there is a third liquid air fraction (k) from the pressure to the low-pressure side. This flow is adjusted such that the operating line and the equilibrium curve, as shown in the McCabe-Thiele diagram of the low-pressure column, move closer together. This reduces the irreversibility of the rectification and increases the O2- and Ar-yield. 

Liquid-Air Fractionation Example. The enthalpy-composition method will be illustrated by several examples. Figure 7-17 gives a schematic diagram of a liquid-air fractionating system. In this particular case the system employed involves... 

Liquid oxygen analyses are customarily made for process control, product purity and to avoid hazards. Usually analytical information required for process control is not extensive. Use of modified Or sat apparatus for manual determination of the oxygen contents of various liquid samples is routine in most plants. Relatively simple thermal conductivity analyzer-controllers govern the flow of liquid air fractions under distillation where differential pressure control is not applicable. Pressure drop and inspection of liquid in a small glass flask are usually sufficient for mechanical filter cycle regulation but a continuous carbon dioxide analysis may be helpful as a check on the overall function. A method which is sufficiently precise for this use is discussed later. 

Industrially, elemental nitrogen is extracted from the air by the fractional distillation of liquid air from which carbon dioxide and water have been removed. The major fractions are nitrogen, b.p. 77 K and oxygen, b.p. 90 K, together with smaller quantities of the noble gases. 

The gas is prepared by fractionation of liquid air because the atmosphere contains 0.94% argon. The atmosphere of Mars contains 1.6% of 40Ar and 5 p.p.m. of 36Ar. 

Carbon tetrafluoride [75-73-0] M 88.0, b -15 . Purified by repeated passage over activated charcoal at solid-C02 temperatures. Traces of air were removed by evacuating while alternately freezing and melting. Alternatively, liquefied by cooling in liquid air and then fractionally distilled under vacuum. (The chief impurity originally present was probably CF3CI). 

Pure ozone is made by fractional distillation of the blue liquid resulting from the cooling of ozonized oxygen in liquid air. Commercially it is often supplied dissolved in chlorofluorocarbons in stainless steel cylinders at ca 475 psig cylinder pressure at 20°C often transported chilled with dry ice. These solutions can be handled safely at vapour concentrations of ca 20% by volume of ozone. 

Ammonia is synthesised from its elements nitrogen and hydrogen. The nitrogen is obtained by the fractional distillation of liquid air. The hydrogen is obtained by the reaction of methane (from natural gas) with steam. 

Claude (2) A process for fractionally distilling liquid air, based on the original Linde process but using two stages. Developed by G. Claude. 

The sources of error indicated above were avoided in a series of experiments carried out by Donnan and Barker, which in principle resemble those made by Lewis, so that only a brief reference to them is necessary. The dissolved substance was nonylic acid, and a drop method. The results could be reproduced with very great accuracy, i.e., to a fraction of one drop in 300—500 drops. Adsorption was produced at a surface air-liquid, air being passed through the solution in bubbles of known size and number, so that the total active surface could be calculated. The bubbles, on reaching the surface, burst, hence the excess of solute carried by them remained in the surface very effective precautions were used to prevent diffusion backwards from this portion into... 

Preparation. The only important large-scale process is the fractional distillation of liquid air. 

The separation section of a gas oil cracker looks like a small refinery, as you can see in Figure 5 or in Figure 5—5. In addition to the fractionators and treaters used in the purification section of the simpler ethane cracker, there are facilities to separate the heavier coproducts. In the front end of the separator facilities in Figure 5-4, the cold box option for handling the liquefaction of the gases is shown. Temperatures as low as -220°F are achieved in this super-refrigerator. At those low temperatures, Freon wont do the job. Liquid air, methane, ethylene, or ammonia are often used as the refrigerant in much the same way Freon has been used in an air conditioner. 

Krypton is the 81st most abundant element on Earth and ranks seventh in abundance of the gases that make up Earths atmosphere. It ranks just above methane (CH ) in abundance in the atmosphere. Krypton is expensive to produce and thus has hmited use. The gas is captured commercially by fractional distillation of liquid air. Krypton shows up as an impurity in the residue. Along with some other gases, it is removed by filtering through activated charcoal and titanium. 

Xenon is found in trace amounts in the atmosphere. It makes up just 0.086 ppm by volume of air. Xenon is the rarest of the noble gases. For every thousand-million atoms of air, there are only 87 atoms of xenon. Even so, it is recovered in commercial amounts by boiling off the xenon from fractional distillation of liquid air. Small amounts of xenon have been found in some minerals and meteorites, but not in amounts great enough to exploit. 

---