Fractional distillation of gases

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. 

Krypton is a rather dense, tasteless, colorless, odorless gas. Its critical temperature is between that of oxygen and carbon dioxide. It is extracted during fractional distillation of liquid oxygen at a temperature of about -63.8°C. At one time it was thought that krypton, as well as the other noble gases, were completely inert. However, in 1967 scientists were able to combine fluorine with krypton at low temperatures to form the compound krypton difluoride (KrFj). In this case krypton has a valence of 2. 

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. 

Coal-Tar Process. The largest quantities of naphthalene are obtained from the coal tar that is separated from the coke-oven gases. The coal tar first is processed through a tar-distillation step where ca the first 20 wt% of distillate, i.e., chemical oil, is removed. The chemical oil contains practically all the naphthalene present in the tar. It is processed to remove the tar acids by contacting with dilute sodium hydroxide and, in a few cases, is next treated to remove tar bases by washing with sulfunc acid. Principal U.S. producers obtain their crude naphthalene product by fractional distillation of the tar acid-free chemical oiL... 

Helium is separated from natural gas by the liquefaction of the other gases. The other noble gases are obtained in large quantities by the fractional distillation of liquid air (Chapter 11, p. 174). 

Air is the major source of oxygen, nitrogen and the noble gases. The gases are obtained by fractional distillation of liquid air but it is a complex process, involving several different steps (Figure 11.8). 

Figure 11.8 Large amounts of gases are obtained from the fractional distillation of liquid air in industrial plants such as this.

Fractional distillation of air The process used to extract individual gases from the air. Air is a major raw material. The mixture of gases is separated by first liquefying the mixture at low temperature and high pressure. The temperature is then allowed to rise and the gases collected as they boil off. The gases so produced have many and varied uses. 

Nitrogen may also be produced directly from air by the fractional distillation of liquid air or by passing air over heated copper. In the latter case, oxygen is removed by combination with the copper to form copper oxide, but the resulting nitrogen is not entirely pure since it is contaminated by the inert gases present in the original sample of air. 

Neon. The second noble gas, neon, occurs in the atmosphere to the extent ol 0.002%. It is obtained, along with the other noble gases (except helium), by the fractional distillation of liquid ait. 

Derivation (1) Gases containing appreciable content of butene-1, along with other butene and butane hydrocarbons, are obtained by fractional distillation of refinery gas. (2) Can be produced directly from ethylene. 

The noble gases are very low-boiling gases. Except for radon, they can be isolated by fractional distillation of liquefied air. Radon is collected from the radioactive disintegration of radium salts. Table 24-1 gives the percentage of each noble gas in the atmosphere. 

Prepared in this way the argon always contains about 0.25 per cent of the other inert gases, chiefly neon. These are best removed either by fractional distillation of the liquid or by the fractional absorption in cold charcoal. 

Referring to the Chemistry in Action essay on p. 319, answer the following questions, (a) Why did it take so long to discover the first noble gas (argon) on Earth (b) Once argon had been discovered, why did it take relatively little time to discover the rest of the noble gases (c) Why was helium not isolated by the fractional distillation of liquid air ... 

The CO/H2 gas (also called water-gas) - depending on the CO H2 ratio - can also be converted (using the Fischer-Tropsch synthesis) to alkanes, alkenes and alcohols. Preferably, substances should be generated for application in known technical systems such as liquefied petroleum gas or low pressure gas (LPG) and gasoline as well as natural gas. The production of solar substitutes for diesels and oils (C > 8), that is petrol products from the fractional distillation of crude oil between 200 °C and 350 °C, is also possible, but offers no advantages in the solar fuel cycle and its stepwise replacement by gases and gasoline should be foreseen. 

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