Since all gases have several states of aggregation and can be liquefied, air consisting of a mixture of gases can also become a liquid. Basically, liquid air is produced to separate pure oxygen, nitrogen and argon from it.
A bit of history
Until the 19th century, scientists believed that gas has only one state of aggregation, but learned to bring air to a liquid state at the beginning of the last century. This was done using the Linde machine, the main parts of which were a compressor (an electric motor equipped with a pump) and a heat exchanger, presented in the form of two tubes rolled into a spiral, one of which passed inside the other. The third component of the design was a thermos, and liquefied gas was collected inside it . Machine parts were covered with insulating materials to prevent access to heat gas from outside. The inner tube located near the neck ended with a throttle.
Gas operation
The technology for producing liquefied air is quite simple. First, the gas mixture is cleaned of dust, water particles, as well as carbon dioxide. There is another important component, without which it will not be possible to produce liquid air - pressure. Using a compressor, air is compressed to 200-250 atmospheres, while cooling it with water. Further, the air goes through the first heat exchanger, after which it is divided into two streams, the larger of which goes to the expander. This term is called a piston machine, which works by expanding the gas. It converts potential energy into mechanical energy, and the gas cools because it does the work.
Then the air, having washed two heat exchangers and thereby having cooled the second stream going towards, comes out and gathers in a thermos.
Turbo expander
Despite the apparent simplicity, the use of an expander is not possible on an industrial scale. The gas obtained by throttling through a thin tube is too expensive; it is not efficient enough and energy-intensive to obtain it, and therefore unacceptable for industry. At the beginning of the last century, the question was about simplifying the smelting of cast iron, and for this a proposal was made to make blowing out of air with a high oxygen content. Thus the question arose about the industrial production of the latter.
The piston expander is quickly clogged with water ice, so the air must first be drained, which makes the process more complicated and more expensive. The development of a turboexpander using a turbine instead of a piston helped solve the problem. Later, turboexpander found application in the process of obtaining other gases.
Application
Liquid air itself is not used anywhere, it is an intermediate in the production of pure gases.
The principle of separation of the components is based on the difference in the boiling of the components of the mixture: oxygen boils at -183 °, and nitrogen at -196 °. The temperature of liquid air is below two hundred degrees, and by heating it, separation can be made.
When liquid air begins to slowly evaporate, nitrogen evaporates first, and after its main part has already evaporated, oxygen boils at a temperature of -183 °. The fact is that while nitrogen remains in the mixture, it cannot continue to heat up, even if additional heating is used, but as soon as most of the nitrogen has evaporated, the mixture will quickly reach the boiling point of the next part of the mixture, that is, oxygen.
Cleansing
However, in this way it is impossible to obtain pure oxygen and nitrogen in one operation. The liquid air in the first stage of distillation contains about 78% nitrogen and 21% oxygen, however, the further the process goes and the less nitrogen remains in the liquid, the more oxygen will evaporate with it. When the concentration of nitrogen in the liquid drops to 50%, the oxygen content in the vapor increases to 20%. Therefore, the vaporized gases are again condensed and subjected to distillation a second time. The more distillations there were, the cleaner the resulting products will be.
In industry
Evaporation and condensation are two opposite processes. In the first, the liquid must expend heat, and in the second, heat will be released. If there is no heat loss, then the heat released and consumed during these processes is equal. Thus, the volume of condensed oxygen will be almost equal to the volume of evaporated nitrogen. This process is called rectification. A mixture of two gases, formed due to the evaporation of liquid air, is again passed through it, and some of the oxygen goes into condensate, giving off heat, due to which some of the nitrogen evaporates. The process is repeated many times.
Industrial production of nitrogen and oxygen occurs in the so-called distillation columns.
Interesting Facts
Upon contact with liquid oxygen, many materials become brittle. In addition, liquid oxygen is a very powerful oxidizing agent, therefore, having got into it, organic substances burn up, releasing a lot of heat. When impregnated with liquid oxygen, some of these substances acquire uncontrolled explosive properties. This behavior is characteristic of petroleum products, which include ordinary asphalt.