Liquid air is the basis for obtaining pure oxygen

Since all gases have several aggregate states 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 aggregate state, but to bring the air to the liquid state have learned at the beginning of the last century. This was done with the Linde machine, the main parts of which were a compressor (electric motor provided with a pump) and a heat exchanger, represented as two coiled tubes, one of which passed inside the other. The third component of the design was a thermos, inside of which liquefied gas was collected . The details of the machine were covered with heat-insulating materials to prevent access to the heat gas from outside. The inner tube located near the neck ended in a throttle.

Gas operation

The technology of obtaining liquefied air is quite simple. First, the mixture of gases is cleaned from dust, water particles, and also from carbon dioxide. There is another important component, without which it will not be possible to produce liquid air - pressure. With the help of the compressor, the air is compressed to 200-250 atmospheres while cooling it with water. Then the air goes through the first heat exchanger, then 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, by washing two heat exchangers and thereby cooling the second flow, coming towards, comes out and is collected in a thermos.

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, obtaining it is not effective enough and energy-consuming, and therefore unacceptable for industry. At the beginning of the last century, there was a question of simplifying the smelting of cast iron, and for this purpose a proposal was made to blow from air with a high oxygen content. Thus, the question arose about the industrial extraction of the latter.

The piston expander is quickly clogged with water ice, so the air needs to be drained beforehand, which makes the process more complicated and expensive. To solve the problem, the development of a turbo-expander using a turbine instead of a piston helped. Later, turboexpanders were used in the process of obtaining other gases.

Application

The liquid air itself is not used anywhere, it is an intermediate product in obtaining pure gases.

The principle of the separation of constituents is based on the difference in the boiling of the constituents of the mixture: oxygen boils at -183 °, and nitrogen at -196 °. The temperature of the liquid air is less than two hundred degrees, and heating it, you can make a separation.

When the liquid air begins to evaporate slowly, the first volatilizes nitrogen, and after its main part has evaporated, at -183 ° the oxygen boils. The fact is that as long as the nitrogen remains in the mixture, it can not continue to heat, even if additional heating is used, but as soon as most of the nitrogen evaporates, 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. Air in the liquid state in the first stage of distillation contains about 78% nitrogen and 21% oxygen, but the further the process proceeds 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 distilled for a second time. The more distillation, the purer the products will be.

In industry

Evaporation and condensation are two opposite processes. At the first, the liquid must spend heat, and at the second - the heat will be released. In case there is no loss of heat, the heat released and consumed during these processes is. Thus, the volume of condensed oxygen will be practically equal to the volume of evaporated nitrogen. This process is called rectification. A mixture of two gases formed by the evaporation of liquid air is again passed through it, and some of the oxygen passes into the condensate, thereby giving off heat, thereby evaporating some of the nitrogen. The process is repeated many times.

Industrial production of nitrogen and oxygen occurs in so-called rectification columns.

Interesting Facts

When exposed to liquid oxygen, many materials become brittle. In addition, liquid oxygen is a very powerful oxidizer, therefore, after getting into it, organic substances burn out, giving off a lot of heat. When impregnated with liquid oxygen, some of these substances acquire uncontrolled explosive properties. This behavior is typical of petroleum products, which include ordinary asphalt.