How can gases be liquefied 1
Elements, school book
138 6 AIR, WATER, SOIL - OUR ENVIRONMENT 6.2 AIR AS TECHNICAL RAW MATERIAL Gas liquefaction • Air separation • Nitrogen • Oxygen • Noble gases • Their use To obtain pure oxygen, air is liquefied and distilled at low temperatures. Since air is now being replaced by pure oxygen in many technical processes, this process is becoming increasingly important. An important market also developed for the by-products nitrogen and noble gases. Gas liquefaction In order to liquefy a gas, you either have to put it under very high pressure or lower the temperature sharply. The liquefaction of gas by increasing the pressure is technically easier. However, it does not work with every gas. The critical temperature is an absolute limit for this method. It is a substance-specific quantity for each gas. If it is exceeded, the interaction forces between the gas particles are not sufficient for liquefaction and the gas remains gaseous at any high pressure. Its density can correspond to that of a liquid - nevertheless it fills every available space equally. Technically, gases can therefore be divided into two groups: permanent gases and gases that can be liquefied under pressure. Pressure-liquefiable gases These gases have a critical temperature that is above the ambient temperature. If such a gas is compressed, it becomes liquid as soon as its saturation vapor pressure is exceeded. The heating gases propane and butane ("liquid gas") are examples of such gases. The substance is in a liquid state in steel cylinders filled with liquid gas. The saturation vapor pressure prevails in the bottle. If you take off gas, the pressure drops and the liquid boils. After removal, so much of the liquid evaporates that the saturation vapor pressure is reached again. Liquid gas cylinders therefore usually do not have a pressure gauge, as this always shows the same pressure down to the last remaining liquid. The gas supply is determined by weighing. Permanent gases Permanent gases have a low critical temperature. At ambient temperature they are above their critical temperature and cannot be liquefied. Even at high pressures, the gas laws still apply with relatively good accuracy. Permanent gas cylinders usually have a manometer that shows the pressure proportional to the filling quantity. The oxygen and hydrogen bottles known from lessons are examples of this. The table (Fig. 138.2) shows that all of the important air components are permanent gases. Air must therefore be cooled to at least below the critical temperature of nitrogen in order to liquefy it. This can be done with the Linde method. The Joule-Thomson effect is used for cooling. If a strongly compressed gas is suddenly released via a throttle valve, its temperature will decrease. In the gas liquefaction machine (Fig. 138.3) the dried air is compressed to around 50 bar. After the heat of compression has been dissipated via a cooler, the air is released at a throttle. The air, which has been greatly cooled by the Joule-Thomson effect, is now used to pre-cool the compressed air in front of the throttle. This countercurrent cooling causes the temperature to drop ever lower until the air after the throttle liquefies. The small amount of helium and neon remains gaseous and escapes with the amount of air required for cooling. Air liquefaction costs considerable amounts of energy to operate the compressors. COOLING COMPRESSION RELAXATION AIR Cold gaseous air Liquid AIR COOLING Kp T crit N 2 –195.8 ° C –147.1 ° CO 2 –183.0 ° C –118.4 ° C Ar –185.9 ° C –122, 5 ° C CO 2 -78.5 ° C (subl.) 31.1 ° C Ne -246.0 ° C -188.8 ° C He -268.9 ° C -268.0 ° C Kr -153, 4 ° C –63.8 ° C Xe –108.1 ° C 16.4 ° C Fig. 138.3: Scheme of air liquefaction Fig. 138.2: Boiling points and critical temperatures of the air components Fig. 138.1: An air separation plant For testing purposes only - property of Verlag öbv
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