Hampson–Linde cycle

Linde method is 1895, developed by Carl von Linde technical method for gas separation, which allows the liquefaction of gas mixtures such as air, and each atmospheric gases such as oxygen, nitrogen and argon ( inert gas) in large quantities and in this sense cooling in the temperature range from 77 to 100 degrees Kelvin ( K ) is used.

Although initially used only for academic purposes, it came in 1902 to the first real industrial application as an important part of the also developed by Carl von Linde air separation plant (technical Abbreviation: LTA ). Even today, air separation systems are used on a large scale to gaseous and liquid oxygen ( GOX and LOX ( liquid oxygen from ) - as technology codes) to win, nitrogen ( GAN LIN) and noble gases. For cooling the Linde process is, however, no longer used in its original construction, as more efficient technical implementations ( Hubkolbenexpander or expansion turbines ) have been developed. However, their cooling is based as the Linde process on the Joule- Thomson effect.

Principle

Relaxing a real gas is accompanied by a change in its temperature. The abstract model of an ideal gas does not show this effect. Whether the temperature change occurs in the form of cooling or heating, depending on whether the inversion temperature (ie the temperature at which the Joule-Thomson coefficient of the gas undergoes a sign change ) is exceeded. If the system is above the inversion temperature, then the gas is heated at expansion (more precisely: adiabatic expansion expansion, the enthalpy does not change by the change in volume ), lower temperatures have a cooling result; This effect is used in the Linde process.

In order to achieve the low boiling point of many gases ( oxygen -183 ° C, -196 ° C nitrogen ), one uses the expanded gas to the countercurrent principle for precooling of the compressed gas.

Application

The Linde process was previously used for cooling of atmospheric gases oxygen, nitrogen and argon and other noble gases to liquefaction.

Air liquefaction

A compressor compresses the air to a pressure of approximately 200 bar. Consequently, their temperature increases by about 45 Kelvin, so for example, from 20 ° C. to about 65 ° C. In a first heat exchanger, the compressed, heated air is then pre-cooled and the temperature returned to the range of the ambient temperature. This heat is transferred from the air liquefaction system in the area. The air is first washed and freed on a molecular sieve of water vapor, dust, hydrocarbons, nitrous oxide and carbon dioxide. Hydrocarbons and nitrous oxide can cause an explosion or even an explosion in the rectification column. Subsequently, the air is expanded through a turbine, wherein the temperature of the air drops to just before the point of liquefaction. Subsequently, the air is not passed through an expansion valve, there being achieved the air to the softening point (about -170 degrees centigrade ).

The Upper Bavarian engineer Fränkl to replace the Gegenstromrekuperatoren by regenerators succeeded. These can be much smaller, cheaper and more efficient to build as a countercurrent shell and tube exchanger. This invention was employed by the company Linde AG and marketed under the name Linde Fränkl process. The method was successfully applied to regenerators with about 1990 until a newer technology emerged, included the re recuperative counter-flow plate heat exchanger with upstream adsorptive drying and cleaning.

In an open vessel at atmospheric pressure, liquid air assumes a temperature of about -190 ° C = 83 K. Here it is boiling, so that their low temperature is maintained, because that of liquid air evaporation heat is extracted. The amount of boiling off air regulates itself so that the supplied by thermal conduction or radiation of heat is equal to the consumed heat of vaporization. Depending on the size and isolation of the container so the liquid can remain airborne for a few hours to many days. Liquid air may be stored in sealed containers without safety devices and corresponding design but not because of rising by gradual heating of internal pressure usually brings them to burst.

Fractionating the liquefied air

Liquid air can be decomposed into its components by fractionation by consequently the different boiling points of the individual components of air are exploited. However, the boiling points of oxygen and nitrogen are very close together. One therefore uses a rectification column: The liquid air runs over several rectification plates in countercurrent to the ascending gas down. It absorbs the oxygen from the gas and emits nitrogen. The rectification is carried out at a pressure of about 5-6 bar. Thus, the liquid is oxygenated, the nitrogen gas.

Liquefaction of hydrogen and helium

To apply the Linde process for hydrogen and helium liquefaction, you have to pre-cool the gases only under the inversion temperature. This usually happens with liquid air. The finally obtained liquid helium boiling at atmospheric pressure at 4.2 K. This is the lowest boiling point of all the elements. By pumping the helium gas above the boiling helium latter evaporation heat is removed, so that its temperature can be further reduced. Since the vapor pressure with temperature but falls off very strong, can be achieved with this method, no lower temperature than 0.84 K; you belong to the vapor pressure of 0.033 mbar.

Physical Basics

The Linde process is based on the Joule- Thomson effect: In an ideal gas, the particles do not interact on each other practice, which is why the temperature of an ideal gas does not depend on the volume. For real gases, however, there are interactions that are described with the help of the Van der Waals equation. The energy content of the real gas also changes in adiabatic (without heat exchange) relaxation without external work was done. That is detectable by the temperature change.

By connecting two gas container having a porous wall and pushes the in room 1, the pressurized gas with a piston slowly through the membrane, which serves to prevent vortices and beam formation in space 2 at a constant, but lower pressure than chamber 1 is, then there is set a smaller temperature difference between the two rooms. He is carbon dioxide at about 0.75 K per bar pressure difference in air is about 0.25 K.

Explainable is it, when you consider that the volume was removed in room 1. The piston of the gas supplied to the work. The amount of gas arises in room 2 and can be required to work against the piston. The difference of this work is beneficial come as internal energy of the gas.

Or

The enthalpy remains constant. At van der Waals gas is the internal energy, the number of degrees of freedom of a particle.

This results in taking into account the van der Waals equation:

Because the enthalpy remains, therefore applies to the total differential:

Transformed by the change in temperature results in:

The counter is positive at a high temperature. It changes its sign at the inversion temperature.

The critical temperature for a van der Waals gas is then.

Above heats a gas at relaxation, below it cools. For carbon dioxide and air is well above the room temperature, hydrogen, however, at -80 ° C.

A high value of the Van der Waals constant therefore causes the temperature greatly decreases upon relaxation of the real gas. This is logical, because with increase in volume, the molecules move apart and have to do work against characterized by attractive forces. This work reduces the kinetic energy of the molecules and hence the temperature of the gas.

Alternative methods

Two other novel methods used for cost-effective production of nitrogen and oxygen in adapted to the needs of purity:

• Gas separation membrane processes # (english membrane gas separation (MGS ) ): diffusion through hollow fiber membrane provides up to high-purity nitrogen and / or oxygen to 40 % enrichment level of compressed air.

• By means of pressure swing adsorption (English pressure swing absorption (PSA ) ) on molecular sieves, CMS - Carbon Molecular Sieve for nitrogen or zeolites - " Zeo molecular sieve" for oxygen can be broken down air through pressure swing in two pressure vessels.