What is super critical pressure

Critical point

Phase diagram of the solid (s), liquid (l), gaseous (g) and supercritical (sc) phase of carbon dioxide (not to scale)

In thermodynamics it is critical point a thermodynamic state of a substance, which is characterized by the equalization of the densities of the liquid and gas phases. The differences between the two states of aggregation cease to exist at this point. In the phase diagram, the point represents the upper end of the vapor pressure curve.


The critical point P.c is characterized by three state variables,

In multi-component systems in particular, gases in systems above their critical temperature, but in the presence of non-critical substances, are referred to as non-condensable components. These can be important for the thermodynamic description of the absorption, for example.

The critical temperature is the temperature below which a gas can be liquefied by pressure; This is no longer possible above the critical temperature. In the case of technical gases that are transported in normal gas cylinders at ambient temperature, only gases with a high critical temperature can be liquefied. Liquid nitrogen is cooled and transported to the consumer, whereas steel cylinders contain gaseous nitrogen, which is under high pressure. Gases such as propane and butane, on the other hand, can be liquefied under pressure and stored at ambient temperature.

Since liquid and gas can no longer be distinguished from one another above the critical point, one speaks of one instead supercritical fluidthat is in a supercritical condition is located. Another name originating from the Anglo-Saxon area is super critical.

As the critical point approaches, the densities of the gaseous state and the liquid state approach each other. The heat of evaporation decreases when approaching and disappears completely when it is reached. Just below the critical point you can see the phenomenon of critical opalescence Observe: Due to the extremely low heat of vaporization, parts of the substance constantly switch back and forth between the liquid and gaseous state, which is visualized by strong streaking.

At the molecular level, the behavior beyond the critical point can be clearly described: If a gas is exposed to ever higher pressure, the distances between the gas molecules decrease continuously. When the critical pressure is reached, the distances are then just as large as between the molecules in the liquid phase; there is no longer any discernible difference.

Experimental observation

1: Subcritical ethane, coexistent liquid and vapor phases
2: critical point, opalescence
3: Supercritical Ethane, Fluid

The transition from the subcritical to the supercritical state can be easily observed, as there is a clearly visible change in the phases at the critical point.

The substances are enclosed under pressure in thick-walled tubes made of quartz glass. Below the critical temperature (for example about 304.2 K for carbon dioxide or 305.41 K for ethane) the tube is partly filled with the liquid substance and partly with the substance's vapor. Both phases are colorless, clear and transparent and separated by the clearly visible liquid surface (phase interface). When heated below the critical temperature, the volume of the liquid initially increases due to thermal expansion, while the volume of the vapor decreases due to compression. Once the substance has reached the critical temperature, a dense fog (critical opalescence) forms briefly, which dissolves again after a few seconds of further heating. This mist can also have distinct colors. Ethane and CO2 are not colored, the fog is white. The tube is then filled with a single homogeneous, clear, transparent phase, the supercritical fluid. When it cools down, fog briefly appears again before the substance divides into a liquid and a gaseous phase.

Estimation and calculation

In addition to a comparatively complex empirical measurement, the critical state variables can also be estimated from the Van der Waals equation, whereby they are also used here to define the reduced variables.

In addition to these equations of state, group contribution methods such as the Lydersen method and the Joback method are often used, with which the critical quantities are determined from the molecular structure.


With the increasing spread of steam engines in the 18th century, the investigation of the boiling behavior of various substances also became a topic of scientific interest. It turned out that as the pressure rises, so does the boiling point temperature. It was assumed that the coexistence of liquid and gas was possible up to arbitrarily high pressures.

This assumption was refuted by the Irish physicist and chemist Thomas Andrews around 1860. Based on studies with CO2 he was able to show that there is a point at which the difference between gas and liquid no longer exists, and which is characterized by a certain temperature, a certain pressure and a certain density. He called this point the "critical point". Shortly afterwards, the Dutch physicist Johannes Diderik van der Waals gave a plausible explanation (see above) for the behavior of substances in the supercritical range.


Supercritical fluids combine the high solvency of liquids with the low viscosity similar to gases. Furthermore, they disappear completely (evaporate) when the pressure is reduced. Thus, they are suitable as ideal solvents, the only disadvantage of which is the high pressure during the process. Supercritical fluids are also used to generate the finest particles. Extractions with supercritical fluids are called distractions.

In supercritical water, SiO2 be solved. When crystallizing on the seed crystal, single crystals are formed from quartz. These are then sawn into small pieces and used as oscillating crystals in quartz watches.

Supercritical water dissolves fats from meat. Since many different substances are deposited in the fat, drugs and other substances are extracted from the meat and detected with this method.

In textile dyeing applications, the good solubility of the dye in the supercritical state can be used to take up the dye and transfer it into the fiber. After the process is complete, the supercritical fluid is relaxed and the remaining dye precipitates out as a solid.

An application of supercritical CO2 is the decaffeination of tea and coffee.

With supercritical CO2 biological specimens can be dried very gently (e.g. for scanning electron microscopy). The samples are first cross-linked, the water is gradually exchanged for a solvent (usually acetone) and the acetone with supercritical CO2 carried out. As a result of this procedure, the structures are largely retained and only a few artifacts occur. The process is called critical point drying or supercritical drying.

Supercritical fluids in internal combustion engines

In a new concept for gasoline internal combustion engines, the fuel is brought into a supercritical state before the injection process. With this method, the fuel is self-igniting and there is no need for external ignition (spark plug). According to initial media reports, this should increase fuel efficiency by around 50% compared to conventional engines. This method should also be used to improve combustion processes with other types of fuel.

See also

Based on an article in: Wikipedia.de
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Date of the last change: Jena, the: 10.02. 2021