One of Over 700 Sites
	with Guide Bernhard Spang Bio | Contact

Subjects

Beginners CAD Software Career CFD Chemicals Corrosion Education Ethics Fluid Mechanics Fundamentals Heat/Mass Transfer Humor Job Market Job Search Journals Labware Mailing Lists Materials Measurement Units Numerical Methods Particle Tech. Process Control Process Economics Process Modeling Process Simulation Renewable Energy Safety Technical Writing Thermodynamics Thermo Software Subject Library  All articles on this topic

More Sites On This Topic

Apply to become a partner for this site.

When a gas is cooled at constant pressure we expect it to condense and become a liquid at some temperature. Likewise, when the pressure is increased at constant temperature the gas should liquefy at some pressure. This is the "normal" behavior of gases. The change of phase from gas to liquid becomes apparent in a jump of the properties (density, specific heat capacity, viscosity etc).

Supercritical fluids don't liquefy. In fact, in the supercritical state it doesn't make sense to speak of gases and liquids because there's no difference. But what are supercritical fluids?

A pure component is in a supercritical state if both its temperature and its pressure are higher than the values at the critical point (see figure below, CP = critical point, TP = triple point). The critical point of a pure component is unique in that the gaseous and the liquid phase become identical. Beyond the critical point there's no difference between gaseous and liquid state and hence no phase change.

The following table shows critical temperatures and pressures of a few pure components. Critical data for many substances can be found in the Appendix of the book The Properties of Gases and Liquids by Reid, Prausnitz and Poling.

Due to their properties some supercritical fluids are excellent solvent extraction media. Especially supercritical carbon dioxide is used as solvent in many separation processes, e.g. for decaffeinating coffee. Recently DuPont announced plans to build a new facility at Fayetteville, North Carolina, USA to make melt-processable fluoropolymers using a process based on supercritical carbon dioxide.

For more information

• Introduction to Supercritical Fluids by phasex corporation.
• About Supercritical Fluids by SEPAREX.
• Supercritical Fluid Database from the Research Group on Supercritical Fluid of The Society of Chemical Engineers, Japan; includes a Java based Critical Properties Calculator.
• JavaScript tour of a Supercritical-Fluid Extractor by Brian M. Tissue.
• Carbon Dioxide by The Wittemann Company.

Figure by Bernhard Spang