What are Supercritical Fluids (SCF) ?

A supercritical fluid is a medium in a state at a pressure and a temperature above the critical point (above critical pressure and critical temperature).
The critical point defines the end of the existence of a phase equilibrium between a boiling liquid and a saturated vapour (end of boiling curve). Above this end point a differentiation between liquid and vapour is impossible; the temperature and the pressure defining the critical point are named critical temperature and critical pressure and are characterised by component specific values. Are temperature and pressure above the critical point (p>pk; T>Tk) the state is called supercritical.

What are the Properties of Supercritical Fluids ?

Supercritical fluids combine properties of liquids (high solubility, high density) and gases (low viscosity). This combination of advantageous properties brings the benefits of the broad field of process applications.

Medium Density
ρ [g/cm3]		 Viscosity
η [mPas]		 Self-diffusion coefficient
D11 [m2/s]
Gases 0,6*10-3 - 2*10-3 1*10-4 - 5*10-5 1*10-5 - 4*10-5
Supercritical fluids 0,2 - 1,0 1*10-4 - 5*10-5 2*10-8 - 7*10-8
Liquids 0,6 - 1,8 1 - 50 2*10-10 - 2*10-9

From the process point of view a supercritical fluid is the ideal solvent. Thus supercritical fluids are the perfect alternative to substitute traditional organic solvents e.g. methanol, n-hexane, dichloromethane, etc.. In contrary to many of the widely used conventional solvents most supercritical fluids are non-toxic and can be easily and completely removed from the dissolved substances.

Advantages of supercritical fluids

  • Low viscosity at relatively high density
  • High solubility and high mass transfer capability
  • Mostly non-toxic
  • High selectivity
  • Residue-free removal of solvent from the extract and raffinate
  • Almost complete recuperation of solvent possible

Since many substances reach their supercritical state at higher pressures the mentioned advantages require special process diligence as well as smart equipment solutions.

Supercritical Fluids

The use of substances in their supercritical state allow for applying solvents which were not used under ambient conditions before. For most supercritical processes CO2 has proven as most suitable. The relatively low critical temperature (31 °C) and the cheap and abundant availability make CO2 particularly attractive for supercritical processing.

Further Advantages of supercritical CO2

  • Gentle treatment of temperature-sensitive substances (e.g. natural substances)
  • Products are obtained solvent-free
  • Simple recovery of solvent
  • Dissolving characteristics can be widely varied by alteration of pressure and temperature
  • Selective separation possible
  • Low-energy solvent recovery due to lower evaporation temperature than conventional solvents
  • High-purity products
  • Germicidal processing
  • Physiologically harmless solvent
  • Non-inflammable solvent

For special processes also different supercritical fluids than CO2 have been in use e.g. laughing gas (N2O), Xenon (Xe) or Propane (C3H8) (see table below). Uhde designed and built plants for a variety of supercritical fluids. Substances not used so far can be studied in lab scale and thus suitable processes can be developed.

Fluid Critical temperature
TC [°C]		 Critical pressure
PC [bar]		 Notes
Carbon dioxide, CO2 31,1 73,8  
Dinitrogen monoxide, N2O 36,6 74,0 unstable
Xenon, Xe 16,8 58,0 very expensive
Ethane, C2H6 32,4 48,8 inflammable, *
Ethylene, C2H4 9,4 50,4 inflammable; *
Propane, C3H8 36,8 42,5 inflammable; *
Water, H2O 374,1 220,5 high temperature, corrosive

* Inflammable and undesirable fluid residues in extract and raffinate