A supercritical fluid is a substance whose temperature and pressure are both over their critical point simultaneously (see diagram for CO2).
The critical temperature is the temperature over which the substance can no longer remain liquid, regardless of the pressure level.
Critical pressure is the pressure over which the substance can no longer form gas, regardless of the temperature.
Supercritical fluids have the density and good dissolving properties of fluids, as well as the viscosity, good compressibility, and diffusability of gases.
Due to their good mixing and mass transfer properties, they are classed as a fluid.
Important advantages of supercritical fluids:
- Safe, cost-effective, environmentally friendly, non-toxic
- They can replace organic solvents
- Good influence over slight changes in operating conditions
- Operating conditions under low temperature and pressure make them attractive for research and development in the pharmaceutical & biotechnology sector.
Selection criteria for the right supercritical fluid:
- Physical-chemical properties of the desired mixture
- Task of the solvent in the process in regards to solubility
- Specific application
- Safety, especially in regards to flammability, phase behavior under operating conditions
- Cost of the fluid
- Cost of the equipment
The most frequently used supercritical fluid is CO2.
Properties of carbon dioxide:
- Low critical temperature (31.1°C)
- Moderate critical pressure (73 bar)
- Non-flammable
- Non-toxic
- Mixable with organic solvents
- Recoverable after processing
- Diffuses faster than conventional fluid solvents
- GRAS status
- Approved by the FDA for use in food and pharmaceuticals
- Environmentally friendly
- Cost-effective
