Controlling Solubility of Supercritical CO2

The Supercritical CO2 extraction process is “geometrically variable.” A supercritical fluid is any compound at a temperature and pressure above its Critical Point.

It can diffuse through solids like a gas, and it can dissolve materials like a liquid. For any pure compound, there is a transition state called “critical” state: for temperatures below the critical temperature (Tc), two phases — liquid and vapor — coexist; for temperatures above Tc, there is only one phase: supercritical fluid. Solubility is a function of pressure and temperature:

  • Solubility increases with increasing pressure at constant temperature.
  • Solubility may increase or decrease when temperatures are raised at constant pressure.

Solubility is related to density. Higher density, higher solubility. This is true from a theoretical point of view, but when applied to a singular compound we may see different results.

The supercritical fluid density always increases with increasing pressure at a fixed temperature, and it always decreases with increasing temperature at a fixed pressure. Solubility depends on this pair of values (pressure/temperature). These values are strictly connected with the solubility of each single compound. For extraction with supercritical fluids, operating conditions are chosen to obtain the selective extraction of compounds of interest, reducing to a minimum the co-extraction of undesired compounds. The selection of the operating conditions depends on the specific compound or compound family to be extracted.