Super Critical Fluid (SCF) can be defined as a substance above its critical temperature and critical pressure. The critical point represents the highest temperature and pressure at which the substance in question can exist as a vapour and liquid in equilibrium.
Generally extraction is usually from a solid matrix but it can also be taken from liquids. CO2 is the most common supercritical fluid albeit occasionally supplemented by co-solvents such as ethanol or methanol. The normal conditions for supercritical CO2 are above a ‘critical’ temperature of 31C and a ‘critical’ pressure of 74 bar. When sub critical the CO2 is in liquid form and when it goes super critical it takes on the appearance of a dense ‘fog’.
In most cases extraction is essentially a diffusion based system. i.e. the solvent is required to diffuse and permeate the raw material enabling the targeted extract to subsequently diffuse from the matrix and into the solvent. The fluid is pumped to a heating zone, where it is heated to supercritical conditions. It then passes into the extraction vessel, where it rapidly diffuses into the solid matrix and dissolves the material to be extracted. The dissolved material is swept from the extraction cell into a separator at lower pressure, and the extracted material settles out. The CO2 can then be cooled, re-compressed and recycled.
In this type of process the advantages of supercritical fluids are twofold; One, it can permeate any raw material much faster and more easily than a liquid. Surface tensions and viscosities are much lesser than with liquids, thus the penetration is more comprehensive allowing access to the smaller pores otherwise inaccessible to liquids; Two, these same qualities facilitate a faster and more efficient recovery of the solvent. This results in no product solvent contamination for the customer and a faster processing speed owing to the speedier recovery of the solvent – normally the slowest common denominator.