Importance of flow control of the supercritical fluid and co solvent in the SCFE process

Byamna shareef

Flow control is a vital parameter in supercritical extraction or supercritical fluid extraction (SCFE) as the more correct technical term is for the process. This is the case regardless of what the supercritical extraction is being used for – cannabis extraction, oleoresin extraction, flavour and fragrance extraction, nutraceutical extraction, phytochemicals extraction and the like. 

Apart from the supercritical fluid (SCF), a co-solvent may also be sometimes used in SCFE to improve the range of extraction.

Flow control over the SCF is important because it determines the resident time – the amount of time that the SCF gets to act on the raw material. This resident time has a direct bearing on the following output parameters of the supercritical extraction process:

  • Yield
  • Efficiency
  • Mass transfer rates 

Dietary supplement, pharma, food, and cosmetic industries use plunger pumps of high work capacity for pumping the SCF at the required rate. There is a difference between SCF pumps and co-solvent pumps as we will now see. 

Co-solvents are sometimes used to expand the range of extraction of SCFE. For many reasons, supercritical carbon dioxide (sCO2) is the most popularly used SCF. This is because it:

  • Has a critical temperature of 31.10C, which is around the ambient temperature improving its compatibility with temperature-sensitive compounds;
  • Has a more manageable critical pressure of 73.9 bar;
  • Is non-flammable and non-toxic;
  • Has a customizable density to upgrade its solvent power;
  • Is available in ample quantities and in pure form; and
  • Has a comparatively low cost.

On the other hand, sCO2 is a non-polar solvent. This makes it naturally good with the extraction of non-polar products. This also makes it naturally less capable of extracting polar products. This limitation comes to the fore during the SCFE of phytochemicals wherein, sCO2:

  • Can extract non-polar phytochemicals as well as those that are volatile and of low molecular weight.
  • Can separate even some polar phytochemicals located outside the cell wall.
  • Cannot extract polar phytochemicals located inside the cell wall.

Addition of the following co-solvents can make sCO2 capable of extracting polar phytochemicals as well:

  • Ethanol
  • Water
  • Methanol

Operators prefer high performance liquid chromatography (HPLC) pumps as co-solvent pumps.

Two methods used to add the co-solvent are:

  • Actuating both the co-solvent pump and the SCF (sCO2 in most cases) pump at the same time while maintaining the restrictor valve in the open position in order to have the correct mixing ratio.

Operationalizing the SCF pump after adding the calculated amount of co-solvent to the sample already placed in the extractor.