Supercritical carbon dioxide (sCO2) is naturally non polar. Therefore, it is more suited for the extraction of non polar compounds from the raw material when used in the supercritical fluid extraction (SCFE) process. This limitation of sCO2 comes to the fore during the extraction of phytochemicals. sCO2:
- Is able to separate the non-polar phytochemicals.
- Can extract some polar phytochemicals located outside the cell walls.
- Is more suited to separate phytochemicals that are volatile and of low molecular weight.
- Unable to separate polar phytochemicals located inside the cell walls.
Co-solvent pumps lend the additional capacity to sCO2 for dissolution of polar compounds during supercritical extraction in India or elsewhere. They do this by adding small quantities of co-solvents such as ethanol, water, and methanol to sCO2. These are added in some fixed percentage (such as 3% or 5%) of the supercritical fluid – which is sCO2 in this case.
Now, the viability of the supercritical fluid extraction process depends on its versatility. If sCO2 can dissolve more than one compound, these can be separated out from it at different pressures in the same nbatch. Scun operations are complex but widely employed by SCFE systems in India and the rest of the world because of the cost savings they deliver.
Please note the distinction between supercritical fluid (SCF) pumps and co-solvent pumps. The former circulates the supercritical fluid – sCO2 in this case, while the latter circulate the co-solvent.
Flow rate is another important parameter for supercritical extraction in India and abroad. It has a direct impact on the resident time – the time for which the SCF gets to act on the raw material. This influences the following deliverables of the SCFE systems in India and elsewhere:
- Mass transfer rate
Supercritical extraction in India and the rest of the world usually rely on plunger pumps of high capacity – particularly for the dietary supplement, pharma, food, and cosmetic industries.
Coming back to co-solvent pumps, SCFE systems in India and abroad use HPLC or High Performance Liquid Chromatography pumps. Two techniques are in vogue for adding the co-solvent in the required proportion to the SCF during the supercritical fluid extraction procedure:
- Both pumps are actuated at the same time and the restrictor valve is maintained at a position that allows the blending of the SCF and co-solvent in the required percentage.
- Co-solvent is added to the raw material already placed in the extractor in the exact amount. Thereafter, the SCF pump is turned on.