Pressure Control in Supercritical Extraction

ByUday P Thakur

Pressure control is perhaps the most important parameter for supercritical fluid extraction (SCFE). This is because the entire mechanism of dissolving the required compound (also called target molecule or simply the product) in its purest possible form and then separating it out, again in the purest possible form, hinges on accurate pressure control. 

Let us first understand the supercritical extraction process. 

SCFE uses a supercritical fluid (SCF), usually carbon dioxide. An SCF is one that is at a:

  • Pressure above its critical pressure; and
  • Temperature above its critical temperature. 

Its properties can be:

  • Similar to that of its liquid state;
  • As that of its gaseous phase; or
  • Somewhere in between these two phases. 

For example, its:

  • Density is high. Liquids have high density.
  • Viscosity and Surface Tension are low. Gases have low values of viscosity and surface tension. 

Greater the density, higher is the solvent power of the SCF – that is, more is its capacity to dissolve the product from the raw material. Most importantly, the solvent power can be customized by changing pressure to dissolve one particular product only. Some amounts of other elements from the raw material do dissolve in the SCF, but these are negligible and can be easily dealt with in the post extraction procedures. 

To benefit from this, the supercritical extraction process:

  • Makes the SCF dissolve the required product at high pressure. This part of the process takes place in the high pressure extractor.
  • Allows the product to come out of solution from the SCF at low pressure. This part of the process occurs in the low pressure separator. 

It is precise pressure control that ensures the purity of extraction and this is why it is so important to the SCFE process. Post extraction processing is necessary to eliminate the trace elements that dissolve unintentionally. But such processing is minimized by the targeted nature of SCFE. 

What is equally important is managing the pressure drop between the extractor and separator. If the SCF is allowed to freely expand from high pressure in the extractor to the low pressure in the separator, its temperature will drop suddenly and result in ice formation. This will block the equipment tubes and introduce discontinuity. 

In order to avoid such freezing, SCFE equipment manufacturers use a back pressure regulation valve. The device facilitates smooth and gradual pressure drop. Temperature in the separator is also maintained so as to avoid ice formation.