Importance of density of supercritical fluids in SCFE

Density is among the most important parameters in the supercritical fluid extraction process (SCFE). This is because density determines the solvent power of the supercritical fluid. Solvent power is the capacity of the supercritical fluid to dissolve the required compound (called target molecule) from the raw material.

The greater the density, the higher the solvent power. The yield and efficiency of extraction depend on the density of supercritical fluid. And since pressure has a large influence on density, pressure control is an inseparable part of all CO2 extraction plants in India or anywhere else.

Supercritical fluid is one that is at a pressure and temperature respectively above its critical pressure and critical temperature. In such a state, the fluid’s properties can be similar to a liquid, a gas, or somewhere in between. Specifically speaking, a supercritical fluid’s:

• Higher density similar to that of a liquid. This endows it with higher solvent power.
• Low viscosity as that of a gas. This helps it diffuse into porous solid raw material and transfer greater mass from the same.
• Low surface tension like that of a gas. As a result, it can seep to greater depths inside a solid raw material.

For the supercritical fluid extraction process, the supercritical fluid is brought into contact with the raw material at higher pressure. At a certain pressure, the supercritical fluid will have one particular density and solvent power to dissolve only one particular compound. That is, it will be involved in the extraction of only one particular target molecule. It will not extract much of the other compounds in the raw material.

Density, controlled as it is by pressure, has a direct bearing on the purity of extraction. Products containing pure ingredients are in great demand at present. And because the supercritical fluid extraction process extracts target molecules in their pure form, it is very popular. Density control is, therefore, at the very heart of SCFE. This is what makes pressure regulation such an important parameter in CO2 extraction plants in India and abroad.

Carbon dioxide (CO2) is the most popularly used fluid for the SCFE process 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.

Although CO2 is a greenhouse gas, CO2 extraction plants in India and the rest of the world that use a CO2 recirculation system do not burden the environment with pollution. This is because, with recirculation, CO2 is unable to escape to the atmosphere where it can contribute to the dreaded process of global warming.

Solvent extraction and hydrodistillation are among the other extraction processes. The heat used in hydrodistillation can and usually does change the chemical composition of the target molecule. Similarly, the solvent used in the solvent extraction process combines with the target molecule. In both cases, the purity of the target molecule suffers.