The viscosity, thermal conductivity, and diffusivity of supercritical fluid are quite different from those in a normal state.
Three advantages of supercritical fluid transport
- The density of the supercritical fluid is hundreds of times greater than that of gas, and the specific value is equivalent to that of liquid;
- Its viscosity is still close to that of gas, but it is 2 orders of magnitude smaller than that of liquid;
- The diffusion coefficient is between gas and liquid, about 1/100 of that of gas, and hundreds of times larger than that of liquid.
Why should supercritical fluid extraction be more efficient than liquid-liquid extraction?
- Liquid-liquid extraction, also known as partitioning, is a separation process consisting of the transfer of a solute from one solvent to another, the two solvents being immiscible or partially miscible with each other.
- Supercritical Fluid Extraction employs a supercritical fluid, most commonly CO2, as the mobile phase solvent for the extraction.
Supercritical fluid not only has the characteristics of relatively large solubility of liquid to solute but also has the characteristics of easy diffusion and movement of gas. Therefore, its mass transfer rate is much higher than that of the liquid phase process, that is to say, supercritical fluid has both the properties of gas and liquid. More importantly, near the critical point, a small change in pressure and temperature can cause a large change in fluid density, which is correspondingly expressed as a change in solubility. Therefore, changes in pressure and temperature can be used to realize the process of extraction and separation.
Six advantages of supercritical fluid extraction over liquid-liquid extraction
Supercritical fluid extraction (SFE) and liquid-liquid extraction (LLE) are both methods of extracting compounds from a mixture, but there are some key differences between the two techniques.
- Solvents Used: In LLE, two immiscible liquids are used as solvents, typically water and an organic solvent such as ether or chloroform. In contrast, SFE uses a supercritical fluid, which is a substance that is heated and pressurized to a state where it has properties of both a gas and a liquid, typically carbon dioxide or ethanol.
- Selectivity: LLE is often used for separating compounds with different polarities, as the two solvents used have different affinities for polar and non-polar compounds. SFE, on the other hand, can be used to extract a wide range of compounds from a mixture with a high degree of selectivity. The selectivity of SFE can be controlled by adjusting the pressure and temperature of the supercritical fluid.
- Solvent Recovery: In LLE, the solvents used are often difficult to recover and can be hazardous to the environment. In contrast, the supercritical fluids used in SFE can be easily removed by simply reducing the pressure and temperature, leaving behind a pure extract without any residual solvents.
- Efficiency: SFE is generally faster and more efficient than LLE, as the supercritical fluid can penetrate the sample more easily and extract the desired compounds more effectively. Additionally, SFE can be automated, making it a more practical option for large-scale extractions.
- Cost: While SFE equipment can be expensive to purchase and maintain, it can be more cost-effective than LLE in the long run due to its higher efficiency and lower solvent usage.
Due to the above-mentioned advantages of supercritical fluid, the extraction efficiency of supercritical fluid should be better than that of liquid-liquid extraction.
Six Differences Between the Supercritical Fluid Extraction and Liquid-Liquid Extraction
Difference 1#: Extraction principle
- Supercritical fluid extraction: The less volatile substances are selectively dissolved in the fluid and extracted to form a supercritical fluid phase
- Liquid-liquid extraction: solvent is added to the mixture to be separated to form a liquid phase
Difference 2#: Extraction ability
- The extraction ability of supercritical fluid is mainly related to density, and it should be controlled by selecting the appropriate pressure and temperature.
- Liquid-liquid extraction: The extraction capacity of the solvent depends on the temperature and the composition of the mixed solvent and has little relationship with the pressure
Difference 3#: Extraction pressure
- Supercritical fluid extraction: It operates under high pressure (5~30Mpa), and can generally be carried out at room temperature, which is beneficial to the treatment of heat-sensitive substances, so it is expected to be applied in pharmaceuticals, food, and biological functional products
- Liquid-liquid extraction: operates at normal temperature and pressure
Difference 4#: Separation method
- Supercritical fluid extraction: The separation between the extracted solute and the supercritical fluid can be decompressed under isothermal pressure or heated up under isothermal pressure.
- Liquid-liquid extraction: The liquid mixture after extraction, the solvent, and solute are usually separated by distillation, which is not good for the treatment of heat-sensitive substances
Difference 5#: Mass transfer capacity
- Supercritical fluid extraction: Due to the superiority of supercritical fluid, the mass transfer capacity of solute is improved
- Liquid-liquid extraction: Mass transfer conditions are often different from supercritical fluid extraction
Difference 6#: Solute concentration
- Supercritical fluid extraction: In most cases, the concentration of the solute in the supercritical fluid phase is very small, and the composition of the supercritical phase is close to that of a pure supercritical fluid
- Liquid-liquid extraction: The extraction phase is the liquid phase, and the solute concentration can be quite large
Due to these special properties of supercritical fluids, supercritical fluids have been more and more widely researched and applied in extraction and separation, environmental protection, material science, reaction engineering, biotechnology, and the cleaning industry.
Among them, supercritical C02 extraction technology has unique advantages compared with traditional extraction methods, has become one of the most extensive supercritical fluid technologies, and has produced huge social and economic benefits.