Supercritical fluid extraction (SFE) is a separation process using supercritical fluids, which have both gas and liquid-like properties. SFE has become increasingly popular in recent years for its high efficiency, high selectivity, and low environmental impact. The general principles, phase equilibria, mass transfer characteristics, and operational conditions of SFE will be discussed in this article.
Basics 1#: Phase Equilibria of Supercritical Fluids
In order to fully utilize the properties of supercritical fluids, it is necessary to understand the phase equilibrium behavior of pure solvents and their mixtures with solutes under supercritical conditions. In general, the solubility of a solute in a solvent depends on the size of the interaction forces between the solute and solvent molecules, which increases as the distance between molecules decreases, i.e., as fluid density increases. Therefore, it can be expected that a supercritical fluid is a “good” solvent in its high-density state (liquid-like), while it is a “poor” solvent in its low-density state (gas-like).
The relationship between the solubility C of a substance in a supercritical fluid and the density ρ of the supercritical fluid is given by the following equation:
lnC=mlnρ+b
Here, the values of m and b are related to the chemical properties of the extracting agent and solute, respectively. According to the principle of similar dissolution, the greater the similarity between the chemical properties of the selected supercritical fluid and the material to be extracted, the greater its solubility.
Basics 2#: Mass Transfer Characteristics of Supercritical Fluids
Supercritical fluids are fluids that exist above their critical temperature and pressure. They exhibit significant advantages over conventional fluids due to their excellent extraction capacity and selectivity. They are ideal extraction solvents because of their excellent mass transfer properties. Parameters such as density, viscosity, and thermal conductivity can have a significant impact on the extraction process.
The diffusion coefficient of a solute in a liquid is generally much less than that in a gas, and temperature and viscosity have a significant effect on the diffusion coefficient. However, as a solute, a supercritical fluid has a larger diffusion coefficient than a gas but smaller than a liquid, with a wide range of variations. The diffusion coefficient of CO2, for example, increases with increasing temperature and is proportional to the square root of temperature.
Basics 3#: Operational Condition Selection of Supercritical Extraction
As the pressure increases, the solubility of the solute in the solvent also increases with rising temperature, while its solubility decreases. Under this pressure, the effect of the decrease in gas density with increasing temperature is greater than that of an increase in the vapor pressure. When the temperature or pressure changes, the extraction capacity of the supercritical fluid will change significantly. Supercritical gas separation and recovery can be performed under constant temperature conditions by varying the pressure or by changing the temperature when the operating pressure must be kept constant.
Basics 4#: Energy Consumption of Supercritical Extraction Process
The equipment used in the SFE process (isothermal static pressure process, adsorption process) mainly consists of an extractor, a separator, a compressor, and a throttling valve. The dissolution of the solute in the supercritical fluid in the extractor is a spontaneous process that does not consume energy. The expansion process of the throttling valve is an isenthalpic process, and some energy can be recovered by replacing the throttling valve with an expansion machine. The operating process in the separator is a mechanical separation process that does not consume energy. The compressor is a device that consumes energy, and its power depends on the solubility of the supercritical fluid to the solute. The greater the solubility, the less circulation is required, and the less energy is consumed.
Comparison of Supercritical Extraction and Traditional Extraction Methods
| Factors | Supercritical Extraction (SFE) | Traditional Extraction Method |
|---|---|---|
| Solvent | Supercritical Fluid | Organic Solvent |
| Purity of Extract | High | Low |
| Extraction Efficiency | High | Low |
| Environmental Impact | Low | High |
| Extraction Time | Short | Long |
| Operating Temperature | Adjustable | Fixed |
| Separation and Recovery | Simple | Complicated |
Basics 5#: Advantages of Supercritical Extraction
- High extraction efficiency: SFE can obtain materials with high purity in a short time, with excellent selectivity and good reproducibility.
- Low environmental impact: The use of supercritical fluids does not produce hazardous waste, and its high efficiency reduces the use of organic solvents, which are polluting and harmful to the environment.
- Adjustable operating conditions: The operational temperature and pressure of SFE can be easily adjusted to optimize extraction conditions.
- Simple separation and recoveryprocess: Compared with traditional extraction methods, the separation and recovery process of SFE is relatively simple, with no need for complex distillation and solvent recovery steps.
Conclusion
Supercritical fluid extraction offers significant advantages over traditional extraction methods in terms of efficiency, selectivity, and environmental impact. Its favorable mass transfer characteristics and adjustable operational conditions allow for optimal extraction and recovery with minimal energy consumption. With ongoing research and development in this field, SFE is expected to continue to gain popularity in various industries, including food, pharmaceuticals, and cosmetics.