Supercritical fluid extraction
Supercritical fluid extraction (SFE) is a separation technique that employs supercritical fluids, primarily carbon dioxide (CO₂), as solvents to isolate target compounds from complex matrices such as plant materials, foods, or environmental samples.[1] It operates by pressurizing and heating the fluid beyond its critical point—31.1°C and 73.8 bar for CO₂—transforming it into a state with gas-like diffusivity and liquid-like density, enabling efficient penetration and selective dissolution of solutes.[2] This process typically involves pumping the supercritical fluid through a sample chamber, where it extracts the desired components, followed by decompression in a separator to recover the extract while recycling the fluid.[3] Developed in the mid-20th century with early commercial applications emerging in the 1970s for processes like coffee decaffeination, SFE gained prominence in the 1980s as a green extraction method due to its ability to replace hazardous organic solvents.[2] The technique's principles rely on tunable solvent properties: increasing pressure enhances density and solvating power, while temperature affects diffusivity and volatility, often optimized with co-solvents like ethanol for polar compounds.[1] Key advantages include environmental sustainability—using non-toxic, recyclable CO₂ leaves no residues—high selectivity for bioactive molecules such as essential oils, carotenoids, and pharmaceuticals, and preservation of heat-sensitive compounds through mild operating conditions (typically 40–100°C).[3][2] SFE finds widespread applications across industries, including food processing for extracting flavors and removing contaminants like caffeine or cholesterol, pharmaceuticals for isolating alkaloids and antioxidants, cosmetics for natural pigments, and environmental analysis for pollutant remediation from soils.[1] Despite its benefits, challenges such as high initial equipment costs and the need for precise parameter control have limited broader adoption, though ongoing advancements in process intensification continue to enhance its efficiency and scalability.[2]Introduction and background
Definition and principles
Supercritical fluid extraction (SFE) is a separation technique that employs a supercritical fluid as a solvent to selectively extract target compounds from a solid or liquid matrix under elevated pressure and temperature conditions.[4] In this process, the supercritical fluid penetrates the matrix, dissolves the solutes based on their solubility in the fluid, and carries them away, after which the extract is recovered by altering the fluid's conditions.[5] The foundational thermodynamic principle of SFE relies on the unique properties of substances in the supercritical state, which occurs when a fluid is maintained above its critical temperature (T_c) and critical pressure (P_c), eliminating the distinction between liquid and gas phases.[4] In this regime, the fluid exhibits liquid-like densities for efficient solvating power while possessing gas-like diffusivities and low viscosities that facilitate rapid mass transfer and penetration into the sample matrix.[5] These hybrid characteristics enable tunable solubility by adjusting pressure and temperature, allowing precise control over the extraction selectivity without phase transitions.[4] In a typical phase diagram, the supercritical region is depicted beyond the critical point, where the vapor pressure curve terminates, and the fluid's density varies continuously with pressure and temperature rather than abruptly across phase boundaries.[4] For carbon dioxide (CO_2), a commonly used supercritical fluid in SFE due to its mild critical parameters, the critical point is at T_c = 31.1^\circ \text{C} and P_c = 73.8 \text{ bar} (7.38 \text{ MPa}).[6] Operating above these values—typically at 40–100^\circ \text{C} and 100–400 bar—ensures CO_2 remains supercritical while optimizing extraction efficiency.[4] SFE aligns with green chemistry principles by enabling solvent-free recovery of the extract through simple depressurization, which causes the supercritical fluid to revert to its gaseous state and evaporate, leaving no residual solvent in the product.[5] This depressurization step, often combined with mild heating, facilitates quantitative solvent removal and recycling, minimizing environmental impact compared to conventional organic solvent-based extractions.[4]Historical overview
The concept of supercritical fluid extraction (SFE) originated in the late 19th century with the pioneering experiments of James B. Hannay and J. Hogarth, who in 1879 demonstrated the solubility of solid inorganic salts, such as calcium, potassium, and sodium chlorides, in supercritical ethanol under high pressure and temperature conditions exceeding the fluid's critical point.[4] Their work, published in the Proceedings of the Royal Society of London, marked the first observation of enhanced solvent properties in supercritical fluids, though practical applications remained unexplored for decades due to technological limitations. Advancements accelerated in the mid-20th century, particularly in the 1960s, when Kurt Zosel at the Max Planck Institute for Coal Research investigated supercritical carbon dioxide (scCO₂) for extracting organic compounds from coal, revealing its potential as a selective solvent for non-polar substances.[7] Zosel's initial patent in 1963 described the general process of SFE using scCO₂, laying the groundwork for industrial applications.[8] By the early 1970s, Zosel extended this to food processing, patenting a method for caffeine decaffeination from coffee beans using moist scCO₂, which selectively removed caffeine while preserving flavor compounds; this process received U.S. Patent 3,806,619 in 1974.[9] Commercialization gained momentum in the 1980s, as scCO₂ was recognized as generally regarded as safe (GRAS) by the FDA for food contact, enabling its approval for extracting food-grade products without residual solvents. The first industrial-scale SFE plant for hops extraction, aimed at isolating bitter acids and essential oils for brewing, became operational in the early 1980s, with significant expansion by 1988 through facilities like those operated by Hopunion in the U.S.[10] During the 1990s and 2000s, SFE expanded into pharmaceuticals and natural products extraction, driven by its tunability for isolating bioactive compounds like essential oils, lipids, and antioxidants from botanicals, with over 100 industrial plants worldwide by the early 2000s focusing on high-value extracts. Recent milestones through 2025 include the integration of computational tools such as density functional theory (DFT) to predict solute-solvent interactions and optimize extraction yields for bioactives; for instance, 2024-2025 studies on rosemary extracts combined SFE with DFT analyses to enhance antioxidant profiling and sustainability.[11]Scientific principles
Properties of supercritical fluids
Supercritical fluids exhibit unique physical properties that bridge the characteristics of liquids and gases, making them particularly suitable as solvents in extraction processes. Their density can be tuned over a wide range, typically from 0.1 to 0.8 g/cm³, approaching liquid-like values while remaining highly responsive to changes in pressure and temperature.[12] This tunability arises from the absence of a distinct liquid-gas phase boundary above the critical point, allowing continuous adjustment without phase transitions. In contrast, the viscosity of supercritical fluids is significantly lower, ranging from 0.005 to 0.01 Pa·s, which is approximately 10 times lower than that of typical liquids (0.05–0.1 Pa·s).[12] Meanwhile, their diffusivity is intermediate but notably higher than liquids, on the order of 10⁻⁷ m²/s, representing 10–100 times greater mobility compared to liquid solvents (∼10⁻⁹ m²/s).[12] These attributes—liquid-like solvating capacity combined with gas-like transport properties—enhance mass transfer efficiency during extractions.[13] The solvating power of supercritical fluids is highly tunable through variations in pressure and temperature, which directly influence density and thus solubility. Density (ρ) as a function of pressure (P) and temperature (T) can be modeled using equations of state such as the Peng-Robinson equation, which provides a thermodynamic framework for predicting phase behavior and solubility in the supercritical regime: P = \frac{RT}{V_m - b} - \frac{a \alpha}{(V_m + b)^2 - b(V_m - b)} where V_m is the molar volume, R is the gas constant, and parameters a, b, and \alpha are substance-specific, enabling computation of ρ from P and T. This adjustability allows precise control over solvent strength, often increasing solubility by orders of magnitude with modest pressure changes near the critical point.[13] Common supercritical fluids include carbon dioxide (CO₂), which is favored for its non-toxicity, low critical temperature (Tc), and moderate critical pressure (Pc), as well as propane, water, and nitrous oxide (N₂O). These fluids are selected based on their critical parameters, which determine the conditions required to achieve the supercritical state. The table below summarizes key critical parameters for these substances:| Fluid | Tc (°C) | Pc (atm) | Critical Density (g/cm³) |
|---|---|---|---|
| CO₂ | 31.3 | 72.9 | 0.47 |
| Propane | 96.8 | 42.4 | 0.22 |
| Water | 374.2 | 218.3 | 0.34 |
| N₂O | 36.5 | 72.5 | 0.45 |