Here’s a clear explanation of the thermodynamics of supercritical fluids, tailored for understanding Supercritical Fluid Extraction (SFE):
1. What Are Supercritical Fluids (SCFs)?
A supercritical fluid is a substance at temperature and pressure above its critical point (Tc and Pc), where liquid and gas phases are no longer distinguishable.
- Exhibits gas-like properties: low viscosity, high diffusivity
- Exhibits liquid-like properties: high density, high solvating power
Supercritical fluids are ideal for SFE because they combine the advantages of gases and liquids.
2. Key Thermodynamic Properties
| Property | Behavior in SCF | Significance in SFE |
|---|---|---|
| Density (ρ) | Tunable with pressure & temperature | Controls solubility of solutes |
| Viscosity (η) | Low (like gas) | Enhances diffusion into plant matrices |
| Diffusivity (D) | High (like gas) | Faster mass transfer |
| Solvent Power | Related to density | Higher density → dissolves heavier compounds |
| Compressibility (κ) | Moderate to high, decreases with pressure | Impacts pressure tuning and process control |
3. Phase Behavior & Supercritical Region
- Below Tc: Distinct liquid and gas phases
- Above Tc and Pc: Supercritical region → density, viscosity, and diffusivity can be tuned
- Density effect: Small changes in pressure → large change in density → large change in solubility
Example (CO₂):
- At 100 bar & 35°C → density ≈ 0.7 g/mL
- At 300 bar & 40°C → density ≈ 0.9 g/mL
- Higher density → extracts heavier or less polar compounds
4. Solubility & Selectivity
- Solubility of solutes in SCFs is highly dependent on density:
- Slight increase in pressure → increased density → higher solubility
- Temperature also affects solubility:
- ↑ Temperature → ↑ solute vapor pressure (positive effect)
- ↑ Temperature → ↓ SCF density (negative effect)
- This balance allows selective extraction of target compounds by tuning T & P.
5. Isotherms & Phase Diagrams
- P–ρ–T diagrams help visualize SCF behavior:
- “Fan-like” region near critical point → small T or P changes drastically alter density and solvating power
- Operators can tune extraction conditions along this curve to maximize selectivity and yield
6. Fractionation & Density Tuning
- Stepwise fractionation: Use sequential T & P settings to extract light, medium, and heavy compounds
- Supercritical CO₂ can also be modified with co-solvents (ethanol, methanol) to increase polarity and dissolve polar compounds
7. Summary
- Supercritical fluids combine gas diffusivity with liquid solvency, enabling efficient extraction
- Density is the key tunable parameter for solubility
- Thermodynamics governs selectivity, yield, and extraction kinetics
- Understanding SCF thermodynamics is essential for process optimization and scale-up in SFE
If you want, I can make a thermodynamics infographic for SFE, showing:
- SCF region above critical point
- How pressure and temperature tuning changes density and solubility
- Flow from raw material → extraction → fractionation
It’s perfect for training or presentations.
Do you want me to create that infographic?