7.3 Supercritical Fluid Extraction

Supercritical fluids exist above critical temperature and pressure, blurring the line between gas and liquid. They combine liquid-like density with gas-like viscosity, enabling efficient extraction of target compounds from various materials.

Supercritical fluid extraction uses these unique properties to selectively extract desired compounds. By adjusting temperature and pressure, the process can be fine-tuned for specific applications, from decaffeinating coffee to purifying pharmaceuticals.

Supercritical Fluids and Extraction

Properties of supercritical fluids

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• Exist above critical temperature and pressure where distinction between gas and liquid phases disappears
• Density similar to liquids enables high solvating power to dissolve target compounds
• Viscosity similar to gases allows for better mass transfer and penetration into porous materials (coffee beans, plant matrices)
• Diffusivity intermediate between gases and liquids enhances mass transfer rates for faster extraction
• Tunable properties by adjusting temperature and pressure enable selective extraction of desired compounds (caffeine, essential oils)

Principles of supercritical fluid extraction

• Separation process using supercritical fluids as solvents to extract target compounds from a matrix
• Solubility of target compounds depends on fluid density controlled by temperature and pressure
• Selective extraction achieved by manipulating solvent properties through changes in temperature and pressure
• Dissolution of target compounds in supercritical fluid based on their solubility
• Diffusion of dissolved compounds through supercritical fluid from matrix to bulk fluid
• Convection carries extracted compounds away from matrix by bulk fluid motion
• Decompression separates extracted compounds from supercritical fluid by reducing pressure or temperature

Design of extraction processes

1. Select appropriate supercritical fluid (CO2, ethane, water) based on target compounds and matrix
2. Determine optimal operating conditions (temperature, pressure, flow rate) through experiments or modeling
3. Choose suitable extraction vessel and equipment based on scale and process requirements
4. Optimize extraction time and cycle to maximize yield and efficiency

• Factors affecting performance: solubility of target compounds, mass transfer rates, matrix properties (particle size, porosity), co-solvents or modifiers
• Applications: natural product extraction (essential oils), food and beverage (decaffeination), pharmaceuticals (drug purification), environmental remediation (soil cleanup)

Supercritical extraction vs other techniques

• Advantages: high selectivity and purity, minimal thermal degradation, environmentally friendly solvents (CO2), easy separation by decompression, potential for continuous operation
• Limitations: high initial capital costs, complexity in design and optimization, limited applicability for polar or high molecular weight compounds, safety concerns with high-pressure
• Comparison:
  • Liquid-liquid extraction: SFE offers higher selectivity and avoids toxic organic solvents
  • Steam distillation: SFE operates at lower temperatures, reducing thermal degradation
  • Soxhlet extraction: SFE has shorter extraction times and uses less solvent
  • Chromatography: SFE can be used as preparative-scale separation with higher throughput