6.3 Extraction equipment and design considerations

Extraction equipment comes in various types, each with unique characteristics. Mixer-settlers, extraction columns, and centrifugal extractors offer different approaches to liquid-liquid separation, balancing factors like flow dynamics, separation efficiency, and space requirements.

Choosing the right extractor involves weighing technical and economic considerations. Physical properties, separation needs, costs, scale, and process integration all play crucial roles in selecting the most suitable equipment for a specific extraction application.

Extraction Equipment Types and Characteristics

Types of extraction equipment

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• Mixer-settlers combine mixing vessel and settling tank for continuous countercurrent flow handles slow separation and long settling times
• Extraction columns utilize vertical cylindrical vessels with internal components for continuous countercurrent flow accommodates multiple theoretical stages
  • Spray columns
  • Packed columns
  • Sieve tray columns
  • Pulsed columns
• Centrifugal extractors employ centrifugal force enhancing phase separation in compact design with high throughput and short residence time suited for similar densities or emulsification-prone systems

Principles of extraction equipment operation

• Mixer-settlers operate through mixing stage creating interfacial area for mass transfer followed by settling stage separating phases by gravity

• Extraction columns function via countercurrent flow of phases with internal components providing staging and enhancing mass transfer

• Centrifugal extractors utilize rapid rotation creating high centrifugal force with continuous feed and product withdrawal

Advantages vs limitations of extractors

• Mixer-settlers offer simple design, easy scalability, and suitability for high viscosity fluids but require large footprint, high solvent inventory, and limited stages per unit

• Extraction columns provide compact design with multiple stages, low solvent inventory, and suitability for moderate to fast kinetics systems but face complex hydrodynamics, potential backmixing, and sensitivity to flow rate variations

• Centrifugal extractors feature small footprint, low holdup volume, and rapid phase separation suitable for thermally sensitive materials but incur high capital and maintenance costs, limited capacity per unit, and scale-up complexity

Selection criteria for extractors

• Technical considerations encompass physical properties (density difference, interfacial tension, viscosity) and separation requirements (theoretical stages, mass transfer rates, residence time constraints)

• Economic considerations involve capital costs (equipment purchase, installation, auxiliary equipment) and operating costs (energy consumption, maintenance, labor requirements)

• Scale of operation factors in production capacity, space limitations, and flexibility for future expansion

• Process integration addresses compatibility with upstream and downstream processes, automation and control requirements, and safety and environmental considerations