Liquid–liquid extraction — For systems with very low density difference between phases, which extractor type is most suitable?

Introduction:
Phase disengagement in liquid–liquid extraction depends strongly on density difference and interfacial tension. When the two liquid phases are nearly the same density, gravitational separation is slow and inefficient. Equipment choice must therefore provide an enhanced driving force for separation.

Given Data / Assumptions:

• Two immiscible (or partially miscible) liquid phases with very small density difference.
• Need both efficient mass transfer and reliable phase separation.
• Industrial-scale continuous operation desired.

Concept / Approach:

Centrifugal extractors (e.g., Podbielniak, disc-stack, or rotor–stator designs) impose high g-forces that dramatically accelerate coalescence and phase separation despite tiny buoyancy forces. They combine mixing and separation within a compact unit, making them ideal for low Δρ systems. Gravity-settling devices (mixer–settlers, packed/spray columns) rely on low g and thus struggle when Δρ is small; columns may also experience flooding or excessive holdup under these conditions.

Step-by-Step Solution:

Verification / Alternative check:

Operational experience in pharma and specialty chemicals shows centrifugal extractors outperform gravity-based columns for closely matched densities, reducing stage volume and residence time.

Why Other Options Are Wrong:

A/D/E: Depend on gravity; with low Δρ, settling becomes impractically slow. C: Pulsed columns improve mass transfer but still rely on gravity for phase disengagement.

Common Pitfalls:

Focusing solely on mass-transfer coefficients and overlooking separation section bottlenecks caused by poor buoyancy.

Final Answer:

centrifugal extractor