Gravity decantation: what phases can a gravity decanter separate effectively?

Introduction / Context:Decanters use gravity to separate liquid phases with a density difference. They are widely used after liquid–liquid extraction, in oil–water separation, and in biodiesel washing.

Given Data / Assumptions:

• Two liquid phases are present.
• Phases are at least partially immiscible and have a density contrast.

Concept / Approach:Separation occurs when immiscible phases form distinct layers under gravity. The denser phase settles; the lighter phase rises. If liquids are miscible, a single homogeneous phase forms and gravity cannot separate them. If immiscible but identical in density, stratification would not occur (a practical impossibility for distinct liquids).

Step-by-Step Solution:Identify requirement: immiscibility + density difference.These conditions define decanter applicability.Select “immiscible liquids of different [density].”

Verification / Alternative check:Chemical engineering textbooks model residence time using Stokes’ law for droplets, highlighting dependence on density difference and interfacial tension.

Why Other Options Are Wrong:Miscible systems do not phase separate under gravity.“Immiscible of same density” would not stratify; separation would be impractical.

Common Pitfalls:Neglecting emulsion formation; even with immiscibility and density contrast, surfactants can stabilize emulsions requiring coalescers.

Final Answer:immiscible liquids of different