Question 1

Why is liquid-liquid extraction used instead of distillation?

Accepted Answer

Liquid-liquid extraction is preferred when compounds have similar boiling points, components are temperature-sensitive, or distillation requires extreme conditions like very low pressure. Mass transfer is driven by solubility differences between immiscible or partially miscible feed and solvent streams, allowing separation without heat-induced degradation of sensitive materials.

Question 2

What is the difference between raffinate and extract in LLE?

Accepted Answer

The raffinate is the solute-depleted feed stream remaining after extraction, while the extract is the solute-rich solvent phase that exits the column. These two phases separate after the solute transfers from the feed to the solvent through mixing, with each phase flowing to different outlets based on density differences.

Question 3

How does counter-current flow improve extraction efficiency?

Accepted Answer

Counter-current systems are more efficient than co-current flow because they maintain a larger concentration gradient throughout the column, promoting better mass transfer. The solvent and feed flow in opposite directions, maximizing contact between phases and improving solute recovery across multiple stages.

Question 4

What are theoretical plates and why do they matter in LLE?

Accepted Answer

Theoretical plates are hypothetical stages where two liquid phases reach equilibrium with no further concentration change occurring. The higher the number of theoretical plates, the more efficient the separation process. This analysis evaluates column performance and helps predict extraction effectiveness and overall system efficiency.

Question 5

How do agitator speed and feed flow rate affect extraction efficiency?

Accepted Answer

Increased agitator speed and feed flow rate improve solute recovery and stage efficiency by enhancing mixing, creating smaller droplets, and improving phase dispersion for better mass transfer. However, both relationships plateau at higher rates due to emulsification and flooding, which prevent clean phase separation between stages.

Question 6

What is a York-Scheibel column and how does it work?

Accepted Answer

A York-Scheibel column is an agitated extraction unit with internal paddle-wheel impellers and wire-mesh packing at each stage. The impellers mix the two liquid phases, while the mesh enables phase separation. Individual partitions create separate stages, allowing counter-current extraction of solutes like isopropanol from organic mixtures.

Question 7

What alternative methods exist when emulsification prevents effective phase separation?

Accepted Answer

Mixer-settler tanks in series provide an alternative when emulsification is problematic. This setup uses an agitator to mix phases in one tank, then allows them to coalesce in a settler tank where the heavy phase settles to the bottom and the light phase rises to the top for separate removal.

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