液-液萃取效率

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.

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.

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.

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.

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.

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.

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.