$13.$ Explain how multistage cross$-$current extraction can be optimized to improve separation efficiency while minimizing solvent usage.

$14.$ Describe the mechanism of solvent recovery in a counter$-$current extraction process and its significance in industrial applications.

$15.$ How do phase equilibria impact the selection of solvents for extraction processes involving complex mixtures?

$16.$ Discuss the challenges associated with scaling up batch extraction processes to continuous operations.