$13.$ In packed columns, the rate of absorption can be optimized by adjusting the solvent flow rate, which affects the $\_\_\_\_\_\_$ of the mass transfer and the height of the $\_\_\_\_$ based on overall $\_\_\_\_\_\_$ coefficients.

$14.$ The minimum solvent flow rate impacts the process design by ensuring $\_\_\_\_\_\_$ use of resources, optimizing the $\_\_\_\_$ efficiency, and determining the absorption column's $\_\_\_\_\_\_$ requirements.

$15.$ The concept of Height Equivalent to a Theoretical Plate $($HETP$)$ aids in evaluating the $\_\_\_\_\_\_$ of packing materials, calculating the column's $\_\_\_\_$ based on mass transfer efficiency, and estimating the $\_\_\_\_\_\_$ of stages.

$16.$ For a packed absorption column, the choice of solvent affects the number of transfer units $($NTU$)$ and height of a transfer unit $($HTU$),$ which together determine the column's total $\_\_\_\_\_\_,\_\_\_\_$ requirements, and $\_\_\_\_\_\_$ efficiency.