$1.$ The $\_\_\_\_\_\_$ is critical in determining the minimum solvent flow rate required for an absorption process, affecting both the $\_\_\_\_$ of the operation and the design of the $\_\_\_\_\_\_$ column.

$2.$ In a gas absorption column, the Height Equivalent to a Theoretical Plate $($HETP$)$ is used to measure the efficiency of the $\_\_\_\_\_\_$ material, which directly impacts the $\_\_\_\_$ height and the overall $\_\_\_\_\_\_$ efficiency.

$3.$ The number of transfer units $($NTU$)$ concept is crucial for designing an absorption column as it helps calculate the required column $\_\_\_\_\_\_,$ based on the mass transfer $\_\_\_\_$ and the solvent $\_\_\_\_\_\_$ rate.

$4.$ For an efficient absorption process, the choice of solvent is determined by its $\_\_\_\_\_\_,$ its ability to $\_\_\_\_$ the target gas, and its impact on the $\_\_\_\_\_\_$ of the column.