$17.$ An optimum reflux ratio seeks to $\_\_\_\_\_\_$ between capital and operational costs, achieving an economical balance for $\_\_\_\_$ while maintaining $\_\_\_\_\_\_.$

$18.$ Relative volatility measures the ease of separating a mixture by distillation and is defined by the ratio of $\_\_\_\_\_\_$ of the components, crucial for $\_\_\_\_$ and $\_\_\_\_\_\_$ design.

$19.$ In flash distillation, a $\_\_\_\_\_\_$ feed results in $\_\_\_\_$ and $\_\_\_\_$ phases in equilibrium at a specific $\_\_\_\_$ and $\_\_\_\_\_\_.$

$20.$ Total reflux in distillation operations means no $\_\_\_\_\_\_$ is withdrawn, and the process is used to establish $\_\_\_\_$ requirements and $\_\_\_\_\_\_$ configurations.