ANS:A - the ratio of number of ideal plates to actual plates.

The overall efficiency of a distillation column is defined as the ratio of the number of actual plates to the number of ideal plates required for the same separation. Therefore, the correct statement is: "The overall efficiency of the distillation column is the ratio of the number of actual plates to ideal plates." Let's briefly explain this concept:

1. Overall Efficiency:
   • The overall efficiency of a distillation column measures how effectively the column performs compared to an ideal theoretical column with the same separation objectives.
   • It takes into account factors such as tray inefficiencies, mixing of vapor and liquid phases, and non-ideal behavior of the components being separated.
   • Mathematically, the overall efficiency (η_overall) is calculated as the ratio of the number of actual plates (N_actual) to the number of ideal plates (N_ideal) required for the same separation: 𝜂overall=𝑁actual𝑁idealηoverall​=Nideal​Nactual​​
2. Comparison with Other Efficiencies:
   • The Murphree efficiency is a measure of the efficiency of individual trays within the column, whereas the overall efficiency considers the entire column.
   • The point efficiency refers to the efficiency of a single theoretical stage in the column.
   • The overall efficiency is always less than or equal to the point efficiency because it accounts for inefficiencies in the entire column, whereas the point efficiency considers only a single stage.

In summary, the overall efficiency of a distillation column provides an indication of how well the column performs compared to an ideal separation. It is calculated as the ratio of the number of actual plates to the number of ideal plates required for the same separation.