ANS:C - both operating and equilibrium lines are parallel.

The absorption factor method is a technique used in the design and analysis of absorption columns in chemical engineering. It's utilized to determine the number of ideal stages required for a given separation process. Here's how it applies to different scenarios:

1. Operating line lies above the equilibrium line:
   • In this case, the absorption factor method is typically employed to calculate the number of ideal stages needed for the separation. This situation often occurs in absorption processes where the solvent (absorbent) is more volatile than the solute. The method helps to find the stages required to achieve the desired degree of separation.
2. Operating line lies below the equilibrium line:
   • Similar to the previous scenario, the absorption factor method is used to determine the number of ideal stages when the operating line is below the equilibrium line. This situation arises in some absorption processes where the solute is more volatile than the solvent. Again, the method assists in calculating the stages necessary for separation.
3. Both operating and equilibrium lines are parallel:
   • When both the operating and equilibrium lines are parallel, it indicates that the separation process is at its limiting condition. In such cases, the absorption factor method can still be applied to determine the number of ideal stages required for the separation, despite the parallelism of the lines.
4. High pressure drop in the column:
   • When there's a significant pressure drop in the column, it can affect the efficiency of the separation process. In this scenario, the absorption factor method can still be used to estimate the number of ideal stages needed, but additional considerations may be required to account for the pressure drop's impact on the column performance.

In summary, the absorption factor method is a versatile tool used to calculate the number of ideal stages in absorption processes, regardless of the relative positions of the operating and equilibrium lines or the presence of significant pressure drops in the column.