ANS:C - both (a) and (b)

In a packed absorption tower, if both the equilibrium and operating lines are straight lines, then the ratio 𝐻𝐸𝑇𝑃𝐻𝑇𝑈𝑂𝐺HTUOG​HETP​ is one at unity value of the absorption factor. Explanation:

1. Packed Absorption Tower:
   • In packed absorption towers, gas and liquid phases are brought into contact with each other to facilitate the absorption of a solute from the gas phase into the liquid phase.
   • Packing materials are used to increase the interfacial area between the gas and liquid phases, promoting mass transfer.
2. Equilibrium and Operating Lines:
   • The equilibrium line represents the relationship between the concentrations of the solute in the gas and liquid phases at equilibrium.
   • The operating line represents the relationship between the actual concentrations of the solute in the gas and liquid phases as they change along the height of the tower.
3. Straight Lines for Equilibrium and Operating Lines:
   • If both the equilibrium and operating lines are straight lines, it indicates that the absorption process follows ideal behavior under certain conditions.
   • Under ideal conditions, the ratio of Height Equivalent to a Theoretical Plate (HETP) to Height of a Theoretical Unit Operation (HTUOG) becomes one at unity value of the absorption factor.
   • The absorption factor is a measure of the efficiency of the absorption process and represents the ratio of the actual change in concentration of the solute to the maximum possible change in concentration.
   • When the absorption factor reaches unity (1), it signifies that the solute has been completely absorbed, and the ratio 𝐻𝐸𝑇𝑃𝐻𝑇𝑈𝑂𝐺HTUOG​HETP​ becomes one, indicating ideal performance.
4. Effect on the Absorption Factor:
   • The absorption factor increases with an increase in the ratio 𝐻𝐸𝑇𝑃𝐻𝑇𝑈𝑂𝐺HTUOG​HETP​ until it reaches unity.
   • Once the absorption factor reaches unity, the ratio 𝐻𝐸𝑇𝑃𝐻𝑇𝑈𝑂𝐺HTUOG​HETP​ becomes one, indicating that further increase in the absorption factor is not possible under ideal conditions.

In summary, if both the equilibrium and operating lines are straight lines in a packed absorption tower, the ratio 𝐻𝐸𝑇𝑃𝐻𝑇𝑈𝑂𝐺HTUOG​HETP​ is one at unity value of the absorption factor, indicating ideal performance. Therefore, the correct answer is: is one at unity value of the absorption factor.