In a packed tower, the value of HETP equals HTU_OG, when the equilibrium and the operating lines are (where, HETP = height equivalant to a theoretical plate HTU_OG = overall gas phase height of a transfer unit)

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

(a) Straight Equilibrium and Operating Lines:

• When the equilibrium and operating lines are straight, it indicates that the system follows ideal behavior, such as Henry's law for gas-liquid equilibrium.
• In a packed tower, straight equilibrium lines imply that the vapor and liquid phases are in equilibrium at each stage of the column. This simplifies the analysis and calculation of mass transfer.
• Straight operating lines indicate that the driving force for mass transfer remains constant along the height of the column. This simplifies the determination of the mass transfer driving force and facilitates the calculation of height equivalent to a theoretical plate (HETP) and overall gas phase height of a transfer unit (HTUOG).

(b) Parallel Equilibrium and Operating Lines:

• When the equilibrium and operating lines are parallel, it indicates that the system exhibits constant relative volatility or separation factor throughout the column.
• In a packed tower, parallel equilibrium and operating lines imply that the mass transfer driving force remains constant along the height of the column, similar to the case with straight lines.
• Parallel lines also indicate that the efficiency of mass transfer in the gas phase (HTUOG) is equal to the efficiency of packing (HETP). This simplifies the analysis by allowing HETP and HTUOG to be directly compared and provides insights into the column's performance.

In summary, both options (a) and (b) relate to conditions that simplify the analysis and calculation of mass transfer in a packed tower. Straight lines indicate ideal behavior, while parallel lines indicate constant driving forces and efficiencies throughout the column.