ANS:B - decreases

In batch distillation with constant reflux, the overhead composition of the light component typically decreases with the lapse of time. Here's why:

1. Initial Composition: Initially, during the early stages of distillation, the overhead vapor composition is richer in the light component because the distillate collected primarily contains the more volatile components.
2. Reflux Effect: As distillation progresses and reflux continues to circulate within the system, the reflux ratio remains constant. This means that a portion of the condensed vapor (reflux) is returned to the column, enriching the liquid in the column with the more volatile components.
3. Re-equilibration: Over time, the continuous recycling of the reflux causes the vapor and liquid phases in the column to re-equilibrate. This results in a decrease in the concentration of the light component in the overhead vapor, as more of it is condensed and returned to the column.
4. Final Composition: As the distillation proceeds, the overhead composition approaches a steady state where it contains lower concentrations of the light component compared to the initial stages of distillation. This is because the reflux ratio and system conditions remain constant, leading to continuous re-equilibration of the vapor and liquid phases.