ANS:D - results due to very low gas velocity.

When the gas velocity in a distillation column is very low, it means that the upward flow of gas through the column is not vigorous enough to adequately interact with the liquid present on the trays. As a result:

1. Insufficient Entrainment: The low gas velocity fails to effectively entrain the liquid present on the trays. This means that the liquid accumulates rather than being carried upward by the gas flow.
2. Liquid Accumulation: Without proper entrainment, the liquid tends to accumulate on the trays or downcomers, forming large pools of liquid.
3. Weeping: This accumulation of liquid leads to a condition known as "weeping," where the liquid flows down the column along the tray surface or downcomers without proper vapor-liquid contact.
4. Reduced Mass Transfer: Weeping reduces the effective interfacial area available for mass transfer between the vapor and liquid phases. As a result, the efficiency of the distillation process decreases, and separation performance may suffer.