ANS:C - is characterised by the presence of foam throughout the space between trays.

Priming in a distillation column is characterized by the presence of foam throughout the space between trays. Priming occurs when liquid entrainment from one tray is carried upwards to higher trays in the column due to high vapor velocities or other factors. This liquid entrainment creates foam, which fills the space between trays and can interfere with the proper operation of the column. The other statements are not accurate:

• Priming typically results from very high gas velocity rather than very low gas velocity. When the gas velocity is too high, it can entrain liquid from the trays, leading to priming.
• Priming is generally undesirable from an efficiency standpoint, as it can cause maldistribution of liquid and vapor throughout the column, leading to reduced separation efficiency and poor column performance.
• Priming does not necessarily reduce the overall pressure drop. In fact, it can increase pressure drop by interfering with proper vapor and liquid flow distribution in the column, leading to inefficiencies and increased resistance to flow.