ANS:D - liquid hold up

The minimum possible diameter of a packed column is mainly decided by flooding. Here's why:

• Flooding: Flooding is a condition in packed columns where the gas velocity becomes so high that it overcomes the liquid's ability to flow downward through the packed bed. This results in the formation of liquid channels, which reduce the effective contact between the gas and liquid phases, leading to decreased mass transfer efficiency.
• Effect on Column Diameter: To prevent flooding, the diameter of the packed column must be large enough to accommodate the required gas and liquid flow rates without exceeding the flooding velocity. If the column diameter is too small, the gas velocity required for the desired mass transfer may exceed the flooding velocity, leading to flooding.
• Gas Viscosity, Liquid Density, and Liquid Holdup: While gas viscosity, liquid density, and liquid holdup can influence the overall performance of the packed column, they do not directly determine the minimum possible diameter. These factors may affect the operating conditions and design considerations of the column, but the critical factor in determining the minimum diameter to prevent flooding is the flooding velocity.