Packed Column Hydraulics: Flooding & Pressure Drop

The Hydraulic Limit of Mass Transfer

In the previous article (HTU/NTU), we calculated the height of the packed column to satisfy the thermodynamic mass transfer requirements. But how do we calculate the diameter of the column?

The diameter is dictated entirely by fluid hydraulics and the threat of Flooding.

Counter-Current Flow Dynamics

In a typical gas scrubber, liquid solvent rains down from the top via gravity, while dirty gas is blown upward from the bottom.

The upward rushing gas exerts an aerodynamic drag force (friction) on the downward falling liquid. As the gas velocity increases, it pushes harder and harder against the liquid, slowing the liquid's descent. This causes liquid to accumulate (holdup) inside the packing voids, increasing the pressure drop ($\Delta P$) across the bed.

The Flooding Point

If the gas velocity is increased too much, the upward aerodynamic drag force perfectly equals the downward force of gravity on the liquid.

At this exact moment, the liquid physically cannot fall. It stops mid-air. Liquid rapidly accumulates, filling the entire column. The column is Flooded. Gas pressure spikes catastrophically, liquid is violently blown out the top of the vent stack, and mass transfer completely ceases.

The Generalized Pressure Drop Correlation (GPDC)

To prevent flooding, engineers size the diameter of the column so that the gas velocity remains safely below the flooding velocity. The industry standard tool for this is the GPDC chart (often called the Leva or Eckert chart).

The chart plots the Capacity Parameter against the Flow Parameter ($F_{lv}$):

$$ F_{lv} = \frac{L}{G} \sqrt{\frac{\rho_G}{\rho_L}} $$

Where $L$ and $G$ are the liquid and gas mass flow rates.

Engineers typically design the column diameter so that the gas velocity operates at 70% to 80% of the Flooding Velocity. At this velocity, the pressure drop across the packing is typically around $0.25$ to $0.50$ inches of water per foot of packing ($200$ to $400 \ Pa/m$).

If you design the column too wide (gas velocity at 30% of flooding), the gas barely touches the liquid, channeling occurs, and mass transfer efficiency ($K_y$) collapses!