Engineering task and calculation objective
This module calculates the pressure drop and flood point in irrigated packed beds according to Section L2.6 of the VDI-Wärmeatlas (VDI Heat Atlas, 12th edition, 2019). In random and structured packing columns for rectification, absorption and desorption, gas and liquid flow countercurrently through the packing. The pressure drop of the irrigated bed and the location of the flood point are the central hydraulic design quantities: they determine the permissible gas throughput and thus the column diameter.
Compared with the dry bed, the liquid film (holdup) narrows the free cross-section, so the pressure drop grows with increasing liquid load. Above the loading point, the gas flow increasingly holds up liquid in the packing; at the flood point, finally, the liquid is no longer transported downward, the column floods and mass transfer breaks down. Anyone who wants to calculate the pressure drop of a packed column must therefore always also report the margin between the operating point and the flood point.
The VDI Heat Atlas method describes these relationships via dimensionless gas and liquid load parameters and packing-specific characteristics, and applies to random packings as well as to structured packings, whose characteristic data (specific surface area, void fraction, resistance constants) come from the manufacturer or from data collections.
Standard and calculation basis: VDI-Wärmeatlas, 12. Auflage 2019
Calculation workflow
- Define packing and material system: The packing type with its specific surface area, void fraction and packing-specific constants, as well as the properties of both phases (densities, viscosities, surface tension where applicable) at operating conditions, are entered.
- Define the loads: The gas load (e.g. as the F-factor, the product of superficial velocity and the square root of the gas density) and the liquid load define the operating point in the loading diagram of the column.
- Calculate the dry pressure drop: First, the pressure drop of the non-irrigated packing is determined from the drag coefficient, the void fraction and the dynamic pressure of the gas flow – it is the reference quantity for the irrigated state.
- Determine liquid holdup and irrigated pressure drop: The liquid content of the packing narrows the free cross-section and increases the effective gas velocity; the pressure drop of the irrigated bed follows from the dry pressure drop with a holdup-dependent amplification factor, which grows strongly toward the loading point.
- Report the flood point and operating margin: The flood point is determined from the condition that the downward transport of the liquid just breaks down; the operating point is usually placed at a defined margin from it (typically 70 to 80 % of the flooding gas load), which at the same time fixes the column diameter.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Inlet temperature | TE | °C |
| Outlet temperature | TA | °C |
| Mean temperature | T | °C |
| Mean gas density | ρg | kg/m³ |
| Mean liquid density | ρfl | kg/m³ |
| Dynamic gas viscosity | ηg | mPa·s |
| Dynamic liquid viscosity | ηfl | mPa·s |
| Kinematic gas viscosity | νg | m²/s |
| Kinematic liquid viscosity | νfl | m²/s |
| Surface tension liquid | σfl | N/m |
| Packing porosity | ψ | - |
| Packing height | H | m |
| Superficial velocity | uG | m/s |
| Superficial velocity (flood point) | uGFl | m/s |
| Liquid flow rate | uL | m/s |
| Particle diameter of packing | dP | m |
| Spec. geometric surface | at | 1/m |
| Reynolds-number (operating point) | ReG | - |
| Reynolds-number (flood point) | ReGFl | - |
| Flood point | Δpt,Flut | - |
| Lower loading point | Δpt,Stau | - |
| Flood factor | Flutfaktor | - |
| Dimensionless liquid flow rate | B | - |
| Liquid phase holdup (flood point) | ⇒ hLFl | - |
Frequently asked questions
How do you recognize flooding of a packed column?
By the steep rise of the pressure drop at nearly unchanged gas load, by liquid accumulating above the packing and by strongly fluctuating differential pressures. In the loading diagram, the pressure drop curve bends sharply upward above the loading point; the flood point is its asymptotic end point. Operation close to the flood point is unstable and leads to a collapse of the separation performance.
Why is the operating point placed at about 70 to 80 % of the flooding load?
Below this margin the hydraulics remain stable against fluctuations in throughput, pressure and foaming tendency, while at the same time the packing is well wetted and mass transfer is effective. An overly conservative design (well below 50 %) increases the column diameter unnecessarily and, at too low a liquid load, can even worsen the wetting.
Do the correlations apply equally to random and structured packings?
The model structure is the same, but the packing-specific characteristics (specific surface area, void fraction, resistance and holdup constants) differ considerably. Structured packings achieve significantly higher loads at the same pressure drop. It is essential that the constants come from measurements on the actual packing type – transferring them between types is a common source of error.
What influence do foaming systems have on the flood point?
Foam formation binds additional liquid in the packing and lowers the practically usable flood point considerably – the purely hydraulically calculated limit is then not reached. For foaming systems (e.g. amine scrubbing), derating factors are applied in practice to the permissible gas load; the correlations themselves do not include this effect.