Packing pressure drop prediction at low operating pressure - Is there anything new?

Accurate prediction of pressure drop at very low operating pressure is of utmost importance. When the operating pressure is below 5 mbar, an error of a single mbar can make a significant difference. At such low operating pressures the gas Reynolds number is low and the column is operating in the laminar or transition regime. In this case, the friction factor can therefore not be seen as constant but is a function of the gas Reynolds number.

The presentation will show experimentally evaluated data determining the friction factor of gauze packing in the laminar and turbulent regime. From such data the critical Reynolds number can be deduced. Examples of industrial applications which were designed in the laminar regime will be shown, indicating typical gas Reynolds numbers.

Hydraulic rating programs which predict pressure drop will be compared and the assumed friction factors are evaluated. It was found that only Sulcol considers the dependency of friction factor on the Reynolds number. Comparison of the calculated pressure drop of packing which can be seen as equivalent, e.g. MellapakPlus 252.Y and Sulzer BXPlus, reveal that the values are indeed very similar for test mixtures (e.g. measured at 50 mbar), however, when comparing the outcome at low gas Reynolds numbers, the predicted pressure drop values can vary by a factor of 2 and more.

Conclusion: extrapolation of pressure drop correlations which were only based on test systems is highly erroneous when used in the laminar regime and when the friction factor is assumed constant. This will lead to a far too low predicted pressure drop.