On packed column hydraulics



Packed columns are normally operated countercurrently in the vapor-continuous regime. At specific combinations of liquid and vapor loads these columns flood. This article proposes that flooding is a form of second order (or “continuous”) phase transition from vapor-continuous to liquid-continuous operation. Thus, flooding is unambiguously defined to be the set of those vapor/liquid flow combinations, {CL, CSf}, that cause a liquid cluster to form that spans the diameter of column. These statements imply that a law of corresponding hydraulic states exists for packed columns. By this, we mean that sets of {CL, CS, (Δp/Z)} data taken with a specific packing and a specific vapor/liquid system exhibit a significant collapse when they are renormalized to {fL, (Δp/ρLgZ)} (where fL is the fractional approach to flood at constant liquid load). The renormalized dataset then applies to any vapor/liquid system using that particular packing. Renormalization thus forms the basis of a method for predicting column pressure drops and flood points for any column using the particular packing being studied. We demonstrate how the renormalization procedure is carried out by analyzing readily available air/water data for a number of different packings. We then show that a version of the Wallis equation can be used to correlate packed column flooding data successfully. Further, we demonstrate that the variation of the Wallis parameters with the equivalent diameters of packings in a geometrically similar family leads to a complete characterization of the effects of the physical properties of liquid on the flooding locus for these packings through the Bond number. This last result is a direct result of generalized homogeneity considerations. Finally, we show that there exists an even more general formulation of the law of corresponding hydraulic states that applies to all packings regardless of type, size, or material of construction. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1671–1682, 2012