Achieve Good Packed Tower Efficiency

Proper design of liquid distributors plays a key role in getting the most from packings.

By Frank Rukovena and Tony Cai, Fractionation Research, Inc.

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Uniform liquid distribution is key to obtaining expected performance from a packed bed. The distributor needs to uniformly allocate the liquid for all anticipated flow rates, with an adequate number of pour points for the size of the packing and sufficient open area for vapor passage. Both liquid orifice point-to-point flow uniformity and uniformity of the orifice drip-point pattern across the superficial tower area are crucial to obtaining good packing performance.

Liquid maldistribution affects packing performance in two ways. The local variations, orifice to orifice, in terms of the L/V ratios may cause the compositional pinch shown in Figure 1. And liquid maldistribution over a large section of the tower cross-sectional area may prompt the liquid to flow unevenly through the packing, concentrating at the wall.

Figure 3 -- Another trough arm distributor: This Raschig DTS trough arm distributor features side orifices with discharge tubes.

When a liquid distributor is operated at rates below its designed turndown ratio, it won’t provide uniform flow point-to-point and across the tower cross-sectional area because of low liquid head above the orifices. As the flow uniformity diminishes the separation deteriorates.

Weir flow and notched trough distributors generally give poor liquid distribution. They also are susceptible to creating liquid entrainment — because they discharge liquid into the vapor riser area where vapor velocity typically is three times higher than the superficial tower vapor velocity even when the packed bed is running at reasonable rates — that can lower separation efficiency by liquid back-mixing. However, this type of distributor may be a good choice if plugging and fouling are a serious problem.

Figure 4 -- Pan distributor: Bottom view shows details of FRI TDP pan distributor.

The spray nozzle distributor poses a similar entrainment problem because it breaks the liquid up into small droplets that can be easily carried back up the tower. But, in this case, no restrictive vapor passage exasperates the entrainment potential.

Key criteria
A well designed liquid distributor should offer:
 • uniformity of flow;
 • appropriate pour-point density;
 • proper irrigation along the column wall area;
 • sufficient open area for vapor flow; and
 • entrainment prevention.

The degree of the uniformity necessary depends upon the service. For example, applications for mass transfer or heat transfer with a close approach to equilibrium require higher flow uniformity than those for general heat transfer purposes. When flow variation is measured by the standard deviation of liquid flow through the pour points, the ratio of the standard deviation to mean liquid flow rates, Cv, should be limited to ± 5% [1].

Adequate pour-point density is important to achieve the optimum efficiency for any packing. A pour point density as low as 3 points/ft2 may yield satisfactory results for some packings [2] (Figures 6 and 7). Packings with high surface area may require a greater pour-point density (>10 points/ft2). We suggest using 6–9 points/ft2 for metal Pall rings.

Figure 6 -- Structured packing: Pour point density causes significant differences in packing efficiency, according to FRI data.

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