1. To maximize the void space per unit column volume: This minimizes resistance to vapor upflow, and therefore, enhances packing capacity. A corollary is that for random packings, capacity increases with particle size (Fig. 8.7); for structured packings, capacity increases with the space between adjacent layers, and for grids, capacity increases as the lattice openings are widened. Comparing to the first objective for maximizing efficiency, this corollary states that the packing size that maximizes capacity also minimizes efficiency. A trade-off therefore exists; the ideal size of packing is a compromise between maximizing efficiency and maximizing capacity.
2. To minimize friction: This favors an open shape that has good aerodynamic characteristics. For instance, the Pall® ring (Fig. 8.2d) is far more open to gas flow compared to the Raschig ring (Fig. 8.1a), and therefore, has greater capacity.
3. To ensure uniform resistance to vapor and liquid flow throughout the bed: Concentrated pockets of aerodynamic resistance can lower the effective tower cross-section area, thus reducing capacity.
4. To permit easy disengagement of vapor from liquid: This is impor tant in high-pressure services and high-liquid-flow-rate services. Vapor disengagement is difficult when the opening available to liquid downflow is narrow (e.g., as in narrow-channel structured packings).
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