1. To maximize the specific surface area, i.e., the surface area per unit volume: This maximizes vapor-liquid contact area, and therefore, efficiency (see Fig. 8.126 and Sec. 8.1.10). A corollary is that for random packings, efficiency generally increases as the particle size is decreased (Fig. 8.7); for structured packings, efficiency generally
'Nomenclature lists and references for Chap. 8 appear at the end of Chap. d.
increases as the space between adjacent layers is decreased, and for grid, efficiency generally increases as the lattice openings are narrowed.
2. To spread the surface area uniformly: This improves vapor-liquid contact, and therefore, efficiency. For instance, a Raschig ring (Fig. 8.1 o) and a Pall® ring (Fig. 8.2d) of an identical size have identical surface areas per unit volume, but the Pall® ring has a superior spread of surface area and is therefore much more efficient.
3. To promote uniform distribution of vapor and liquid throughout the packed bed: Uniform distribution improves packing efficiency. For instance, random packing particles that "interlock" with, or "nest" inside other particles can lead to channeling and therefore to lower efficiency.
4. To freely drain any liquid, so that stagnant liquid pockets are minimized: Stagnant liquid contributes little to mass transfer and wastes packing surface,
5. To maximize wetting of packing surfaces: Dewetting of packing surfaces at low liquid rates reduces efficiency and restricts turndown. Although the wetting characteristics are primarily a function of the packing material, the size and geometry of the packing are also important.
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