1. To maximize resistance to mechanical deformation and/or breakage and, especially, to deformation under the weight of the bed: For instance, the partition in the center of the Lessing ring (Fig. 8.16) gives it a superior resistance to deformation and breakage than that of the Raschig ring (Fig. 8. la).
2. To minimize cost: Packing cost, as well as the requirements for packing supports and column foundations, generally rise with the weight per unit volume of packing. A corollary is that packings become cheaper as the particle size increases (random packing), as the space between layers increases (structured packing), or as the lattice openings widen (grids).
3. To maximize fouling resistance: Packings become more fouling resistant as the particle size increases (random packing), or the space between layers increases (structured packing) or the lattice opening widens (grids). Geometric shapes that resist trapping of sediment or polymer are advantageous.
4. To minimize liquid holdup (where degradation or polymerization is encountered): The more liquid is held at high temperatures, the more it degrades and polymerizes.
5. To minimize deterioration in service: Packing geometry and size affect the sensitivity of a packing to corrosion, erosion, chemical attack, and migration through the support grid openings.
6. To minimize damage during abnormal operation: Packing geometry and size affect the ability of a packed bed to weather pressure surges or to catch fire at shutdown (when containing adhered pyrophoric material or coated with hot flammable liquid).
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