Intermittently operated packed towers can develop dry-out, crystallization, poor restart wetting, stagnant corrosion, thermal cycling, and drainage problems that are not obvious from steady-state design data. Shutdown and restart conditions should therefore be included in packing and internal-system design.

Tower-packing material selection should include CIP and cleaning conditions as well as normal process chemistry. Strong acids, alkalis, oxidizers, solvents, hot water, or steam may create more severe material exposure than normal operation and can affect packing, distributors, supports, demisters, fasteners, and gaskets.

Exothermic absorption can create an internal temperature peak inside a packed bed even when gas and liquid inlet temperatures appear acceptable. Reaction heat, liquid circulation, distribution quality, contaminant loading, and upset conditions should be considered when selecting temperature-sensitive packing materials.

Different packing types can be used in separate beds when tower conditions vary significantly by elevation. Each bed should be evaluated for hydraulics, mass transfer, support compatibility, material, pressure drop, and liquid redistribution. Clearly separated engineered sections are preferable to uncontrolled mixing of different packings.

When a project specifies a nonstandard structured-packing corrugation angle, determine whether the requirement is exact geometric duplication or equivalent hydraulic and mass-transfer performance. Standard alternatives may be acceptable, but changes in corrugation geometry should be clearly evaluated and documented.