A hybrid column intentionally uses trays and structured packing in different sections of the same tower. Trays may be retained where side draws, liquid inventory, fouling tolerance or specific process functions are important, while structured packing can be installed where lower pressure drop, higher capacity or greater separation efficiency creates more value. Successful hybrid designs require careful transition internals and section-by-section hydraulic checks so that the remaining trays do not become the new column bottleneck.

Structured packing can be used in formaldehyde absorbers, but formaldehyde absorption combines multicomponent mass transfer, liquid-phase reactions and significant heat release. Packed sections often operate with external circulation, cooling and redistribution, while concentrated formalin can create polymerization or deposition risks under unsuitable conditions. Packing selection should therefore consider section-by-section temperature, formaldehyde concentration, liquid distribution, cooling capacity and fouling tendency rather than maximizing specific surface area alone.

Structured packing can be used in Rectisol and Selexol physical-solvent acid-gas absorbers, particularly where high pressure, large gas throughput and low hydraulic resistance are important. Unlike amine systems, physical solvents remove CO₂ and H₂S mainly by dissolution, making gas partial pressure, solvent temperature and circulation rate critical to packing performance. Rectisol service also requires attention to low-temperature materials. Packing, liquid distribution and section-by-section hydraulics should be evaluated against the approved licensor or EPC process design rather than selected from a generic structured packing model alone.

Structured packing can be used in nitric acid absorption towers, but NOx absorption is a reactive and exothermic process rather than a simple gas-scrubbing duty. Packing selection should consider NOx composition, oxidation, absorber pressure, acid concentration, temperature control, cooling, liquid distribution and material compatibility. A structured packing retrofit is most useful when the actual limitation is gas-liquid contacting or hydraulic performance; it cannot correct inadequate cooling or unfavorable process chemistry by itself.

Structured packing can be used in liquid-liquid extraction columns, but the design differs fundamentally from vapor-liquid distillation or absorption. One liquid normally forms the continuous phase while the other moves as dispersed droplets, making droplet size, phase holdup, density difference, interfacial tension, axial back-mixing and phase distribution key design parameters. Standard distillation packing should not automatically be substituted for specialized extraction packing without evaluating the actual two-liquid system.