Phosgene–hydrogen chloride separation in isocyanate production can use structured packing as part of a pressure-coupled recovery system. Modern BASF technology compresses the HCl-rich column overhead, partially condenses it at higher pressure, then depressurizes the liquid to create cold reflux at the distillation-column pressure. Structured packing helps provide the required rectification with relatively low pressure drop, but the packing, compressor, condenser and reflux inlet must be considered as one thermodynamic system. Startup can create a different hydraulic condition because an absorbing solvent may temporarily be introduced to control ascending phosgene before normal HCl reflux is established. Packing selection therefore requires process-stream composition, pressure profile, reflux phase condition and the surrounding overhead-system arrangement—not only tower diameter and packed height.

Maleic anhydride recovery absorbers can require different internals in different tower sections. A published organic-solvent recovery process uses structured packing or metal grids in two recirculating gas-cooling sections, where high liquid flow, heat removal, low pressure drop and short liquid residence time are important. The main and final absorption sections instead favor specially designed trays because local liquid loading becomes much lower. Temperature control is critical: excessive temperature reduces absorption and promotes maleic-to-fumaric-acid conversion, while excessive cooling can contribute to water condensation and fumaric-acid solid deposits. Structured packing should therefore be selected by local tower duty rather than specified across the entire absorber.

A cyclohexanone vacuum column can suffer bottom or reboiler fouling partly because pressure drop in the upper tray section raises the pressure and boiling temperature of the lower column. In a documented industrial retrofit, only the 15 trays above the feed were replaced with 350Y structured packing. At an 80 mbar top pressure, the reported bottom pressure fell from about 205 to 135 mbar while the required upper-column separation was maintained, helping reduce bottom temperature and fouling. The key retrofit question is therefore not whether structured packing is generally better than trays, but which column section is consuming the vacuum pressure budget and whether replacing that section can reduce bottom thermal stress without sacrificing separation.

Furfural purification must remove light impurities such as water, methanol, acetone and acetic acid while also separating heavier compounds such as 2-acetylfuran and 5-methylfurfural. Furfural and water form a heterogeneous azeotrope, allowing condensed overhead to split into organic and aqueous phases, while vacuum operation reduces the high temperatures that can promote furfural resinization. A recent industry-oriented pilot process used BX500 wire-mesh structured packing in the upper sections and 252Y corrugated-sheet packing below, achieving furfural purity above 99.5% at about 5 kPa condenser pressure. Packing selection should therefore balance high-stage efficiency, low pressure drop and section-specific resistance to heavy/resin-forming material.

Hydrogen cyanide purification can use structured packing to provide efficient HCN-water separation with relatively low pressure drop. The application is unusual because conventional reflux can concentrate higher nitriles such as acetonitrile, acrylonitrile and propionitrile inside the tower. At sufficiently high concentration, two liquid phases may form, creating polymerization and foaming risks that can disturb structured-packing hydraulics. Packing selection should therefore balance theoretical-stage efficiency with channel openness, liquid distribution, reflux strategy, pressure-drop stability and the plant's actual higher-nitrile impurity profile.