Reducing product cross-contamination between campaigns requires attention to residual liquid in packing, distributors, collectors, feed piping, and low points. Retrofit should improve drainability and flushing while maintaining the hydraulic function required during normal production.

Unknown legacy tower-internal materials should be investigated through historical records, appropriate material verification, condition inspection, and new process-compatibility review. Critical components should not be reused based solely on visual assumptions, and the retrofit should create a reliable material record for the future.

When the bottom sump becomes the capacity bottleneck, retrofit should evaluate total liquid balance, outlet nozzle, control valve, downstream piping, working volume, solids accumulation, gas-inlet interaction, and clearance below the packing support. Higher-capacity packing cannot solve insufficient liquid removal from the tower bottom.

Strong gas swirl entering an existing packed tower can bias flow toward the wall and reduce use of the full cross-section. Retrofit should identify the upstream swirl source, evaluate inlet-plenum length, use low-resistance flow straightening where justified, and consider interaction with the packing support and wall region.

A high-velocity side inlet can create severe gas maldistribution even when average tower velocity is acceptable. Retrofit should consider nozzle velocity, jet direction, distance to the bed, low-resistance flow conditioning, drainage, structural jet load, and evidence of localized hydraulic stress.