Structured packing has been applied to the cryogenic distillation of air. The specific surface areas of packing are higher than that of conventional sieve trays. The lubrication oils are used for manufacturing of the packing and some oils remain on the packing surface, which results in introducing a larger amount of the total remained oil in the structured packed columns than that in the sieve trays. In the oxygen enriching section of the low pressure column of the air separation unit, the accumulation of the oil should be avoided for safety. High level local accumulation may be caused by the migration which is due to evaporation and recondensation of the oil during the defrosting operation, which occurs in start-up and shut-down procedures of the air separation unit. An allowable oil film limit of 50–100mg/m² is recommended by CGA. It is confirmed that oil migration by gravity and cold chipping does not occur at this oil level. However, there is no experimental or theoretical study of the migration mechanism by evaporation and recondensation.

The purpose of this study is to determine whether the evaporation and recondensation causes a high accumulation level in the structured packed column under the initial condition of 100mg/m² oil level. The correlation on mass transfer for evaporation and recondensation of oil (Shell Talpa 30) was developed by the laboratory-size experiments. By using the mass transfer correlation, the behavior of the oil migration was simulated for the low pressure column. The simulation results showed that the accumulation of the lubrication oil (Shell Talpa 30) was less than 1 mg/m² during the defrosting operation, hence an unfavorable oil accumulation did not occur under the initial condition of 100mg/m² oil level. On the other hand, it was found that the accumulation of hexadecane (light oil) was higher than that of Shell Talpa 30 (heavy oil) at a same time within the defrosting operation.