Evaluation of ignition mechanisms for aluminium trays and packing in the low-pressure column of air separation units

Experimental work has shown that trays and packing fabricated from aluminium are flammable under certain conditions normally encountered in oxygen distillation columns. Therefore, the safety of trays and packing depends entirely on the exclusion of ignition mechanisms.

This paper presents an evaluation of ignition mechanisms with respect to their relevance to the adiabatic section of the column. The combustion of hydrocarbons has been identified as the only realistic ignition mechanism. Therefore, most of the experiments described in the paper refer to this mechanism. It is shown that the combustion of an oil film on packing (approximately 5 g m⁻²) is not able to ignite aluminium packing, but that accumulated oil (a few grams) is able to ignite aluminium trays and packing as well as copper packing. This result stresses the importance of controlling any hydrocarbon accumulation.

A short review of accumulation mechanisms for lubrication oil from fabrication and hydrocarbons from air shows that accumulation within the adiabatic section of the distillation column is practically impossible.

Finally, it is concluded that no ignition sources for aluminium trays and packing are present in the low-pressure column of oxygen distillation columns. This explains the very good safety record of aluminium trays for more than 30 years of industrial experience characterized by no single case of tray combustion. Aluminium packing, now in operation for more than five years, can be expected to be as safe as trays.