Pyrophoric iron sulphides formation in Oil and Gas

A pyrophoric material is a liquid or solid that, even in small quantities and without an external ignition source, can ignite within 5 minutes after coming in contact with air.

Pyrophoric iron sulphides form when iron is exposed to hydrogen sulphide, or any other compound that contains sulphur, in an oxygen deficient atmosphere. They are found frequently in vessels, storage tanks, and sour gas pipelines. Pyrophoric iron sulphides present a hazard when equipment and tanks are opened for cleaning, inspection, and maintenance. As the iron-sulphide compounds dry out and come in contact with air, they react with the oxygen and spontaneously ignite.

In oil and petrochemical industry, this only partially defines the concern.
We also need to be concerned with the fact the pyrophoric material can
create heat which can ignite residual hydrocarbons associated with the
equipment containing the pyrophoric material. 

  • Pyrophoric compound + oxygen (typically air) Oxide of the compound + heat
  • Sometimes with several intermediate reaction steps
  • Can be very reactive or very slow to react
  • Can vary with conditions, humidity, temperature, particle size, degree of
  • disbursement in air, etc. 

Pyrophoric form mostly on

  • Crude oil tanks
  • Asphalt tanks
  • Sour water tanks
  • Vessels in sour service such as coke drums, distillation columns, inlet
  • separators, pig receiver / launchers
  • Reactors
  • API Separators
  • Marine tankers and barges
  • Portable tanks and tote bins 

General Precautions to Avoid Pyrophoric Iron Fires

1.    The scraps and debris collected from cleaning of filters in naphtha / crude service must be kept wet and disposed of underground. 

2.    Tanks, reactors, columns, and exchangers in high-sulfur feed service must be kept properly blanketed with N2 during idle periods. 

3.    All equipment and structured packing must be properly water washed and kept wet when exposed to the atmosphere. 

4.    In processes where catalyst handling is required (such as in Hydrotreating and fluid catalytic cracking) caution must be taken during catalyst recharge or disposal. When unloading any spent coked catalyst, the possibility exists for iron sulfide fires. If the spent catalyst is warm and contacts oxygen, iron sulfide will ignite spontaneously and the ensuing reaction may generate enough heat to ignite carbon deposited on the catalyst. Therefore catalyst must be stripped of all hydrocarbons, cooled to about 50 o C and wetted with water to prevent it from igniting vapors. Once cooled, the used catalyst may be emptied into drums for later shipment to a regenerator or a disposal site. As the catalyst may be highly pyrophoric (containing iron sulfide, etc.), it should be dumped into drums containing an internal liner for shipment. The drum and liner should first be filled with inert gas, which is then displaced by the catalyst. The liner should be tied off and a small chunk of dry ice placed inside the drum before sealing. These precautions should protect against catalyst auto ignition.”

In Shell refinery in the 1960s where the flare line had been opened for maintenance.  Several staff were standing directly downstream of the line and suddenly there was a huge ball of flame that shot out of the flare line and caused several extremely severe, disfiguring burns (including, ironically, the side of the body and face of the fire and safety department chief).  The cause of the flame was was determined to be pyrophoric iron, burning spontaneously on contact with air, which set off some hydrocarbon liquids still in the line.

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