PYROPHORIC IRON FIRES

At one time or another, most refineries experience spontaneous ignition of iron sulfide Either on the ground or inside equipment. When this occurs inside equipment like Columns, vessels, and tanks and exchangers containing residual hydrocarbons and air, the results can be devastating. Most commonly, pyrophoric iron fires occur during shutdowns. when equipment and piping are opened for or maintenance.

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Instances of fires in crude columns during turnarounds, explosions in sulfur, crude or asphalt storage tanks, overpressures in vessels, etc., due to pyrophoric iron ignition are not uncommon. Often the cause of such accidents is a lack of understanding of the phenomenon of pyrophoric iron fires. This article aims to explain the basics of pyrophoric iron fires.

What is Pyrophoric Iron Oxidation?

The word "pyrophoric" is derived from the Greek for "fire-bearing". According to Webster's dictionary, "pyrophoric material" means "any material igniting spontaneously or burning spontaneously in air when rubbed, scratched, or struck. Iron sulfide is one such pyrophoric material that oxidizes exothermically when exposed to air. It is frequently found in solid iron sulfide scales in refinery units. It is formed by the conversion of iron oxide (rust) into iron sulfide in an oxygen-free atmosphere where hydrogen sulfide gas is present (or where the concentration of hydrogen sulfide (H2S) exceeds that of oxygen). The individual crystals of pyrophoric iron sulfides are extremely finely divided, the result of which is that they have an enormous surface area-to-volume ratio. When the iron sulfide crystal is subsequently exposed to air, it is oxidized back to iron oxide and either free sulfur or sulfur dioxide gas is formed. This reaction between iron sulfide and oxygen is accompanied by the generation of a considerable amount of heat. In fact, so much heat is released that individual particles of iron sulfide become incandescent.

This rapid exothermic oxidation with incandescence is known as pyrophoric oxidation and it can ignite nearby flammable hydrocarbon-air mixtures.

Basic chemical reactions:

Iron sulfide is one of the most common substances found in

refinery distillation columns, pressure vessels, etc. It is formed by the reaction of rust

or corrosion deposits with hydrogen sulfide as shown below:

Fe2O3(rust)+3H2S→2FeS+3H2O+S

There is a greater likelihood of this reaction occurring when the process involves a

feedstock with high sulfur content. This pyrophoric iron sulfide (PIS) lays dormant in

the equipment until the equipment is shut down and opened for service, exposing the

PIS to air, allowing the exothermic process of rapid oxidation of the sulfides to oxides

to occur, as shown in the equations below:

4FeS+3O2→2Fe2O3+4S+heat

4FeS+7O2→2Fe2O3+4SO2+heat

The heat usually dissipates quickly unless there is an additional source of combustible material to sustain combustion. The white smoke of SO2 gas, commonly associated with pyrophoric fires, is often mistaken for steam.

Pyrophoric iron oxidation in Distillation Columns

In petroleum refineries, the equipment most prone to pyrophoric combustion induced fires is the distillation columns in crude and vacuum distillation units. Deposits of iron sulfide are formed from corrosion products that most readily accumulate at the trays, pump around zones, and structured packing. If these pyrophoric iron sulfide (PIS) deposits are not removed properly before the columns are opened up, there is a greater likelihood of PIS spontaneous ignition. The trapped combustible hydrocarbons, coke, etc. that do not get adequately removed during washing steaming often get ignited, leading to fires and explosions inside the equipment. These fires not only result in equipment damage but can also prove fatal for the personnel who are performing inspection and maintenance work inside the columns. The accidents due to pyrophoric iron oxidations are entirely avoidable if safe

procedures for column handover are followed. The targets of these procedures should be twofold:

• First, to remove all the combustibles

• Second, to remove or neutralize pyrophoric iron sulfide deposits

The basic distillation column oil-cleanup procedure is discussed in steps below.

1. Steaming

2. Hot Water Washing

3. Blinding

4. Cold Water Washing:

5. Chemical Injection for Removal and Neutralization of PIS Deposits