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رحیم بهوندپور HSE 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, e.g. finely divided metals". 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 makes no difference whether these pyrophoric sulfides exist as pyrite, troilite, marcasite, or pyrrhotite. 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: 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 shutdown 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 è 2Fe2O3 + 4S +heat 4FeS + 7O2 è 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. References "Pyrophoric Materials Handbook, Flammable Metals and Materials", By Charles R. Schmitt, P.E., C.H.C.M., Edited By Jeff Schmitt “Basic Technology of Zyme-Flow Process”, Bevan Collins, International Technical Director, United Laboratories, LLC , www.zymeflow.com |
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رحیم بهوندپور کارشناس HSE و دانشجوی کارشناسی ارشد HSE-MS علوم و تحقیقات تهران و شاغل در شرکت ملی نفت ایران -
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