[ Petrochemical ]
[ Petrochemical ]
1 . Catalytic cracking- Large hydrocarbon molecules are broken up or ‘ cracked ’ into smaller , more valuable hydrocarbon molecules . The cracking process takes place in the presence of a catalyst that promotes the breaking of large molecules in such a way as to maximise yield . Catalytic cracking produces a full range of hydrocarbons , from methane to residue .
2 . Fractional distillation – The function of fractional distillation is to refine and separate ( as well as join or split ) the various lengths of hydrocarbon chains to create various petroleum products from different distillates .
The efficient functioning of equipment in the refinery is closely dependent on its design , correct material specification , and timely identification of potential trouble areas . Vital equipment prone to mild , medium , and severe corrosion includes vessels , tanks , pumps , pipeline fittings , heat exchangers , compressors , and fired heaters .
Corrosion at the processing stages Various types of corrosion mechanisms may occur to equipment in the various processing stages . These include :
• Cracking ,
• Corrosion fatigue ,
• Low / high-temperature corrosion ,
• Hydrogen flaking ,
• Corrosion under insulation ,
• Metal dusting ,
• Carburisation ,
• Graphitisation .
The extent of damage can lead to catastrophic failures in oil refineries . Key design factors to consider when designing equipment include the geometry of the components , and the fluid flow and velocity in the refinery , which are controlled to prevent the accumulation of sour water and other corrosive substances . Choosing a construction material that can withstand the harsh environment of the refinery process is critical . Corrosion in refineries can be mainly classified into low and high-temperature corrosion , causing :
• Erosion-corrosion - In the equipment body .
Corrosion problems faced at processing stages in petroleum refining .
Refinery Wastewater
Air Emission ( e . g . H 2 S )
Crude Oil
Desalting
Distillation
Downstream Processing
Purification
Vital equipment prone to mild , medium and severe corrosion includes vessels , tanks , pumps , and heat exchangers .
Removal of corrosive salts and suspended solids
Separation into major distillation fractions
Change of molecular structure of hydrocarbon molecules
Denitrogenation / Desulfurization / Demetallation
• Weight loss - The density of the metal in the equipment is likely to be reduced for several reasons . The presence of oxygen can result in a molecular state where metallic oxides can easily slough off the parent body .
• Stress corrosion cracking - Usually occurs in certain specific alloy environments , which are induced by the combined effect of tensile strength and environment .
• Corrosion fatigue - Premature fractures due to cyclic stresses are imposed on equipment / components in a corrosive environment , causing pitting corrosion and initiating fractures .
• Galvanic corrosion - Occurs when two metallic materials in the equipment with different nobilities ( electrochemical potential ) are in contact and are exposed to an electrolytic environment . The difference in metallic potential causes corrosion .
Areas affected by corrosion The most sensitive and corrosion-prone equipment , such as pipelines , valves , storage tanks and vessels exposed to medium to high temperatures , should be constructed from stainless steel . Equipment in areas where lower temperature products are handled may be fabricated using carbon steel or low alloy steels containing chromium – molybdenum . Grades such as AISI 4130 / 35 / 40 coated with anti-corrosion
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