FCC feed streams may contain relatively high levels of sulfur and nitrogen. During catalytic cracking these compounds break down to form hydrogen sulfide, ammonia and cyanides, which can condense with the water vapor to form very aggressive sour water. Bisulfide corrosion and hydrogen induced cracking and blisters result in elevated pH due to ammonia and the formation of corrosive species from the hydrogen sulfide and cyanide relative to this pH; these are problems that must be addressed.
Combating Bisulfide Corrosion and Hydrogen-Induced Corrosion
The presence of ammonia, hydrogen sulfide and varying levels of cyanides in FCC main fractionator overhead systems means there is potential for corrosion. Corrosion mechanisms normally found are bisulfide corrosion and hydrogen-induced corrosion due to cyanides.
Bisulfide corrosion is caused by the conversion of hydrogen sulfide in high pH water to bisulfide, and the direct reaction of bisulfide with iron in the process materials. The application of Nalco Champion filming inhibitors specially designed to operate in high pH environments minimizes this corrosion.
The presence of cyanide ions can accelerate bisulfide attack and promote hydrogen induced cracking mechanisms, such as:
Sulfide Cracking (SSC)
Hydrogen-Induced Cracking (HIC)
Stress-Oriented Hydrogen-Induced Cracking (SOHIC)
Refiners employ water washing, chemical treatment to reduce hydrogen generation and chemical treatment to reduce hydrogen cyanide in the system to prevent or minimise corrosion and hydrogen activity attack. Nalco Champion can help minimise the impacts of corrosion by reducing the formation of hydrogen atoms through the use of uniquely designed filming inhibitor chemistries.