Ammonium chloride (NH4Cl) underdeposit corrosion in hydrotreater reactor effluent coolers can produce extremely high corrosion rates and may lead to leaks or fires. There are generally no cost effective, reliable materials to resist this attack; hence a goal of any design is to prevent formation of deposits. This paper will give a general description of the corrosion mechanism and contrast it with ammonium bisulfide corrosion which is also a major concern in reactor effluent air coolers (REAC's).
Case histories of NH4Cl corrosion are given along with theories on why chloride salt problems seem to be increasing in the industry. Methods of calculating NH4Cl deposition temperatures are presented which allow estimation of the chloride levels which would cause concern at various conditions. Mitigation methods are then presented which involve
minimizing the sources of chlorides, using better materials and/or optimizing critical design details. In many units with chloride salt concerns, proper contacting of the water wash with the inlet stream vapor phase is essential.
Keywords: ammonium chloride, hydrotreater, reactor effluent, air cooler, underdeposit corrosion, water wash, ammonium bisulfide, material selection