Ammonium Bisulfide Corrosion

The intensification of ammonium bisulfide corrosion is a ‘byproduct’ of processing crude slates with increasing concentrations of sulfur and nitrogen compounds. This chapter provides an overview of NH₄HS corrosion problems, focusing on the impact of process parameters and material selection

General Information

Ammonium bisulfide corrosion, also known as alkaline sour water corrosion, is a prevalent issue in hydroprocessing units, which encompass hydrocracking (HC), hydrotreating (HT), and hydrodesulfurization (HDS). Catalytic hydroprocessing involves desulfurization and denitrification reactions (see Equations 1 and 2), resulting in the production of ammonia (NH ₃) and hydrogen sulfide (H₂S). These compounds readily react to form ammonium bisulfide, or formally, ammonium hydrosulfide (NH₄HS), through a simple, reversible reaction as shown in Equation 3.

Ammonium bisulfide is thermally unstable, and at high temperatures in the hydrogenation reactor effluent (>300°C / >572°F), the equilibrium of the reaction in Equation 3 shifts to the left. Upon cooling in the reactor effluent air cooler (REAC), the formation of solid ammonium bisulfide becomes favorable. This solid salt can deposit inside the REAC tubes, leading to fouling and plugging. Water washing upstream of the REAC prevents the formation of solid deposits, as NH₄HS dissolves in water, creating the so-called alkaline sour water (see Figure 1). The phenomenon of ammonium bisulfide corrosion can be triggered in two ways: by the sour water itself and by wet, incompletely dissolved ammonium bisulfide deposits. The latter scenario is the most severe, capable of rapidly degrading many Corrosion Resistant Alloys (CRAs).

Schematic diagram of HT/HK unit with areas of high potential to ammonium bisulfide corrosion.[1](#reference1)

Figure 1: Schematic diagram of HT/HK unit with areas of high potential to ammonium bisulfide corrosion.¹

Ammonium bisulfide corrosion is not exclusive to hydroprocessing units; it also plays a significant role in amine units (specifically the regenerator overhead) and sour water strippers (the entire unit). Table 1 lists the most common areas where alkaline sour water corrosion may occur.

Table 1 Potential locations for ammonium bisulfide corrosion in refinery units.¹ ²

Process Unit	Affected Areas
Delayed Coking Unit (DCU)	Fractionator overhead (OVHD) loop from condensers to accumulator
Fluid Catalytic Cracking (FCC)	• Fractionator OVHD loop from condensers to accumulator. • Wet gas compression system (after 1st and 2nd stage compressors: lines and KO drums).
Hydrotreating/Hydrocracking (HP/HT/HC)	See Figure 1
Visbreaker Unit (VBU)	VB fractionator OVHD loop (condensers and OVHD separator)
Amine Treating Unit (ATU)	Regenerator OVHD loop (condensers, OVHD separator and assisted pipelines)
Sour Water Stripper (SWS)	• Virtually whole unit is susceptible to ammonium bisulfide corrosion: - feed lines, drums, and exchangers, - upper section of stripping column, - OVHD section (lines, condensers, separator, reflux lines)

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References

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