The pH Paradox: Corrosion Dynamics and FeS Layer Behaviour in Hydrotreating and Hydrocracking.

In the oil refining industry, particularly in hydrotreating and hydrocracking units, managing corrosion is an ongoing challenge. Carbon steel, commonly used in these environments, is prone to corrosion. However, in the presence of hydrogen sulphide (H?S), a protective iron sulphide (FeS) layer often forms on metal surfaces. This FeS layer acts as a barrier to reduce corrosion rates, but its stability is influenced by operational conditions, especially the pH of the water in contact with metal surfaces.

This article examines the chemical and metallurgical behaviour of FeS layers in hydrotreating and hydrocracking units. We investigate the conditions that lead to FeS layer dissolution, how pH and other factors impact layer integrity, and methods for controlling corrosion. The discussion dives into the chemical processes that destabilise FeS at a microscopic level, offering practical recommendations for maintaining a stable and protective FeS layer.

Formation of the FeS Layer in Hydrotreating and Hydrocracking Units

During desulphurisation in hydrotreating and hydrocracking units, H?S is commonly present. This sulphur-containing gas reacts with iron in carbon steel to form iron sulphide (FeS):

Fe (s) + H?S (g) → FeS (s) + H? (g)

The FeS layer forms as an adherent film on the metal surface, serving as a protective barrier that limits further reactions between steel and corrosive agents. This film plays a critical role in reducing corrosion rates, particularly in acidic or mildly acidic environments where FeS remains relatively stable.

Influence of pH on FeS Layer Stability

The stability of the FeS layer in hydrotreating and hydrocracking units is highly sensitive to the pH of the water in contact with metal surfaces. Understanding the behaviour of Fe2? and S2? ions within the FeS lattice under different pH conditions—especially in slightly acidic or near-neutral environments—is key.

In slightly acidic environments, the low solubility of Fe2? ions helps maintain FeS stability. Here, Fe2? ions do not easily dissolve, even in the presence of H? ions. This slight acidity stabilises the FeS structure by attracting H? ions to Fe2? ions within the lattice, creating an equilibrium that favours the solid FeS lattice:

FeS (s) ? Fe2? (aq) + S2? (aq) (shifted towards the solid)

The sulphide chemistry further supports this stability, as sulphur primarily exists as hydrogen sulphide (H?S) or bisulphide (HS?) in slightly acidic conditions. HS? is less dissociative into S2?, keeping sulphur levels low and reducing any chemical disruptions that could weaken Fe-S bonds.

As pH shifts toward neutral or slightly alkaline values, Fe2? solubility increases, allowing Fe2? ions to leave the FeS lattice more readily. At these higher pH levels, abundant OH? ions interact with Fe2? to form soluble iron hydroxides:

Fe2? (aq) + 2 OH? (aq) → Fe(OH)? (aq)

This reaction drives the dissociation of Fe2? from FeS, promoting protective layer dissolution. The equilibrium shifts, releasing Fe2? and S2? ions into the water, weakening the FeS structure and exposing underlying metal to further corrosion:

FeS (s) ? Fe2? (aq) + S2? (aq) (shifted towards the aqueous phase)

As Fe2? is depleted from FeS, the layer’s integrity weakens, increasing vulnerability to corrosive species like H?S and chlorides.

Chemical Equilibria in FeS Dissolution and Sulphide Chemistry

The dissolution of FeS at near-neutral or slightly alkaline pH results from changes in chemical equilibria, particularly in Fe2? and sulphide species solubility. In slightly acidic pH, the limited presence of OH? ions favours FeS stability:

FeS (s) ? Fe2? (aq) + S2? (aq) (favouring the solid)

Here, HS? dominates sulphide chemistry, and minimal dissociation into S2? keeps the sulphur content low, reducing the likelihood of free S2? ions reacting with FeS and destabilising the layer.

HS? (aq) ? H? (aq) + S2? (aq)

As pH nears neutral or slightly alkaline values, HS? dissociation increases, introducing more S2? ions, and creating a more aggressive chemical environment. The abundance of OH? ions also facilitates Fe2? dissociation from FeS, promoting dissolution:

FeS (s) ? Fe2? (aq) + S2? (aq) (shifted towards the aqueous phase)

This dissociation further destabilises the FeS lattice. The S2? ions may form soluble complexes with other metal cations, compounding the erosion of the FeS layer and weakening its protective capability.

At a microscopic level, the loss of Fe2? creates voids within the FeS lattice, weakening Fe-S bonds and leading to particle detachment. Turbulent flow can then physically strip FeS particles from the surface, leaving metal exposed to aggressive species such as H?S and chlorides, accelerating corrosion.

Role of Temperature and Pressure

Temperature and pressure significantly affect FeS stability in hydrotreating and hydrocracking processes. Understanding these effects is essential for managing corrosion and equipment longevity.

Increasing temperature raises Fe2? solubility, shifting FeS dissolution equilibrium:

FeS(s) ? Fe2?(aq) + S2?(aq)

Higher temperatures increase ionic movement, accelerating FeS dissolution and corrosion product formation. This depletes the FeS layer, exposing metal to aggressive environments. Elevated temperatures may also degrade organic inhibitors, further increasing corrosion risks.

Pressure influences FeS stability, especially in hydrocracking units, where higher pressures enhance gas solubility in liquids. Greater H?S solubility affects concentrations of free and bound species on metal surfaces. Under certain conditions, increased pressure can stabilise FeS due to solvation effects altering the environment around corrosion products. This complex interplay between temperature and pressure must be well-understood to predict FeS behaviour and develop effective corrosion control strategies.

Influence of Ammonia on pH and Corrosion Dynamics

Ammonia (NH?) introduces another variable in hydrotreating and hydrocracking units. Ammonia, dissolved from nitrogen-containing feedstocks, influences pH by establishing an equilibrium:

NH?(g) + H?O(l) ? NH??(aq) + OH?(aq)

As ammonia (NH?) dissolves in water, it reacts to form ammonium (NH??) and hydroxide (OH?) ions, increasing the pH and creating a more alkaline environment. This shift in pH can destabilise FeS layers, as discussed, leading to increased Fe2? solubility and potential layer dissolution.

Additionally, NH?? ions can combine with hydrogen sulphide ions (HS?) to form ammonium bisulphide (NH?HS):

NH??(aq) + HS?(aq) ? NH?HS(aq)

Ammonium bisulphide (NH?HS) poses a significant corrosion risk, particularly because it creates a corrosive salt that can deposit on metal surfaces. In areas where NH?HS accumulates, such as low-flow regions or at metal interfaces, this salt can facilitate under-deposit corrosion (UDC). The NH?HS deposits trap moisture, concentrate acidic species, and lead to localised pitting. This is especially problematic in high-temperature environments, where NH?HS deposits are more likely to form and accelerate corrosion rates.

The presence of NH?HS also promotes a unique threat by breaking down the protective FeS layer. As FeS dissolves, the exposed carbon steel is directly attacked by NH?HS, increasing both general and pitting corrosion. In severe cases, NH?HS corrosion can lead to wall thinning, causing rapid loss of metal and jeopardising equipment integrity.

In alkaline environments, increased OH? ions promote HS? dissociation, producing more free S2? ions. This enhances the aggressiveness of the environment, leading to FeS dissolution:

FeS(s) ? Fe2?(aq) + S2?(aq)

As pH rises, Fe2? solubility also increases, weakening the FeS layer. The combined effect of pH, ammonia, and sulphide species necessitates close monitoring and control of operating conditions to preserve FeS stability and corrosion resistance.

Corrosion Mechanisms in Hydrotreating and Hydrocracking Units

The pH of water in contact with metal surfaces is a critical factor in corrosion control, particularly where protective FeS layers are present. When pH is slightly acidic, FeS remains stable, limiting dissolution and offering corrosion protection. Slight acidity retains Fe2? and S2? ions within the FeS matrix, reinforcing the solid FeS layer and preventing Fe2? dissolution.

As pH shifts towards neutral or slightly alkaline, Fe2? solubility rises, promoting FeS dissociation. This compromises FeS protection, exposing carbon steel to aggressive conditions, and increasing corrosion rates. The loss of FeS leaves metal vulnerable to corrosive ions, like chlorides and sulphides, risking premature failure in critical components.

Management Strategies for Corrosion Prevention

To effectively manage corrosion in hydrotreating and hydrocracking units, it is essential to monitor and control the pH levels within operational ranges that favour the stability of the FeS layer. Implementing chemical treatments to adjust pH and control the concentration of aggressive species can enhance the protective capacity of FeS.

1. Use of Corrosion Inhibitors: Incorporating suitable corrosion inhibitors can further protect carbon steel surfaces. Inhibitors can form films that enhance the stability of existing FeS layers or create additional barriers against corrosive species.
2. Regular Monitoring: Regular monitoring of pH and sulphide concentrations is vital to maintain optimal conditions for FeS layer stability. This can include sampling water phases and deploying online monitoring systems to adjust chemical treatments in real time.
3. Optimisation of Operational Conditions: Optimising operational parameters such as temperature, pressure, and flow rates can also contribute to maintaining the integrity of the FeS layer. Minimising turbulence in areas where corrosion is likely can help preserve the protective FeS layer.

By implementing these strategies, it is possible to manage corrosion effectively in hydrotreating and hydrocracking units, ensuring that FeS layers remain stable and protective over extended operational periods.

Conclusion

In summary, the stability of the FeS layer in hydrotreating and hydrocracking units is highly dependent on pH levels and the presence of sulphide species. The interplay between chemical equilibria and corrosion mechanisms underscores the importance of maintaining appropriate operational conditions. By implementing effective management strategies, the oil refining industry can mitigate corrosion risks and enhance the longevity and reliability of hydrotreating and hydrocracking equipment.

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