Apr-2025

Synergistic multifunctional corrosion inhibitors for high-acid crudes

Mitigate high-temperature naphthenic acid corrosion in enabling reliable and economical processing of high acid crudes.

Amutha Nagarajan, Ghousya Gulzareen Khannam, Kannan Perumal, Sathees Kesavan, Hitesh Bagaria and Nimeshkumar Patel
Veolia Water Technologies and Solutions

Article Summary

Processing ‘opportunity’ crude oil brings various process challenges, from high total acid number (TAN) to high concentrations of sulphur, nitrogen, and aromatic compounds. Nevertheless, these crude oils remain economically attractive to refiners due to their substantial price discounts, which can significantly improve refinery profit margins. Among these opportunity crude oils, high-acid crude represents a distinct category characterised by substantial concentrations of naphthenic acids.

Naphthenic acids comprise a complex mixture of cyclopentyl and cyclohexyl carboxylic acids. They exhibit a broad molecular weight distribution ranging from 120 to 700 g/mol and are quantified as naphthenic acid number (NAN, mg of KOH/g oil), which is a fraction of TAN. In refinery operations, naphthenic acid-induced high-temperature corrosion is particularly pronounced in atmospheric and vacuum distillation units operating within 175-400°C (350-750°F).

The corrosivity of such crudes is influenced by multiple critical parameters beyond temperature. These parameters include the concentration, molecular weight, and boiling point distribution of naphthenic acid constituents, as well as the quantity and nature of sulphur compounds present. Additionally, operational factors such as shear stress and material of construction significantly impact corrosion intensity.

To minimise high-temperature corrosion, the use of corrosion inhibitors is typically a viable approach, particularly where the unit is not constructed with high-grade metallurgies. The phosphate/phosphonate-based chemistries are relatively effective for such services. However, there is a risk of downstream fouling and catalyst poisoning. Against this backdrop, work started on the development of a synergistic multifunctional corrosion inhibitor composition that consists of an organophosphate corrosion inhibitor and an organic dispersant chemistry free of any metals, sulphur, or phosphorus.

Several potential candidates were extensively evaluated for high-temperature corrosion inhibition and fouling tendency, including their ability to keep iron phosphate dispersed. Surface characterisation, using the X-ray photoelectron spectroscopy (XPS) technique, has been carried out to show the good protective layer formed by the corrosion inhibitor product. A synergistic formulation of a primary corrosion inhibitor and a dispersant was found to provide superior performance at 50% lower phosphorus concentration. The formulation was made into a marketable product and positioned for field trials in several refineries.

Naphthenic acid corrosion and its mitigation
Naphthenic acid corrosion represents a critical asset integrity challenge for crude oil refineries processing high naphthenic acid content crude oils and blends. These acids typically concentrate within the distillation operation’s temperature range of approximately 175-400°C, exhibiting particularly corrosive behaviour under high shear stress conditions. The corrosion mechanism associated with naphthenic acid interaction on metallic surfaces exhibits considerable complexity, rendering precise prediction challenging.

Understanding of corrosion mechanisms suggests that naphthenic acids interact with iron present on metallic surfaces, forming iron naphthenates. These compounds are oil-soluble and readily detach from metallic surfaces, subsequently exposing additional bare metal to acid attack. Prolonged exposure to these acids results in substantial metal degradation.

Furthermore, iron naphthenates can undergo subsequent reactions with sulphur species present within the system, leading to various corrosion scenarios such as:
- Isolated naphthenic acid corrosion, characterised by minimal presence of non-reactive sulphur compounds, which has negligible impact on overall corrosion phenomena.
- Combined naphthenic acid and reactive sulphur component corrosion, resulting in accelerated sulphidic deterioration.
- Naphthenic acid corrosion modulated by natural inhibitors (hydrogen sulphide), wherein iron naphthenates react with hydrogen sulphide to produce iron sulphide, subsequently releasing naphthenic acid that may initiate additional downstream corrosion.¹

Refineries may try to implement various methodologies to control and mitigate naphthenic acid corrosion, including modification and monitoring of operational parameters or integration of high TAN feedstock with low TAN feedstock. Note that the latter approach may present economic challenges and potentially negatively impact refinery margins. Enhancement of metallurgical specifications (see Figure 1) can be effective in improving corrosion resistance but may not always be feasible as it involves a significant investment of capital and time.

Another approach to mitigating risks associated with high-acid crude processing is the utilisation of high-temperature corrosion inhibitors (CI) and the establishment of optimised chemical dosage protocols. Furthermore, the selection and implementation of appropriate inhibitors, coupled with strategic application methodologies, enables refineries to process feedstock containing elevated acid concentrations. This capability enhances operational flexibility and potentially yields improved financial performance through the processing of more economically advantageous feedstocks.

High-acid crudes corrosion inhibition by chemical treatment
Phosphate or phosphonate esters, sulphur (S) compounds, and combinations of sulphur and phosphorus (P)-based chemistries tend to inhibit corrosion at high temperatures and are commonly used as naphthenic acid corrosion inhibitors. The corrosion inhibitor is expected to form a protective layer of insoluble phosphates and iron sulphide, protecting against the formation of hydrocarbon-soluble and corrosive iron naphthenate.2 An analysis of a metal surface to which an inhibitor treatment is applied, demonstrating corrosion mitigation, is discussed in the following section.