Petroleum refineries often encounter naphthenic acid corrosion when processing opportunity crudes with a high total acid number (TAN). The formation of oil soluble iron carboxylates in refinery units operating at high temperature regimes (200 °C to 400 °C) with corresponding high fluid velocities, can lead to high rates of corrosion where susceptible metallurgies are used in the affected zones.
To mitigate naphthenic acid corrosion, the application of chemical additives (i.e., corrosion inhibitors) can be implemented. The type of inhibitor traditionally used to mitigate this type of corrosion are largely composed of phosphorus-based compounds. However, P-based inhibitors have come under scrutiny due to their potential impact on refinery process units. Consequently, a variety of new inhibitors, where the amount of phosphorus has been reduced or eliminated have been developed to mitigate naphthenic acid corrosion.
In the present study, a variety of corrosion inhibitors, phosphate esters (mono, di, or triesters), thiophosphate esters, and organo-polysulfides were used to inhibit naphthenic acid corrosion in hydrotreated base oils with a TAN of 10 mg KOH/g or higher. The results presented herein not only describe the mechanism by which such molecules inhibit naphthenic acid corrosion, but also provides insight into the relationship between the chemical functionalities of these inhibitors and their ability to successfully mitigate naphthenic acid corrosion. These findings serve as a blueprint for determining the effectiveness of a naphthenic acid corrosion inhibitor with respect to the structural features of the inhibitor.
Opportunity crudes are generally defined as petroleum crudes bearing a high level of sulfur, metals, or total acid number (TAN). These crudes are typically offered at a discounted value. Thus, refining such crudes carries with it a lucrative incentive. However, due to the above-mentioned characteristics, processing such crudes presents numerous operational challenges as well, such as naphthenic acid corrosion, which is commonly associated with the high TAN content in these crudes. In refinery units such as the crude distillation tower, these carboxylic acids react with the iron atoms of the metal surfaces to produce oil soluble iron carboxylates.1,2 The continued formation of such complexes would then erode the metal surface. Equipment failure due to such corrosion results in shutting down a large segment or the entire refinery. Therefore, establishing a means for mitigating this type of corrosion is paramount for processing crude oils with an elevated TAN due to naphthenic acids.