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ABSTRACT
Naphthenic (NAP) acids and sulfur compounds are important corrosive species contained in low quality crudes and can cause significant equipment damage when such crudes are processed. Although the two corrosive species have a synergistic effect at high temperatures their corrosion products can influence the extent of the damage. Thus the iron sulfide (FeS) scales formed as sulfur reacts with iron can hinder the NAP acid diffusion to the metal limiting their corrosive effect. The FeS scale properties are influenced by different factors such as the types of sulfur compounds in oil (sulfides, disulfides, mercaptans, thiophenes), NAP acid interactions, temperature, flow conditions - all factors that are difficult to control. This experimental work intended to evaluate the properties of FeS scales formed from model sulfur compounds (sulfides and mercaptans) in interaction with NAP acids using the "pretreatment - challenge" test protocol. According to this protocol FeS scales were formed on metal samples from mercaptans/mercaptans and sulfides on different ratios/concentrations in the presence of NAP acids at high temperature. Further, preformed scales were exposed to a constant NAP acid attack under high temperature and continuous flow conditions. Samples metal losses and scale SEM/EDS analysis were used to characterize the protective properties of FeS scales.
Keywords: sulfur, naphthenic acid, Total Acid Number (TAN), corrosion, model compounds, high temperature
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INTRODUCTION
Highly corrosive effects at high temperature of some heavy crude oils are the result of complex interactions between naphthenic (NAP) acid and sulfur (S) compounds they contain.1,2 The mechanism of NAP acid and sulfidic corrosion is not well understood but it is generally accepted by refinery engineers and scientists that it is described by three chemical reactions.3
In Reaction 1 NAP acids react with the iron forming the iron naphthenates that are oil soluble and as a consequence they are constantly removed from the metal by the oil flow. The S compounds thermally decompose forming hydrogen sulfide (H2S) that reacts almost instantly with the iron forming iron sulfide (FeS), a solid corrosion product which builds up on metal surfaces as multilayered scales (Reaction 2). The corrosion byproducts iron naphthenates and H2S react with each other rebuilding the NAP acids and forming more solid FeS (Reaction 3).
... (1)
... (2)
... (3)