A Review of Naphthenic Acid Corrosion and Sulfidic Corrosion in Crude Oil Refining Applications

• Published in: NACE International Corrosion Conference Proceedings pp. 1 - 15
• Main Authors: Yoon, Yuhchae, Srinivasan, Sridhar
• Format: Conference Proceeding
• Language: English
• Published: Houston NACE International 01.01.2019
• Subjects: Acids; Baffles; Carbon steel; Carbon steels; Carboxylic acids; Chains; Chemisorption; Chromium steels; Construction materials; Corrosion; Corrosion prevention; Corrosion resistance; Corrosion resistant alloys; Corrosion resistant steels; Crude oil; Flow velocity; Heat exchangers; High temperature; Inspection; Kinetics; Liquid phases; Molecular structure; Molecular weight; Neutralizers; Optimization; Paraffins; Petroleum refineries; Reaction kinetics; Refining; Sodium hydroxide; Stainless steels; Sulfur compounds; Vacuum distillation

Reducing crude oil costs have continuously forced refineries to look for so-called "opportunity" or "alternate" crudes, which are usually low quality corrosive crude oils with high concentrations of naphthenic acids and sulfur compounds.5 Processing of these highly acidic and sulfurcontaining crudes at high temperatures in refineries has promoted corrosion in hot oil distillation units and associated piping systems, and continues to be an important issue for the refining industry. Mitigating this corrosion involves several strategies:5 9'10 * Improvements in refinery crude blending, injection of inhibitors, and deacidification of stream distillation (neutralizers) practices * Selection of better materials of construction utilizing corrosion resistant alloys (CRAs) and surface modification (coatings) * Control of flow velocity and associated wall shear stress produced by the flow media * Optimization of in-service inspection and monitoring in oil refineries (process adjustments: total sulfur, TAN, temperature) Crude Corrosivity issues occur at locations where fluid velocity is relatively high, in atmospheric distillation units, vacuum distillation units, associated units, transfer lines, valves, baffles, heat exchangers, and side cut piping. Some examples of naphthenic acid structure are illustrated in Figure 1 n Corrosivity of Naphthenic acids Naphthenic acids in the refining industry often include all organic acids with thermal stability relevant to crude oil distillation.3'7'13-15 Derungs reported that naphthenic acid corrosion had been observed from the 1920s, and wrote that naphthenic acid corrosion was differentiated from sulfur corrosion on the basis of different corrosion phenomena.3'16 He suggested that naphthenic acid corrosion was dependent on the concentration of the naphthenic acids in the crude oils, operation temperature, fluid velocity and activity of the naphthenic acids.3 Derungs observed corrosion of mild steel by naphthenic acids became apparent above 221°C (430°F) with a maximum at 282°C (540°F), and didn't observe the corrosion above 399°C (750°F).3 Finally, Derungs suggested two principal ways to reduce naphthenic acid corrosion through removal of active naphthenic acids or neutralizing of the naphthenic acids with either lime or caustic soda, by modifying fluid velocity condition, and by selecting proper materials.3 However he defined naphthenic acids as saturated carboxylic acids.3 In later studies, the side chains connecting the carboxylic group are not limited to saturated chains but are also aromatic and polycyclic.13 The number of naphthenic acid species in the refining industry is large for any given crude oil fraction and can differ between different crude oils. The study found that the observation of Derungs was valid.4 He also tried to correlate the corrosion kinetics of naphthenic acid corrosion to refinery corrosion data.3 4 Gutzeit concluded that carbon steel and 12 Cr steel corrosion rates directly related to the increasing temperature and naphthenic acid concentration, and suggested that the kinetics of naphthenic acid corrosion may conform to the Arrhenius equation, and chemisorption controls the rate of corrosion.4 He added that corrosion only occurs in the liquid phase, and pressure has little impact.4 He also concluded that alloys with Mo was beneficial to 18/8 stainless steel, and corrosion by condensation was worst.4 Piehl presented a correlation of naphthenic acid corrosion to temperature, total acid number (TAN), velocity, alloys, composition, and locations within the crude distillation units.7 He mentioned that heavy vacuum gas oil fraction was the fraction most commonly found with high naphthenic acid content. 7 Yepez studied the reaction of naphthenic acid with iron by using iron powder test method to determine the dissolved iron levels of a number of carboxylic acid solutions in liquid paraffin including naphthenic acids.17 Figure 2 shows the dissolved iron against the temperature in different TAN values of naphthenic acids.