Corrosion Behavior of Naphthenic Acids Isolated from Vacuum Gas Oil Fractions

• Published in: NACE International Corrosion Conference Proceedings pp. 1 - 15
• Main Authors: Kurapati, Yathish, Robbins, Winston, Bota, Gheorghe, Young, David
• Format: Conference Proceeding
• Language: English
• Published: Houston NACE International 01.01.2018
• Subjects: Acetic acid; Acids; Aminopropyl silica; Carboxylic acids; Corrosion tests; Crude oil; Crude oil prices; Evaluation; Gas oil; High temperature; Laboratories; Mineral oils; Naphthenic acids; Pretreatment; Scanning electron microscopy; Silicon dioxide; Solid phases; Sulfidation; Sulfur; Sulfur compounds; Toluene

Most opportunity crudes have increased acidity and reactive sulfur compounds, making high temperature (~220-400°C) sulfidation-naphthenic acid (SNAP) corrosion a key concern for process and corrosion engineers.1'2 Naphthenic acids (NAP) are naturally occurring carboxylic acids in crude oil that are typically measured by Total Acid Number (TAN - mg of KOH required to neutralize acid in one gram of oil). Crude oil distillates' matrix interactions, especially those due to sulfur, complicate interpretation of corrosion by NAP acids.2-4 Even if isolated from petroleum sources, commercial NAP acids may not be representative of the boiling range (molecular weight) of acids found in vacuum gas oils.2'8 The majority of naphthenic acids from commercial sources boil at temperatures lower than those of vacuum gas oils, where SNAP is of greatest interest.8 Carboxylic acids, available as model compounds for laboratory corrosion tests, generally lack the structural characteristics of naphthenic acids: short -CH2COOH side chains attached to 1-4 saturated rings.9-11 Many of these characteristics of NAP were identified after their isolation for analytical evaluation. The isolation of these acids from a HVGO matrix was achieved using solid phase extraction with aminopropyl silica (APS) and the evaluation of the "isolated NAP acids" with respect to corrosion in an inert mineral oil (spiked to native TAN) as per the "Pretreatment - Challenge" protocol developed at the Institute for Corrosion and Multiphase Technology (ICMT) at Ohio University.6,7 STAGE 1 - NAP ACID ISOLATION The bulk quantities of NAP acids needed for corrosion testing were isolated from the HVGO fractions by an SPE procedure using an amine-functionalized silica - aminopropyl silica (APS) that has been used previously on an analytical scale.15 In this procedure, NAP acids present in HVGO are adsorbed onto the APS while the bulk of the oil is removed with solvents and then the NAP acids are displaced with excess acetic acid in toluene, as detailed below (Figure 1). With the existing apparatus, only 500g - 550g of HVGO can be processed. [...]the acid isolation procedure had to be performed in two batches to obtain enough NAP acid extract to conduct the corrosion tests.