| Because of increasing oil prices and the continuous need for improving business margins, the refining industry faces new challenges by processing cheaper oils, such as "opportunity crudes". These usually contain higher amounts of corrosive sulfur compounds and naphthenic acids (NAP) which, at high velocity multiphase flow conditions, can impair the integrity of transfer lines in crude distillation units. Previous studies have shown that NAP corrosion is particularly aggressive in the presence of a liquid phase at the metal surface. Although the effect of flow was previously assessed in single-phase flow conditions, a high velocity multiphase flow has been suggested to occur in the transfer lines. Therefore, this study investigates the multiphase flow effect on NAP corrosion of carbon steel in the presence or absence of an iron sulfide corrosion product layer, at elevated temperature and fluid flow velocities, using a small-scale annular flow rig (AFR). In parallel, it examines the hydrodynamics of a multiphase flow mixture at room temperature and high flow velocities in a large-scale cold flow rig (CFR), in order to identify the flow patterns and their characteristics. A mechanistically derived gas-liquid two-phase flow model is additionally developed to predict the flow regimes and related characteristics, and for its application to the operating conditions in the AFR to understand what may control the NAP corrosion rate.;Results show a multiphase flow effect on the NAP corrosion rate at superficial gas velocities in the range of 1--10 m/s, where a decrease in the corrosion rate by 60% occurred at a constant liquid velocity of 0.1 m/s and a total acid number of 2 mg KOH / g oil. The iron sulfide scale built during sulfidation conferred some protection in single phase flow, but none in multiphase flow. The main predicted flow patterns were: annular, stratified, intermittent and bubble. The oil wetting is suggested to be the main mechanism controlling pure NAP corrosion in multiphase flow; the dominant pattern in the AFR was most likely a mist flow, leading to lower wetted wall fractions and, consequently, reduced corrosion rates. |
