STUDY OF PHYSICO-CHEMICAL PARAMETERS CHARACTERIZING NAPHTHENIC ACID CORROSION

The recent increase in crude oil cost arose interest in cheaper oils, usually called “opportunity crudes”. These oils are difficult to transport and treat, mainly because of their sulphur content, acidity and high density, but their cheap price makes them attractive, because refineries capable of processing them could gain a higher profit. Opportunity crudes composition affects process parameters and they can also cause corrosion and failures in plant equipment. It is well known that the main corrosive agents are the naphthenic acids (NAs), but the mechanism is complex and very little is known about it since there are many factors affecting it such as type of acids, temperature, sulphur compounds and fluid velocity. Thus damage process has not been clarified yet and at this time there is no accurate model available for predicting crude oil corrosiveness. Though considerable work has been done to understand NA corrosion in refineries, it turned out to be unsuccessful when the interdependence of NAs type and corrosion effects is considered. The objective of the research activity was to get more insight into the relationships between physico-chemical properties of crude oils and corrosion, in order to build a model aimed at identification of a corrosivity index which might help operators to determine the aggressiveness of processed fluids. For this purpose an analytical protocol was identified in order to determine the main molecular characteristics, such as number of carbon atoms, rings and average molecular weight. After some preliminary high temperature and pressure tests, a new equipment was developed and designed capable of reproducing real plant conditions. The small-scale plant was first used with a reference fluid consisting of gasoil and a commercial mixture of NAs, and then tested with actual crudes. The use of experimental design and of proper analytical techniques allowed to build a model for the corrosion of two different materials.