1. INTRODUCTION

Refining is a very elaborate operation, by which crude oil is transformed into a series of products such as, gases, fuels, solvents, lube oils, etc. Crude oil is a complex mixture of hydrocarbons (HC) of different C/H ratio and molecular structures. The different classes of HC molecules comprise paraffins, olefins, cycles, aromatics, resins, asphaltenes, and other poly-unsaturated molecules. In addition to hydrocarbons, crude oils also contain some other compounds composed by other atoms (heteroatoms) than carbon and hydrogen. Those moieties consist of sulfur (S), nitrogen (N), oxygen (O), and heavy metals (mainly iron, nickel, and vanadium). Crudes are usually classified in terms of their specific gravity as very light, light, median, heavy, and extra heavy. An empirical set of units for the crude gravity, defined by the American Petroleum Institute (API), is currently used in oil industry. Their appearance varies from transparent liquids to black solids, going from light to heavy. Light oils have lower specific gravity and larger API gravity, while for heavy oils vice versa. Their composition also changes, and so the concentration of those heteroatomic compounds typically increases from light to heavy. The crude oils are also categorized in terms of their chemical composition, as for instance, sour crude oils, those presenting high acidity (in the past associated with the presence of H ₂S and other sulfur compounds, but more recently directly measured), paraffinic (those mostly composed by paraffins), naphthenic, and aromatic (a high proportion of cyclic or aromatic compounds,...