Petroleum Residues, Characterization of
Authors: Bacaud Robert 
Published: September 2006

Petroleum residues are the remaining fraction left after the distillation of crude oil. According to the conditions of the primary distillation, a distinction is made betweenatmospheric and vacuum residues (VRs). Since the simplest, lighter constituents of petroleum have been separated by distillation, residues constitute the most complex fraction of crude oil and a detailed inventory of the individual compounds is impossible. Thus characterization methods will aim at giving an adequate description of the pertinent properties for a given purpose. Practically all of the existing analytical techniques have been applied: chromatographic, spectroscopic, solvent extraction, fractionation, etc. The analyst's strategy will consist in choosing the procedure that will be able to provide limited but pertinent information, at an acceptable cost, in a reasonable time. Some easily obtained physical parameters correlate with chemical composition data. This is the case with specific gravity, viscosity, and the content of carbon residue obtained after pyrolysis. Standard test methods for the determination of these parameters have been developed.

The direct determination of compositional parameters such as aromatic carbon and hydrogen content is obtainable through nuclear magnetic resonance (NMR) spectroscopy without previous fractionation. The quantitative distribution of saturated, aromatic, and polar compounds can be measured by thin‐layer chromatography (TLC) coupled with flame ionization detection (FID). The parameters obtained through various analytical techniques are generally hardly correlated since they refer to different units (e.g. mole %, weight %, or volume %) that cannot be interconverted.

A more detailed characterization requires previous separation for the preparation of simpler fractions. Although the definition of residues implies they have been produced after a preliminary distillation, short‐path molecular distillation, simulated distillation by high‐temperature gas chromatography (GC), and supercritical fluid extraction (SFE) apply to residues and provide the distribution of fractions possessing equivalent boiling point up to about 720°C at atmospheric pressure. The fractions resulting from distillation cuts can subsequently be characterized. Simulated distillation by GC can be coupled with other analytical tools such as element‐specific detectors or a mass spectrometer.

The separation of components in residues can be performed according to their solubility in solvents possessing increasing polarity or by chromatographic solvent elution after preliminary adsorption of the dissolved sample on a solid adsorbent. Among a wide variety of separation schemes, some have been standardized and the corresponding fractions are designated according to solubility: oils or maltenes, resins, and asphaltenes. The yield of the fractions is indicative of composition and behavior during further processing of residues.

Further characterization of fractions involves either chromatographic or spectroscopic techniques. Chromatographic methods include thin‐layer, liquid‐phase, andsize‐exclusion modes and provide information related with composition of fractions in terms of hydrocarbon group types (saturated, aromatics, polars) and molecular weight distribution. Spectroscopic methods [ultraviolet (UV), mass, NMR] are suitable for compositional evaluation.

Physical and analytical parameters can be correlated with the distribution of structural groups existing in the hydrocarbon fraction of residues: paraffinic, naphthenic, olefinic, and aromatic (single or fused rings).