A geochemical framework for understanding residue properties

摘要

Crude oils, and the residual fractions thereof, vary widely in many relevant properties. Thus, a detailed characterization of a single residue stream or even a number of residue streams from one region may not be very relevant to a refiner in a different part of the world. We have therefore attempted to develop a framework for understanding residue properties in a generic fashion. A detailed analysis of 11 vacuum residue fractions from a wide variety of crude oils resulted in a general model for understanding the composition and processability of residue streams. This model is based on the geochemical origin (kerogen type) and the maturity of the crude oil. The different origin of kerogen Ⅰ (paraffinic) residues, compared with the more conventional kerogen Ⅱ residues, is reflected in a large number of properties and also in a larger variability in these properties. However, upon maturation, the average properties of kerogen Ⅰ residues and kerogen Ⅱ residues converge so that mature residues of both kerogen types have much in common (low S content, low metals content, high H/C ratio etc). On a molecular basis, kerogen Ⅰ residues contain more paraffins in the form of wax compounds. Upon maturation, residues tend towards lower average molecular weights, higher H/C ratio's (lower density), lower viscosities, lower concentrations of hetero-atoms (sulphur, nitrogen, V and Ni), asphaltenes and MCRT, and higher wax contents. Maturation is found to have a negative effect on the stability and coking tendency of the residue fraction. NMR data reveal that the asphaltenes become more aromatic upon ageing, whereas the maltenes (non-asphaltenes) become less aromatic, thus causing an increasing gap in aromaticity between these fractions. Significant differences in asphaltene molecular structure between kerogen Ⅰ and kerogen Ⅱ residues were observed. Kerogen Ⅲ crudes were not included in this study as they generally contain insufficient vacuum bottoms to be of practical interest. The analytical data suggest that the concept of kerogen type and maturity may be a useful tool in describing and understanding residue characteristics and assist in optimizing feedstock selection for residue conversion processes and other residue applications (fuel oils, bitumen), as well as in understanding fouling and coking phenomena.