| |
[摘要]：Most crude oils contain traces of vanadium, which cause significant detrimental impact during catalytic conversion and combustion. The concentrations in bitumen and vacuum residue are generally much higher, which poses a problem for the economical upgrading of these feedstocks. These problems are relevant since the world reserves of conventional light oils are dwindling and being replaced by an increasing amount of heavier feedstocks. In this article, the current understanding of the distribution and form of vanadium compounds within vacuum residue is critically discussed. The implications of this chemistry on prospects for new separation methods, other than deasphalting, are considered. Although only a small fraction of the vanadium is contained within the polar resin fraction (i.e., maltenes), this fraction has been most often characterized to determine the chemical form of the vanadium compounds. Various spectroscopic techniques have been used to determine that these resin soluble vanadium compounds exist as metalloporphyrins characterized by their intense absorption of UV/visible radiation. In the case of the asphaltene bound metals, this UV/visible absorbance is not observed and has historically led to their distinction as "non-porphyrins". However, more recent results using X-ray spectroscopies (EXAFS and XAN.ES), as well as a more in depth analysis of the UV/vis response of metalloporphyrins, indicates that, although these asphaltene-bound vanadium compounds do not exhibit the characteristic UV/visible absorption, they are indeed still bound in a porphyrinic structure. The fact that the majority of the vanadium is contained within the highly aromatic, highly polar asphaltene fraction also poses additional roadblocks to their selective removal. This fraction has been shown to associate/aggregate significantly in most (if not all) solvents. Recent work on the nature and possible mechanisms of the molecular association of asphaltenes in solution can be extended to help elucidate the molecular interactions occurring between asphaltenes and metalloporphyrins, and hence the nature of the inclusion of metalloporphyrins within the asphaltene fraction. Many different solvent systems including aromatics, chloro-carbons, alcohols, ketones, as well as other polar solvents have been used to extract the metalloporphyrins from the asphaltene fraction with limited success. In most cases, the effect of asphaltene solubility and aggregation in the given solvent were not considered. The selective separation of metalloporphyrins is clearly hampered by gaps in the basic understanding of the metalloporphyrin properties and behavior in solution. |
|
