The vacuum residue fraction of petroleum is one of the most complex mixtures of compounds known, one which continues to defy comprehensive analysis. The combination of complexity, diversity, and aggregation of the asphaltenes has enormous implications for upgrading reactions and phase behaviour. One of the challenges is that residue molecules fall into a range of molecular weight that is intermediate between commonly available compounds and the high polymers. Extrapolation of thermodynamics and reaction chemistry from these well understood domains to describe residue processing has some merit, but also significant risks.
Our approach is to attach the problem from both directions, working downward from the whole material and building upward by chemical synthesis. We are using modern instrumental methods to quantitatively characterize the pendant groups and building blocks that comprise the residue components. The asphaltene fraction still defies satisfactory direct analysis, but analyses of the constituent building blocks are yielding quantitative results. Working from the bottom up, we are synthesizing families of components to represent key structural features of residue molecules. We then investigate the reactions of these molecules under cracking conditions, and define the molecular basis for coke formation. This research is completely revising our understanding of how cracking reactions proceed in vacuum residue materials, because when cracking occurs in the liquid phase, addition or retrograde reactions are far more important that anyone has recognized previously. This presentation will offer a synthesis of these two approaches, defining the molecular features that control residue behaviour, the main pathways for coke formation, and the implications for advanced residue processing in the future. Controlling the unwanted liquid phase reactions is the key to improving yields of distillate products from the bottom of the barrel.
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