High temperature behaviors of asphaltene aggregates in heavy feed stocks and in mixtures with diluents.

摘要

The propensity of asphaltenes to aggregate can cause severe coke deposition problems during catalytic hydroprocessing of heavy oils. Numerous studies on this subject have been performed. However, asphaltene aggregate size is an uncontrolled and unexplored and potentially important variable.;Athabasca bitumen and Maya crude were filtered through commercial nano ceramic membranes to separate asphaltene aggregates by size in their native media without dilution. Two distinct nanostructures, one enriched in pentane asphaltenes and one enriched in mineral matter, were identified. The asphaltene-rich nanostructures are polydispersed with a broad size distribution extending from less than 5 nm to more than 100 nm. Their composition is independent of size and they do not associate preferentially at 473 K with other constituents, such as mineral matter or resins in native feeds, as prevalent notions suggest.;Coke deposition experiments were conducted in a 15 mL batch reactor on a commercial hydroprocessing catalyst NiMo/gamma-Al2O3 at 653 K for 2 h at a 30:1 feed to catalyst ratio (weight basis). Two sets of experiments were performed: one with feedstock, permeates, and retentates of Athabasca bitumen and Maya crude oil focused on the impact of asphaltene aggregate size on coke deposition; the other with Athabasca bitumen diluted with n-dodecane, n-decane or 1-methylnaphthalene to investigate the interplay among thermophysical properties in mixtures at diverse length scales in hydroprocessing environment.;Coke deposition outcome is shown to be a combination of mixture properties at the macroscopic scale (the number, nature, and composition of phases present), the nano scale (asphaltene nanoaggregation within phases) and the molecular scale (hydrogen solubility by phase). Asphaltene-rich nanoaggregate size and maltene composition are shown to play secondary roles in coke deposition. Elemental analyses of sectioned catalyst pellets showed that coke deposition within pellets was diffusion limited, while vanadium deposition, arising primarily from the maltene fraction, was not diffusion limited. The findings suggest that prevalent asphaltene-rich nanostructure models and their chemistry be re-evaluated and underscore the need for the explicit incorporation of physical phenomena in the development of coke deposition models.