Narrow-cut characterization reveals resid process chemistry

Abstract

Supercritical fluid extraction, narrow-cut characterization of bitumen pitch, hydrocracker and once-through coker vacuum resids revealed the unexpected process chemistry of bitumen hydrocracking. Hydrocracking appears to prolong the coke formation process of the end-cut material of bitumen, by converting it to coke precursors. The conversion (or removal) mechanism for most key species (microcarbon residue (MCR), metals, N) in hydrocracking is by partitioning, which is similar to coking. Sulfur species that convert in hydrocracking are from the front-end of bitumen pitch; there is only a small sulfur reduction in the end-cut. Bitumen pitch and resid products have similar MCR distribution which is dependent on the “depth” of resid, not the conversion processes. Coke yield did not correlate with the MCR content of narrow-cut feed; coke yield was insignificant at the front-fractions and high at the end-cut. The sub-fractions of bitumen derived resid products have lower H/C ratios than those of bitumen pitch, but have the same MCR content. This can be explained by the pendant-core model, in which bitumen pitch and resid products have the same amount of aromatic cores (coke precursors). The inverse correlation of key species with H/C ratio is process dependent. The reaction products of end-cut (mainly asphaltenes) remained in resid product slate (524°C+ materials) in once-through coking and hydrocracking operations. The implications for the current commercial bitumen upgrading flowsheet are also discussed.