(712i) Direct Cracking of Raw Oil Sands Bitumen - A New Approach

Abstract

The objective of this study is to
determine the viability of direct fluid catalytic cracking (FCC) of the
resids-free portion of the raw Athabasca oil sands bitumen in the presence of the
easily-cracked synthetic crude oil (SCO) after removing its light 343°C?
fraction. A 343?525°C cut heavy gas oil (HGO) was prepared by distilling a
vacuum topped bitumen from the vacuum distillation unit at Syncrude commercial
upgrading plant¹ (Figure 1) in northern Alberta, Canada. It was then
blended with various amounts of   343°C+ fraction of commercial SCO, called Syncrude
Sweet Premium (SSP). Five HGO blends were prepared containing 100, 75, 64, 61,
and 48v% of SCO HGO. The two blends with 64 and 48v% were referred to here the
synsynbit and synbit HGOs, respectively. An earlier marketability study²on oil sands products in Asian
countries predicted that (1) synsynbit, blended to approximate the quality of
Middle East light sour crudes, would have the largest exporting volume and
diversity of marketability, compared with those for the sweet SCO, and (2)
Asian refineries would be able to accept synsynbit without any major modifications of the plant structure.

Each HGO blend was catalytically cracked at 500 and 520°C
using a bench-scale Advanced Cracking Evaluation (ACE) unit. The results showed
the acceptable performance of bitumen HGO when adequate amount of SCO HGO was
added. The conversion, defined as the summation of dry gas (H2, H2S, C1?C2),
liquefied petroleum gas (LPG, i.e., C3+C4), gasoline (C5?221°C), and coke, and selectivities
of gasoline, light cycle oil (LCO, a diesel pool component, 221?343°C), and
coke were either improved or little affected, in comparison with those of 100%
SCO HGO used as a bench mark (Table 1). In-house analyses supported by those
reported in the literature³ showed that the bitumen HGO, which
accounted for 40.6wt% vacuum topped bitumen, actually contained less nitrogen
than its counterpart and had relatively low Ni+V and CCR. These imposed no serious
threat to the cracking catalysts which were vulnerable to these poisons.
Perhaps, the major challenge was the mild increase in the dry gas yield notably
due to the augmentation of H2S, and higher sulfur levels in gasoline and diesel
which might need desulfurization prior to being sent to gasoline and diesel
pools. As sulfur species in bitumen HGO exist mostly as sulfides, mercaptans,
and small thiophenes, they should be easily removed by mild hydrotreating or
other low cost process.

In summary, the proposed route provided an interesting
and promising option to directly crack the raw bitumen leading to a substantial
saving in energy and reduction in green house gases by eliminating the costly
upgrading of the bitumen HGO.

Figure 1. Syncrude upgrading process flow

Table 1. Product distributions at 72 wt% conversion for HGO blends

             ^a heavy cycle oil, sometimes
used as heavy fuel oil

             ^b the amount of coke on catalyst

[1]
Yui, S., Chung, K. ?Syncrude Upgrader Revamp Improves Product Quality?, Oil & Gas Journal,  December 17, 2007, pp. 52?59.

[2] du Plessis, D., Wu, N. ?Marketability of Oil Sands Products
in Asian Countries?, presentation to HUTF, June 20, 2007.

[3] 
Chung, K.H., Xu, C.M., Hu, Y., Wang, R. ?Supercritical Fluid Extraction
Reveals Resid Properties?, Oil & Gas Journal, January 20, 1999, pp.
66?69.