(535d) Incorporating Oil-Sands Heavy Oils into Fluid Catalytic Cracking Feedstocks—Some Observations from a Comprehensive Study

The Canadian oil sands resource is immense with about 174 billion barrels recoverable using current technologies. Oil-sands bitumen is a mixture of immature and complex hydrocarbons with a relatively low hydrogen-to-carbon ratio (i.e., very aromatic) and an abundance of chemical impurities. Because of high viscosity, the bitumen can only be transported through pipelines after being mixed with suitable diluents such as natural gas condensates or naphthas from oil-sands processing plants. Alternatively, the bitumen can be upgraded to a light and bottomless (resides-free) synthetic crude oil (SCO), which is pipelineable. There are several open-pit upgraders in Alberta producing SCOs of different qualities. At present, both de-watered bitumens and SCOs are commodities in the North American oil market. In Canadian and US refineries, the major and heavy fractions of these materials, i.e., heavy gas oils (HGOs) or vacuum gas oils (VGOs) and some resids in the bitumens, end up as feeds to the fluid catalytic cracking (FCC) units. FCC is traditionally the dominant refinery conversion process for producing high-octane gasoline and other valuable products. It is known that bitumen-derived crude (BDC), including SCO, is less superior to produce FCC feed than stocks from conventional sources. For this reason, refiners limit the use of BDC (less than ~25 % addition) in their conventional FCC-based operations.

Over the years, the National Centre for Upgrading Technology (NCUT), in cooperation partly with Syncrude Canada, the largest oil-sands operator in the world, examined the cracking characteristics of various BDCs. These included the bitumens (regular bitumen and the partially deasphalted bitumen after the paraffinic froth treatment) and VGOs representing products from current or potential upgrading processes. The objective of this comprehensive study was to understand bitumen chemistry, thereby improving the qualities of BDCs as FCC feeds. Cracking experiments were conducted on different occasions at 500?540 °C using one of, or the combination of, the following reactors: the ARCO-type riser, the fluid- and fixed-bed microactivity test (MAT) units, and the Advanced Cracking Evaluation (ACE) unit. The study involved two commercial equilibrium FCC catalysts, catalyst A (cat-A)?an octane-barrel catalyst containing rare-earth ultra-stable Y zeolite (REUSY) mixed with a small amount of shape-selective ZSM-5, and catalyst HRO-610?a large-pore bottoms-cracking catalyst containing rare-earth exchanged Y zeolite (REY). Total liquid products (TLPs) from selected runs were characterized (without prior separation) for: (a) hydrocarbon types of gasoline and (b) boiling-point distributions of aromatics, nitrogen, and sulfur.

The comprehensive FCC study featuring a large number and variety of feedstocks, with complete analyses, cracked in a wide range of catalyst/oil (C/O) ratios in several reaction systems, created a huge data base from which useful and intersting cracking characteristics were observed. Two case studies are selected and presented below.

1. Improved Cracking Characteristics of Bitumen through Paraffinic Froth Treatment. In oil sands open-pit operations, bitumen is extracted by means of a hot-water process. The resulting bitumen froth contains, on average, approximately 60, 30, and 10 wt % bitumen, water, and solids, respectively. The froth must be demulsified in the treatment plant where a solvent is added to remove water and solids. In this study, either a paraffinic solvent (n-heptane) or an aromatic solvent (toluene) was added to the froth feed (in 3:1 w/w ratio) resulting in a ?dry? bitumen called ?P-? or ?A-bitumen?, depending on if it has been treated with paraffinic or aromatic solvent. The recoveries were roughly 90 and 98 wt % for P- and A-bitumen, respectively. Each bitumen was successively added, in 0, 10, 25, 50, 75, and 100 wt % concentrations, to a heavy gas oil from the conventional sweet crude Rainbow Zama obtained from a refinery. This was followed by catalytic cracking in a fluid-bed MAT reactor loaded with catalyst HRO-610 at 540 °C. The following highlight the key observations.

· Froth treatment with the paraffinic solvent removed a significant amount of asphaltenes originally in P-bitumen (by 46 wt % relative to that of A-bitumen), resulting in a considerable quality improvement reflected by reductions of Conradson Carbon Residue (CCR), Ni, and V (by 30 wt % each). Consequently, the cracking performance was also improved. At a given bitumen addition and conversion, P-bitumen gave more premium products, i.e., gasoline, liquefied petroleum gas (LPG), and light cycle oil (LCO) used as diesel fuel, and less low-value products, i.e., dry gas, coke, and heavy cycle oil (HCO) used as heavy fuel oil.

· Using a 50-wppm Ni+V economic limit as a guide for commercial resid cracking, a refiner can accept, in the resid FCC (RFCC) process, 43 wt % more bitumen after being treated with this paraffinic solvent. The resulting product slate is quite acceptable for RFCC from a refiner's point of view.

· Some synergetic effects existed in catalytic cracking a blend composed of Rainbow Zama HGO and a bitumen. This was reflected by the nonlinear relationship of conversion or product yield with the bitumen addition at a given C/O ratio.

2. Variation of Product Qualities with Conversion. This case study involved a deasphated VGO and its hydrotreated product, both cracked with cat-A at 510 °C in MAT reactors. The results showed interesting variation as conversion increases.

· The aromatics-enrichment effect prevailed in three liquid product fractions, with LCO being the highest in aromatics concentration at a given conversion;

· The unique sulfur or nitrogen concave yield curves indicated the balances, in three liquid product fractions, between the sulfur or nitrogen removal and augmentation effects.