Oil sands bitumen and heavy oil contain approximately 50 % residuum which has a boiling point above 525 C. Delayed coking and fluid coking are currently the major processes used to convert this residuum to useful hydrocarbons of lower molecular weight. Both of these processes produce substantial amounts of coke, an undesirable by-product. It appears that the present industrial approach is to diminish this problem by using a medium severity hydrocracking process followed by a coking process. The total amount of coke per unit of original residuum decreases when this combination of processes is used. However, when the coking processes are considered in isolation, the coke yield is substantially greater when the feedstock is unconverted hydrocracked residuum rather than the original bitumen. No matter how high the severity of the hydrocracking process, some unconverted residuum is always produced. Furthermore the coke yield from the unconverted hydrocracked residuum increases with the severity used to hydrocrack the original residuum.
Work in our laboratory has been performed to examine other uses for the unreacted residuum produced by a hydrocracking process. One possibility is the combination of catalytic cracking and gasification (1). The distillate hydrocarbons produced by catalytic cracking can be used in fuel products and the hydrogen produced by gasification can be used in the hydrocracking process.
Vacuum residuum from Cold Lake bitumen was the feedstock used in the experiments described here. A special technique was used to ensure that the residuum made good contact with the catalyst. First the residuum was dissolved in tetrahydrofuran (THF) to form a solution of 0.29 g Cold Lake residuum/mL solution. The catalyst was dried at 110 C for 6 hours, calcined at 500 C for 2 hours, and then used to constitute a bed fluidized with nitrogen. The solution of Cold Lake residuum was added dropwise to the fluidized bed of catalyst, as shown in Figure 1. Subsequently the THF solvent was evapourated by heating the catalyst to 80 C. From a visual inspection, this technique appeared to produce a relatively even distribution of the residuum on the catalyst surface.
Catalytic cracking experiments were performed in the microbalance reactor shown in Figure 2. The catalyst was supported on one arm of the weighing mechanism. Nitrogen gas flowed continuously into both the weighing mechanism chamber and the side arm of the hangdown tube.
Nitrogen from both entrances flowed past the catalyst within the hangdown tube and finally into a condenser that was immersed in a liquid nitrogen bath. When the cracking reaction occurred, the product molecules entered the vapour phase and were swept away by the nitrogen carrier gas. The decrease in weight of the catalyst caused by vaporization of low molecular weight product molecules formed during the cracking reaction, was recorded. The amount of coke on the catalyst was obtained as the difference between the original weight of residuum and the final weight loss during the cracking reaction.
Two types of experiments were performed. In the first type of experiment the temperature was increased in a series of steps.