Quality of Distillates from Repeated Recycle of Residue

The yields of gas, liquid, and coke from the coking of Athabasca vacuum residue were determined as a function of conversion of the residue. High conversion was achieved by subjecting the unconverted residue to repeated coking steps. The cumulative yield of liquid increased by 12% from stage 1 of coking, where 80% residue conversion was obtained, to stage 3 where over 97% cumulative residue conversion was obtained. The quality of the distillates obtained at each stage deteriorated based on the concentrations of nitrogen and polyaromatic hydrocarbons. Although coking reactions resulted in a significant decrease in the number of paraffinic chains, the average chain length remained constant. A mass balance showed that the aromatic carbon increased in the first coking step, then remained constant.