Coupling of mass transfer and reaction in coking of thin films of an Athabasca vacuum residue

The coupling of reaction and mass transfer was investigated for the thermal cracking of thin films of an Athabasca vacuum residue. Thin films of the v acuum residue were coated on the interior of a stainless steel tube and the n heated to 530 degreesC using induction heating. Vapor products were swept out of the reaction zone with nitrogen, and reactions in the liquid phase were allowed to proceed to completion. Coke yield, total liquid product, an d gas make were determined for initial film thicknesses ranging from 10 to 80 mum. Product qualities of the liquid samples were characterized by high- temperature simulated distillation and microcarbon residue concentration. T he coke yield decreased as the film thickness was reduced, while the yield of distillate products (< 524 degreesC boiling point) increased. As the fil m thickness was reduced, the mode of mass transport shifted from bubbling i n thick films to diffusion through a stagnant thin film. These results were consistent with a decrease in the trapping of cracked products in coke as the film thickness was reduced, because of more effective transport of prod ucts out of the liquid phase. A mathematical model was developed to analyze the results and to estimate the diffusivity of gases in the reacting liqui d phase.