(341c) Downer-to-Riser Coupling Technique for Heavy Oil Catalytic Cracking

Fluid catalytic cracking (FCC) unit converts low value heavy hydrocarbons into a series of more valuable products such as gasoline and light olefinic compounds. Many recent developments in the FCC technique have focused on achieving cleaner gasoline with less content of olefins to meet the growing environmental concerns and maximizing some high-added value products such as ethylene and propylene. It is helpful to improve the product quality and product distribution by carrying out the main primary and secondary reactions in two different reaction zones with different hydrodynamics and process conditions. Consequently, the FCC reactions can be operated in coupled reactors, such as the riser-riser coupled reactor and the riser-downer coupled reactor, under a controllable and selective mode. A coupled high-density downer-to-riser reactor was proposed and demonstrated feasible operation at high flux conditions in a pilot-scale experimental setup. This coupled reactor aims to utilize both downer and riser advantages to improve the gasoline quality and efficiently control the distribution of FCC products. Downer accommodates high severity cracking reactions at the initial stage with the benefit of near plug flow operation; while riser is operated at a lower temperature with longer contacting time to promote hydrogen shift reactions. In this study, a 2D convection-diffusion-reaction model incorporated with hydrodynamics and a 14-lump kinetic model is established to predict the conversion and yields of different products in the coupled design. The kinetic model is based on a molecular description of cracking and hydrogen shift reactions. Promising results are achieved for the coupled downer-to-riser reactor compared with the conventional riser and other coupled reactors.