(70e) Studies on the Mechanism and Regulation-Control Method of a High Density Gas-Solids Circulating Fluidized Bed Downer

The efficient conversion and optimal utilization of heavy oil has great practical significance to the development of national economy. Fluid catalytic cracking (FCC) is a critical approach to deal with heavy oil. Riser, as the key part of the FCC unit, is characterized by the non-uniform “core-annuals” flow structure with relatively significant gas and solids backmixing which may result in low reactant conversion and product selectivity. Downer reactor, in which gas and solids move downward co-currently, has unique features such as the plug-flow reactor performance and relatively uniform gas-solids distribution compared to the riser reactor. Downer is therefore acknowledged as a novel multiphase flow reactor with great potential in dealing with heavy oil. However, low solids holdup and poor initial gas-solids contacting hinder the promotion and application of downer reactor. Mechanism of increasing solids holdup and approaching high density downer have not been well understood until now. This study starts from the entrance of the downer to in-deep investigate its effects on high density operating condition and to study the contributions of the downer entrance to accomplishing high solids holdup and good gas-solids contacting and in downer reactor. On the other hand, all parts of the entire circulation system have been optimized in order to develop a regulation and control method to obtain and maintain a stable operating of the high density downer. Based on the above work, the mechanism of achieving the high density downer has been demonstrated, a multi-scale computational model has also been established to preciously describe the hydrodynamic characteristics and the scale-up of high density downer has been illustrated all of which can provide the fundamental data and basic theoretical support to develop a technology of downer catalytic cracking of heavy oils and extend the applications of high density downer reactors.