The efficient conversion and optimal utilization of heavy oil has great practical significance to the national economic development. Fluid catalytic cracking (FCC) is an effective vehicle for heavy oil upgrading. However

the riser

as a key component 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. The downer reactor

where gas and solids move downward co-currently has unique features such as the plug-like flow structure and much more uniform gas-solid distribution compared to the riser reactor. The downer is therefore acknowledged as a novel multiphase flow reactor with great potential in upgrading heavy oil. However

low solids holdup and poor initial gas-solid contacting hinder the promotion of the downer reactors. In this paper

previous studies on increasing solids concentration and initial gas-solid mixing in the downer reactors are summarized. Studies on the mechanism of increasing solids concentration and promoting initial mixing will not only enrich the fundamental research but also benefit the promotion of the downer reactor in its applications.