Radial Distribution of Particle Concentration in Downer

Fluid catalytic cracking(FCC) is one of the most critical oil secondary processing units in the refinery and it undoubtedly plays a vital role in China petroleum processing industry. More than 70% gasoline as well as about 30% diesel oil is produced via the FCC process. In a modern FCC unit, the catalytic cracking reaction mainly occurs in the riser reactor.However, the core-annular structure and gas-solid backmixing property of the riser reduced the conversion rate of heavy oil and product selectivity.Compared with the riser, the downer has the advantages of close to horizontal thrusting flow pattern, uniform gas-solid distribution, short gas-solid contact time, small gas-solid axial backmixing and narrow residence time distribution, etc. It has a wide application prospect in the rapid catalytic cracking process of heavy oil and rapid cracking of coal powder.

In downer reactor, the flow and distribution of oil & catalysts will influence the whole reaction directly. However, the actual condition is far from the ideal uniform contact of gas with solids phase.The particle concentration distribution of downer has obvious non-uniform structure. This non-uniform radial concentration distribution of particle/gas may result in an uneven contact of two phases and lead to undesirable effects on the cracking reaction.

In these years, there have been many theoretical explanations for the distribution of particle concentration in circulating fluidized bed.However, most of the theories are only qualitative descriptions without a quantitative analysis.At present, there is no ideal explanation for the cause of the radial distribution of particle concentration. Most studies focus on describing the phenomenon or quantifying the distribution of particle concentration based on experiment or numerical simulation.The current explanations include boundary layer theory, gas-solid phase drag theory, local pressure drop theory and turbulent diffusion theory.However, the main problem with these explanations is that they only stay at the qualitative description stage and fail to provide convincing quantitative analysis.

In this paper, the aerodynamic theory and field theory is “transplanted” into the two-phase flow. A lateral force, namely the Kutta-Joukowski force, is used to explain and quantify the formation of non-uniform structure in a downer.

Both the theoretical analysis and analyse others data were employed in this paper. The method to calculate local Kutta-Joukowski force is proposed via theoretical analysis. This force is the function of particle velocity curl (The particle velocity curl does not mean the actual rotation, but a function of velocity gradient), slip velocity and gas density.

Combined with that proposed in the published literatures, the distribution of Kutta-Joukowski lateral force in the downer is given. On the one hand, and effects of operating parameters on the Kutta-Joukowski lateral force distribution profile is analysed. The result shows Kutta-Joukowski lateral force is the main reason of the non-uniform structure occurence. The direction of the force is the lower part of the concentration point to the higher part of the concentration; while its valus were mainly determined by the particle physical property, the particle flux, the superficial gas velocity, the radial/axial position.The empirical correlation of the radial distribution of Kutta-Joukowski lateral force in the descending bed was obtained by regression of the experimental data.Finally,On the other hand, the particle concentration gradient force is introduced and quantified. This force balances the Kutta-Joukowski lateral force in the downer. The quantitave correlation of these two forces is established.The empirical correlation formula of concentration gradient force coefficient K is regression.

Key Words: Downer; non-uniform structure; Particle Velocity Curl; Slip Velocity; Kutta-Joukowski Lateral Force