A DEM Numerical Simulation Of Gas-solid Two-phase Flow In Circulating Fluidized Bed Riser

As an efficient, bubble-free gas-solid contact technology, circulating fluidized bed hasbeen widely applied in the field of catalytic cracking, drying, power generation, combustion,desulfurization, etc. Riser is the main reactor of circulating fluidized bed, the gas-solid flowbehavior within the riser has a great impact on momentum, mass, and energy transfer betweenthe two phases. The Discrete Element Method (DEM) directly tracks the trajectory of each ofthe solid particles, it can reflect the more realistic interactions between the particles. Therefore,it has been widely applied in the study of the gas-solid flow problem. In this paper, gas-solidflow behavior in the riser is numerically simulated by using DEM. Gas turbulence isintroduced by the standard k-ε two-equation model, rolling friction between particles is alsoconsidered, and van der Waals force and liquid bridge force are respectively introduced in dryand wet particles system.The discrete element method is applied to simulate gas-solid flow behavior in the riser.Firstly, simulation results are compared with experimental findings of literature for verifyfeasibility of the code. Then gas-solid flow behavior of uniform sized particles andpolycomponent of particles in the riser are investigated. Furthermore, the effect of thesuperficial gas velocity and solid flux to particle flow are also studied. The results indicatethat the numerical predictions are in good agreement with the experimental results except foranalog values are slightly lower than the experimental dates. By simulating to the uniformsized particles flow, the results indicate that gas-solid flow shows significant heterogeneity inthe riser, and there is a set amount of backmixing of both gas and particle phase. Furthermore,it is demonstrated that particle velocity increasing and particle concentration decreasing withincreasing superficial gas velocity, and particles had more homogeneous distribution. Largersolid flux lead to more heterogeneous particle distribution and higher particle concentration,while particle velocity is insensitive to changes of solid flux. By simulating to polydisperseparticle flow, it is found that particles of different diameters are appeared different flowbehaviors. Concentration of large particles is higher than small particles; large particles showa typlical core-annulus flow structure with a dense phase near the walls and a dilute phase atthe center of riser, small particles in contrast. Compared with large particles, small particleshave a larger velocity and granular temperature. Mikami model is introduced to add liquid bridge force between wet particles, and vander Waals force is ignored by analysis. Flow behavior of wet particles in riser is simulatedwith the discrete element method. First, the flow behavior of wet particles with differentmoisture content is researched. Then, agglomeration characteristics of wet particles isanalysed quantitatively. The results show that because of existence of the liquid bridge force,it appears wet particles agglomeration phenomenon, and the motions of wet particles in theriser have two ways with single particles and agglomerates. Agglomerate duration time andagglomerate time fraction increase along the radial direction, and with increasing of moisturecontent, agglomerate duration time and agglomerate time fraction decrease at the center andincrease near the walls. Frequency of agglomerate occurrence decrease along the radialdirection, and greater moisture content lead to lower frequency of agglomerate occurrencenear the wall.