Particle Simulation of Gas Fluidized Beds

The results presented so far in this section correspond to the regime of fully developed riser flow. Kuipers and van Swaaij (1996) applied the KTGF-based model developed by Nieuwland et al. (1996b,c) to study the effect of riser inlet configuration on the (developing) flow in CFB riser tubes and found that the differences in computed radial profiles of hydrodynamic key variables (i.e., gas and solids phase mass fluxes) rapidly disappear with increasing elevation in the riser tube. 

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Wang, X., Rhodes, M., 2003. Determination of particles residence time at the wall of gas fluidized beds by discrete element method simulation. Chem. Eng. Sci. 58 387-395. 

A 2D soft-sphere approach was first applied to gas-fluidized beds by Tsuji et al. (1993), where the linear spring-dashpot model—similar to the one presented by Cundall and Strack (1979) was employed. Xu and Yu (1997) independently developed a 2D model of a gas-fluidized bed. However in their simulations, a collision detection algorithm that is normally found in hard-sphere simulations was used to determine the first instant of contact precisely. Based on the model developed by Tsuji et al. (1993), Iwadate and Horio (1998) incorporated van der Waals forces to simulate fluidization of cohesive particles. Kafui et al. (2002) developed a DPM based on the theory of contact mechanics, thereby enabling the collision of the particles to be directly specified in terms of material properties such as friction, elasticity, elasto-plasticity, and auto-adhesion. 

The application of large scale computer simulations in modeling fluidized bed coal gasifiers is discussed. In particular, we examine a model wherein multidimensional predictions of the internal gas dynamics, solid particle motion and chemical rate processes are possible. 

Fig. 20. Snapshots of particle configurations for the simulation of slug formation with homogeneous inflow conditions in a 2D gas fluidized bed. Top Nonideal particles e = 0.9 and...

Xu, B.H. and Yu, A.B. (1997), Numerical simulation of gas solid flow in a fluidized bed by combining discrete particle method with computational fluid dynamics, Chem. Eng. Sci., 52, 2785. 

Fig. 10.13. Particle circulation patterns at various fluidizing velocities for a gas fluidized bed consisting of 0.42 — 0.6 (mm) diameter glass beads [90]. Simulated flow patterns obtained with the CPV model of Lindborg [91]. (a) = 32 (cm/s) and...

The cooling of copper spheres at different initial locations in a gas-fluidized bed was examined (Zhou et al., 2009). In physical experiments, the temperature of hot spheres is measured using thermocouples connected to the spheres (CoUier et al., 2004 Scott et al., 2004). The cooling process of such hot spheres can be easily traced and recorded in the CFD-DEM simulations, as shown in Fig. 14A. The predicted temperature is comparable with the measured one. The cooHng curves of nine hot spheres are slightly different due to different local fluid flow and particle structures. 

The topic of the drag coefficient in multiple particle systems has regained interest because of the CFD simulations of turbulent multiple particle systems such as agitated sohd suspensions, aerated stirred vessels, solid—liquid fluidized beds, and gas-sohds cyclones. A recent review on this topic can be found in Ghatage et al (2013) who also presented new experimental data on the shp velocity of a foreign particle in sohd—Uquid fluidized beds. 

Wang J A review of Eulerian simulation of Geldart A particles in gas-fluidized beds, Ind Eng Chem Res 48 5567-5577, 2009. http //dx.doi.org/10.1021/ie900247t. 

Table 1 gives the values of design and operating parameters of a scale model fluidized with air at ambient conditions which simulates the dynamics of an atmospheric fluidized bed combustor operating at 850°C. Fortunately, the linear dimensions of the model are much smaller, roughly one quarter those of the combustor. The particle density in the model must be much higher than the particle density in the combustor to maintain a constant value of the gas-to-solid density ratio. Note that the superficial velocity of the model differs from that of the combustor along with the spatial and temporal variables. 

Subsequently, simulations are performed for the air Paratherm solid fluidized bed system with solid particles of 0.08 cm in diameter and 0.896 g/cm3 in density. The solid particle density is very close to the liquid density (0.868 g/ cm3). The boundary condition for the gas phase is inflow and outflow for the bottom and the top walls, respectively. Particles are initially distributed in the liquid medium in which no flows for the liquid and particles are allowed through the bottom and top walls. Free slip boundary conditions are imposed on the four side walls. Specific simulation conditions for the particles are given as follows Case (b) 2,000 particles randomly placed in a 4 x 4 x 8 cm3 column Case (c) 8,000 particles randomly placed in a 4 x 4 x 8 cm3 column and Case (d) 8,000 particles randomly placed in the lower half of the 4x4x8 cm3 column. The solids volume fractions are 0.42, 1.68, and 3.35%, respectively for Cases (b), (c), and (d). 

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