Wurster velocity distributions

Fig. 7.50 Instantaneous particle positions and velocity distributions inside the Wurster tube at t= 1.4s colors indicate the velocity magnitude.

Summarizing, it can be stated that a homogenous velocity distribution of the particles inside the Wurster tube was achieved at a high spout velocity of 160 m s and at low gap distances between the tube and distributor of 5 times the particle... 

Fig. 7.56 Instantaneous position and velocity distribution of the particles in the Wurster zone for different gap distances at time t= 1.4s.

Fig. 4.19 I nstantaneous particle positions and particle velocity distributions at identical process conditions, (a) Conventional fluidized bed with top spray (b) Wurster equipment (c) ProCell spouted bed (Fries, 2012). Colors indicate the particle velocities.

First simulations were performed for the empty apparatus without the sohd particles. The atomizer gas flow velocity was set to 80 m/s. The Wurster gap distance was 22 mm. Figure 18 illustrates the simulated steady-state flow field in the empty apparatus. F re 19 shows the time-averaged steady-state flow profile obtained with CFD simulation of the empty apparatus at a volume flow of 143 m /h. In the vicinity of the atomizer, the gas velocity reaches its maximum. Figure 19 shows the distribution of the gas velocity depending on the distance to the bottom plate. The flow becomes wider and slower with the apparatus height. At the small distances to the bottom the distribution of the air velocity is bimodal due to high differences in the air velocity in different zones of the bottom plate. At the height H = 820 mm the velocity distribution shows only a small wide peak. 

Figure 19 The simulated velocity distribution on the different distances from the bottom plate in the empty Wurster coater.

Figure 23 Time-averaged particle-particle collision velocity distribution for entire apparatus and inside the Wurster tube (for the case study 1).

A spouted bed process is similar to the Wurster process. The principal difference is that an air velocity differential needed to properly circulate particles through the nozzle spray zone is created by a tapered coat chamber design rather than an air distribution pattern created by the fluidization plate. An illustration of a spouted bed is provided in Figure 7.5. Process ciir entering through the fluidization plate passes upward past the nozzle, Ccurying particles with it as they feed in from the periphery of the plate and lower chamber. Spray is directed upward with the direction of pmticle flow. Vertical increase in cross-sectional area reduces the air and particle velocity to allow pmticles... 

Figure 21 shows the radial distribution of the time-averaged particle velocities in the Wurster coater for at different heights from the bottom. Inside the Wurster tube the flow velocities are significandy higher than outside the tube. The particle velocities inside the tube clearly increase with the height because of the acceleration of the particles due to process gas flow and atomizer gas flow. 

Inside the Wurster tube, the particles show a high rotation (average rotational velocity equals 253.2 rad/s for the study case 1), which is very desirable for the homogenous distribution of the spray droplets on the particle surface to produce a homogenous and closed film. However in a moment only 3.48% of aU particles are situated inside the tube. This results in a smaller number of collisions in comparison with the dense bed in the annulus... 

A quantitative comparison of the particle dynamics in the Wurster tube for all three case studies is presented in Table 8. The time-averaged distributions of the vertical component of translational velocity and angular velocity of particles inside the tube are shown in Figs. 25 and 26, respectively. 

The radial distribution of the particle velocities depends significandy on the height, as it can be seen in Fig. 27. Here, the time-averaged total velocities and their vertical components are presented for different gap distances. The time-averaged total velocity of particles v, which are moved on a radial distance r from the central axis of the Wurster-coater apparatus, is given as ... 

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