Fluidization Behavior

Fluid bed processes have been subject to many problems and uncertainties in development and scale up from bench-scale reactors. The fluidization behavior of each process seems different and very often does not meet expectations based on experience with earlier plants. With hindsight fluid cat cracking seems to be an ideal system from the point of view of easy operation and straightforward scale up. 

Fig.3. Fluidization behaviors of nano particle (TiOj 21nm)...

The fluidization quality significantly decreased when the reaction involving a decrease in the gas volume was carried out in a fluidized catalyst bed. In the present study, we carried out the hydrogenation of CO2 and used relatively large particles as the catalysts. Since the emulsion phase of the fluidized bed with these particles does not expand, we expected that the bed was not affected by the gas-volume decrease. However, we found that the fluidization quality decreased and the defluidization occurred. We studied the effects of the reduction rate of the gas volume and the maximum gas contraction ratio on the fluidization behavior. 

FCB using these catalyst particles. We investigated the effects of the gas-volume reduction rate and the maximum contraction ratio on the fluidization behavior during the reaction. 

The gas composition at the outlet of the reactor was determined using gas chromatography. The selectivity of methane was almost 100%. We directly observed the fluidization behavior and photographed it using a video camera with recording onto a videotape. We also measured the expansion of the emulsion by the bed collapse method [10] during the reaction. 

Numerical simulation of particle fluidization behaviors in a rotating fluidized bed... 

So far, some researchers have analyzed particle fluidization behaviors in a RFB, however, they have not well studied yet, since particle fluidization behaviors are very complicated. In this study, fundamental particle fluidization behaviors of Geldart s group B particle in a RFB were numerically analyzed by using a Discrete Element Method (DEM)- Computational Fluid Dynamics (CFD) coupling model [3]. First of all, visualization of particle fluidization behaviors in a RFB was conducted. Relationship between bed pressure drop and gas velocity was also investigated by the numerical simulation. In addition, fluctuations of bed pressure drop and particle mixing behaviors of radial direction were numerically analyzed. 

In this study, particle fluidization behaviors in a RFB were numerically analyzed by using a DEM-CFD coupling model [3]. The particle motion was calculated by DEM, which calculates the motion of each particle by integrating the Newton s equations for individual particle step by step, allowing for the external forces acting on a particle. Equations of transitional and rotational motions for individual particles are as follows ... 

Fig. 3 shows the calculated and experimental results of particle fluidization behaviors in a RFB. A high-speed video camera (FASTCAM MAX, Photoron CO., Ltd.) was used for visualization of actual particle fluidization behavior. The bubbling fluidization behaviors, such as the bubble formation, eruption and particle circulation with rotational motion, could be well simulated, and these behaviors were also observed in the experimental results. 

Fig. 6 shows the FFT spectrum for calculated bed pressure drop fluctuations at various centrifugal accelerations. The excess gas velocity, defined by (Uo-U ,, was set at 0.5 m/s. Here, 1 G means numerical result of particle fluidization behavior in a conventional fluidized bed. In Fig. 6, the power spectrum density function has typical peak in each centrifugal acceleration. However, as centrifugal acceleration increased, typical peak shifted to high frequency region. Therefore, it is considered that periods of bubble generation and eruption are shorter, and bubble velocity is faster at hi er centrifugal acceleration. 

Baeyans, J., and Geldart, D., Predictive Calculations of Flow Parameters in Gas Fluidized Bed and Fluidization Behavior of Various Powders, Proc. Int. Symp. on Fluidization anditsAppl., p. 263 (1973)... 

Group D products generally are of large particle size and/or very high solids particle density. In some respects, fluidization behavior is similar to Group B, although higher gas velocities are required for fluidization. 

Products lying close to a particular boundary may exhibit fluidization behavior from either one of the adjoining categories (e.g., a product which is in close proximity of the A-B boundary may exhibit characteristics from either Group A or B). It is difficult to estimate the error or tolerance associated with each boundary (e g., between Groups A and B, or B and D). 

Irrespective of the method used to produce the granules and the consequent batch size, a signiflcant factor in scale-up is the increase in drying capacity in larger equipment. The heat delivered to the bed of fluidizing granules comes from a combination of inlet air temperature and volume, and, to a lesser extent, inlet air dew point. As previously mentioned, the process air volume is also responsible for fluidization behavior, and this will be the first variable to be considered. 

When scaling-up the fluid-bed process, a major requirement is to produce fluidization behavior on the larger machines equivalent to that used on the scale that provided the basis for process development. To achieve this goal, and minimize attritional effects, the same air velocities for each scale of equipment are required. Thus, the overall increase in air volume required during scale-up will be related to the increase in area of the perforated base plate, and, in the case of the Wurster process, the open area of the partition plate immediately beneath each of the inner partitions. Such calculations are simplified when scaling-up from an 18" pilot scale machine to, say, a 32" machine, since the latter represents a three-multiple of the former, and thus would require a threefold increase in airflow. 

The following criterion could also be used to find the expected fluidization regime for a specified system. Experiments on particulate fluidization show that particle and fluid densities and fluid viscosity are the most significant factors affecting fluidization behavior. On the basis of this, a dimensionless discrimination number /)n has been suggested to... 

These parts are used in fluidized beds for various purposes. For example, gas distributors and various types of baffles are installed to decrease the size of the bubbles. Moreover, draft tubes are used to enhance gas or solid circulation. Other devices such as horizontal baffles limit circulation and backmixing of solids and gas. Horizontal or vertical tubes are used for heat management. Devices used to control or improve fluidization behavior, to improve fluidization of cohesive particles or to achieve solids recovery are within the various internals met in fluidized bed reactors (Kelkar and Ng, 2002). Immersed tubes in small diameter beds may lead to slugging. Furthermore, attrition of particle breakage may change the size distribution and possibly change the fluidization behavior. 

This set of equations is sufficient to characterize a particulate matrix which should be used in fluidized bed adsorption regarding its fluidization behavior. It has to be noted, however, that the correlations have been developed for the fluidization of spherical particles of uniform diameter. In reality, most adsorbents are provided with a certain distribution of particle diameter. In this case, classified fluidization occurs and a modified equation should be used to describe the hydrodynamics of bed expansion [21]. For an estimation of the suitability of a certain matrix for fluidized bed adsorption the correlations shown above are convenient to use and provide sufficient information. The minimum fluidization velocity may be calculated using an average particle diameter as recommended by Couderc [22], In the next section, conventional as well as new matrices shall be described under this respect. 

Particles can be classified into four groups (i.e., Groups A, B, C, and D) based on their fluidization behavior [Geldart, 1973], This classification, known as Geldart s classification, is shown in Fig. 9.1, where particles are classified in terms of the density difference between the particles and the gas (pp — p), and the average particle diameter dp. 

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