Fluidizing characteristics

The effectiveness of a fluidized bed as a ehemical reactor depends to a large extent on the amount of convective and diffusive transfer between bubble gas and emulsion phase, since reaction usually occurs only when gas and solids are in contact. Often gas in the bubble cloud complex passes through the reactor in plug flow with little back mixing, while the solids are assumed to be well mixed. Actual reactor models depend greatly on kinetics and fluidization characteristics and become too complex to treat here. 

The fluidization characteristics of an FCC catalyst largely depend on the unit s mechanical configuration. The percentage of less than 40 microns in the circulating inventory is a function of cyclone efficiency. In units with good catalyst circulation, it may be economical to minim the fraction of less than 40-micron particles. This is because after a few cycles, most of the 0-40 microns will escape the unit via the cyclones. 

PSD is an important indicator of the fluidization characteristics of the catalyst, cyclone performance, and the attrition resistance of the catalyst. A drop in fines content indicates the loss of cyclone efficiency. This can be confirmed by the particle size of fines collected downstream of the cyclones. An increase in fines content of the E-cat indicates increased catalyst attrition. This can be due to changes in fresh catalyst binder quality, steam leaks, and/or internal mechanical problems, such as those involving the air distributor or slide vah es. 

Effect of Uniformity of Gas Distribution on Fluidization Characteristics in Conical Gas Fluidized Beds... 

Catalyst powders with carefully specified particle size distribution have been known to possess good fluidization characteristics. Generally, addition of fine particles to coarse particles tends to improve the latter s fluidization characteristics. Experiments were thus conducted on binary particle mixtures, each consisting of a fairly close particle size distribution. 

Figure 50 illustrates the fluidizing characteristics of a set of Group B-A mixtures of relatively coarse (212 microns) polyethylene spheres mixed with a relatively fine (27 microns) FCC catalyst. As the fine FCC catalyst is added, there is a progressive predominance of the Group A fluidizing characteristics of the finer component. 

Figure 51 shows the effect of adding talc (7 microns), Group C, to a coarse sea sand (165 microns), Group B. The beneficial effect of the fines on the fluidizing characteristics is shown to increase to a fines composition of around 25%, and then it diminishes with further fines addition. 

Figure 52 plots the dimensionless subsidence time for the six sets of Group A-A and Group B-A binary mixtures for different fractions of fines, Xp showing that the improvement of fluidizing characteristics by addition of fine particles increases monotonically with increasing percentage of the fines. 

Figure 52. Improving fluidizing characteristics by fines addition, solid groups A-AandB-A. (Yang, Tung, andKwauk, 1985.)...

This synergism of Group B-C mixtures testifies to the significance of particle size selection and particle size distribution design, in order to tailor a solid particulate material to certain desired fluidizing characteristics. 

The larger is the value of Sy, the stronger is the mutual synergistic interaction between the coarse and fine particles. Also, an effective fine particle additive to improve the fluidizing characteristics of coarse particles calls for a large value of m produced with minimal amount of the fine material, that is, a small value of x2. 

Tung, Y., Yang, Z., Xia, Y., Zheng, W., Yang, Y., and Kwauk, M., Assessing Fluidizing Characteristics of Powders, Sixth Int. Fluidization Conf, Banff, Canada (1989)... 

In some cases, especially for very fine powders, researchers have looked at other properties to explain or classify product behavior. For example, Gel dart et al. (1984) have found that the ratio of tapped to aerated bulk density provides a good indication of the likely fluidization characteristics of fine and cohesive powders. 

In an early attempt to remove mists from a gas stream, by using a fluidized bed as a kind of filfer (Meissner and Mickley, 1949), if was found that the operation worked well if the bed particles had a porous structure, and that when non-porous particles were used fluidization ceased (i.e. the bed quenched) at very low moisture contents. This view is supported in work on the effect of bed moisture on the fluidization characteristics of fine powders (D Amore et al., 1979) in which if was shown that porous materials can tolerate considerably more liquid than non-porous particles (such as glass ballotini, sand and limestone) before what these workers call bed compaction occurs. Smith and... 

DAmore, M., Donsi, G. and Massimilla, L., The influence of bed moisture on fluidization characteristics of fine powders. Powder Tech., 23 (1979) 253-259. 

The catalyst samples were screened to give a 200- by 325-mesh particle size sample. This was done to remove fines and coarse particles and hence assure good fluidization characteristics with minimal entrainment of fines. As a result, the majority of the particles in the sample has a particle size between 44 and 53 microns. Dry ice was used to eliminate plugging of the screen due to static electricity. 

Table 2. Fluidization characteristics of different matrices suitable for protein adsorption...

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