Fluidization quality

Fluidization quality of fluidized catalyst beds involving a decrease in gas volume... 

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. 

It is reported [1] that the fluidization quality was drastically decreased when the hydrogenation of CO2 was carried out in a fluidized catalyst bed (FCB). Recently, the phenomena occurring in the bed were directly observed [2] and it was found that the upper part of the emulsion phase was defluidized and this packed particles was lifted up through the column like a moving piston. 

In the case of a FCB with small particles, the emulsion phase expands [5, 6, 7] when the bed is fluidized. This would make the bed sensitive to the decrease in the gas volume in the emulsion phase. If this assumption is true, we can postulate that the fluidization quality is hardly affected by the gas-volume reduction when the particles, which induce a small emulsion phase expansion, are used. The emulsion phase expansion decreases with increasing particle size and density [6]. In the present study, therefore, the particles used were larger and heavier than that generally used in the FCB. We carried out the hydrogenation of CO2 in a... 

When a=8.8 at 523 K, similar phenomena were observed. On the other hand, when the value of a was 37, the good fluidization state was maintained even at 523 K. There was no direct relationship between the conversion and fluidization quality. This is because that the effect of the gas-volume reduction was low at high a values. 

The apparent reaction rate constant for the first order reaction, k, was calculated from the conversion of CO2. Since the gas-volume reduction rate increased with k, a poor fluidization was induced by high reaction rate. We investigated the effect of the rate of the gas-volume change on the fluidization quality. The rate of the gas-volume change can be defined as rc=EA(dxA/dt), where Sa is the increase in the number of moles when the reactants completely react per the initial number of moles. This parameter is given by 7-1. When the parameter, Ea, is negative, the gas volume decreases as the reaction proceeds. 

As the fluidizing quality of the powder deteriorates from Group A to Group B, however, the range for fast fluidization dwindles, until, for sandy materials, TURBULENT often jumps to TRANSPORT without the intermediate FAST stage. This is shown in Fig. 25 for a titanomagnetite concentrate, which is heavy and comparatively coarse. 

The Wurster bottom spray system has also been used successfully to coat particles as small as 100 microns. Attempting to coat smaller particles may result in the same difficulties as discussed in the previous segment. Batch capacities range from a few hundred grams to approximately 600 kg. Because fluidization quality is affected by batch size, at least 50% of the volume outside of the partition should be occupied by the uncoated product. Finished product batch size (for fine and intermediate particles) can be determined by the following equation . 

At the high-velocity end of fast fluidization, the transition from dense to dilute phase operation noted for coarse solid particles is still possible, though the better the solid material is designed for improved fluidization quality, the less sudden the transition will be. 

Also, the fluidizing quality of a powder can be quantified by using a dimensionless time , ranging from 0 to oo the higher the value of 0, the better the powder. It should be noted from Fig. 4 that a powder with good fluidizing quality is identified by the following characteristics ... 

Yang, Z., "Effect of Particle Size Distribution on Fluidization Quality, M. S. Thesis, Inst. Chem. Metall. (1982). 

Fluidization quality in terms of material properties, particle characteristics, and particle group behavior thus needs to be assessed on three scales gross scale of the fluidized bed (macro scale), aggregate scale of gas bubbles, and particle clusters (meso scale), and scale of the discrete, individual particles (micro scale), as described in Chapter 4. 

Zheng, W. Influence of primary particle properties on fluidization quality, M.S. thesis, Institute of Chemical Metallurgy (1987). 

A related problem occurs during the fluidized-bed combustion of low-rank coals. The fluidized-bed combustion of high-sodium coals, particularly in silica beds, leads to the formation of agglomerates of ash and bed material. The agglomerates decrease heat transfer and fluidization quality of the bed in severe cases the formation of... 

Stable, long-term operations of fluidized-bed reactors can be compromised by bed material-to-polymer interactions, which can generate worsening of fluidization quality. Several investigations carried out with monopolymeric feeds of packaging wastes (PE, PP, PET and PVC)... 

Apart from density and particle size, several other solid properties, including angularity, surface roughness and composition may also signiflcantly affect the quality of fluidization. However, in many cases Geldart s classiflcation chart is still a useful starting point to examine fluidization quality of a speciflc gas-solid system. 

The choice to work under a pressure of 10 Torr was a compromise between the concentration of the active species which increases when the pressure decreases and the fluidization quality which was better when the pressure increases. 

Fluidization quality of fine and large particles can be enhanced by the assistance of external means such as vibration or agitation. Moreover, these particles can be immersed in a bed of fluidizable inert particles to improve their fluidization quality [67]. 

Daleffe et al. [68] reported that vibrated FBDs can improve solids circulation and reduce particle agglomeration as well as channeling in the bed. This in turn improves the powder fluidization quality. Improved solids circulation and interaction have also been reported in the drying of whole milk powder [69]. Reduction in airflow rate when vibration is applied reduces product entrainment and reduces the size and cost of gas-cleaning equipment without adversely affecting the drying kinetics. 

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