Defluidization

Figure 13 shows two pipe distributors, one in a branched and one in a ring configuration. These distributors minimize weeping, have good turndown, may requite the lowest pressure drop, and avoid the need for a plenum chamber. They are also well suited to multiple-level fluid injection. The disadvantages of these distributors are that there are defluidized soHds beneath the distributor and the mechanical design is more complex. 

Fig. 22. Standpipe and standpipe pressure profiles showiag (—) fluidized flow and (--------) packed bed or defluidized flow.

FIOR Process. In the FIOR process, shown in Figure 5, sized iron ore fines (0.04—12 mm) are dried in a gas-fired rotary dryer. A skip hoist dehvers the dry fines to lock hoppers for pressurizing. The fines pass through four fluidized-bed reactors in series. Reactor 1 preheats the ore to 760°C in a nonreducing atmosphere. Reactors 2, 3, and 4 reduce the ore at 690—780°C. At higher (ca 810°C) temperatures there is a tendency for the beds to defluidize as a result of sticking or hogging of the reduced material. 

Air atomizing nozzles are commonly used to control the droplet-size distribution independently of the liquid feed rate and to minimize the chances of defluidization due to uncontrolled growth or large droplets. 

Aside from proper aeration, the flowing catalyst must coniain sufficient 0-40 micron fines to avoid defluidization. 

ABB Lummus s RTD consists of a two-stage reactor cyclone system (see Figure 9-2). The riser cyclones (the first stage) are hard-piped to the riser. Attached to the end of each riser cyclone dipleg is a conventional trickle valve as shown in Figure 9-3. Each trickle valve has a small opening to prevent catalyst defluidization, which can be a problem, especially during start-ups. 

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, the gas volume decreases when the reaction involving a decrease in the volume is carried out at constant temperature and under constant pressure. If the gas in the emulsion phase cannot be compensated by the gas supply from bubbles, the emulsion phase is condensed and bubbles cannot rise through the emulsion phase. Finally, defluidization in the bed occurs. This part of the packed bed will be lifted up like a moving piston. 

Fig. 2. Schematic time-series pictures of the formation and movement of a defluidized part.

Fig. 2. Photos for a) before fluidization, b) good fluidization, c) onset of defluidization at the entrance to the introducing particles and d) linkage of large particles of the bed surface...

Fig. 4. Signal indication for (a) good fluidization, (b) local defluidization...

The effect of gas velocity on the bed pressure drop (-APbed) with a uniform distributor (Fopen = 1.68 %) in the beds with decreasing and increasing Ug is shown in Fig. 5. As can be seen, -APbed maintains almost a constant value rmtil the minimum velocity of full fluidization (Unur) and then it decreases with decreasing Ug. As shown, Umfd is the maximum velocity of full defluidization, Umpf is the minimum velocity of partial fluidization, and Umu is the minimum velocity of full fluidization [6]. 

Compo (1987) also investigated the effect of temperature on the velocity required to prevent defluidization for (i) materials which agglomerated rapidly after their sintering temperature (Ts) was reached (coal, plastics, etc.), and (ii) materials which agglomerated slowly after reaching Ts (calcium chloride, etc.) For the rapidly agglomerating materials, the gas velocity had to be increased rapidly to prevent defluidization after Ts was reached. For the weakly agglomerating materials, a much smaller increase in gas velocity was required to prevent defluidization after Ts was reached. 

Main Advantages Canminimizeweep- Possible Disadvantages Defluidized sol-... 

Compo, P., Pfeffer, R., and Tardos, G. I., Minimum Sintering Temperatures and Defluidization Characteristics of Agglomerating Particles, Powder Technology, 51(1) 87-103 (1987)... 

Compo, P., Tardos, G., and Pfeffer, R., Thermally Induced Agglomeration and Defluidization Characteristics of Fluidizable Particles, Proceedings of the Second World Congress in Particle Technology, Kyoto, Japan (1990)... 

Siegell, J. H., High Temperature Defluidization, Powder Technol., 38 13-22 (1984)... 

Broadly speaking, for G/S systems, three modes of particle-fluid contacting may be recognized to take place simultaneously as shown in Fig. 43 bubbles containing sparsely disseminated particles, emulsion of densely suspended particles, and defluidized (transient as well as persistent) particles not fully suspended hydrodynamically by the flowing gas. For all intents and purposes, it is desirable to suppress bubbles and to prevent defluidization. 

A number of driers may be used in series. For example, a cascade of well-mixed driers approximafes fo fhe performance of a plug flow drier. However, of more pracfical significance is the combination of a well-mixed drier followed by a plug flow drier for drying very wet solids which may defluidize if fed directly to a PF unit. The CSTR drier is better able to cope with the high moisture content feed and the plug flow drier allows a low final moisture content to be obtained. 

Vibro-fluidization is used for cohesive, sticky solids or friable foods (Bahu, 1997) and for materials which would defluidize in a conventional plug flow drier (Reay and Baker, 1985). Vibration of the bed increases the drying rate due to an increase in the surface-to-bed heat transfer coefficient (Reay and Baker, 1985), particularly below minimum fluidizing velocity. A detailed treatment of the mechanisms of vibration fluidization is given by Reay and Baker (1985). 

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