Fluidized beds particle size

Fluidized-bed design procedures requite an understanding of particle properties. The most important properties for fluidization are particle size distribution, particle density, and sphericity. 

In a fluidized bed reactor, entrained particles leaving in a dilute phase stream are conventionally and desirably either partially or wholly condensed into a bulk stream and returned to the bed via a centrifugally driven cyclone system. At equilibrium, or when steady state operation is attained, any particle loss rate from the cyclones, as well as the remaining bed particle size distribution, are functions of (a) the rate of any particle attrition within the system and (b) the smallest particle size that the cyclone system was designed to completely collect (i.e., with 100% efficiency), or conversely the largest size which the system cannot recover. These two functions result in an interdependency between loss rate and bed particle size distribution, eventually leading to an equilibrium state (Zenz Smith, 1972 Zenz, 1981 Zenz Kelleher, 1980). 

As noted already, the bed particle size changes inversely with the intensity of fluidization. Hence reduced gas velocity increases granule size and coalesced structures are favoured, while layer growth and smaller particles result at high velocities. A lower limit exists for the gas velocity below which particle movement is inadequate and defluidization occurs. 

During operation waste is blended with required feed additives and fed by air lift and gravity to the calciner. The feed is atomized by air through spray nozzles located on the wall of the calciner vessel. The primary solidification mechanism is the evaporation of atomized liquid droplets on the fluidized-bed particles. A portion of the atomized liquid also evaporates to a dry powder before striking the surface of a bed particle. Therefore, the calciner produces a mixture of powdery solids and granules in the size range 0.05 to 0.5 mm. 

Wiman J, Almstedt AE. Aerodynamics and heat transfer in a pressurized fluidized bed Influence of pressure, fluidization velocity, particle size and tube geometry. Chem Eng Sci 52 2677-2696, 1997. 

Hence the pure attrition tests are not sufficient when a quantitative prediction of the attrition-induced material loss or a prediction of the effect on the bed particle size distribution is required. This is for instance the case in the design procedure of a process, where the capacity of the dust collection system and the lifetime of the bed material must be evaluated (Vaux and Fellers, 1981 Zenz, 1974) and where—above all—the hydrodynamics and thus the bed particle size distribution must be clear (Ray et al., 1987a Zenz, 1971). Another example is when a new generation of catalyst is to be developed for an existing fluidized bed reactor here it might not be sufficient to know the relative hardness in comparison to the previous catalyst generation. For a comprehensive cost-benefit analysis, it is rather necessary to predict the attrition-induced loss rate and to be sure about the hydrodynamics and thus the particle population of the bed material. The same is valid when a fuel or a sorbent is to be changed in a combustion process. 

As mentioned in the introduction, the effect of attrition on the particle size distribution is quite often as relevant as the attrition-induced loss is. The reason is quite obvious it is the strong dependence of the process performance on the bed particle size distribution. In the chemical industry, for example, the content of fines, i.e., the mass of particles below 44 microns, has often been observed to have a strong effect on the fluidized bed reactor performance, de Vries et al. (1972) reported an increase in the conversion of gaseous hydrogen chloride in the Shell chlorine process from 91 to 95.7% with an increase of the fines content in the bed material from 7 to 20%. The same effect was observed by Pell and Jordan (1988) with respect to the propylene conversion during the synthesis of acrylonitrile. They reported on an increase of the conversion from 94.6 to 99.2% as the fines content was changed from 23 to 44%. 

Figure 31 Simulation of the bed particle size distribution in a fluidized bed system. (Reppenhagen and Werther, 2001.)... 

Andersson B-A. Effect of bed particle size on heat transfer in circulating fluidized bed boilers. Powder Technol... 

The catalyst mechanical resistance is a critical property for the success of fluidized beds. Particle—particle collisions at the gas spargers and abrasion in the cyclones tend to attrit the powder and thus, to maintain constant production, makeup catalyst must be added at regular time intervals. Particle size management is another key issue fines (powder with a diameter between 20 and 44 fim) are carried upwards with the gas to cyclones and must be returned to the bed through diplegs. Under certain conditions (e.g. start-up), the powder may become cohesive and can block the diplegs. As a consequence, the fines are carried through the cyclone and accumulate in the filters. This will eventually result in an unscheduled shutdown. 

Because in several applications of three-phase fluidized beds a size distribution is commonly encountered. Fan et al. [75] analyzed the conditions of particle mixing and particle stratification using a binary mixture of solids. Using a 7.62 cm. diameter plexiglass column and two types of binary solid mixtures - 3mm and 4 mm glass particles, and 3 mm and 6 mm glass particles three possible mixing states were observed. These three states... 

Particle Size. The soHds in a fluidized bed are never identical in size and foUow a particle size distribution. An average particle diameter, is generally used for design. It is necessary to give relatively more emphasis to the low end of the particle size distribution (fines), which is done by using the surface mean diameter, to calculate an average particle size ... 

Group D particles are large, on the order of 1 or more millimeters (1000 fim) in average particle size. In a fluidized bed, they behave similarly to Group B particles. Because of the high gas velocities required to fluidize Group D particles, it is often more economical to process these particles in spouted or in moving beds, where lower gas rates suffice. 

Whereas Geldart s classification relates fluidized-bed behavior to the average particle size in a bed, particle feed sizes maybe quite different. For example, in fluidized-bed coal (qv) combustion, large coal particles are fed to a bed made up mostly of smaller limestone particles (see Coal conversion processes). 

Bubble size control is achieved by controlling particle size distribution or by increasing gas velocity. The data as to whether internal baffles also lower bubble size are contradictory. (Internals are commonly used in fluidized beds for heat exchange, control of soflds hackmixing, and other purposes.)... 

Bubbles can grow to on the order of a meter in diameter in Group B powders in large beds. The maximum stable bubble size is limited by the size of the vessel or the stabiUty of the bubble itself. In large fluidized beds, the limit to bubble growth occurs when the roof of the bubble becomes unstable and the bubble spHts. EmpidcaHy, it has been found that the maximum stable bubble size may be calculated for Group A particles from... 

Flue particles ia a fluidized bed are analogous to volatile molecules ia a Foiling solution. Therefore, the concentration of particles ia the gas above a fluidized bed is a function of the saturation capacity of the gas. To calculate the entrainment rate, it is first necessary to determine what particle sizes ia the bed can be entrained. These particles are the ones which have a terminal velocity less than the superficial gas velocity, assuming that iaterparticle forces ia a dilute zone of the freeboard are negligible. An average particle size of the entrainable particles is then calculated. If all particles ia the bed are entrainable, the entrained material has the same size distribution as the bed material. 

The fluidized-bed system (Fig. 3) uses finely sized coal particles and the bed exhibits Hquid-like characteristics when a gas flows upward through the bed. Gas flowing through the coal produces turbulent lifting and separation of particles and the result is an expanded bed having greater coal surface area to promote the chemical reaction. These systems, however, have only a limited abiUty to handle caking coals (see Fluidization). 

Fluidized-bed reaction systems are not normally shut down for changing catalyst. Fresh catalyst is periodically added to manage catalyst activity and particle size distribution. The ALMA process includes faciUties for adding back both catalyst fines and fresh catalyst to the reactor. 

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