Operation of the Fluidized Bed

The gas distributor has a considerable effect on the proper operation of the fluidized bed. Basically, there are two types (Perry and Green, 1999) (a) for use when the inlet gas contains solids and (b) for use when the inlet gas is free of solids. In the latter case, the distributor is designed to prevent the backflow of solids during normal operation, and in many cases, it is designed to prevent backflow during shutdown. To provide distribution, it is necessary to restrict the gas or the gas and solids flow so that the pressure drops across the restriction amount of 0.5 to 20 kPa. 

The operation of the fluidized bed can be enhanced by the addition of electrostatic equipment. With this addition, the powder will actually be propelled from the bed to the metallic part brought close to the bed, as illustrated in Figure 4. 

The fluidization properties of a powder in air may be predicted by establishing in which group it lies. It is important to note that at operating temperatures and pressures above ambient a powder may appear in a different group from that which it occupies at ambient conditions. This is due to the effect of gas properties on the grouping and may have serious implications as far as the operation of the fluidized bed is concerned. Table 7.1 presents a summary of the typical properties of the different powder classes. 

When heavier refractories are required because of operating conditions, insulating brick is installed next to the shell and firebrick is installed to protect the insulating brick. Industrial experience in many fields of application has demonstrated that such a hning will success-billy withstand the abrasive conditions for many years without replacement. Most serious refractory wear occurs with coarse particles at high gas velocities and is usually most pronounced near the operating level of the fluidized bed. 

As an example of the chemical signihcance of the process technology, the products of die Fischer-Tropsch synthesis, in which a signihcant amount of gas phase polymerization occurs vary markedly from hxed bed operation to the fluidized bed. The hxed bed product contains a higher proportion of straight chain hydrocarbons, and the huidized bed produces a larger proportion of branched chain compounds. 

A salient feature of the fluidized bed reactor is that it operates at nearly constant temperature and is, therefore, easy to control. Also, there is no opportunity for hot spots (a condition where a small increase in the wall temperature causes the temperature in a certain region of the reactor to increase rapidly, resulting in uncontrollable reactions) to develop as in the case of the fixed bed reactor. However, the fluidized bed is not as flexible as the fixed bed in adding or removing heat. The loss of catalyst due to carryover with the gas stream from the reactor and regenerator may cause problems. In this case, particle attrition reduces their size to such an extent where they are no longer fluidized, but instead flow with the gas stream. If this occurs, cyclone separators placed in the effluent lines from the reactor and the regenerator can recover the fine particles. These cyclones remove the majority of the entrained equilibrium size catalyst particles and smaller fines. The catalyst fines are attrition products caused by... 

The fluidized-bed process for this reaction has several advantages over a fixed-bed process. First, the process is highly exothermic, and the selectivity to C3H3N is temperature dependent. The improved temperature control of the fluidized-bed operation enhances the selectivity to acrylonitrile, and substantially extends the life of the catalyst, which readily sinters at temperatures in excess of 800 K. Furthermore, since both the reactants and products are flammable in air, the use of a fluidized bed enables the moving particles to act to quench flames, preventing combustion and ensuring safe operation. 

In principle, the experimental protocol of fluidized bed adsorption does not deviate from packed bed operations, the main difference being the direction of liquid flow. The standard sequence of frontal chromatography, equilibration, sample application, wash, elution, and cleaning (CIP) is performed with an upward direction of flow as shown in Fig. 3. During equilibration of the matrix the stabilization of the fluidized bed occurs, in case of size and/or density distribution of the adsorbent particles the classification within the bed may be detected by visual observation of the bed. As discussed below, bed stability may... 

Kunii, D. and Levenspiel, 0. (1990). Entrainment of Solids from Fluidized Beds. I. Holdup of Solids in the Freeboard, n. Operation of Fast Fluidized Beds. Powder Tech., 61,... 

---