Fluidization purposes

For non-spherical particles, values of sphericity lie in the range 0 < < 1. Thus, the effective particle diameter for fluidization purposes is the product of the surface-volume mean diameter and the sphericity (Kunii and Levenspiel, 1991). The sphericity of regular-shaped particles can be deduced by geometry whilst the sphericity of irregular-shaped... 

A wide variety of special-purpose incinerators (qv) with accompanying gas scmbbers and soHd particle collectors have been developed and installed in various demilitarisation faciUties. These include flashing furnaces that remove all vestiges of explosive from metal parts to assure safety in handling deactivation furnaces, to render safe small arms and nonlethal chemical munitions fluidized-bed incinerators that bum slurries of ground up propellants or explosives in oil and rotary kilns to destroy explosive and contaminated waste and bulk explosive. 

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.)... 

The most innovative photohalogenation technology developed in the latter twentieth century is that for purposes of photochlorination of poly(vinyl chloride) (PVC). More highly chlorinated products of improved thermal stabiUty, fire resistance, and rigidity are obtained. In production, the stepwise chlorination may be effected in Hquid chlorine which serves both as solvent for the polymer and reagent (46). A soHd-state process has also been devised in which a bed of microparticulate PVC is fluidized with CI2 gas and simultaneously irradiated (47). In both cases the reaction proceeds, counterintuitively, to introduce Cl exclusively at unchlorinated carbon atoms on the polymer backbone. 

Heterogeneous hydrogenation catalysts can be used in either a supported or an unsupported form. The most common supports are based on alurnina, carbon, and siUca. Supports are usually used with the more expensive metals and serve several purposes. Most importandy, they increase the efficiency of the catalyst based on the weight of metal used and they aid in the recovery of the catalyst, both of which help to keep costs low. When supported catalysts are employed, they can be used as a fixed bed or as a slurry (Uquid phase) or a fluidized bed (vapor phase). In a fixed-bed process, the amine or amine solution flows over the immobile catalyst. This eliminates the need for an elaborate catalyst recovery system and minimizes catalyst loss. When a slurry or fluidized bed is used, the catalyst must be separated from the amine by gravity (settling), filtration, or other means. 

During World War II, production of butadiene (qv) from ethanol was of great importance. About 60% of the butadiene produced in the United States during that time was obtained by a two-step process utilizing a 3 1 mixture of ethanol and acetaldehyde at atmospheric pressure and a catalyst of tantalum oxide and siHca gel at 325—350°C (393—397). Extensive catalytic studies were reported (398—401) including a fluidized process (402). However, because of later developments in the manufacture of butadiene by the dehydrogenation of butane and butenes, and by naphtha cracking, the use of ethanol as a raw material for this purpose has all but disappeared. 

At high ratios of fluidiziug velocity to minimum fluidizing velocity, tremendous solids circulation from top to bottom of the bed assures rapid mixing of the solids. For aU practical purposes, beds with L/D ratios of from 4 to 0.1 can be considered to be completely mixed continuous-reaction vessels insofar as the sohds are concerned. 

Classification The separation of fine particles from coarse can be effected by use of a fluidized bed (see Drying ). However, for economic reasons (i.e., initial cost, power requirements for compression of fluidizing gas, etc.), it is doubtful except in special cases if a fluidized-bed classifier would be built for this purpose alone. 

Several patents exist on carrying out exothermic reactions for manufacture of reactive intermediates where high selectivity is essential. Even this author has a patent to make ethylene oxide in a transport line reactor (Berty 1959). Yet no fluidized bed technology is in use today. Mostly fixed bed, cooled tubular reactors are used for that purpose. 

Applicability/Limitations Fluidized beds require frequent attention for maintenance and cleaning purposes. This treatment is ideal for slurries and sludges but not for bulky or viscous wastes. The waste particles should be of a certain size and be homogeneous. Wastes must have a low sodium content and a low heavy metal content. Some refractory wastes may not be fully destroyed since these units operate at low combustion temperatures (750 to 1000°C). 

There is approximately a 22% deviation between the experimental and the distribution mean residenee time. However, the main purpose was to use the information from the RTD eurve to improve the reaetor operation. The results of the RTD provided vital information eoneern-ing the effeets of operating eonditions and struetural designs on solid-mixing patterns in fluidized systems. The perfeet mixing funetion was generated by e , where 6 = t/f. Figure 8-19 shows plots of these funetions against dimensionless residenee time 6. 

The calcium bisulfite acid used in the manufacture of sulfite cellulose is the product of reaction between gaseous sulfur dioxide, liquid water, and limestone. The reaction is normally carried out in trickle-bed reactors by the so-called Jenssen tower operation (E3). The use of gas-liquid fluidized beds has been suggested for this purpose (V7). The process is an example of a noncatalytic process involving three phases. 

Fluidized-bed CVD was developed in the late 1950s for a specific application the coating of nuclear-fuel particles for high temperature gas-cooled reactors. PI The particles are uranium-thorium carbide coated with pyrolytic carbon and silicon carbide for the purpose of containing the products of nuclear fission. The carbon is obtained from the decomposition of propane (C3H8) or propylene... 

The average pore size and the pore size distribution should be such that physical limitations are not placed on the conversion of reactants to products. The particle size of the carrier must also be suitable for the purpose intended (i.e., small for fluidized bed reactors and significantly larger for fixed bed applications). 

For most purposes, the correlations for jD presented in Tables 12.1 and 12.2 also suffice for estimating jH. There is, however, one additional correlation for fluidized beds that is worth noting. On the basis of data for the fluidization of 20 to 40 mesh silica and alumina gel particles in air at Reynolds number values (DpG/fi) ranging from 9 to 55, Kettenring et al. (92) suggest that... 

On the basis of different assumptions about the nature of the fluid and solid flow within each phase and between phases as well as about the extent of mixing within each phase, it is possible to develop many different mathematical models of the two phase type. Pyle (119), Rowe (120), and Grace (121) have critically reviewed models of these types. Treatment of these models is clearly beyond the scope of this text. In many cases insufficient data exist to provide critical tests of model validity. This situation is especially true of large scale reactors that are the systems of greatest interest from industry s point of view. The student should understand, however, that there is an ongoing effort to develop mathematical models of fluidized bed reactors that will be useful for design purposes. Our current... 

Most investigations of fluidization parameters take place at ambient temperature and pressure. Yet, nearly all processes operate at elevated temperature, and many at elevated pressure. Therefore, it is necessary to know how increasing temperature and pressure affect the operation of fluidized systems. However, the operation of fluidized test facilities at temperature and pressure is much more difficult and costly than operating them at ambient conditions. It is not surprising then that information on how temperature and pressure affect the operation of fluidized beds is not as prevalent as would be desired. However, many researchers have undertaken the difficult task of building and operating units to obtain these badly needed data. The purpose of this chapter is to present what is known about operating fluidized beds at elevated temperature and pressure. 

Friability tests can be used for various purposes. They are widely used in quality control. Here, samples of produced material are subjected to a more or less arbitrary but well defined stress. The attrition extent is assessed by comparison with a standard value and a decision is reached whether the material meets the standard. Moreover, friability tests are often used for comparison of different materials to select the most attrition-resistant one. This is a usual procedure in the case of catalyst development. For example, Contractor et al. (1989) tested anew developed fluidized bed VPO-catalyst in a submerged-jet attrition test (described below). Furthermore, the specific attrition rate of a material in a certain process can be roughly estimated by friability tests. In this case the stress must be similar to that occurring in the process and the obtained degradation extent must be compared with those of other materials from which the process attrition rate is known. 

For the general purpose of minimizing air flow, transport velocity, wear and power, the fluidized dense-phase mode of flow is preferred for long-distance applications. Efficient blow tank feeders, rotary-screw compressors, refrigerated dryers and stepped-diameter pipelines also are recommended. For products that are not suited to fluidized dense-phase, the possible modes of flow include dilute-phase (suspension flow) or bypass conveying (Wypych, 1995a). 

When evaluating a material for the purpose of establishing dense-phase and long-distance suitability, it is important to undertake all the necessary tests (e.g., particle sizing, particle and bulk densities, fluidization and deaeration). Also, if possible, it is useful to compare such results with those obtained on previously conveyed similar materials (e.g., fly ash). However, it should be noted that such an evaluation only is a qualitative one and it is not possible to predict say, minimum air flows or pipeline pressure drop based on such data (i.e., pilot-scale tests normally are required to confirm minimum velocities, friction factors, etc., especially over long distances and for large-diameter pipes). 

The purpose of this section is to anticipate and consider risks inherent in fluidization and also to summarize the more thoroughly documented and well-understood risks associated with the kinds of peripheral systems and facilities likely to be found in manufacturing plants using fluidization processes. 

Heterogeneous catalytic gas-phase reactions are most important in industrial processes, especially in petrochemistry and related fields, in which most petrochemical and chemical products are manufactured by this method. These reactions are currently being studied in many laboratories, and the results of this research can be also used for synthetic purposes. The reactions are usually performed [61] in a continuous system on a fixed catalyst bed (exceptionally a fluidized bed). 

A fluidized-bed reactor consists of three main sections (Figure 23.1) (1) the fluidizing gas entry or distributor section at the bottom, essentially a perforated metal plate that allows entry of the gas through a number of holes (2) the fluidized-bed itself, which, unless the operation is adiabatic, includes heat transfer surface to control T (3) the freeboard section above the bed, essentially empty space to allow disengagement of entrained solid particles from the rising exit gas stream this section may be provided internally (at the top) or externally with cyclones to aid in the gas-solid separation. A reactor model, as discussed here, is concerned primarily with the bed itself, in order to determine, for example, the required holdup of solid particles for a specified rate of production. The solid may be a catalyst or a reactant, but we assume the former for the purpose of the development. 

A fluidized bed reactor has a substantial free space above the main level of the catalyst for purpose of disengaging entrainment. In this region plug flow may be assumed to prevail. An overall appropriate model accordingly will consist of well mixed and bypass zones in parallel followed by a plug flow zone. The fraction of flow in bypass is 1-a and the fraction of vessel volume in plug flow is 2. Find the transfer function and equations for the responses to step and impulse inputs of tracer. 

In fluidized beds, the temperature is uniform within a few degrees even in the largest vessels, but variation of comnposition is appreciable in large vessels, and is not well correlated for design purposes. One currently successful moving bed process is the UOP "Stacked Reactor" platforming where the catalyst is transported and regenerated in a separate zone. When the activity of the catalyst declines fairly rapidly, its variation with time and position must be taken into account by the mathematical formulation. 

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