Fluidization mechanics

Our approach to the problem of predicting the performance of fluidized bed filters involves logically coupling models that describe the flow behavior of the fluidized state with models that describe the mechanisms of particle collection. The collection mechanisms analysis leads to expressions for determining the collection efficiency of a single filter element. An example of a collection mechanism is inertial impaction by which a particle deviates from the gas stream lines, due to its mass, and strikes a collector. It should be noted that because particle collection mechanisms are functions of the fluid flow behavior in the vicinity of a collector, there exists an interdependency between fluidization mechanics and particle collection mechanisms. 

In a previous paper, the importance of fluidization mechanics on the performance of fluidized bed filters was demonstrated (13). To accomplish this, classical methods were employed for evaluating... 

Fig. 6.n-13 Comparison between the fluidization mechanisms in a conventional fluid bed and the rotating Omnitex FB processor (courtesy Nara, Tolcyo, Japan)... 

Figure 4.18 Comparison of the various PET-SSP processes (F), fluidization mechanical agitation...

The fluidizing mechanism behaves in response to oscillations as follows. In the case of low oscillation intensity, powder particles covering the vibrator surface become compacted. With increasing intensity the particles start to separate from the surface and become mobile, thus expanding the powder layer. The amplitude of oscillations and acceleration of the particles attenuate exponentially with distance from the vibrator surface. There is a condition of vibration-induced fluidizing of polydispersed polymer dispersions [52]... 

Fluidization Mechanics. Regenerators and reactors behave veiy much the same, and common flow rates and densities are indicated in Table 21-20. Fluidization, may be maintained (see fig. 21-1) at veloci ties of 1 to 2 ft per sec, but at some limiting velocity (1.5 to 2.2) fine catalyst tends suddenly to be carried upwards out of the bed to the cyclones. If downward movement is desired as in standpipes, a higher gas velocity is maintained (2 to 7) so that packing of the catalyst does... 

Pure PVA dissolves in water but does not fluidize by melting. Commercial production of PVA fiber is therefore carried out by wet spinning or dry spinning, utilizing aqueous PVA solution. In either case, purified PVA is dissolved in hot water and the solution is extmded through fine holes of a spinneret the extmded streams are coagulated to form continuous filaments, which are then heat-treated to have adequate mechanical properties. 

Circulating fluidized beds (CFBs) are high velocity fluidized beds operating well above the terminal velocity of all the particles or clusters of particles. A very large cyclone and seal leg return system are needed to recycle sohds in order to maintain a bed inventory. There is a gradual transition from turbulent fluidization to a truly circulating, or fast-fluidized bed, as the gas velocity is increased (Fig. 6), and the exact transition point is rather arbitrary. The sohds are returned to the bed through a conduit called a standpipe. The return of the sohds can be controUed by either a mechanical or a nonmechanical valve. 

The first of these reactions takes place at temperatures of about 150°C, the second reaction proceeds at about 550—660°C. Typical furnaces used to carry out the reaction include cast-iron retorts the Mannheim mechanical furnace, which consists of an enclosed stationary circular muffle having a concave bottom pan and a domed cover and the Laury furnace, which employs a horizontal two-chambered rotating cylinder for the reaction vessel. The most recent design is the Cannon fluid-bed reactor in which the sulfuric acid vapor is injected with the combustion gases into a fluidized bed of salts. The Mannaheim furnace has also been used with potassium chloride as the feed. 

Fluidized-bed reactor systems put other unique stresses on the VPO catalyst system. The mixing action inside the reactor creates an environment that is too harsh for the mechanical strength of a vanadium phosphoms oxide catalyst, and thus requires that the catalyst be attrition resistant (121,140,141). To achieve this goal, vanadium phosphoms oxide is usually spray dried with coUoidal siUca [7631-86-9] or polysiUcic acid [1343-98-2]. Vanadium phosphoms oxide catalysts made with coUoidal sUica are reported to have a loss of selectivity, while no loss in selectivity is reported for catalysts spray dried with polysUicic acid (140). 

Classifiers can be grouped into horizontal current and vertical current types, or into mechanical, nonmechanical, sedimentation, and hydrauHc or fluidized-bed types depending on the design of the equipment. The available equipment, their sizes, capacities, and their uses are given in Table 5. 

Polymerization in the Gas Phase. Many polymerization catalysts can be adapted for use in the gas phase. A gas-phase reactor contains a bed of small PE particles that is agitated either by a mechanical stirrer or by employing the fluidized-bed technique. These processes are economical because they do not requite solvent tecitculation streams. 

Sohd—sohd blending can be accompHshed by a number of techniques. Some of the most common iaclude mechanical agitatioa which iacludes devices such as ribboa Headers, impellers, paddle mixers, orbiting screws, etc a rotary fixed container which iacludes twia-sheU (Vee) and double-cone blenders and fluidization, ia which air is used to Head some fine powders. 

Manufacture. Titanium chloride is manufactured by the chlorination of titanium compounds (1,134—138). The feedstocks usually used are mineral or synthetic mtile, beneficiated ilmenite, and leucoxenes. Because these are all oxygen-containing, it is necessary to add carbon as well as coke from either coal or fuel oil during chlorination to act as a reducing agent. The reaction is normally carried out as a continuous process in a fluid-bed reactor (139). The bed consists of a mixture of the feedstock and coke. These are fluidized by a stream of chlorine iatroduced at the base (see Fluidization). The amount of heat generated in the chlorination process depends on the relative proportions of CO2 or CO that are formed (eqs. 1 and 2), and the mechanism that... 

SNR s fluidized-bed cogeneiation system is an early example of the commercial development of AFBC technology. Foster Wheeler designed, fabricated, and erected the coal-fired AFBC/boHer, which generates 6.6 MWe and 37 MW thermal (also denoted as MWt) of heat energy. The thermal energy is transferred via medium-pressure hot water to satisfy the heat demand of the tank farm. The unit bums 6.4 t/h of coal and uses a calcium to sulfur mole ratio of 3 to set the limestone feed rate. The spent bed material may be reiajected iato the bed as needed to maintain or build bed iaventory. The fly ash, collected ia two multicyclone mechanical collectors, may also be transferred pneumatically back to the combustor to iacrease the carbon bumup efficiency from 93%, without fly ash reiajection, to 98%. 

Activated alumina and phosphoric acid on a suitable support have become the choices for an iadustrial process. Ziac oxide with alumina has also been claimed to be a good catalyst. The actual mechanism of dehydration is not known. In iadustrial production, the ethylene yield is 94 to 99% of the theoretical value depending on the processiag scheme. Traces of aldehyde, acids, higher hydrocarbons, and carbon oxides, as well as water, have to be removed. Fixed-bed processes developed at the beginning of this century have been commercialized in many countries, and small-scale industries are still in operation in Brazil and India. New fluid-bed processes have been developed to reduce the plant investment and operating costs (102,103). Commercially available processes include the Lummus processes (fixed and fluidized-bed processes), Halcon/Scientific Design process, NIKK/JGC process, and the Petrobras process. In all these processes, typical ethylene yield is between 94 and 99%. 

Contactive (Direct) Heat Transfer Contactive heat-transfer equipment is so constructed that the particulate burden in solid phase is directly exposed to and permeated by the heating or cooling medium (Sec. 20). The carrier may either heat or cool the solids. A large amount of the industrial heat processing of sohds is effected by this mechanism. Physically, these can be classified into packed beds and various degrees of agitated beds from dilute to dense fluidized beds. 

When a stationary vessel is employed for fluidization, all sohds being treated must be fluidized nontluidizable fractions fall to the bottom of the bed and may eventually block the gas distributor. The addition of mechanical vibration to a fluidized system offers the following advantages ... 

Vibrofluidizatlon It is possible to fluidize a bed mechanically by imposing vibration to throw the particles upward cychcaUy. This enables the bed to operate with either no gas upward velocity or reduced gas flow. Entrainment can also be greatly reduced compared to unaided fluidization. The technique is used commercially in drying and other applications [Mujumdar and Erdesz, Drying Tech., 6, 255-274 (1988)], and chemical reaction applications are possible. See Sec. 12 for more on diying applications of vibrofluidization. 

Size Reauction Three major size-reduction mechanisms occur in the fluidized bed. These are attrition, impact, and thermal decrepitation. 

All these mechanisms will cause completion of fractures that were started before the introduction of the solids into the fluidized bed. 

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