Fluidization physical processes

Fluidization Physical process that converts the state of solid particles to a fluidlike state as a fluid is sent through the solid material. 

Fluidized-bed appHcations in the 1990s may be separated into catalytic reactions, noncatalytic reactions, and physical processes. Examples of fluidized-bed appHcations include the foUowing ... 

Fluidized-bed process incinerators have been used mostly in the petroleum and paper industries, and for processing nuclear wastes, spent cook liquor, wood chips, and sewage sludge disposal. Wastes in any physical state can be applied to a fluidized-bed process incinerator. Au.xiliary equipment includes a fuel burner system, an air supply system, and feed systems for liquid and solid wastes. The two basic bed design modes, bubbling bed and circulating bed, are distinguished by the e.xtent to which solids are entrained from the bed into the gas stream. 

Fluidized beds are widely used to achieve either chemical reactions or physical processing that require interfacial contact between gas and particles. Heat transfer is important in many of these applications, either to obtain energy transfer between the solid and gas phases or to obtain energy transfer between the two-phase mixture and a heating/cooling medium. The latter case is particularly important for fluidized bed reactors which require heat addition or extraction in order to achieve thermal control with heats of reaction. 

Liquid hydrazine, 13 586 Liquid hydrocarbons, in fluidized-bed processes, 20 169-170 Liquid hydrogen delivery of, 13 853 energy density of, 13 839 physical and thermodynamic properties of, 13 762-763t as a rocket fuel, 13 800 storage of, 13 785-786 Liquid hydrogen sulfide, 23 630, 633 Liquid hydrogen tank levitation system, 23 866... 

Our objective here is to study quantitatively how these external physical processes affect the rate. Such processes are designated as external to signify that they are completely separated from, and in series with, the chemical reaction on the catalyst surface. For porous catalysts both reaction and heat and mass transfer occur at the same internal location within the catalyst pellet. The quantitative analysis in this case requires simultaneous treatment of the physical and chemical steps. The effect of these internal physical processes will be considered in Chap, 11. It should be noted that such internal effects significantly affect the global rate only for comparatively large catalyst pellets. Hence they may be important only for fixed-bed catalytic reactors or gas-solid noncatalytic reactors (see Chap. 14), where large solid particles are employed. In contrast, external physical processes may be important for all types of fluid-solid heterogeneous reactions. In this chapter we shall consider first the gas-solid fixed-bed reactor, then the fluidized-bed case, and finally the slurry reactor. 

In contrast to fixed- and fluidized-bed reactors, several physical processes must occur in series before a reactant gas can reach the catalytic surface. Some of these are likely to affect the global rate appreciably, and others are not. Various investigators have considered the relative importance of the several steps. With hydrogenation as an illustration, the process may be visualized as occurring in the following sequence (see Fig. 10-6) ... 

The mechanical and physical pretreatment processes include screening and size-reduction, density and particle separation in settling ponds, as well as hydrocyclone and fluidized bed processes, dewatering by... 

In general, nonuniform structures, in both time and space, is widespread in bubbling, turbulent, and fast fluidization regimes. On the one hand, such nonuniformity can enhance the mass and heat transfer of a bed. On the other hand, it decreases the contact efficiency of gas and solids and makes the scale-up rather difficult. Internals are usually introduced not to eliminate the nonuniform flow structure completely but to control its effect on chemical reactions. The function of internals varies in different fluidization regimes, as do the types and parameters of internals. Taking these purposes into consideration, internals may be successfully applied to catalytic reactors with high conversion and selectivity, and some other physical processes. 

The results show that the D PM method offers great potential to describe fluidized bed processes on the scale of individual particles using only physically based input parameters. Especially, the exact description of partide-particle interactions using a sophisticated contact model is a strong feature of this method. However, limitations due to the high numerical effort are still important. Process scale granulators caimot be described vdthin a reasonable simulation time. 

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. 

Physical models of commercial fluidized bed equipment provide an important source of design information for process development. A physical model of a commercial fluidized bed processor provides a small-scale simulation of the fluid dynamics of a commercial process. While commercial processes will typically operate at conditions making direct observation of bed fluid dynamics difficult (high temperature, high pressure, corrosive... 

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