Other Applications of Fluidization

Other applications of fluidization have been made to such materials as sodium chloride liable salt), soda ash. sodium phosphate, sodium sulfate, starch, talc, magnesium oxide, dry clay, bone acid, hydrated lime, and various high polymers in powdered or head" form. Fluidization is especially effective in loading and unloading materials from railroad cars and trucks, as well as in moving them aboul within the plant. 

Other applications of microparticles include spray drying, stack gas scrubbing, particle and droplet combustion, catalytic conversion of gases, fog formation, and nucleation. The removal of SO2 formed in the combustion of high-sulfur coal can be accomplished by adding limestone to coal in a fluidized bed combustor. The formation of CaO leads to the reaction... 

In considering heat transfer in gas-solid fluidization it is important to distinguish between, on the one hand, heat transfer between the bed and a heat transfer surface (be it heated bed walls or heat transfer coils in the bed) and, on the other hand, heat transfer between particles and the fluidizing gas. Much of the fluidization literature is concerned with the former because of its relevance to the use of fluidized beds as heterogeneous chemical reactors. Gas-particle heat transfer is rather more relevant to the food processing applications of fluidization such as drying, where the transfer of heat from the inlet gas to the wet food particle is crucial. 

Other applications of liquid-solid fluidized beds that have been suggested or put into practice include the leaching of vegetable oils from seeds (Epsfein, 2003), fhe freeze concentration of solutions (Rios et al., 1985) and osmotic drying (Marouze et al., 2001). Fluidization is also the basis of fhe hydraulic fransport of vegetables (McKay et al., 1987). Three-phase fluidized beds have been employed for the fermentation of cocoa beans (Jacquef et al, 1981 Rios et al, 1985). 

Table 10.3 presents a few examples of industrial applications of fluidized beds for synthesis reactions. Other examples are given by [146, 82, 53, 58]. 

The theory and food applications of fluidized bed drying have been discussed in many textbooks and articles [5,22-24,26,27]. Apart from the commercial drying of peas, beans, and diced vegetables, it is also used for drying potato granules, onion flakes, and fruit juice powders. It is often used as a secondary dryer to finish the drying process initiated in other types of dryers. It can be carried out as a batch or continuous process with a number of modifications. 

In fluidized beds there are favorable conditions for rapid heat and mass transfer between the solids and the fluid. Very rapid mixing of the solids generally occurs so that the coefficients for the transfer of heat to boimdary surfaces are quite high. Fluidized beds are, therefore, used both as heat exchangers and chemical reactors particularly where close control of temperature is required and where large amounts of heat must be added to, or removed from the system. There are, of course, many other relevant applications of fluidized beds in the materials processing industry. A proper knowledge of the flow patterns of both fluid and particles is, therefore, necessary. 

This chapter provides an overview of some of the more important commercial applications of fluidized bed technology not covered in the other sections. Recent developments with high potentials of commercialization in the near future are also discussed. 

Table 10.3 contains a few examples of industrial applications of fluidized beds for synthesis reactions. Other examples are given by [32, 37, 48, 86]. Other fluid bed applications have also used CFBs in preference to dense phase fluidized beds, but the use of CFBs is limited to situations where the higher capital and operational costs of higher gas velocity can be justified by significant process advantages. In many applications, a well designed dense phase fluidized bed may suffice and be less costly to construct and operate than a CFB. 

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. 

There are many uses of fluidized beds. A number of applications have become commercial successes others are in the pilot-plant stage, and others in bench-scale stage. Generally, the fluidized bed is used for gas-solids contac ting however, in some instances the presence of the gas or sohd is used only to provide a fluidized bed to accomplish the end result. Uses or special (maracteristics follow ... 

The most widespread biological application of three-phase fluidization at a commercial scale is in wastewater treatment. Several large scale applications exist for fermentation processes, as well, and, recently, applications in cell culture have been developed. Each of these areas have particular features that make three-phase fluidization particularly well-suited for them Wastewater Treatment. As can be seen in Tables 14a to 14d, numerous examples of the application of three-phase fluidization to waste-water treatment exist. Laboratory studies in the 1970 s were followed by large scale commercial units in the early 1980 s, with aerobic applications preceding anaerobic systems (Heijnen et al., 1989). The technique is well accepted as a viable tool for wastewater treatment for municipal sewage, food process waste streams, and other industrial effluents. Though pure cultures known to degrade a particular waste component are occasionally used (Sreekrishnan et al., 1991 Austermann-Haun et al., 1994 Lazarova et al., 1994), most applications use a mixed culture enriched from a similar waste stream or treatment facility or no inoculation at all (Sanz and Fdez-Polanco, 1990). 

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