Turbulent and Fast Fluidization

In a bubbling bed, gas bypassing is often the main culprit in lowering efficiency of gas-solid contacting, but two other phenomena sometimes play roles  

Each mechanism exposes the particles to a range of gas atmospheres, the atmosphere varying rapidly in time. Either of the two mechanisms can give rise to reaction kinetic effects, which at times can be hurtful (Squires, 1982), and at other times, helpful (Squires, 1961, 1973). 

In a fluid bed of a fine powder, increasing velocity beyond the transition reported by Lanneau (1960) gives rise to bubbleless fluidization, thereby greatly reducing both gas bypassing and the gradient in gas composition between emulsion phase and dilute phase. 

Because Lanneau had worked in steel equipment, it remained for Kehoe and Davidson (1971) to describe the transition from bubbling to what they dubbed the turbulent regime of fluidization. 

It remained for Massimilla (1973) to provide laboratory research demonstrating the higher contacting efficiency that a turbulent fluid bed affords. Wainwright and Hoffman (1974) reported excellent contacting for oxidation of orthoxylene in what was probably a fast fluid bed. 

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Eor turbulent and fast-fluidized beds, bubbles are not present as distinct entities. The following expression for bed voidage, bed occupied by gas, where U is in m/s, has been suggested (17) ... 

With increasing fluid velocity, a particle-fluid system starts with the particle-dominated fixed bed terminating at UmC, spans the particle-fluidcompromising regimes of particulate, bubbling, turbulent and fast fluidization,... 

This class of reactions, carried out in fluidized beds, involves parallel and series reactions, with reaction intermediates being the desired products. Industrial examples include partial oxidation of n-butane to maleic anhydride and o-xylene to phthalic anhydride. The vigorous solid mixing of fluidized beds is valuable for these reactions because they are highly exothermic. However, gas backmixing must be minimized to avoid extended gas residence times that lead to the formation of products of total combustion (i.e., CO2 and H2O). For this reason, fluidized bed catalytic partial oxidation reactors are operated in the higher velocity regimes of turbulent and fast-fluidization. 

Obviously, more research is needed on the turbulent and fast fluidized-bed reactors. At present, tests on a series of pilot plant reactors ending with a semicommercial unit are unavoidable. 

Horio, M., More, K., Takei, Y., and Ishii, H., Simultaneous Gas and Solid Velocity Measurements in Turbulent and Fast Fluidized Beds , in Fluidization VII (O. E. Potter and D. J. Niclin, eds.). Engineering Foundation, New York,... 

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. 

Horio M, Mori K, Takei Y, Ishii H. Simultaneous gas and solid velocity measurements in turbulent and fast fluidized beds. In Potter OE, Nicklin DJ, eds. Fluidization VII. New York Engineering Foundation,... 

Models similar to those described above for the bubbling regime may also be used to characterize beds operated in the slugging, turbulent and fast fluidization regimes, with appropriate changes in the relationships used to describe the mass transfer and other model parameters. The conditions needed to ensure that the bed is operating in these regimes are presented in Section 2. 

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