Fluidization parameter

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. 

Ehrlich, S., and Drenker, S. Performance results and fluidization parameters of large FBC boilers, presentation at International Conference on Fluidization VII, Australia, 1992. 

Calculating the Fluidization Parameters. The porosity at minimum fluidization is obtained from the Broadhurst and Becker correlation (1975) ... 

Minimum Fluidizing Velocity U,nj, the minimum fluidizing velocity, is frequently used in fluid-bed calculations and in quantifying one of the particle properties. This parameter is best measured in small-scale equipment at ambient conditions. The correlation by Wen audYu [A.l.Ch.E.j., 610-612 (1966)] given below can then be used to back calculate d. This gives a particle size that takes into account effects of size distribution and sphericity. The correlation can then be used to estimate U, at process conditions, if U,nj cannot be determined experimentally, use the expression below directly. 

Apphcation Type Reactor volume, nP TABLE 25-47 Typical Design Parameters for Fluidized Beds ... 

Design criteria for satisfactoiy biological fluidized bed treatment systems include the major parameters given in Table 25-47. 

Reactors may be operated batchwise or continuously, e.g. in tubular, tubes in shell (with or without internal catalyst beds), continuous stirred tank or fluidized bed reactors. Continuous reactors generally offer the advantage of low materials inventory and reduced variation of operating parameters. Recycle of reactants, products or of diluent is often used with continuous reactors, possibly in conjunction with an external heat exchanger. 

Equation 6-108 is also a good approximation for a fluidized bed reactor up to the minimum fluidizing condition. However, beyond this range, fluid dynamic factors are more complex than for the packed bed reactor. Among the parameters that influence the AP in a fluidized bed reactor are the different types of two-phase flow, smooth fluidization, slugging or channeling, the particle size distribution, and the... 

M. Belcu, D. Turtoi. Simulation of the fluidized-bed crystallizers (I). Influence of parameters. Cryst Res Technol 57 1015, 1996. 

Values for the various parameters in these equations can be estimated from published correlations. See Suggestions for Further Reading. It turns out, however, that bubbling fluidized beds do not perform particularly well as chemical reactors. At or near incipient fluidization, the reactor approximates piston flow. The small catalyst particles give effectiveness factors near 1, and the pressure drop—equal to the weight of the catalyst—is moderate. However, the catalyst particles are essentially quiescent so that heat transfer to the vessel walls is poor. At higher flow rates, the bubbles promote mixing in the emulsion phase and enhance heat transfer, but at the cost of increased axial dispersion. 

The apparent reaction rate constant for the first order reaction, k, was calculated from the conversion of CO2. Since the gas-volume reduction rate increased with k, a poor fluidization was induced by high reaction rate. We investigated the effect of the rate of the gas-volume change on the fluidization quality. The rate of the gas-volume change can be defined as rc=EA(dxA/dt), where Sa is the increase in the number of moles when the reactants completely react per the initial number of moles. This parameter is given by 7-1. When the parameter, Ea, is negative, the gas volume decreases as the reaction proceeds. 

Y. T, Shah, Design Parameters for Mechanically Agitated Reactors Mooson Kwauk, Particulate Fluidization An Overview... 

Figure 26, shown earlier, is a simple form of input mapping called table lookup. A more complicated inference mechanism is illustrated in Fig. 30. Here we see a simple example from a fluidized catalytic cracking unit in which multiple product quality attributes can be explained by multiple operating parameters (Ramesh et al., 1992).

Different reactor networks can give rise to the same residence time distribution function. For example, a CSTR characterized by a space time Tj followed by a PFR characterized by a space time t2 has an F(t) curve that is identical to that of these two reactors operated in the reverse order. Consequently, the F(t) curve alone is not sufficient, in general, to permit one to determine the conversion in a nonideal reactor. As a result, several mathematical models of reactor performance have been developed to provide estimates of the conversion levels in nonideal reactors. These models vary in their degree of complexity and range of applicability. In this textbook we will confine the discussion to models in which a single parameter is used to characterize the nonideal flow pattern. Multiparameter models have been developed for handling more complex situations (e.g., that which prevails in a fluidized bed reactor), but these are beyond the scope of this textbook. [See Levenspiel (2) and Himmelblau and Bischoff (4).]... 

In later sections, the use of the scaling relationships to design small scale models will be illustrated. For scaling to hold, all of the dimensionless parameters given in Eqs. (36), (37) or (39) must be identical in the scale model and the commercial bed under study. If the small scale model is fluidized with air at ambient conditions, then the fluid density and viscosity are fixed and it will be shown there is only one unique modeling condition which will allow complete similarity. In some cases this requires a model which is too large and unwieldy to simulate a large commercial bed. 

The first term in the list multiplied by the third term has been shown by Glicksman (1988) to be equivalent to the ratio of superficial and minimum fluidization velocities in the viscous limit. The controlling parameters can therefore be written as... 

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