Contacting fast fluidization

A well-defined bed of particles does not exist in the fast-fluidization regime. Instead, the particles are distributed more or less uniformly throughout the reactor. The two-phase model does not apply. Typically, the cracking reactor is described with a pseudohomogeneous, axial dispersion model. The maximum contact time in such a reactor is quite limited because of the low catalyst densities and high gas velocities that prevail in a fast-fluidized or transport-line reactor. Thus, the reaction must be fast, or low conversions must be acceptable. Also, the catalyst must be quite robust to minimize particle attrition. 

At gas velocities higher than those used for BFBs we successively enter the turbulent (TB), fast fluidized (FF), and the pneumatic conveying (PC) regimes. In these contacting regimes solids are entrained out of the bed and must be replaced. Thus in continuous operations we have the CFB, shown in Fig. 20.1. Flow models are very sketchy for these flow regimes. Let us see what is known. 

The high frequency of cluster formation and dissolution in fast fluidization is reflected in high-frequency random voidage fluctuations as shown at the top subfigure of Fig. 11. Such a change in two-phase behavior promotes efficient gas/solids contacting. 

The residual carbon contents at different axial locations of the combustor were measured in the pilot plant tests (Li et al., 1991), as shown in Fig. 18. These data show that axial variations in carbon content with temperature (from 810 °C-923 °C) are as a whole rather slight, but mean carbon content increases with decreasing excess air ratio. Besides, for excess air ratios greater than 1.2, the carbon content at the top of the combustor is somewhat less than that at the bottom, while for excess air ratio less than 1.2, the opposite tendency is evident. In conclusion, for this improved combustor, an excess air ratio of 1.2 is considered enough for carbon burn-out, leading to reduced flue gas and increased heat efficiency as compared to bubbling fluidized bed combustion. That is probably attributable to bubbleless gas-solid contacting for increased mass transfer between gas and solids in the fast fluidized bed, as explained by combustion kinetics. 

The fast fluidized bed operates with high gas velocity and high solid mass flow rate as compared to the ordinary turbulent bed, thus intensifying gas/solid contacting and improving the mass transfer of oxygen from the gas phase to the solid phase, thereby resulting in a severalfold increase in CBI. 

Fast fluidization was first studied in China in the early 1970s, but because of limited contact with the rest of the world, the results have not circulated much outside of its borders. It is the purpose of this book, therefore, to acquaint our international peers with the essential results of our efforts in this research. 

While many engineers on the international front are turning to FF for better gas-solids contacting (though each is primarily concerned with his or her own process needs), Z. Yu of Tsing Hua University (THU) has studied the common features of different processes. In Chapter 2 she provides a scheme for collocating the multifarious applications of fast fluidization that enables a researcher interested in using FF to have quick access to comparable techniques in different professions. The collocation chart she has prepared is supported by personal computer software. 

In general, the high operating gas velocities for lean phase fluidization yield a short contact time between the gas and solid phases. Fast fluidized beds and co-current pneumatic transport are thus suitable for rapid reactions, but attrition of catalyst may be serious. 

For pneumatic conveying all the particles are evenly dispersed in the gas. This makes contacting ideal or close to ideal. The plug flow model is thus well suited for the dilute transport reactors, but has also been used for the denser fast fluidization regime neglecting gradients in the solids distribution. For first order reactions the model can be written as ... 

Kagawa H, Mineo H, Yamazaki R, Yoshida K (1991) A gas-solid contacting model for fast-fluidized bed. In eds Basu P, Horio M, Hasatani M Circulating Fluidized Bed Technology III, Pergamon, Oxford, pp 551-556... 

Fast-fluidized retorted shale is cooled from 990°F (482°C) to about 175°F (79°C) by contacting it countercurrently with balls from the preheat section. Since the conveying (flue) gas is cooled and contracts as it rises, it may be desirable to reduce the vessel size accordingly in the upper portion to maintain the desired flow rate of gas. 

Kagawa, H., Mineo, H., Yamazaki, R., and Yoshida, K., A Gas-Solid Contacting Model for Fast Fluidized Bed , in Circulating Fluidized Bed Technology III (P. Basu, M. Horio and M. Hasatani, eds.), Pergamon Press, Oxford, 1991. 

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. 

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