Fluidized catalyst beds fines effect

From the foregoing dicussion it is apparent that residuum hydroconversion processes can be influenced adversely by pore diffusion limitations. Increasing the catalyst porosity can alleviate the problem although increased porosity is usually accompanied by a decrease in total catalytic surface area. Decreasing the catalyst particle size would ultimately eliminate the problem. However, a different type of reaction system would be required since the conventional fixed bed would experience excessive pressure drops if very fine particles were used. A fluidized system using small particles does not suffer from this limitation. However, staging of the fluidized reaction system is required to minimize the harmful effects that backmixing can have on reaction efficiency and selectivity. 

It is also possible to use finely divided catalyst in the expanded-bed reactor. If the catalyst is a suitable size (50 to 200 pm, i.e. 50 x 10 4 to 200 x 10 4 cm), it is possible to operate the expanded-bed reactor without recycling the liquid products to maintain the catalyst in the fluidized state. In addition, the finely divided catalyst has a relatively larger number of external pores on the surface than the extruded catalyst and is less likely to have metal contaminants plugging up these pores because of their size. The overall effect of a finely divided catalyst in this reactor is more efficient sulfur removal for a given set of process conditions. 

The effectiveness of the gas-solid mass transfer in a circulating fluidized bed (see Chapter 10) can be reflected by the contact efficiency, which is a measure of the extent to which the particles are exposed to the gas stream. As noted in Chapter 10, fine particles tend to form clusters, which yield contact resistance of the main gas stream with inner particles in the cluster. The contact efficiency was evaluated by using hot gas as a tracer [Dry et al., 1987] and using the ozone decomposition reaction with iron oxide catalyst as particles [Jiang etal., 1991], It was found that the contact efficiency decreases as the particle concentration in the bed increases. At lower gas velocities, the contact efficiency is lower as a result of lower turbulence levels, allowing a greater extent of aggregate formation. The contact efficiency increases with the gas velocity, but the rate of increase falls with the gas velocity. 

Reddy, Karri S. B., and Knowlton, T. M. The Effect of Fines Content on the Flow of FCC Catalyst in Standpipes, in Circulating Fluidized Bed Technology IV (Amos A. Avidan, ed.), pp. 589-594. Somerset, Pennsylvania (1993). 

Ikeda pt al. (15, 110), investigating the effect of fines on bed expansion, pressure fluctuation, and discharge rate of particles from a 8-cm-i.d. fluid bed, found that bed expansion and discharge rate increase and pressure fluctuations decrease with increasing content of fines (<44 /am). The range of particles smaller than 44 /am is called "fines fraction its role has been summarized by Ikeda (12) as shown in Table IV. The desirable properties of fluidized particles utilized as catalyst were given by Ikeda (12) as... 

Fluidized beds give relatively higher performance, but within a narrow operating window. Another type of reactors, the slurry reactor, effectively utilizes the catalyst because of their small particle size in the micrometer range. However, catalyst separation is difficult and a filtration step is required to separate fine particles from the product. Moreover, when applied in the continuous mode, backmixing lowers the conversion and usually the selectivity [2]. Conventional continuous tubular reactors are used as falling film or wall reactor with catalyst coated on the wall however, supply/removal of heat and often broad residence time distribution because of large reactor diameters are two main drawbacks commonly encountered with such reactors. 

The effectiveness factor r] of the catalyst pellets, expressing the fraction of the intrinsic rate of reaction being exploited in the fixed-bed configuration, is extremely small (rj = 10 -10 ). This is due to the large catalyst pellet sizes used to avoid excessive pressure drop along the length of the reactor (18-20m). This severe limitation can be broken by using a fluidized-bed reactor with fine catalyst particles (effectiveness factor = 1.0) thus the full intrinsic activity of the catalyst is utilized. 

These empirical correlations were mostly determined for fluidized beds of fine sand or fluid cracking catalyst. It is advised to use these relations only for applications of these materials. For applications of other powders it is better to measure mass transfer rates in small scale fluidized b s, and use empirical equations for predicting the scale-up effects. When one wants to use a new catalyst in really large reactors, e.g., with a diameter and a height of several meters, one should preferably do some pilot tests with the catalyst in a "cold flow model reactor of at least 1 m diameter. Measurements of mass transfer rates under realistic flow conditions may be sufficient. 

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