Design of Fluidized Bed Catalytic Reactors

Within a fluidized bed, low-density regions or gas bubbles are formed at apparently unpredictable rates. 

The velocity at which gas flows through the dense phase corresponds approximately to the velocity that produces incipient fluidization. The bubbles rise, however, at a rate that is nearly an order of magnitude greater than the minimum fluidization velocity. In effect, then, as a consequence of the movement of solids within the bed and the interchange of fluid between the bnbbles and the particle rich regions of the bed, there are wide disparities in the residence times of various fluid elements within the reactor and in the times that the flnid elements are effectively in contact with the catalyst particles. 

From the standpoint of attempting to develop mathematical models for the simulation of fluidized-bed reactors, one must determine if the phenomena mentioned above and other aspects of the behavior of finidized beds can 

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In designing fixed and ideal fluidized-bed catalytic reactors, we have assumed up to now that the activity of the catalyst remains constant throughout the catalyst s life. That is, the total concentration of active sites, C accessible to the reaction does not change with time. Unfortunately, Mother Nature is not so kind as to allow this behavior to be the case in most industrially significant catalytic reactions. One of the most insidious problems in catalysis is the loss of catalytic activity that occurs as the reaction takes place on the catalyst. A wide variety of mechanisms have been proposed by Butt and Petersen, to explain and model catalyst deactivation. 

Program for design of fluidized bed two phase catalytic reactor % particulate fluidization (liquid solid fluidization)... 

Our treatment of Chemical Reaction Engineering begins in Chapters 1 and 2 and continues in Chapters 11-24. After an introduction (Chapter 11) surveying the field, the next five Chapters (12-16) are devoted to performance and design characteristics of four ideal reactor models (batch, CSTR, plug-flow, and laminar-flow), and to the characteristics of various types of ideal flow involved in continuous-flow reactors. Chapter 17 deals with comparisons and combinations of ideal reactors. Chapter 18 deals with ideal reactors for complex (multireaction) systems. Chapters 19 and 20 treat nonideal flow and reactor considerations taking this into account. Chapters 21-24 provide an introduction to reactors for multiphase systems, including fixed-bed catalytic reactors, fluidized-bed reactors, and reactors for gas-solid and gas-liquid reactions. 

Finally, possible causes for deactivation of catalytic membranes are described and severad aspects of regenerating catalytic membrane reactors are discussed. A variety of membrane reactor configurations are mentioned and some unique membrane reactor designs such as double spiral-plate or spiral-tube reactor, fuel cell unit, crossflow dualcompartment reactor, hollow-fiber reactor and fluidized-bed membrane reactor are reviewed. 

Our objective here is to study quantitatively how these external physical processes affect the rate. Such processes are designated as external to signify that they are completely separated from, and in series with, the chemical reaction on the catalyst surface. For porous catalysts both reaction and heat and mass transfer occur at the same internal location within the catalyst pellet. The quantitative analysis in this case requires simultaneous treatment of the physical and chemical steps. The effect of these internal physical processes will be considered in Chap, 11. It should be noted that such internal effects significantly affect the global rate only for comparatively large catalyst pellets. Hence they may be important only for fixed-bed catalytic reactors or gas-solid noncatalytic reactors (see Chap. 14), where large solid particles are employed. In contrast, external physical processes may be important for all types of fluid-solid heterogeneous reactions. In this chapter we shall consider first the gas-solid fixed-bed reactor, then the fluidized-bed case, and finally the slurry reactor. 

Tarhan, M. O., Catalytic Reactor Design. New York McGraw-Hill. 1983. Yates. J. G.. Fundamentals of Fluidized-Bed Chemical Processes. 3rd ed. London Butterworth, 1983,... 

The intense research effort carried out into the study of catalyst properties for the conversion of plastic wastes is in contrast with the few studies that have addressed reactor design. Thus, most of the studies use batch or simple fixed bed reactors despite the heat transfer and flow problems associated with the low thermal conductivity and high viscosity of the molten plastics. Various alternatives have been proposed to solve these problems the use of fluidized bed reactors, dissolution of the plastics in heavy oil fractions previously fed into the reactor, and a combination of thermal and catalytic treatments. However, all these processes present a number of difficulties, which makes further work on the reactor design necessary. 

The term in situ literally means in place. The ultimate in situ spectroscopic experiment would be conducted in place inside of a standard catalytic reactor. This extreme is rarely practical or even desirable, though, and an in situ experiment is generally understood to be one in which an attempt has been made to recreate some characteristics of the industrial process in an environment suitable for spectroscopy. Since reactor conditions such as temperature, pressure, and flowrate or reactor design (e.g., fluidized bed) may not be optimal for spectroscopy, some compromise is inevitable. 

Previous sections of tbis chapter have provided an overview of key issues affecting the performance of catalytic fluidized-bed reactors. In this section, we address more directly key challenges which affect the design, scale-up, and implementation of fluidized-bed processes. 

The reactor is the core and is generally the most researched part of the pyrolysis technology. Extensive literature is available for catalytic pyrolysis that has been carried out at both bench/laboratory scale (ie, bubbling and circulating fluidized beds, auger reactors, and conical spouted bed reactors) and analytical scale reactors (ie, analytical pyrolysis or py-GC/MS either tubular quartz micro reactor or packed bed reactor). Specific reactor designs are not discussed in this work. Catalytic fast pyrolysis can be split into two different operation modes defined by the location of the catalyst in the process in situ and ex situ (Tan et al., 2013) (Fig. 14.2). 

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