Catalytic Fluidized Beds

Luss and Amundson (1968) have studied the dynamics of catalytic fluidized beds. The system is a good example of a stiff set of differential equations. Catalytic fluidized beds are utilized for a variety of reactions such as oxidation of naphthalene and ethylene and the production of alkyl chlorides. A batch fluidization reactor is usually built as a cylindrical shell with a support for the catalyst bed. The reactants enter from the bottom through a cone and cause the catalyst particles to be fluidized in the reactor. The reactants leave through a cyclone in which the entrained solids are separated and returned to the bed. 

The catalyst particles are assumed to be small enough so that heat and mass transfer resistances can be lumped at the particle surfaces, and the reaction takes place in the porous volume of the catalyst. All particles are assumed to have the same temperature and partial pressure of the reactants. It is also assumed that one irreversible reaction A B occurs in the bed. The dynamic model for the interstitial gas includes a mass balance  

The left-hand-side is the rate of accumulation of reactant in the gas phase, the first term on the right-hand-side is the rate into the reactor minus the rate out of the reactor, and the second term on the right-hand-side is the mass transfer rate between the gas phase and the particles. 

The last term is the reaction rate mechanism. The energy balance gives, dT 

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Additional catalytic processes. Nitrobenzene is hydrogenated to aniline (U.S. Patent 2,891,094). Melamine and isophthalouitrile are produced in catalytic fluidized-bed readers. Badger has announced a... 

The attrition rate, i.e., the rate of generation of fines, 0-d microns, at the submerged jets in a fluidized bed, tends to fall off asymptotically with time to a steady-state rate as shown in Fig. 9. Initially the attrition rate is high due to the wearing off of angular comers. Typically, it takes long time, hours to days, for the particles to reach steady-state (equilibrium) where the particles tend to be more rounded. For most catalytic fluidized bed processes, the bed operates at equilibrium. That means the most significant part of the attrition rate curve is the steady-state rate. 

The Econ-Abator system is a fluidized-bed catalytic oxidation system. Catalytic fluidized beds allow for destruction of volatile organic compounds (VOCs) at lower temperatures than conventional oxidation systems (typically 500 to 750°F). The technology uses a proprietary catalyst consisting of an aluminum oxide sphere impregnated with chromium oxide. 

The production of butadiene is discussed in the diene section Polybutadiene. Although several routes have been developed to produce acrylonitrile, almost all now is produced by the catalytic fluidized-bed ammoxidation of propylene. 

Peters, M.H. Fan, L.-S. Sweeney, T.L. Reactant Dynamics in Catalytic Fluidized Bed Reactors with Flow Reversal of Gas in the Emulsion Phase, presented at 1980 AIChE Meeting, Chicago, 111. 

Abashar, M. (2004). Coupling of steam and dry reforming of methane in catalytic fluidized bed membrane reactors, hit. ]. Hydrogen Energy 29, 799-808. 

Consider the dynamics of a catalytic fluidized bed in which an irreversible gas phase reaction takes place (Aiken and Lapidus, 1974 [17] Cutlip and Shacham, 1999). [9] Partial pressure of reactant in fluid (P), temperature of reactant in fluid (T), partial pressure of reactant at the catalyst surface (Pp) and partial pressure of reacfanf at the catalyst surface (Tp) are governed by the following equations ... 

It should be mentioned that the combustion technology is not limited to these major designs. In catalytic fluidized bed combustion of low-sulfur natural gas, for example, powder catalysts are operated in the turbulent flow regime where the gas-solid contact is optimal so as to maintain a high combustion efficiency [46]. 

The dynamics of bubble formation and growth and of solids movement within the bubbling fluidized beds have been analyzed in great detail, and elaborate computer simulations have been developed for all regimes of flnidization. The reader is referred to the specialized literatnre for details on snch models. Here we describe a fairly simple model that is applicable to the bnbbling regime and that treats a catalytic fluidized bed much like a gas-liquid reactor. 

A new process to convert glycerol to acrolein uses a catalytic fluidized bed reactor ... 

For comprehensive treatments of gas-solid fluidization, see Hetsroni [1], Yates [2], Davidson et al. [3], Geldart [4], Pell [5], Kunii and Levenspiel [6], Grace et al. [7], and Yang [8]. While fluidization can be applied to liquid-solid and three-phase (gas-liquid-solid) systems, the great majority of applications, especially in terms of catalytic processes, are for gas-solid systems, and this chapter is limited to this case. Readers interested in three-phase catalytic fluidized-bed reactors should consult Fan [9]. For a detailed review of spouted bed catalytic... 

The catalyst particles used in most catalytic fluidized-bed reactor processes are small enough, typically less than 100 pm in diameter, that catalyst effectiveness factors are close to 1. As a result, intraparticle difliisional resistances tend to play a minor role in fluidized-bed catalytic processes, unlike in packed-bed reactors where catalyst particles are more than an order of magnitude larger. 

Table 4.1 Some typical key characteristics of catalytic fluidized-bed reactors compared with those of alternative types of reactor.

One of the earliest successful catalytic fluidized-bed operations was to produce phthalic anhydride by partial oxidation... 

Many other catalytic fluidized-bed processes have been tested at various scales. These include catalytic low-temperature oxidation, catalytic gasification and pyrolysis of biomass and waste plastic, production of carbon nanotubes, dry reforming of methane, hydrogenation and dehydrogenation of hydrocarbons, methanol-to-gasoline (MTC) process, synthesis of dimethyl ether (DME), and selective catalytic reduction of nitrogen... 

Table 4.3 Typical operating ranges and features of catalytic fluidized-bed reactors,...

Table 4.4 compares the typical properties of major catalytic fluidized-bed processes, like those identified in Table 4.2, with corresponding practice for major gas-solid processes like combustion, gasification, roasting, incineration, and calcination. 

Figure 4.4 Pictorial representation of particles and void regions in flow regimes of principal interest for catalytic fluidized-bed reactors.

Figure 4.3 Diagrammatic representation of flow regimes of gas fluidization with increasing superficial gas velocity, with transition velocities indicated on the arrows between adjacent flow regimes. Dashed rectangular box encloses the five flow regimes of primary interest with respect to catalytic fluidized-bed reactors.

Other important properties of catalytic fluidized-bed reactors— mixing characteristics, heat and mass transfer, entrainment, attrition, agglomeration, and wear—are outlined in this section. 

These two equations mechanistically accoimt for contributions from both particle convection to the fixed surface (large first term on right side of Eq. 4.12) and gas convection (second term commencing with 0.165), the former term being the dominant term for the relatively fine particles of interest in catalytic fluidized-bed reactors. For typical bubbling bed operating conditions at atmospheric pressure and for particles of mean size 50-100 pm, the overall bed-to-immersed-surface heat transfer... 

In considering what to incorporate in models, one needs to keep in mind the long list of factors that can influence overall performance of catalytic fluidized-bed reactors ... 

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. 

When catalytic fluidized-bed reactors are scaled up or scaled down, there is no universally accepted scaling procedure. One approach is to maintain the HID ratio constant another is to hold H constant and only vary the diameter or cross-sectional area. In addition, there is no consensus on whether the distributor of a small facility should be a fully scaled geometrically identical version of the distributor intended for the full-scale reactor, or a portion of the full-scale version. Often the latter is impractical. 

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