Bed plug-flow catalytic reactor

PACKED BED PLUG-FLOW CATALYTIC REACTOR 9.8.1 Laboratory Scale Reactor... 

FIGURE 9.15 Schematic representation of a packed bed plug-flow catalytic reactor. 

The physical arrangement involves a method of containing the solid reactant while the fluid phase is passed over the solid in such a way that intimate contact is possible between the flowing fluid and the resident solid. This physical arrangement is familiar from the static bed plug flow catalytic reactor and is now applied here to a reactor where solid interactions with a fluid phase, including adsorption, are studied. The interactions between the phases involved in a catalytic TS-PFR have been described above. The non-catalytic interactions are now considered as they take place in a TS-PF-SSR. The differences between the two reactor types lie in hardware requirements and in the mode of operation. 

Series and series-parallel reactions in a packed-bed plug-flow membrane reactor have been analyzed by Lund and his co-investigators [Agarwalla and Lund, 1992 Lund, 1992 Bernstein and Lund, 1993]. First consider the following consecutive catalytic reactions ... 

Under steady-state conditions in a plug-flow tubular reactor, the onedimensional mass transfer equation for reactant A can be integrated rather easily to predict reactor performance. Equation (22-1) was derived for a control volume that is differentially thick in all coordinate directions. Consequently, mass transfer rate processes due to convection and diffusion occur, at most, in three coordinate directions and the mass balance is described by a partial differential equation. Current research in computational fluid dynamics applied to fixed-bed reactors seeks a better understanding of the flow phenomena by modeling the catalytic pellets where they are, instead of averaging or homogenizing... 

Packed-bed micro-reactor model (powdered catalyst)—For both the SCR and the PGM components of the ASC, experimental data collected over the powdered catalysts have been analyzed according to a heterogeneous one-dimensional plug-flow dynamic reactor model that assumes the catalytic bed to be isothermal and isobaric [22]. 

In the three idealized types of reactors just discussed (the perfectly mixed batch reactor, the plug-flow tubular reactor [PFRj), and the perfectly mixed con-tinuous-stirred tank reactor [CSTR]), the design equations (i.e., mole balances) were developed based on reactor volume. The derivation of the design equation for a packed-bed catalytic reactor (PBR) will be carried out in a manner analogous to the development of the tubular de.sign equation. To accomplish this derivation, we simply replace the volume coordinate in Equation (1-10) with the catalyst mass (i.e., weight) coordinate W (Figure 1-14). 

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. 

It is not practical to stir all reaction systems, for example, bulk polymerizations, postpolymerization reactions, fixed-bed catalytic reactors, and plug-flow reactors. Although multipoint temperature sensing is often used as a key solution to determine a runaway in nonagitated vessels, the occurrence of hot spots may not always be detected. 

Consider the catalytic reaction of Example 18.2. Using the rate equation found for this reaction determine the amount of catalyst needed in a packed bed reactor (assume plug flow) for 35% conversion of A to R for a feed of 2000 mol/hr of pure A at 3.2 atm and 117°C. 

Kjaer (K9) gives a very comprehensive study of concentration and temperature profiles in fixed-bed catalytic reactors. Both theoretical and experimental work is reported for a phthallic anhydride reactor and various types of ammonia converters. Fair agreement was obtained, but due to the lack of sufficiently accurate thermodynamic and kinetic data, definite conclusions as to the suitability of the dispersed plug flow model could not be reached. However, the results seemed to indicate that the... 

If the radial diffusion or radial eddy transport mechanisms considered above are insufficient to smear out any radial concentration differences, then the simple dispersed plug-flow model becomes inadequate to describe the system. It is then necessary to develop a mathematical model for simultaneous radial and axial dispersion incorporating both radial and axial dispersion coefficients. This is especially important for fixed bed catalytic reactors and packed beds generally (see Volume 2, Chapter 4). 

Industrial fixed-bed catalytic reactors have a wide range of different configurations. The configuration of the reactor itself may give rise to multiplicity of the steady states when other sources alone are not sufficient to produce the phenomenon. Most well known is the case of catalytic reactors where the gas phase is in plug flow and all diffusional resistances are negligible, while the reaction is exothermic and is countercurrently cooled. One typical example for this is the TVA type ammonia converter [38-40]. 

For the special case of a packed bed catalytic reactor with plug flow, the equation is rewritten in terms of catalyst weight,... 

The trickle bed reactor allows for plug flow reactor assumptions even at extremely low liquid-flow rates. The trickle bed is classified as a continuous heterogeneous catalytic reactor. 

Plug flow reactors are often used to investigate heterogeneously catalysed reactions. Typically 0.1-10 g of catalyst with a pellet diameter smaller than 1 mm is loaded into a tube of 1 cm diameter and a few dm long. A central thermocouple well allows the measurement of the temperature inside the catalytic bed. 

The fluidized-bed reactor involves a rapid movement of the solid catalytic particles throughout the bed so that the operation can come close to one of uniform temperature throughout the reactor. The actual flow pattern for the operation of a fluidized bed is very complex and is between that for the ideal back-mix reactor and the ideal plug-flow reactor so that special methods for design may be required to approximate the real situation. 

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