Fixed-bed gas-solid catalytic reactors

Froment, G.F., Hofmann, H., Design of fixed-bed gas-solid catalytic reactors, in Chemical Reaction and Reaction Engineering, Marcel Dekker, New York, USA, 1987... 

Radial dispersion of mass and heat in fixed bed gas-solid catalytic reactors is usually expressed by radial Peclet number for mass and heat transport. In many cases radial dispersion is negligible if the reactor is adiabatic because there is then no driving force for long range gradients to exist in the radial direction. For non-adiabatic reactors, the heat transfer coeflScient at the wall between the reaction mixture and the cooling medium needs also to be specified. 

The risks of using space velocity for scale-up of fixed-bed (gas-solid) catalytic reactors are well known [12]. Trickle-bed... 

Froment, G.F. and Hofmann, H.P.K. (1987), Design of Fixed Bed Gas-Solid Catalytic Reactors, in JJ.Carberry and A.Varma, Eds., Chemical Reaction and Reactor Engineering, 373-440 (Marcel Dekker). [235]... 

Duducovic MP (1999) Trends in catalytic reaction engineering. Cattil Today 48(1 ) 5-15 Froment GF, Hofmann HPK (1987) Design of fixed-bed gas-solid catalytic reactors. In Ctir-berry JJ, Varma A (eds) Chemical reaction and reaction engineering. Marcel Dekker Inc, New York and Basel, pp 373-440... 

A semicontinuous reactor is a reactor for a multiphase reaction in which one phase flows continuously through a vessel containing a batch of another phase. The operation is thus unsteady-state with respect to the batch phase, and may be steady-state or unsteady-state with respect to the flowing phase, as in a fixed-bed catalytic reactor (Chapter 21) or a fixed-bed gas-solid reactor (Chapter 22), respectively. 

Types of reactors a. Gas-solid catalytic reactors Fixed bed reactors... 

As in the case of gas-solid catalytic reactors, here also it is common practice to use fixed-bed, fluidized-bed, or moving-bed reactors. However,... 

The countercurrent-flow fixed-bed operation is often used for gas-liquid reactions rather than gas-liquid-solid reactions. Examples of reactions using this type of reactor are given by Danckwerts.29 A comparison between a gas liquid-solid (catalytic) fixed-bed reactor and a gas-liquid-solid (inert) fixed-bed reactor is shown in Table 1-7. The major difference between packed-bed gas-liquid reactors and gas-liquid-solid catalytic reactors is in the nature and size of the packing used and the conditions of gas and liquid flow rates. The packed-bed gas-liquid reactors use nonporous, large-size packing, so that they can be operated at high gas and liquid flow rates without excessive pressure drop. The shape of... 

Magnifying the section of a SILP surface, we should obtained a picture similar to that hypothesized in Figure 38. The IL film is physically adsorbed on the surface of the solid support and contains the dissolved catalyst. Since the film has the size of the diffusion layer, all metal complexes are involved in the catalytic reaction. When SILP particles are used as the fixed bed of a flow reactor, reagents enter the IL film, they react under homogeneous conditions (the thin IL film) and products, eventually, are desorbed into the carrier gas stream. 

Two gas-solid catalytic reactions, (1) and (2), are studied in fixed-bed reactors. Rates of reaction per unit mass of catalyst, at constant composition and total pressure, indicate the variations with mass velocity and temperature shown in the figure. The interior pore surface in each case is fully effective. What do the results shown suggest about the two reactions ... 

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. 

For gas-soM catalytic reactors also, fixed-bed, fluidized-bed, and moving-bed reactors are commonly employed. However, since all gas-solid noncatalytic reactions are inherently time dependent, time becomes an unavoidable parameter in the analysis. We briefly outline the procedures for the three reactor types mentioned and also touch upon a few other types. In view of the special importance of fluidized-bed reactors for these systems, two case studies involving the use of this reactor are presented in Section 11.5 (Case Studies 11.6 and 11.12). Dutta and Gualy (1999) give a comparative evaluation of fixed- and fluidized-bed reactors. 

Case Study 11.4 Gas-Solid (Catalytic) Reaction in Fixed-Bed NINA and Adiabatic Reactors Reduction oe Nitrobenzene to Aniline... 

Figure 4.10.6 Adiabatic fixed bed reactors for gas-solid catalytic reactions (a) simple fixed bed (b) rack type reactor with interstage injection of gas (c) rack type reactor with interstage cooling or heating (ErtI, Knoezinger, and Weitkamp, 1997).

Gas mixing in laboratory internal recycle reactors used for gas-solid catalytic studies may be assessed from pressure drop (Berty, 1974 Berty, 1979), temperature drop measurements across the bed (Mahoney, 1984), or from mass transfer coefficient estimations (Caldwell, 1983). For a given impeller speed, the first method involves comparing the bed pressure drop of the recycle reactor v/iih pressure drop of a calibrated fixed catalyst bed conducted in a separate unit. Then knowing the fluid velocity versus pressure drop for the calibrated bed, the impeller speed versus fluid velocity can be drawn. The recycle rate can also be determined from thermodynamics based on the ratio of the adiabatic temperature change and the measured temperature difference. This method requires the measurements of temperatures across the bed and the mass flow rate. 

The reliability of a model is the function of the validation method such as testing it with independent data and experimental transport and thermodynamic properties at the reaction conditions. For the case of hydrotreating of heavy petroleum, the reactor involves three phases the nonvaporized hydrocarbon (liquid), the vaporized hydrocarbon plus the hydrogen (gas), and the fixed-bed catalyst (solid). Hence, the system to be modeled is a three-phase fixed-bed catalytic heterogeneous reactor. Some assumptions can be made in order to represent the real experimental reactor. 

Catalytic testings have been performed using the same rig and a conventional fixed-bed placed in the inner volume of the tubular membrane. The catalyst for isobutane dehydrogenation [9] was a Pt-based solid and sweep gas was used as indicated in Fig. 2. For propane oxidative dehydrogenation a V-Mg-0 mixed oxide [10] was used and the membrane separates oxygen and propane (the hydrocarbon being introduced in the inner part of the reactor). 

In a fixed-bed catalytic reactor for a fluid-solid reaction, the solid catalyst is present as a bed of relatively small individual particles, randomly oriented and fixed in position. The fluid moves by convective flow through the spaces between the particles. There may also be diffusive flow or transport within the particles, as described in Chapter 8. The relevant kinetics of such reactions are treated in Section 8.5. The fluid may be either a gas or liquid, but we concentrate primarily on catalyzed gas-phase reactions, more common in this situation. We also focus on steady-state operation, thus ignoring any implications of catalyst deactivation with time (Section 8.6). The importance of fixed-bed catalytic reactors can be appreciated from their use in the manufacture of such large-tonnage products as sulfuric acid, ammonia, and methanol (see Figures 1.4,11.5, and 11.6, respectively). 

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