Heterogeneous Laboratory Reactors

Steady, continuous basis. Then ideal stirred-tank behavior is approached as the recycle rate is increased. Recycle ratios (recycle rate to feed rate) of 15 or more are usually sufficient to achieve results close to stirred-tank operation. 

Instead of rotating the catalyst. Ford and Perlmutter inserted a cylinder containing a deposited catalyst in a stirred vessel. Studies on the vapor-phase dehydrogenation of sec-butyl alcohol indicated that stirred-tank performance was achieved.,  

Many speciahzed laboratory reactors and operating conditions have been used. Sinfelt has alternately passed reactants and inert materials through a tubular-flow reactor. This mode of operation is advantageous when the activity of the fixed bed of catalyst pellets changes with time. A system in which the reactants flow through a porous semiconductor catalyst, heated inductively, has been proposed for studying the kinetics of high-temperature (500 to 2000°C) reactions. An automated microreactor 

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Although our major interest in this chapter is heterogeneous laboratory reactors, for the sake of completeness we shall briefly review homogeneous systems. Experimental measurements in homogeneous reactors represent... 

It is a good idea to run the laboratory reactor without catalyst to check for homogeneous reactions. However, this method does not work when the homogeneous reaction involves reactants that do not occur in the feed but are created by a heterogeneous reaction. It then becomes important to maintain the same ratio of free volume to catalyst volume in the laboratory reactor used for intrinsic kinetic studies as in the pilot or production reactors. 

A variety of laboratory reactors have been developed for the determination of the kinetics of heterogeneous reactions, all with specific advantages and disadvantages. Several reviews of laboratory reactors are available [28-33]. The evaluations of the available methods in these reviews are different because of the variation of chemical reactions and catalysts investigated and the different viewpoints of the authors. It is impossible to choose a best kinetic reactor because too many conflicting requirements need to be satisfied simultaneously. Berty [34] discussing an ideal kinetic reactor, collected 20 requirements as set forward by different authors. From these requirements it is easy to conclude, that the ideal reactor, that can handie all reactions under all conditions, does not exist For individual reactions, or for a group of similar reactions, not all requirements are equally important. In such cases it should be possible to select a reactor that exhibits most of the important attributes. 

To analyze reaction mechanisms in complex catalytic systems, the application of micropulse techniques in small catalytic packed beds has been used. Christoffel [33] has given an introduction to these techniques in a comprehensive review of laboratory reactors for heterogeneous catalytic processes. Mtlller and Hofmann [59,61] have tested the dynamic method in the packed bed reactor to investigate complex heterogeneous reactions. Kinetic parameters have been evaluated by a method, which employs concentration step changes and the time derivatives of concentration transients at the reactor outlet as caused by a concentration step change at the reactor inlet. 

A laboratory reactor system was constructed for the investigation of tar conversion under real fixed-bed conditions. The basic idea behind the reactor design was the spatial separation of primary pyrolysis and secondary tar reactions. A scheme of the system is shown in fig, 2. After the primary pyrolysis stage, the tar containing pyrolysis gas is swept to the conversion reactors where homogeneous and/or heterogeneous tar reactions can be investigated. 

In conclusion, a few comments about the practical importance of heterogeneous-homogeneous reactions from kinetic viewpoint. Usually industrial reactors have a relatively large free volume and the packing material is also present in addition to the catalyst. If homogeneous reactions are beneficial for the overall process, then they will be retarded due to deactivation of radicals and the rate will be lower compared to laboratory reactors with more advantageous reactor geometries. 

Fig. 3.1.3-1 Laboratory reactors for determining the kinetics of heterogeneously catalyzed reactions [Luft 1987a, Forni 1997, Cavalli 1997] ...

Equation I l.S.a-1 is obtained from a material balance on a reference component, say A, over an elementary cross section of the tubular reactor, containing an amount of catalyst dW. Indeed, as previously mentioned, rate equations for heterogeneously catalyzed reactions are generally referred to unit catalyst weight, rather than reactor volume, in order to eliminate the bed density. Obviously, different packing densities between the laboratory reactor in which kinetic data were determined and the industrial reactor, calculated on the basis of these data would lead to different results. 

The two t5q)es of laboratory reactor shown in Figure 13-3 have proved to be the most suitable for reaction engineering investigations on heterogeneously catalyzed gas-phase reactions [27]. 

Figure 4.14 Main types of laboratory reactors for studying various homogeneous and heterogeneous reactions. RDC = rotating disk contactor.

A steady-state one-dimensional heterogeneous plug-flow model has been used to represent the behavior of the laboratory reactor. The internal and external mass transfer resistances are taken into account. For different operating conditions, very small Prater numbers were calculated. For this reason, the thermal gradients inside the washcoat were not included in the reactor model (Froment and Bischoff, 1990). The axial solid temperature profiles were measured for each experiment, therefore only the mass balances for gas and solid phases are needed to model the reactor. The equations that represent the system are the following ... 

E. G. Christoffel, Laboratory reactors and heterogeneous catalytic processes, Catalysis Reviews Science and Engineering, voL 24, pp. 159-232, 1982. 

Recent advances in h h-speed screening technologies for heterogeneous catalysts have been recently summarized by Zheng and Zhou (2011). Laboratory reactors and various aspects associated with catalyst testing from the perspective of obtaining intrinsic reaction kinetics has been the... 

Different laboratory reactors are used for kinetic studies. For studies of liquid-liquid reactions and homogeneously catalyzed reactions, a batchwise operated stirred tank reactor is frequently used. Tubular reactors loaded with catalyst (fixed bed) are more common for studies of heterogeneously catalyzed gas reactions. The tubular reactor displays a simple design and is easy to operate. A simultaneous integral and differential mode of operation can be achieved by a tap reactor for measuring concentration and temperatures at defined axial positions. Gradientless operation with respect to temperature and concentration can be obtained by an external or internal recycle. 

Kapteijn, F. and Moulijn, ]A, (1999) Laboratory reactors, in Hattdbook cf Heterogeneous Catalysis, vol. 3 (eds G. Erd, H. Knoetzinger, and J. Weitkamp), Wiley-VCH Verlag GmbH, Weinheim. 

The most appropriate laboratory reactor for detailed kinetic investigations is the continuously operated, gradientless recycle reactor. A large number of different constructions is described in the literature /44/. We have developed and successfully used for many years for heterogeneously catalysed vapor phase reactions such a reactor with internal recirculation (Fig. 21) It can be operated up to 800 K and 50 bars (catalyst volume 10 cm ). 

Sunderland, P., "An assessment of laboratory reactors for heterogeneously catalyzed vapour phase reactions", Trans. Instn. Chem. Engrs., 54, 135 (1976). 

Figure 1. Types of laboratory reactors used in the studies of heterogeneous catalysis.

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