Real Catalytic Reactors

A demonstration of this approach has been reported to evaluate the ability of a lattice-Boltzmann code to predict both spatially resolved flow fields and MR propagators characterizing flow through random packings of spheres (model fixed beds) for flows defined by Peclet (Pe) and Reynolds numbers in the range 182 < Pc <3 50 and 0.4 < Re <0.77 (85). Excellent agreement was found between the numerical predictions and experimental measurements. Current interest in this field addresses the validation and development of numerical codes predicting flows at Reynolds numbers more appropriate to real catalytic reactors. 

Because of their low pressure drop, structured reactors in practice dominate the field for treating tail gases. Figure 2 presents the major types of reactor. The monolithic reactor represents the class of real structured catalytic reactors, whereas the parallel-passage reactor and the lateral-flow reactor are based on a structured arrangement of packings with normal catalyst particles. 

After the type and strength of interaction of a potential poison with the catalyst surface has been studied and the number of adsorption sites estimated, its effect on the rate of a given catalytic reaction can be studied. Any kind of catalytic reactor may in principle be used for these studies, that is, static as well as dynamic methods are suitable, and the various forms of pulse techniques are applicable. The real distinction between the two types of poisoning experiments that have been performed lies in the fact that the poison is either fed together with the reactant and is present in the gas phase throughout the run or the surface is poisoned by irreversibly preadsorbing the poison while the gas phase is kept free of it. If the poison is present in the gas phase, a larger number of modes of interaction with different surface sites may be possible than for the... 

Fundamental deactivation data are more difficult to obtain than fundamental catalytic reaction rate data because the latter must be known before the nature of the deactivation function can be determined. This is largely due to the kinds of reactors that are used to study deactivation. Many of the usual difficulties experienced in trying to get fundamental deactivation data can be obviated by using a reactor system in which the conversion and hence the compositions of the major components remain constant both in time and in space within the reactor. A description of an apparatus of this type and its utilization to study the deactivation of a real catalytic reaction are presented in this paper. The problem of determining the initial activity in a rapidly deactivating system is also discussed. 

Direct experimental studies on catalytic reactions using real atmospheric aerosols are just beginning. Therefore, the anticipated role of such reactions in the Earth s atmosphere is based mainly on estimates from experiments made with model catalysts, together with known data that characterizes the atmosphere as a sort of global catalytic reactor. For example, a drastic acceleration of chemical transformations in the atmosphere after volcanos eruptions has been observed [1]. Also, the possible... 

In the framework of this description an attempt to model an effect of spatial non-uniformity of real catalytic systems was made (Bychkov et al., 1997). It was assumed that reaction proceeds in a heterogeneous system represented by two active infinite plane surfaces and in the gas gap between them. Surface chemistry was treated as for the Li/MgO catalyst (see Table III). Because of substantial complexity of the kinetic scheme consisting of several hundred elementary steps, the mass-transfer was described in this case as follows. The whole gas gap was divided into several (up to 10) layers of the same thickness, and each of them was treated as a well-stirred reactor. The rate of particle exchange between two layers was described in terms of the first-order chemical reaction with a rate constant ... 

There are, of course, some limitations. An obvious one is that this method cannot be applied yet to the preparation of industrial catalysts, another one is its cost because ultra-high-vacuum (UHV) equipment is required. This drawback explains why this method is usually coupled to surface techniques such as XPS, UPS, RHEED, and AES, which also require UHV. The last disadvantage is that the best suited supports are those that are flat, i.e., oxide single-crystal faces, oxides produced by oxidation of a metal single crystal, or compressed powder oxides. There have been several examples where the preparation chamber also serves as sample chamber for surface techniques and is coupled to a catalytic reactor. Whereas there are a number of works using this approach for bulk metals (80), there are, by contrast, few studies dealing with metals supported on either single crystals (81) or polycrystalline supports (78, 79,82, 83). The latter type of system appears to be the model catalysts closest to the real catalyst. 

By neglecting intraparticle thermal gradients and both axial and radial dispersion phenomena, a fixed - bed catalytic reactor can be simulated by a heterogeneous one - dimensional plug-flow model. This provides a picture, at least from a qualitative point of view, reasonably close to the real one [cf. Carberry, 1975 Pereira et al., 1979]. Let us introduce new dimensionless temperatures ... 

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. 

Figure 10 Real-time plot of reactor component concentrations in catalytic hydrogenation step illustrating early detection of baseline upset by on-line micro-HPLC.

Figure 3.1 shows a typical laboratory flow reactor for the study of catalytic kinetics. A gas chromatograph (GC, lower shelf) and a flow meter allow the complete analysis of samples of product gas (analysis time is typically several minutes), and the determination of the molar flow rate of various species out of the reactor (R) contained in a furnace. A mass spectrometer (MS, upper shelf) allows real-time analysis of the product gas sampled just below the catalyst charge and can follow rapid changes in rate. Automated versions of such reactor assemblies are commercially available. 

In previous chapters, we deal with simple systems in which the stoichiometry and kinetics can each be represented by a single equation. In this chapter we deal with complex systems, which require more than one equation, and this introduces the additional features of product distribution and reaction network. Product distribution is not uniquely determined by a single stoichiometric equation, but depends on the reactor type, as well as on the relative rates of two or more simultaneous processes, which form a reaction network. From the point of view of kinetics, we must follow the course of reaction with respect to more than one species in order to determine values of more than one rate constant. We continue to consider only systems in which reaction occurs in a single phase. This includes some catalytic reactions, which, for our purpose in this chapter, may be treated as pseudohomogeneous. Some development is done with those famous fictitious species A, B, C, etc. to illustrate some features as simply as possible, but real systems are introduced to explore details of product distribution and reaction networks involving more than one reaction step. 

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