Catalysts and catalytic reactors

Traditionally the technique of the medical physicist, magnetic resonance imaging (MRI) has long been used to investigate the internal structure of the human body and the transport processes occurring within it for example, MRI has been used to characterize drug transport within damaged tissue and blood flow within the circulatory system. It is therefore a natural extension of medical MRI to implement these techniques to study flow phenomena and chemical transformations within catalysts and catalytic reactors. 

Gas-phase MR will undoubtedly find more widespread use in studies of catalysts and catalytic reactors initial studies have been done with thermally polarized gases. Clearly, it will be of interest to image gas flows in reactors in this application, the measurement strategies used to image gas and liquid flows will be similar. However, gas- and liquid-phase species diffusing within a porous catalyst will be influenced to differing extents by the physical and chemical characteristics of the catalyst. These... 

Catalyst deactivation is always a problem in catalyst and catalytic reactor design. Empirical equations to represent deactivation rates in design calculations are reported by Weekman (1968), Sadana and Doraiswamy (1971), and Doraiswamy and Sharma (1984). Here, we briefly touch upon the... 

Catalyst deaetivation is one of the most vexing problems in catalyst and catalytic reactor design. We shall not be concerned with this in the present ehapter beyond using empirical equations to represent deactivation rates for use in design calculations (see, e.g., Weekman, 1968 Sadana and Doraiswamy, 1971 Doraiswamy and Sharma,... 

In graduate school at the Massachusetts Institute of Technology, I had the privilege of studying catalysis with Professor Charles Satterfield, who became my thesis advisor. Professor Satterfield had a profound influence on my interest in, and understanding of, catalysts and catalytic reactors. My years with Professor Satterfield at MIT were one of the high points of my journey. 

Whilst the basic process for generation and conversion of syngas is well established, production from biomass poses several challenges. These centre on the co-production of tars and hydrocarbons during the biomass gasification process, which is typically carried out at 800 °C. Recent advances in the production of more robust catalysts and catalytic membrane reactors should overcome many of these challenges. 

From these experiments, one can see that the direct partial oxidation of CH4 to synthesis gas over catalytic monoliths is governed by a combination of transport and luetic effects, with the transport of gas phase species governed by the catalyst geometry and flow velocity and the lanetics determined by the nature of the catalyst and the reactor temperature. Under the conditions utilized here, the direct oxidation... 

On each of these, random and structured reactors behave quite differently. In terms of costs and catalyst loading, random packed-bed reactors usually are most favorable. So why would one use structured reactors As will become clear, in many of the concerns listed, structured reactors are to be preferred. Precision in catalytic processes is the basis for process improvement. It does not make sense to develop the best possible catalyst and to use it in an unsatisfactory reactor. Both the catalyst and the reactor should be close to perfect. Random packed beds do not fulfill this requirement. They are not homogeneous, because maldistributions always occur at the reactor wall these are unavoidable, originating form the looser packing there. These maldistributions lead to nonuniform flow and concentration profiles, and even hot spots can arise (1). A similar analysis holds for slurry reactors. For instance, in a mechanically stirred tank reactor the mixing intensity is highly non-uniform and conditions exist where only a relatively small annulus around the tip of the stirrer is an effective reaction space. 

Structured reactors and catalysts are encountered in a large variety (3,6). Structured catalytic reactors can be divided into two categories. The first involves a structured catalyst, whereas the second one involves normal catalyst particles arranged in a nonrandom way. In the first category, the catalyst and the reactor are essentially identical entities. 

The field of chemical kinetics and reaction engineering has grown over the years. New experimental techniques have been developed to follow the progress of chemical reactions and these have aided study of the fundamentals and mechanisms of chemical reactions. The availability of personal computers has enhanced the simulation of complex chemical reactions and reactor stability analysis. These activities have resulted in improved designs of industrial reactors. An increased number of industrial patents now relate to new catalysts and catalytic processes, synthetic polymers, and novel reactor designs. Lin [1] has given a comprehensive review of chemical reactions involving kinetics and mechanisms. 

Catalysis and Catalytic Reactors Chac Separable kinetics -r = (Past history) X -r (Fresh catalyst)... 

Because silica is volatile (Si(0H)4 from Reaction R42 in Table 4.1) at high temperatures in high-pressure steam, it is now excluded from catalysts for steam reforming [85] [389], unless it is combined with alkali. For the same reason, silica-free materials are applied for the brick-lined exit gas collector and in autothermal reformers. Silica would be slowly removed from the catalyst (or brickwork) and deposited in boilers, heat exchangers and catalytic reactors downstream of the reformer. 

Many researchers pointed out the multiscale character of catalytic processes (Krishna and Sie, 1994 Tunca et al., 2006). For example, the temporal and length scales involved in functioning heterogeneous catalysts and the reactors in which they are used span several orders of magnitude as described by Centi and Perathoner (2003). Hence, the difference in length scale between an active catalyst site and the reactor in which it is expected to operate efficiendy is approximately 10 orders of magnitude. Accordingly,... 

The basic problem in the design of a heterogeneous reactor is to determine the quantity of catalyst and/or reactor size required for a given conversion and flow rate. In order to obtain this, information on the rate equaiion(s) and their parameter(s) must be made available. A rigorous approach to the evaluation of reaction velocity constants has yet to be accomplished for catalytic reactions at this time, industry still relies on the procedures set forth in the previous chapter. For example, in catalytic combustion leac-tioas, the rate equation is extremely complex and cannot be obtained either analytically or numerically. A number of equations may result and some simplification is often warranted. As mentioned earlier, in many cases it is safe to assume that the expression may be satisfactorily expressed by the rate equation of a single step. 

Aerosol reactors are used to synthesize nano-size ptirticles. Owing to their size, shape, and high specific surface area, nanopartides can be used in a number of applications such as in pigments in cosmetics, membranes, photo-catalytic reactors, catalysts and ceramics, and catalytic reactors. 

Overview. The objectives of this chapter are to develop an understanding of catalysts, reaction mechanisms, and catalytic reactor design. Specifically. after reading this chapter one should be able to... 

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