Catalytic internals, characteristics

Table 10.3 Characteristics of the applied structured catalytic internals for the ethyl acetate system.

All schemes presented are similar and conventional to a great extent. It is characteristic that the epoxidation catalysis also results in the heterolytic decomposition of hydroperoxides (see Section 10.1.4) during which heterolysis of the O—O bond also occurs. Thus, there are no serious doubts that it occurs in the internal coordination sphere of the metal catalyst. However, its specific mechanism and the structure of the unstable catalyst complexes that formed are unclear. The activation energy of epoxidation is lower than that of the catalytic decomposition of hydroperoxides therefore, the yield of oxide per consumed hydroperoxide decreases with the increase in temperature. 

The essential feature of an autothermal reactor system is the feedback of reaction heat to raise the temperature and hence the reaction rate of the incoming reactant stream. Figure 1.6 shows a number of ways in which this can occur. With a tubular reactor the feedback may be achieved by external heat exchange, as in the reactor shown in Fig. 1.6a, or by internal heat exchange as in Fig. 1.6b. Both of these are catalytic reactors their thermal characteristics are discussed in more detail in Chapter 3, Section 3.6.2. Being catalytic the reaction can only take place in that part of the reactor which holds the catalyst, so the temperature profile has the form... 

Paul Weisz suggested in a lucid note published in 1973 that cells, and indeed even entire organisms, have evolved in a way that maintains unity effectiveness factor [24]. That is, the size of the catalytic assembly is increased in nature as the overall rate at which that assembly operates decreases, and the relationship between characteristic dimension and activity can be well approximated by the observable modulus criterion for reaction limitation. It is possible that Weisz s arguments may fail under process conditions, and internal gradients within a compartment or cell may be important. However, at present it appears that the most important transport limitations and activities in cells are those that operate across cellular membranes. Therefore, to understand and to manipulate key transport activities in cells, it is essential that biochemical engineers understand these membrane transport processes and the factors influencing their operation. A brief outline of some of the important systems and their implications in cell function and biotechnology follows. 

The reaction rate per unit volume of catalyst as well as its selectivity depend on both the specific catalytic activity and the surface area of the active component per unit catalyst volume, as well as on its pore structure. These characteristics are determined by the conditions of catalyst preparation. Therefore, when developing a new catalyst, it is extremely important to be able to determine in advance the required internal surface area and the most suitable pore structure of the catalyst for the given reaction. 

Madon and Boudart propose a simple experimental criterion for the absence of artifacts in the measurement of rates of heterogeneous catalytic reactions [R. J. Madon and M. Boudart, Ind. Eng. Chem. Fundam., 21 (1982) 438]. The experiment involves making rate measurements on catalysts in which the concentration of active material has been purposely changed. In the absence of artifacts from transport limitations, the reaction rate is directly proportional to the concentration of active material. In other words, the intrinsic turnover frequency should be independent of the concentration of active material in a catalyst. One way of varying the concentration of active material in a catalyst pellet is to mix inert particles together with active catalyst particles and then pelletize the mixture. Of course, the diffusional characteristics of the inert particles must be the same as the catalyst particles, and the initial particles in the mixture must be much smaller than the final pellet size. If the diluted catalyst pellets contain 50 percent inert powder, then the observed reaction rate should be 50 percent of the rate observed over the undiluted pellets. An intriguing aspect of this experiment is that measurement of the number of active catalytic sites is not involved with this test. However, care should be exercised when the dilution method is used with catalysts having a bimodal pore size distribution. Internal diffusion in the micropores may be important for both the diluted and undiluted catalysts. 

Montanaro L. and Saracco G., Influence of some precursors on the physico-chemical characteristics of transition aluminas for the preparation of ceramic catalytic filters. Ceramics International 21 A3 (1995). 

Almost 3200 different enzymes have been listed and categorized by the International Union of Biochemistry and Molecular Biology in its last report in 1992. An encyclopaedic description of more than 7000 commercially available enzymes can be found in Ref [36]. Table 10.2 collects some industrial enzymes suppliers. Enzymes exhibiting the same catalytic function are known as homologous enzymes and they fall into two classes heteroenzymes and isoenzymes. The first group includes enzymes derived from different sources but which catalyse identical reactions, yet show different chemical and kinetic characteristics. A comprehensive enzyme information system, termed BRENDA, is available via the Internet (http //www.brenda.uni-koeln.de). 

It was a sad moment for his friends and colleagues in chemistry departments throughout the world, to read of the death of Professor Garner on March 4th, 1960. He was particularly well known to catalytic chemists, by his papers over some thirty years, and by his effective contributions to the series of conferences on catalysis which were initiated by the Faraday Society Discussion at Liverpool in 1950. Many readers of this notice will remember hearing his paper presented to the First International Conference on Catalysis at Philadelphia in 1956, and will recall with pleasure his characteristically modest, yet persuasive contributions in discussion. Garner s influence extended far beyond his own research group at Bristol, his leadership and inspiration being felt over a wide circle of scientists. 

These kinds of processes lead to the internal olefinic (alkene) and the branched paraffinic (alkene) and other branched olefinic products which are characteristic of catalytic cracking processes. 

At /e > 25 internal diffusion control is reached. Any substrate molecule diffusing into the enzyme layer is converted therein only part of the enzyme is acting catalytically. Diffusion controlled sensors exhibit the following characteristics ... 

The catalytic importance of zeolites from an industrial standpoint resides both in the ability to subtly tailor their properties to described characteristics and in the consequent high activities and selectivities( l). These last two attributes are primarily a result of the large internal surface area of the zeolites and their microporosity, respectively. The aspect of tailoring zeolites to desired characteristics demands an intimate knowledge of both their structural and chemical properties. 

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