Catalytic process variables

Inert gas pressure, temperature, and conversion were selected as these are the critical variables that disclose the nature of the basic rate controlling process. The effect of temperature gives an estimate for the energy of activation. For a catalytic process, this is expected to be about 90 to 100 kJ/mol or 20-25 kcal/mol. It is higher for higher temperature processes, so a better estimate is that of the Arrhenius number, y = E/RT which is about 20. If it is more, a homogeneous reaction can interfere. If it is significantly less, pore diffusion can interact. 

Deep catalytic cracking (DCC) is a catalytic cracking process which selectively cracks a wide variety of feedstocks into light olefins. The reactor and the regenerator systems are similar to FCC. However, innovation in the catalyst development, severity, and process variable selection enables DCC to produce more olefins than FCC. In this mode of operation, propylene plus ethylene yields could reach over 25%. In addition, a high yield of amylenes (C5 olefins) is possible. Figure 3-7 shows the DCC process and Table 3-10 compares olefins produced from DCC and FCC processes. ... 

This chapter contains a discussion of two intermediate level problems in chemical reactor design that indicate how the principles developed in previous chapters are applied in making preliminary design calculations for industrial scale units. The problems considered are the thermal cracking of propane in a tubular reactor and the production of phthalic anhydride in a fixed bed catalytic reactor. Space limitations preclude detailed case studies of these problems. In such studies one would systematically vary all relevant process parameters to arrive at an optimum reactor design. However, sufficient detail is provided within the illustrative problems to indicate the basic principles involved and to make it easy to extend the analysis to studies of other process variables. The conditions employed in these problems are not necessarily those used in current industrial practice, since the data are based on literature values that date back some years. 

Like in any catalytic process, process variables crucially impact reaction kinetics, conversion efficiency and catalyst stability. Increasing temperature favors cracking, thus decreasing the isomerate yield. It is preferred to have a high-activity catalyst and operate at the lowest possible temperature to achieve the highest RONC. Hydrogen shifts the equihbrium concentrations of olefins and carbenium ions. 

The basic idea is to examine operating parameters to find the optimum combination of them for optimum performance. A short list of the most important might include the following Fj, Cjo, Cj, v, V, T, Tq, u, P, and, of course. For catalytic processes additional variables include D, d, Sg, e, shape, and catalyst chemical properties such as chemical composition, activity, and selectivity. Most catalytic reactors operate with significant mass transfer limitations because one usually wants to raise the temperature until mass transfer becomes noticeable in order to attain the highest rate possible. In all cases one determines the effects of these variables on reactor performance. 

The two variables change their role with respect to their dependent versus independent, intensive versus extensive nature. This is also true of e.g. calorimetric, conductometric and spectrophotometric titrations using UV-, IR- or NMR-spectrosco-py We additionally have to consider that in the titration of the catalytic process only the external dynamics are measured a direct comparison with the actual metal fraction of the related intermediate complexes is generally not possible We call this analysis of homogeneous catalytic systems by a metal-ligand titration the method of inverse titration and for the resulting diagrams we use the term li nd-concentration control maps ([L]-control maps) . 

With the financial and technical help of those two oil companies, extensive development work on the catalytic cracking process was carried out on a laboratory and semiplant scale. This included the study of catalysts and the process variables, as well as the development of new engineering concepts which led to the first commercial application of this process in 1936. 

For a catalyzed surface reaction like the exchange of H2 with D2 we cannot talk about a single mechanism for the reaction. We must specify the experimental conditions (pressure, surface coverage, temperature, and surface structure) as the reaction mechanism is likely to change with changing conditions of the experiments. Also, since there are several reaction paths available at the various surface sites, even under specified experimental conditions it is likely that the experimental technique utilized to monitor the reaction rate and product distribution may not detect products that form along the various reaction branches with equal probability. Thus, a combination of techniques that are employed over a wide range of experimental variables is necessary to reveal the nature of the complex catalytic process. 

Recent work by Weber86 87 has shown that it is possible to determine the activity of catalysts from a completely practical point of view by correlating the more important actions of the catalyst as a function of the process variables (temperature, pressure, nature of catalyst) by means of simple equations, the validity of which has been proved for a large number of catalytic processes. These equations are especially useful in the study of simultaneous chemical reactions in which selectivity problems play a part. They are of great value for more systematical research on catalysts. 

V max has limited utility because it is related to [Et], an experimentally controlled variable. In contrast, k2 is a property of the enzyme. For the reaction model in Scheme 4.7, k2 is the rate-determining step in the catalytic process and is frequently labeled kcaV Like k2, kcat is a first-order rate constant with units of inverse time. kcat is often called the turnover frequency (TOF), the maximum number of substrate molecules an enzyme can convert to product per unit time. For an enzyme to achieve a maximum kcat, reaction conditions, namely temperature and pH, must be optimal. 

The preceding discussion has concentrated on the selection of catalytically active components. Although this is an essential task, this is just one aspect of the whole catalytic process, which also includes selection of catalyst support and the design of the overall catalyst in relation to reaction engineering requirements, so that not only activity and selectivity but also mechanical and chemical stability are ensured. For catalyst supports, the design variables are the degree and the form of the dispersion of the catalytic active components, and the porosity of the support. 

Time consuming kinetic studies are indispensible for the design, operation and process control. A description is needed of the catalytic rate as a function of the process variables, i.e. temperature, pressure, and composition of the reaction mixture. 

Effect of reaction variables on the different stages of a three phase catalytic process.24... 

---