Reactor kinetic approach

The kinetics of a mixed platinum and base metal oxide catalyst should have complementary features, and would avoid some of the reactor instability problems here. The only stirred tank reactor for a solid-gas reaction is the whirling basket reactor of Carberry, and is not adaptable for automotive use (84) A very shallow pellet bed and a recycle reactor may approach the stirred tank reactor sufficiently to offer some interest. 

This paper deals mainly with the condensation of trace concentrations of radioactive vapor onto spherical particles of a substrate. For this situation the relation between the engineering approach, the molecular approach, and the fluid-dynamic approach are illustrated for several different cases of rate limitation. From these considerations criteria are derived for the use of basic physical and chemical parameters to predict the rate-controlling step or steps. Finally, the effect of changing temperature is considered and the groundwork is thereby laid for a kinetic approach to predicting fallout formation. The relation of these approaches to the escape of fission products from reactor fuel and to the deposition of radon and thoron daughters on dust particles in a uranium mine is indicated. 

An accurate design of unsteady-state catalytic processes requires knowledge about the catalyst behavior and reaction kinetics under unsteady-state conditions, unsteady-state mass and heat transfer processes in the catalyst particle and along the catalyst bed, and dynamic phenomena in the catalytic reactor. New approaches for reactor modeling and optimization become necessary. Together, these topics form a wide area of research that has been continuously developed since the 1960s. 

In the 1970s Sawagushi et al. [21] presented resnlts and validated a kinetic approach to the intensity function in the case of steam gasification of PE in a fixed-bed reactor. These results are presented in Fignre 10.4. 

FIG. 21-147 Changes in state as applied to granulator and reactor kinetics and design. [Ennis, Theory of Granulation An Engineering Approach, in Handbook of Pharmaceutical Granulation Technology, 2d ed., Farikh (ed.), Taylor h- Francis, 2005. With permission.]... 

Kinetic approaches represent realistic and comprehensive description of the mechanism of network formation. Under this approach, reaction rates are proportional to the concentration of unreacted functional groups involved in a specific reaction times an associated proportionality constant (the kinetic rate constant). This method can be applied to the examination of different reactor types. It is based on population balances derived from a reaction scheme. An infinite set of mass balance equations will result, one for each polymer chain length present in the reaction system. This leads to ordinary differential or algebraic equations, depending on the reactor type under consideration. This set of equations must be solved to obtain the desired information on polymer distribution, and thus instantaneous and accumulated chain polymer properties can be calculated. In the introductory paragraphs of Section... 

For practical reactor calculations, however, it is generally sufficient to use a kinetic approach based on the rate-determining elementary step. In many cases, an empirical rate equation that describes the influence of the most important variables wth sufficient accuracy in the chosen operating range is adequate. 

Figure 16.1 illustrates that for low conversions, for example, Xai, the CSTR and PFR volumes are similar, since the respective areas AEFG and under AT curve are similar. As the desired conversion is decreased, the areas for the two reactor types approach the same value. Small reactors lead to small conversions and are useful for kinetic studies. 

There are several questions that can be raised about the operation of a reactor. They form the basis of a classification and define ideal conditions that are desirable for proper measurements of reaction rates. In practice, industrial reactors will approach these idealized limits more or less closely and the corresponding situations can be handled more or less successfully by the rapidly developing methods of chemical engineering kinetics (alias chemical reaction engiruering). 

An early model describing the pervaporative reaction of acetic acid with ethanol was presented by Krupiczka and Koszorz (1999). It was a simple, three-parameter model describing the concentration profiles in the process (a kinetic approach was considered) in the form of three differential equations. The activity coefficients were calculated using the UNIFAC property method. A hydrophilic membrane PERVAP 1005 GFT was used. Differently, Tanna and Mayadevi (2007) developed a two-step series model to study the performance of the membrane reactor. In this case, the goal was the assessment of the parameters that drive the process. As a conclusion of this work, the authors suggested using a low-flux membrane with a sufficient surface area when designing a PVMR. 

The two solutions are identical. Hence, for a long time no importance was attributed to the use of a kinetic approach for describing batch polycondensations starting from monomers, and the statistical approach was preferred. Of course, chemical engineers had to deal with semi-batch and continuous stirred tank reactors where the statistical approach, although possible, is cumbersome and error-prone, so a few papers appeared in the 1960s dealing with kinetically controlled linear polycondensations [274—276]. 

Figures 3 to 8 show the results of calculation as a plot of concentration of a reactant species against bed height. Figure 3 shows one extreme, a high chemical rate constant (K = 50 s ) and a high rate of interphase gas exchange (ig/Vg = 2.0 s ). The reactor performance approaches the plug flow case (but with rate controlled by the exchange coefficient, not the chemical kinetics) and the prediction not very sensitive to the proportion of gas flowing interstitially. Even so, the bed hei t needed to achieve a given level of conversion is materially reduced as the interstitial flow proportion increases.

Continuous reactors are commonly used for producing synthetic polymers. In many cases, they offer certain advantages over batch reactors in terms of product quality and ease of handling reagents and products. Because reactions can reach a steady state in continuous reactors, this approach can also be of fundamental value in studying kinetics and mechanisms of reactions. 

Classical reactor analysis and design usually assume one of two idealized flow patterns plug flow or completely backmixed flow. Real reactors may approach one of these however, it is often the nonidealities and their interaction with chemical kinetics that lead to poor reactor design and performance (Levenspiel, 1998). Nonidealities include channeling, bypassing, and dead zones, among others. 

There are in general several steps of refinement to model a gasification system. Zero-dimensional models show the lowest complexity, and rely on empirical correlations or thermodynamic equilibrium calculations. The next step is a onedimensional model that usually requires kinetic expressions either to resolve the space or time coordinate using idealized chemical reactor models. Approaching two- or three-dimensional calculations provokes the use of computational fluid dynamics (CFD) that may incorporate either equiUbriiun or kinetics-based turbulence chemistry interactions. Each step of modeling adds significant complexity and calculation time. 

The Argonaut is introduced to students by tour and lecture. Next, the students perform a complete reactor checkout and the approach-to-critical experiment using a control rod. This is followed by a brief study of some reactor kinetics and an experimental determination of some delayed-neutron parameters. 

The aim of the first part of the experiment day is to acquaint the student with some of the details of the reactor design through a tour and lecture, and to introduce the operational and safety aspects of the reactor by means of a complete reactor checkout. The second part of the day is devoted to the approach-to-critical experiment using a control rod and to reactor kinetic studies. 

---