Reactor thermal aspects

Dente and Ranzi (in Albright et al., eds.. Pyrolysis Theory and Industrial Practice, Academic Press, 1983, pp. 133-175) Mathematical modehng of hydrocarbon pyrolysis reactions Shah and Sharma (in Carberry and Varma, eds.. Chemical Reaction and Reaction Engineering Handbook, Dekker, 1987, pp. 713-721) Hydroxylamine phosphate manufacture in a slurry reactor Some aspects of a kinetic model of methanol synthesis are described in the first example, which is followed by a second example that describes coping with the multiphcity of reactants and reactions of some petroleum conversion processes. Then two somewhat simph-fied industrial examples are worked out in detail mild thermal cracking and production of styrene. Even these calculations are impractical without a computer. The basic data and mathematics and some of the results are presented. 

In the following chapters, an example reaction system will be used for illustrating purposes. In order to focus on thermal aspects of reactor safety, no explicit chemistry will be used, but a general reaction scheme is used instead ... 

Part Three presents key related topics essential to the design, development, deployment, and acceptance of the Generation IV and advanced nuclear reactor concepts, which include the safety of advanced reactors, nonproliferation for advanced reactors (political and social aspects), thermal aspects of conventional and alternative fuels, hydrogen co-generation with Generation IV nuclear power plants, and advanced small modular reactors. Correspondingly, Part Three consists of five chapters (Chapters 16—20) written by top international experts working within these areas. 

Grande, L., Villamere, B., Allison, L., Mikhael, S., Rodriguez-Prado, A., Pioro, I., 2011. Thermal aspects of uranium carbide and uranium di-carbide fuels in supercritical water-cooled nuclear reactors. Journal of Engineering for Gas Turbines and Power 133 (2). February, 7 pp. 

Deposition of TiN by the thermal decomposition of tetrakis(dimethylamido)titanium (TDMAT) in a nitrogen atmosphere (as opposed to ammonia) was characterized by a simple Arrhenius rate expression. Adequate deposition rates and good step coverage were achieved for 3/1 aspect ratio holes, 0.40 micron in size. A reactor model was designed,... 

Jones, O. C., Jr., and S. G. Bankoff, 1977, Two-Phase Flow in Light Water Reactors, Proc. Symp. on Thermal and Hydraulic Aspects of Nuclear Reactors, ASME, Atlanta GA. (3)... 

Y. S. Tang. Ph.D has more than 35 years of experience in the field of thermal and fluid flow. His research interests have covered aspects of thermal hydraulics that are related to conventional and nonconventional power generation systems, with an emphasis on nuclear reactor design and analysis that focuses on liquld-meta -cooled reactors. Dr. Tang is co-author of Radioactive Waste Management published by Taylor 8 Francis, and Thermal Analysis of Liquid Metal Fast Breeder Reactors, He received a B5. from National Central University In China and an MS. in mechanical engineering from the University of Wisconsin. He earned his Ph.D. 

In addition to flow, thermal, and bed arrangements, an important design consideration is the amount of catalyst required (W), and its possible distribution over two or more stages. This is a measure of the size of the reactor. The depth (L) and diameter (D) of each stage must also be determined. In addition to the usual tools provided by kinetics, and material and energy balances, we must take into account matters peculiar to individual particles, collections of particles, and fluid-particle interactions, as well as any matters peculiar to the nature of the reaction, such as reversibility. Process design aspects of catalytic reactors are described by Lywood (1996). 

During recent years experimental work continued actively upon the macroscopic aspects of thermal transfer. Much work has been done with fluidized beds. Jakob (D5, J2) made some progress in an attempt to correlate the thermal transport to fluidized beds with transfer to plane surfaces. This contribution supplements work by Bartholomew (B3) and Wamsley (Wl) upon fluidized beds and by Schuler (S10) upon transport in fixed-bed reactors. The influence of thermal convection upon laminar boundary layers and their transition to turbulent boundary layers was considered by Merk and Prins (M5). Monaghan (M7) made available a useful approach to the estimation of thermal transport associated with the supersonic flow of a compressible fluid. Monaghan s approximation of Crocco s more general solution (C9) of the momentum and thermal transport in laminar compressible boundary flow permits a rather satisfactory evaluation of the transport from supersonic compressible flow without the need for a detailed iterative solution of the boundary transport for each specific situation. None of these references bears directly on the problem of turbulence in thermal transport and for that reason they have not been treated in detail. 

The CSTR is, in many ways, the easier to set up and operate, and to analyse theoretically. Figure 6.1 shows a typical CSTR, appropriate for solution-phase reactions. In the next three chapters we will look at the wide range of behaviour which chemical systems can show when operated in this type of reactor. In this chapter we concentrate on stationary-state aspects of isothermal autocatalytic reactions similar to those introduced in chapter 2. In chapter 7, we turn to non-isothermal systems similar to the model of chapter 4. There we also draw on a mathematical technique known as singularity theory to explain the many similarities (and some differences) between chemical autocatalysis and thermal feedback. Non-stationary aspects such as oscillations appear in chapter 8. 

Maintenance of proper temperature is a major aspect of reactor operation. The illustrations of several reactors in this chapter depict a number of provisions for heat transfer. The magnitude of required heat transfer is determined by heat and material balances as described in Section 17.3. The data needed are thermal conductivities and coefficients of heat transfer. Some of the factors influencing these quantities are associated in the usual groups for heat transfer namely, the Nusselt, Stanton, Prandtl, and Reynolds dimensionless groups. Other characteristics of particular kinds of reactors also are brought into correlations. A selection of practical results from the abundant literature will be assembled here. Some modes of heat transfer to stirred and fixed bed reactors are represented in Figures 17.33 and 17.18, and temperature profiles in... 

There are several aspects of thermal sensitivity and instability which are important to consider in relation to reactor design. When an exothermic catalytic reaction occurs in a non-isothermal reactor, for example, a small change in coolant temperature may, under certain circumstances, produce undesirable hotspots or regions of high temperature within the reactor. Similarly, it is of central importance to determine whether or not there is likely to be any set of operating conditions which may cause thermal instability in the sense that the reaction may either become extinguished or continue at a higher temperature level as a result of fluctuations in the feed condition. We will briefly examine these problems. 

These points are explained in detail in this chapter. In a first section, the general aspects of reaction engineering for batch reactors are briefly presented. The mass and heat balances are analysed and it is shown that a reliable temperature control is central to the safety of batch reactors. The different strategies of temperature control and their consequences on reactor safety are explained in the following sections. For each strategy, the design criteria and the safety assessment procedure are introduced. The chapter is closed by recommendations for the design of thermally safe batch reactions. 

Besides these purely static aspects, the dynamic behavior of an adiabatic batch reactor must also be considered. The adiabatic temperature course is a function of the thermal properties of the reaction mixture. The adiabatic temperature increase influences the final temperature as well as the rate of the temperature increase. For highly exothermal reactions, even for small increase in conversion, the increase in temperature is important (see Section 2.4.3). 

The thermal time constant is only one aspect of reactor dynamics. In practice, the heat carrier temperature cannot be adjusted instantaneously at industrial scale, as it has its own dynamics, depending on the equipment and the temperature control algorithm. These aspects of the dynamics of the heat exchange and temperature control systems are considered in the next sections. 

Part I gives a general introduction and presents the theoretical, methodological and experimental aspects of thermal risk assessment. The first chapter gives a general introduction on the risks linked to the industrial practice of chemical reactions. The second chapter reviews the theoretical background required for a fundamental understanding of mnaway reactions and reviews the thermodynamic and kinetic aspects of chemical reactions. An important part of Chapter 2 is dedicated to the heat balance of reactors. In Chapter 3, a systematic evaluation procedure developed for the evaluation of thermal risks is presented. Since such evaluations are based on data, Chapter 4 is devoted to the most common calorimetric methods used in safety laboratories. 

In the second part of the chapter, the mathematical model of the BR has been augmented by considering its behavior in the presence of significant thermal effects and of a proper heat exchange apparatus. In particular, modeling these aspects brings the reader to understand the need for considering the thermal stability of batch reactors (Chap. 4) and the need for adequate systems of automatic temperature control (Chap. 5). 

Like other salt melts ionic liquids are characterized by a specific combination of physicochemical properties high ionic conductivity, low viscosity, high thermal stability compared to conventional liquid solvents, wide electrochemical windows of up to 7 V and - in most cases - extremely low vapor pressures. Due to their low vapor pressure ionic liquids are not only well suited for the application of UHV-based analytical techniques (e.g. photoelectron spectroscopy [3]), but also for use in plasma reactors with typical pressures of the order of 1 Pa up to 10 kPa. Moreover, due to their high electrical conductivity, ionic liquids may even be used as electrodes for plasmas. To date there are just a few reports on the combination of low-temperature plasmas and ionic liquids available in the literature [4—6]. Therefore, the essential aspects of experiments with ionic liquids in typical plasma reactors are discussed in this section. 

A fundamental aspect in the reactor design is the contribution of different thermal resistances in achieving a highly efficient heat transfer. The overall heat coefficient U is given by the relation ... 

When dealing with the design of the equipment for carrying out a photochemical reaction, several aspects must be considered. Some of them are common to the design of conventional thermal reactors, such as the kinetic characteristics of the reactions involved, the phases of the system, the necessity... 

Various aspects of the effect of process scale-up on the safety of batch reactors have been discussed by Gygax [7], who presents methods to assess thermal runaway. Shukla and Pushpavanam [8] present parametric sensitivy and safety results for three exothermic systems modeled using pseudohomogenous rate expressions from the literature. Caygill et al. [9] identify the common factors that cause a reduction in performance on scale-up. They present results of a survey of pharmaceutical and fine chemicals companies indicating that problems with mixing and heat transfer are commonly experienced with large-scale reactors. 

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