Chemical reactors Subject

High-pressure models applicable to pressure vessels and chemical reactors subjected to more than 15 psig... 

KEYWORDS catalyst deactivation, coke formation, kinetics of coke formation, diffusional limitations, chemical reactors subject to catalyst deactivation. 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

In a chemical production process at least one of the unit operations (the chemical reactor) is the place in which chemical conversion takes place. However, the chemical upstream reactor is proceeded by a series of unit operations in which the new materials are downstream prepared (the upstream operations). After conversion has taken place, the products are operations subjected to a further series of unit operations (the downstream operations). These downstream operations include product recovery and purification steps. A typical example of a production process is illustrated in Figure 1.1. 

One feature that distinguishes the chemical engineer from other types of engineers is the ability to analyze systems in which chemical reactions are occurring and to apply the results of his analysis in a manner that benefits society. Consequently, chemical engineers must be well acquainted with the fundamentals of chemical kinetics and the manner in which they are applied in chemical reactor design. This textbook provides a systematic introduction to these subjects. 

One feature that distinguishes the education of the chemical engineer from that of other engineers is an exposure to the basic concepts of chemical reaction kinetics and chemical reactor design. This textbook provides a judicious introductory level overview of these subjects. Emphasis is placed on the aspects of chemical kinetics and material and energy balances that form the foundation for the practice of reactor design. 

Simpler optimization problems exist in which the process models represent flow through a single pipe, flow in parallel pipes, compressors, heat exchangers, and so on. Other flow optimization problems occur in chemical reactors, for which various types of process models have been proposed for the flow behavior, including well-mixed tanks, tanks with dead space and bypassing, plug flow vessels, dispersion models, and so on. This subject is treated in Chapter 14. 

Chemical reactions will take place only when the reactant molecules are in intimate contact. In some cases, especially with very fast reactions or viscous liquids, segregation of the reactants can exist, which make the reaction rates and selectivities dependent on the mixing intensity. In chemical reactor engineering, the assumption is usually made that only mean concentrations need be considered. In reality, concentration values fluctuate about a mean, and in some cases these fluctuations must be considered in detail. This field is very complex and is still the subject of much research. This example serves only to introduce these concepts and to show how simulations can be made for certain simple situations. 

Summary. In this chapter the control problem of output tracking with disturbance rejection of chemical reactors operating under forced oscillations subjected to load disturbances and parameter uncertainty is addressed. An error feedback nonlinear control law which relies on the existence of an internal model of the exosystem that generates all the possible steady state inputs for all the admissible values of the system parameters is proposed, to guarantee that the output tracking error is maintained within predefined bounds and ensures at the same time the stability of the closed-loop system. Key theoretical concepts and results are first reviewed with particular emphasis on the development of continuous and discrete control structures for the proposed robust regulator. The role of disturbances and model uncertainty is also discussed. Several numerical examples are presented to illustrate the results. 

In Fig. 1, various elements involved with the development of detailed chemical kinetic mechanisms are illustrated. Generally, the objective of this effort is to predict macroscopic phenomena, e.g., species concentration profiles and heat release in a chemical reactor, from the knowledge of fundamental chemical and physical parameters, together with a mathematical model of the process. Some of the fundamental chemical parameters of interest are the thermochemistry of species, i.e., standard state heats of formation (A//f(To)), and absolute entropies (S(Tq)), and temperature-dependent specific heats (Cp(7)), and the rate parameter constants A, n, and E, for the associated elementary reactions (see Eq. (1)). As noted above, evaluated compilations exist for the determination of these parameters. Fundamental physical parameters of interest may be the Lennard-Jones parameters (e/ic, c), dipole moments (fi), polarizabilities (a), and rotational relaxation numbers (z ,) that are necessary for the calculation of transport parameters such as the viscosity (fx) and the thermal conductivity (k) of the mixture and species diffusion coefficients (Dij). These data, together with their associated uncertainties, are then used in modeling the macroscopic behavior of the chemically reacting system. The model is then subjected to sensitivity analysis to identify its elements that are most important in influencing predictions. 

In any real reactor, the flow will not follow the plug-flow pattern precisely. Non-ideal flow in chemical reactors is the subject of Chap. 6 where the various models used to predict the performance of industrial reactors are discussed at some length. 

Chemical reactions carried out on an industrial scale are subject to control by processes such as mass transfer and heat transfer, which, in small-scale work, may often be reduced to negligible proportions by judicious choice of conditions. Volume 23 deals with these aspects of reaction kinetics, which must always be considered when significant scale-up is contemplated and which may be expected to be of paramount importance in industrial operations. The principles of chemical reactor design are treated in a form digestible by chemists and there is some emphasis on the way in which available kinetic data may be utilised by the chemical engineer. 

In the United States two major textbooks helped define the subject in the early 1960s. The first was a book by Levenspiel that explained the subject pictoriaUy and included a large range of applications, and the second was two short texts by Aris that concisely described the mathematics of chemical reactors. While Levenspiel had fascinating updates... 

Logically, the subject of designing a chemical reactor for a given process might proceed as shown in the following sequence of steps. 

The chemical reactor is the unif in which chemical reactions occur. Reactors can be operated in batch (no mass flow into or out of the reactor) or flow modes. Flow reactors operate between hmits of completely unmixed contents (the plug-flow tubular reactor or PFTR) and completely mixed contents (the continuous stirred tank reactor or CSTR). A flow reactor may be operated in steady state (no variables vary with time) or transient modes. The properties of continuous flow reactors wiU be the main subject of this course, and an alternate title of this book could be Continuous Chemical Reactors. The next two chapters will deal with the characteristics of these reactors operated isothermaUy. We can categorize chemical reactors as shown in Figure 2-8. 

This is the fun (and frustration) of chemical reaction engineering. While thermodynamics, mass and heat transfer, and separations can be said to be finished subjects for many engineering apphcations, we have to reexamine every new reaction system from first principles. You can find data and construct process flowsheets for separation units using sophisticated computer programs such as ASPEN, but for the chemical reactors in a process these programs are not much help unless you give the program the kinetics or assume equihhrium yields. 

It is our belief that a course in chemical reaction engineering should introduce all undergraduate students to aU these topics. This is an ambitious task for a one-semester course, and it is therefore essential to focus carefully on the essential aspects. Certainly, each of these subjects needs a full course to lay out the fundamentals and to describe the reaction systems peculiar to them. At the same time, we beheve that a course that considers chemical reactors in a unified fashion is essential to show the common features of the diverse chemical reactors that our students will be called on to consider. 

We regard the essential aspects of chemical reaction engineering to include multiple reactions, energy management, and catalytic processes so we regard the first seven chapters as the core material in a course. Then the final five chapters consider topics such as environmental, polymer, sohds, biological, and combustion reactions and reactors, subjects that may be considered optional in an introductory course. We recommend that an instmctor attempt to complete the first seven chapters within perhaps 3/4 of a term to allow time to select from these topics and chapters. The final chapter on multiphase reactors is of course very important, but our intent is only to introduce some of the ideas that are important in its design. 

In a method proposed by Booth et al. (141) for the determination of phylloquinone in various food types, extracted samples are subjected to silica solid-phase extraction followed, in the case of meat or milk samples, by further purification using reversed-phase solid-phase extraction or liquid-phase reduction extraction, respectively. The final test solution is analyzed by NARP-HPLC, and the fluorescent hydroquinone reduction products of phylloquinone and the internal standard are produced online using a postcolumn chemical reactor packed with zinc metal. 2, 3 -Dihydrophylloquinone, a synthetic analog of phylloquinone, is a suitable internal standard for the analysis of vegetable juice, whole milk, and spinach. Another synthetic analog, Ku23), is used for the analysis of bread and beef, because a contaminant in the test solution coelutes with dihydro-phylloquinone. 

The research subject in the given problem is the process of cementation based on squeezing out mercury from salt-acidic solution by means of a less useful metal, such as aluminum. A study of kinetics of the given chemical reaction shows that this process may be effectively conducted in a continuous chemical reactor. Process efficiency is measured by mercury concentration in the solution after refinement. This is simultaneously the system response as it may be measured quite accurately and quantitatively. These three factors influence the cementation process significantly Xi-temperature of solution, °C X2-solution flow rate in reactor, ml/1 and X3-quantity of aluminum g. The factor space is defined by these intervals 50[Pg.341]

The area of reactor design has been widely studied, and there are many excellent textbooks that cover this subject. Most of the emphasis in these books is on steady-state operation. Dynamics are also considered, but mostly from the mathematical standpoint (openloop instability, multiple steady states, and bifurcation analysis). The subject of developing effective stable closedloop control systems for chemical reactors is treated only very lightly in these textbooks. The important practical issues involved in providing reactor control systems that achieve safe, economic, and consistent operation of these complex units are seldom understood by both students and practicing chemical engineers. 

Mac Mullin and Weber [1] introduced the concept of the RTD in the analysis of chemical reactors, and Danckwerts [2] developed this concept further in his classical paper, which has since formed the basis of various investigations involving flow systems in chemical and biochemical reactors. Levenspiel [3], Levenspiel and Bischoff [4], Himmelblau and Bischoff [5], Wen and Fan [6], and Shinnar [7] have given extensive treatments of this subject. 

That mixing in chemical reactors is a subject of very active research is attested by the recent publication of several review... 

The RTD concept is now well established and widely used for designing, scaling-up and optimizing chemical reactors. This subject was recently reviewed by Nauman (1). Most classical textbooks (55) mainly deal with the simple case of steady state incompressible flow, single inlet and outlet, and purely convective motion... 

This chapter gives an introduction to the subject of chemical reaction engineering. The first part introduces basic definitions and concepts of chemical reaction engineering and chemical kinetics and the importance of mass and heat transfer to the overall chemical reaction rate. In the second part, the basic concepts of chemical reactor design are covered, including steady-state models and their use in the development... 

An excellent reference is Deckwer (Bubble Column Reactors, Wiley, 1992). Two complementary reviews of this subject are by Shah et al. [AlChE J. 28 353-379 (1982)] and Deckwer [in de Lasa (ed.), Chemical Reactor Design and Technology, Martinus Nijhoff, 1985, pp. 411-461]. Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions,Wiley 1984). 

Lerou and Froment [10] found by calculations that a reactor may ignite under non constant flow conditions while it is still stable if constant flow is assumed. Kalthoff and Vortmeyer [11],(Figure 4) found an improved agreement between measured and calculated ranges of multiple solutions for non -uniform flow. From the previous work therefore can be concluded that non-uniform porosity and flow distributions effect the chemical reactor performance. The question however, whether real improvements are obtained has to be subject to a comparison of experimental results with calculations. 

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