Reactor for complex reaction

Design of a batch reactor for complex reactions of the type A-----—>C... 

Chapter 4 outlines the elements of stoichiometry, rates, and reaction and reactor analysis. The reader is introduced to the concept of ideal reactors and the principles of their design for simple reactions. Extensions of these ideal reactors form the subject matter of Part III but are anticipated at this stage as an introductory setting for that part. Chapter 5 extends the analysis to complex reactions, but the design of reactors for complex reactions is deferred to Part III. 

Then the compositions of A, B, C, D, R, and S can be expressed in terms of the reaction coordinates and 2 defined by Equations 5.7 and 5.8, respectively. Hence the number of equations would be (a) six if written in terms of the rates of formation/disappearance of the individual components or (b) two if written in terms of the extent of reaction in each step. This concept is applied in Chapter 11 to the actual design of reactors for complex reactions... 

We now turn to the design of reactors for complex reactions. We will focus on the ethylation reaction, using the following less formal nomenclature A = aniline, B = ethanol, C = monoethylaniline, D = water, E = diethyl-aniline, F = diethyl ether, and G = ethylene. The four independent reactions then become... 

Chemical engineering students take a course entitled reaction engineering during their junior year this course teaches both analytical and graphical methods for designing plug flow reactors for complex reaction kinetics and heterogeneous physical systems. 

The search for Turing patterns led to the introduction of several new types of chemical reactor for studying reaction-diffusion events in feedback systems. Coupled with huge advances in imaging and data analysis capabilities, it is now possible to make detailed quantitative measurements on complex spatiotemporal behaviour. A few of the reactor configurations of interest will be mentioned here. 

Graphical Approach to the Analysis of Batteries of Stirred Tank Reactors Operating at Steady State. Even in reaction systems where it is not possible to determine the algebraic form of the reaction rate expression, it is often possible to obtain kinetic data that permit one to express graphically the rate as a function of the concentration of one reactant. Laboratory scale CSTR s are particularly appropriate for generating this type of kinetic data for complex reaction... 

Our treatment of Chemical Reaction Engineering begins in Chapters 1 and 2 and continues in Chapters 11-24. After an introduction (Chapter 11) surveying the field, the next five Chapters (12-16) are devoted to performance and design characteristics of four ideal reactor models (batch, CSTR, plug-flow, and laminar-flow), and to the characteristics of various types of ideal flow involved in continuous-flow reactors. Chapter 17 deals with comparisons and combinations of ideal reactors. Chapter 18 deals with ideal reactors for complex (multireaction) systems. Chapters 19 and 20 treat nonideal flow and reactor considerations taking this into account. Chapters 21-24 provide an introduction to reactors for multiphase systems, including fixed-bed catalytic reactors, fluidized-bed reactors, and reactors for gas-solid and gas-liquid reactions. 

Kinetic Treatment and Reactor Performance for Complex Reactions... 

As will be seen, the rate at which the potential is changed (i.e., the sweep rate) becomes veiy important. For complex reactions, it may have to be so slow (0.01 mV s 1) that cyclic voltammetry approaches a potentiostatic (rather than a potentiody-namic) technique. On the other hand, too large a sweep rate may yield parameters that are not those of the steady state and hence are difficult to fit into a mechanism of consecutive reactions in which the attainment of a steady state (d6/dt = 0) at each potential is a basic assumption. Thus, determining the mechanisms of reactions that are to function in steady-state devices such as fuel cells or reactors is more likely to... 

Table 6. Overall Conversions in a Fixed-Bed Reactor for the Reaction of Tet-rachloromethane with Complex Fluorides at Various Temperatures10...

A laboratory catalytic reactor that is a closed system, is said to be a static system. The kinetic model for complex reactions taking place in the reactor is of the form... 

There have been many hybrid multiscale simulations published recently in other diverse areas. It appears that the first onion-type hybrid multiscale simulation that dynamically coupled a spatially distributed 2D KMC for a surface reaction with a deterministic, continuum ODE CSTR model for the fluid phase was presented in Vlachos et al. (1990). Extension to 2D KMC coupled with ID PDE flow model was described in Vlachos (1997) and for complex reaction networks studied using 2D KMC coupled with a CSTR ODEs model in Raimondeau and Vlachos (2002a, b, 2003). Other examples from catalytic applications include Tammaro et al. (1995), Kissel-Osterrieder et al. (1998), Qin et al. (1998), and Monine et al. (2004). For reviews, see Raimondeau and Vlachos (2002a) on surface-fluid interactions and chemical reactions, and Li et al. (2004) for chemical reactors. 

Heterogeneously catalyzed reactions are usually studied under steady-state conditions. There are some disadvantages to this method. Kinetic equations found in steady-state experiments may be inappropriate for a quantitative description of the dynamic reactor behavior with a characteristic time of the order of or lower than the chemical response time (l/kA for a first-order reaction). For rapid transient processes the relationship between the concentrations in the fluid and solid phases is different from those in the steady-state, due to the finite rate of the adsorption-desorption processes. A second disadvantage is that these experiments do not provide information on adsorption-desorption processes and on the formation of intermediates on the surface, which is needed for the validation of kinetic models. For complex reaction systems, where a large number of rival reaction models and potential model candidates exist, this give rise to difficulties in model discrimination. 

For complex reactions and with multistage CSTRs, more than three steady states can exist (as in Fig. 23-17c). Most of the work on multiplicities and instabilities has been done only on paper. No plant studies and a very few laboratory studies are mentioned in the comprehensive reviews of Razon and Schmitz Chem. Eng. Set, 42, 1,005-1,047 [1987]) and Morbidelli et al. (in Carberry and Varma, Chemical Reaction and Reactor Engineering, Dekker, 1987, pp. 973-1,054). 

As for reactions with a single reactant, rate methods are not well suited for evaluation of results from batch, tubular, and differential recycle reactors. For complex mechanisms, however, no convenient concentration methods might exist, so that a rate method could be the lesser of two evils. 

Bilous, 0., and Piret, E. L. 1955. Continuous stirred tank reactors A new graphical method for complex reactions and reflux designs. A.I.Ch.E. Journal 1, 480-487. 

Conceptually, the framework of the theory permits description of interphase heat and mass transfer with reaction occurring in either or both phases. In theory one can use this approach to study the affects of partial mixing of the dispersed phase on extent of reaction for non-first-order reactions which occur in the droplets. Analyses can be made for mass-transfer-controlled reactions and selectivity for complex reactions. Difficulties in the solution of the resulting integro-diflferential equations have restricted applications at present to partial solutions. For example, the effects of partial droplet mixing on extent of reaction were studied for uniform drops. Mass transfer from nonuniform drops for various reactor geometries was studied for dispersions with drop breakage only or drop coalescence only. 

Chemical reactions, which proceed extremely fast and without considerable heat of reaction, should not be carried out in stirred tanks, but in pipe reactors. This particularly applies for complex reactions of the type competitive-consecutive reactions, in which care must be taken, so that the desired product formed does not come into contact with the educt. Otherwise an undesired secondary reaction would take place, whereby the selectivity would be reduced. 

Initial Rate Method For reversible reactions, we use a modified differential method—the initial rate method. In this case, a series of experiments are conducted at selected initial reactant compositions, and each run is terminated at low conversion. From the collected data, we calculate (by numerical differentiation) the reaction rate at the initial conditions. Since the reaction extent is low, the reverse reaction is negligible, and we can readily determine the orders of the forward reaction from the known initial compositions. The rate of the reversible reaction is determined by conducting a series of experiments when the reactor is charged with selected initial product compositions. The initial rate method is also used to determine the rates for complex reactions since it enables us to isolate the effect of different reactants. 

More tests are needed comparing measured conversions and temperature profiles with model predictions for tubular reactors. Comparisons will be easier for reactions with simple kinetics than for complex reactions such as partial oxidations. Tests should be made over a wide range of Reynolds numbers, which may require high velocities and long reactors. If kinetic data are uncertain or unavailable, the overall heat transfer coefficient for the 1-D model can be obtained from the axial temperature profile and the total heat removal [41] ... 

This latter technique of Himmelblau, Jones, and Bischoff (H-J-B) has proved to be efficient in various practical situations with few, scattered, data available for complex reaction kinetic schemes (see Ex. 1.6.2-1). Recent extensions of the basic ideas are given by Eakman, Tang, and Gay [48,49, 50]. It should be pointed out, however, that the problem has been cast into one of linear regression at the expense of statistical rigor. The independent variables , X jp, do not fulfill one of the basic requirements of linear regression that the Xi p have to be free of experimental error. In fact, the X p are functions of the dependent variables C/tf) and this may lead to estimates for the parameters that are erroneous. This problem will be discussed further in Chapter 2, when the estimation of parameters in rate equations for catalytic reactions will be treated. Finally, all of the methods have been phrased in terms of batch reactor data, but it should be recognized that the same formulas apply to plug flow and constant volume systems, as will be shown later in this book. 

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