REACTOR KINETICS Objectives

If there are two or more reactants involved in the reaction, both can be converted completely in a single pass only if they are fed to the reactor in the stoichiometric proportion. In many cases, the stoichiometric ratio of reactants may be the best, but in some instances, where one reactant (especially water or air) is very much cheaper than the other, it may be economically advantageous to use it in excess. For a given size of reactor, the object is to increase the conversion of the more costly reactant, possibly at the expense of a substantial decrease in the fraction of the cheaper reactant converted. Examination of the kinetics of the reaction is required to determine whether this can be achieved, and to calculate quantitatively the effects of varying the reactant ratio. Another and perhaps more common reason for departing from the stoichiometric proportions of reactants is to minimise the amount of byproducts formed. This question is discussed further in Section 1.10.4. 

One of the most difficult decisions that a textbook writer has to make is to select what material to cover and what topics to leave out. This is especially difficult in chemical reaction engineering because of the wide scope of the field and the diversity of topics that it covers. As the title indicates, this book focuses on the analysis and design of chemical reactors. The objective of the book is to present a comprehensive, unified methodology to analyze and design chemical reactors that overcomes the deficiencies of the current pedagogy. To concentrate on this objective, some topics that are commonly covered in chemical reaction engineering textbooks (chemical kinetics, catalysis, effect of diffusion, mass-transfer limitation, etc.) are... 

This chapter discussed esterification mechanisms, and evaluated the kinetics objectively and quantitatively, which provided a most effective way to select catalyst and design reactor for different esterification systems. It is discovered that some new catalysts (such as lipases, room temperature ionic liquids) have being used in esterification nevertheless, there are few research on the case. Herein, it is worthy to be investigated deeply. 

With the TAP approach reactions also occur at a vacuum-solid interface. However, unlike the surface science approach, the solid surface in a TAP experiment is generally a disordered technical material such as industrial catalyst particles that are generally studied in flow reactors. The objective of a TAP experiment is to measure nonsteady-state kinetic properties and track how changes in reaction conditions and sample composition alter these properties. Incrementally increasing or decreasing the amount of a single component while simultaneously making kinetic measurements charts the correspondence between a component... 

For proper design and simulation of HDT reactors, kinetic and reactor modeling are aspects that need to be deeply studied however, this is not a trivial task due to the numerous physical and chemical processes that occur simultaneously in the reactor phase equilibrium, mass transfer of reactants and products between the gas-liquid-solid phases, diffusion inside the catalyst particle, a complex reaction network, and catalyst deactivation. Ideally, the contribution of all these events must be coupled into a robust reactor performance model. The level of sophistication of the model is generally defined based upon the pursued objectives and prediction capability [4]. 

To(r,0 = some convenient reference temperature The immediate objective is to obtain a solution of the reactor-kinetics equations [(9.84) and (9.85)] which also satisfy the energy-balance relation (9.125). As in the previous problems in time dependence, we look for a solution which is separable in time and space. This treatment is limited, therefore, to bare systems and to reflected systems which meet the requirements set forth in Sec. 9.Id. On this basis we can use the relations (9.88) for the flux and the precursor concentrations. In addition, we select... 

The reactor director objects to this question and answer, in that it is defining prompt jump as the magnitude of the flux increase rather than the ratio of flux after and before the reactivity change. (The ratio is that used in analyses, and always appears as a ratio in the kinetic equations see Hetrick equation 2-47 or Keepin equations 8-9, 8-10). Answer b is equally valid. 

The gas motion near a disk spinning in an unconfined space in the absence of buoyancy, can be described in terms of a similar solution. Of course, the disk in a real reactor is confined, and since the disk is heated buoyancy can play a large role. However, it is possible to operate the reactor in ways that minimize the effects of buoyancy and confinement. In these regimes the species and temperature gradients normal to the surface are the same everywhere on the disk. From a physical point of view, this property leads to uniform deposition - an important objective in CVD reactors. From a mathematical point of view, this property leads to the similarity transformation that reduces a complex three-dimensional swirling flow to a relatively simple two-point boundary value problem. Once in boundary-value problem form, the computational models can readily incorporate complex chemical kinetics and molecular transport models. 

Chemical vapor deposition (CVD) of carbon from propane is the main reaction in the fabrication of the C/C composites [1,2] and the C-SiC functionally graded material [3,4,5]. The carbon deposition rate from propane is high compared with those from other aliphatic hydrocarbons [4]. Propane is rapidly decomposed in the gas phase and various hydrocarbons are formed independently of the film growth in the CVD reactor. The propane concentration distribution is determined by the gas-phase kinetics. The gas-phase reaction model, in addition to the film growth reaction model, is required for the numerical simulation of the CVD reactor for designing and controlling purposes. Therefore, a compact gas-phase reaction model is preferred. The authors proposed the procedure to reduce an elementary reaction model consisting of hundreds of reactions to a compact model objectively [6]. In this study, the procedure is applied to propane pyrolysis for carbon CVD and a compact gas-phase reaction model is built by the proposed procedure and the kinetic parameters are determined from the experimental results. 

The large-scale spread of DAFCs is closely related to the development of efficient anodic and cathodic materials, characterized by very fast electrochemical kinetics, stability at the high current densities in alkaline environments and modest cost. This objective requires cathodes without noble metals and anodes with very low amounts of noble metals. In order to improve the cheapness and sustainability of the processes described above, the most accepted opinion is the possibility of using solar light by means of the introduction of Ti02, pure or doped, into the electrode material formulation. Figure 4.15 shows a typical laboratory-scale photoelectrocatalytic reactor. 

The solution of problems in chemical reactor design and kinetics often requires the use of computer software. In chemical kinetics, a typical objective is to determine kinetics rate parameters from a set of experimental data. In such a case, software capable of parameter estimation by regression analysis is extremely usefiil. In chemical reactor design, or in the analysis of reactor performance, solution of sets of algebraic or differential equations may be required. In some cases, these equations can be solved an-... 

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. 

When the objective of the modeling effort is to develop and validate a reaction mechanism, the major uncertainty in the model must reside in the detailed chemical kinetic mechanism. Under these conditions, the process must be studied either under transport-free conditions, e.g., in plug-flow or stirred-tank reactors, or under conditions in which the transport phenomena can be modeled very precisely, e.g., under laminar flow conditions. This way. 

The Flow Reactor In flow reactor experiments designed for chemical kinetic interpretation, the objective is to achieve a plug-flow situation, where composition and temperature are uniform over the cross section of the reactor. This condition may be approximated both in the turbulent [442] and the laminar [233] flow regimes. In the turbulent flow regime, a high linear flow rate secures negligible recirculation flow. Each element of gas reacts as it moves, with the characteristic time scale for heat and mass transfer by... 

The primary objective of this chapter is to develop low-dimensional representations of chemically reacting flow situations. Specifically these include batch reactors (corresponding to homogeneous mass-action kinetics), plug-flow reactors (PFR), perfectly stirred reactors (PSR), and one-dimensional flames. The derivations also serve to illustrate the approach that is taken to derive appropriate systems of equations for other low-dimensional circumstances or flow situations. 

B. Flow Reactors. Laboratory-scale catalytic reactors and reactors for the reaction of solids with gases arc often constructed from metal. One of the principal objectives in the use of laboratory-scale catalytic reactors is the determination of rate data which can be associated with specific physical and chemical processes in a catalytic reaction. Descriptions are available for these kinetic analyses as they relate to reactor designs and reaction conditions. ... 

At APCI experiments were conducted in a fixed-bed reactor and product gas compositions were monitored Typical results of this study are given in Table III (8). According to APCI equilibrium calculations very little elemental sulfur yields were predicted. However, high sulfur yields were obtained, particularly with high steam concentrations. This was an interesting observation, and detailed studies on this system were warranted As before, the objectives of the studies at WVU are to identify the reactions, to elucidate the mechanisms and then to study their kinetics. 

The American consortium NeSSI [23] (New Sampling Sensor Initiative) is a CPAC-sponsored (Center for Process Analytical Chemistry) open initiative formed in 2000 to create a standard for process analysis. The object is to implement modular and smart process analytics. It also aims at the future integration of micro analytical devices and micro structured reactors. The concept is derived from the semiconductor industry and follows the ISA SP76 standard. Companies such as Parker-Hannifin, Kinetics and Swagelok are involved. Further information is also given on the web site [23],... 

The main ideas of chemical kinetics are reviewed in the next section for the sake of completeness, a brief account is given here of the performance of continuous reactors as compared to BR, which is the object of the present book. 

The first objective of flowsheeting is to obtain a consistent description of the mate-rial-balance envelope. The reactor model should be of the kinetic type, at least for the main reactions, in order to account for the effect of variable flow rates and composition of recycles coming from separations. Stoichiometric or yield reactor models can be employed for describing secondary reactions and formation of impurities. In a first attempt the separators may be black-boxes provided with appropriate specifications. 

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