Reaction modeling reactor concepts

The first study on the oxidation of arylmethanes used this reaction as a model to show the general advantages of electrochemical micro processing and to prove the feasibility of an at this time newly developed reactor concept [69]. Several limits of current electrochemical process technology hindered its widespread use in synthetic chemistry [69]. As one major drawback, electrochemical cells stiU suffer from inhomogeneities of the electric field. In addition, heat is released and large contents of electrolyte are needed that have to be separated from the product. 

The cyclohexene hydrogenation is a well-studied process especially in conventional trickle-bed reactors (see original citations in [11,12]) and thus serves well as a model reaction. In particular, flow-pattern maps were derived and kinetics were determined. In addition, mass transfer can be analysed quantitatively for new reactor concepts and processing conditions, as overall mass transfer coefficients were determined and energy dissipations are known. In lieu of benchmarking micro-reactor performance to that of conventional equipment such as trickle-bed reactors, such a knowledge base facilitates proper, reliable and detailed comparison. 

We have used CO oxidation on Pt to illustrate the evolution of models applied to interpret critical effects in catalytic oxidation reactions. All the above models use concepts concerning the complex detailed mechanism. But, as has been shown previously, critical. effects in oxidation reactions were studied as early as the 1930s. For their interpretation primary attention is paid to the interaction of kinetic dependences with the heat-and-mass transfer law [146], It is likely that in these cases there is still more variety in dynamic behaviour than when we deal with purely kinetic factors. A theory for the non-isothermal continuous stirred tank reactor for first-order reactions was suggested in refs. 152-155. The dynamics of CO oxidation in non-isothermal, in particular adiabatic, reactors has been studied [77-80, 155]. A sufficiently complex dynamic behaviour is also observed in isothermal reactors for CO oxidation by taking into account the diffusion both in pores [71, 147-149] and on the surfaces of catalyst [201, 202]. The simplest model accounting for the combination of kinetic and transport processes is an isothermal continuously stirred tank reactor (CSTR). It was Matsuura and Kato [157] who first showed that if the kinetic curve has a maximum peak (this curve is also obtained for CO oxidation [158]), then the isothermal CSTR can have several steady states (see also ref. 203). Recently several authors [3, 76, 118, 156, 159, 160] have applied CSTR models corresponding to the detailed mechanism of catalytic reactions. 

Cyclohexene hydrogenation is a well-studied process that serves as model reaction to evaluate performance of gas-liquid reactors because it is a fast process causing mass transfer limitations for many reactors [277,278]. Processing at room temperature and atmospheric pressure reduces the technical expenditure for experiments so that the cyclohexene hydrogenation is accepted as a simple and general method for mass transfer evaluation. Flow-pattern maps and kinetics were determined for conventional fixed-bed reactors as well as overall mass transfer coefficients and energy dissipation. In this way, mass transfer can be analyzed quantitatively for new reactor concepts and processing conditions. Besides mass transfer, heat transfer is an issue, as the reaction is exothermic. Hot spot formation should be suppressed as these would decrease selectivity and catalytic activity [277]. 

Some of the above concepts will be used below in the description of reaction models (Sect. 2) and reactor models (Sect. 3). 

Turbulence is the most complicated kind of fluid motion. There have been several different attempts to understand turbulence and different approaches taken to develop predictive models for turbulent flows. In this chapter, a brief description of some of the concepts relevant to understand turbulence, and a brief overview of different modeling approaches to simulating turbulent flow processes is given. Turbulence models based on time-averaged Navier-Stokes equations, which are the most relevant for chemical reactor engineers, at least for the foreseeable future, are then discussed in detail. The scope of discussion is restricted to single-phase turbulent flows (of Newtonian fluids) without chemical reactions. Modeling of turbulent multiphase flows and turbulent reactive flows are discussed in Chapters 4 and 5 respectively. 

Oxidative coupling of isobutene suffers from severe deep oxidation. As in many other partial oxidation reactions selectivity remains low, despite intensive optimization of catalysts and reaction conditions. Among various new reactor concepts, the separation of catalyst reduction and reoxidation is very promising (two step process). Reaction engineering investigations of the two step process have been done. The influence of reaction conditions and reversibility of reduction/reoxidation cycles have been investigated. Based on the reaction engineering results a first approach to a kinetic model of both reaction steps has been developed. 

Here the basic concepts of system theory and principles for mathematical modelling are presented for the development of diffusion reaction models for fixed bed catalytic reactors. 

The conception, optimization, extrapolation or mastering of a GPTR necessitates first of all the elaboration of a reaction model, then its incorporatation into a code of Computational Fluid Dynamics (CFD) in order to carry out simulations of the phenomenon being studied under different operating conditions of composition, of pressure, of temperature, with different reactant loads, and for different reactor geometries. The analysis of the results of the simulations then allows the optimum reactant-reaction-reactor-product configuration to be determined or the phenomenon to be extrapolated. 

If the reaction system considered is fast, the process can be satisfactorily described assuming reaction equilibrium. Here, a proper modeling approach is based on the nonreactive equilibrium-stage model, extended by the chemical equilibrium relationship. An alternative approach proposed by Davies and Jeffreys [13] includes two separate steps. First, the concentrations and flow rates of the leaving streams are calculated with the simple nonreactive equilibrium-stage model. Afterwards, the leaving concentrations are adapted by using an additional equilibrium reactor concept. However, the latter approach does not consider direct interactions between the chemical and thermodynamic equilibrium. 

Reaction modelling of a microstructured billing film reactor incorporating staggered herringbone structures using eddy diffusivity concepts. Chem. Eng. 227, 66. 

A model-based performance analysis of fixed-bed, fluidized-bed, and porous-membrane reactors has been reported, concluding that fluidized-bed reactors can improve the yield up to 26% (which is still below the industrial requirements), while fixed-bed reactors cannot be used industrially. However, membrane reactors offer the possibility of increasing the yield by fine oxygen distribution through the membrane. In this way, it has been suggested that it is possible to optimize the reaction conditions in all investigated reactor concepts. This alternative operation mode using a PBMR, is proposed in order to increase the efficiency of the OCM with a performance of 23.2% yield, 53.9% selectivity, and 42.7% methane conversion. 

In this chapter, the basic concepts needed for prediction of reactor performance are reviewed. Multiple reaction systems, reactor models, and heat transfer considerations are enphasized. Case studies are presented based on reactors found in the processes used as exanples throughout the text and in the appendixes. 

A hybrid CFD-reaction engineering framework for multiphase reactor modelling basic concept and application to bubble column reactors. Chem. Eng. Sci., 58, 3077 -3089. 

Slesser and Highet (S15) have proposed a theoretical model for the case of a second-order chemical reaction taking place in a slurry reactor. This model is based on concepts very similar to those employed by Sherwood and Farkas, apart from the obvious complications resulting when one treats a second-order reaction. 

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