Simulation of industrial reactor

Zuflerey, B. (2006) Scale-down Approach Chemical Process Optimisation Using Reaction Calorimetry for the Experimental Simulation of Industrial Reactors Dynamics, EPFL, n°3464, Lausanne. 

Two books deal almost exclusively with the subject of mass transfer with chemical reaction, the admirably clear expositions of Astarita (A6) and Danckwerts (D2). Since then a flood of theoretical and experimental work has been reported on gas absorption and related separations. The principal object of this chapter is to present techniques, results, and opinions published mainly during the last 6 or 7 years on mass-transfer coefficients and interfacial areas in most types of absorbers and reactors. This necessitates some review of mass transfer with and without chemical reaction in the first section, and comments about the simulation of industrial reactors by laboratory-scale apparatus in the concluding section. Although many gas-liquid reactions are accompanied by a rise in temperature that may be great enough to affect the rate of gas absorption, our attention here is confined to cases where the rise in temperature does not affect the absorption rate. This latter topic (treated by references B20, TIO, S3, T3, V5) could justify another complete chapter. 

In reaction engineering investigations it is generally not sufficient to draw conclusions about the activity and selectivity of a catalyst on the basis of conversion and yield. Transport limitations and hence the structure of the individual catalyst particles (shell catalyshbulk catalyst, molded catalystfextruded catalyst, etc.) must also be taken into account. The determination of the parameters and the selection of models for the quantitative kinetic description of the catalyst should be followed by the simulation of industrial reactors in order to obtain more information on the practical suitability of the chosen catalyst. 

Shah, J. J. and R. O. Fox (1999). CFD simulation of chemical reactors Application of in situ adaptive tabulation to methane thermochlorination chemistry. Industrial Engineering Chemistry Research 38, 4200 4-212. 

We have presented a general reaction-diffusion model for porous catalyst particles in stirred semibatch reactors applied to three-phase processes. The model was solved numerically for small and large catalyst particles to elucidate the role of internal and external mass transfer limitations. The case studies (citral and sugar hydrogenation) revealed that both internal and external resistances can considerably affect the rate and selectivity of the process. In order to obtain the best possible performance of industrial reactors, it is necessary to use this kind of simulation approach, which helps to optimize the process parameters, such as temperature, hydrogen pressure, catalyst particle size and the stirring conditions. 

The design or simulation of FCC units involves numerically solving the above 21 equations and relations (7.25) to (7.45). The solution process will be discussed later. For the simulation of industrial units and the verification of this model for industrial data, the majority of these 21 equations are used to calculate various parameters in the 10 equations numbered (7.29) to (7.38). Specifically, equations (7.33) to (7.35) compute the concentration and temperature profiles in the bubble phase of the reactor and equation (7.38) computes the temperature profile in the regenerator. This leaves the main equations (7.29) to (7.32), (7.36), and (7.38) as six coupled equations in the six state variables xid, X2D, Yrd, d e, and Yqd-... 

The collaboration is still going on. The full-loop, 3D simulations of MIP reactors are being performed to help further scale-up. To some extent, the multi-scale CFD is beginning to take the place of virtual experiment for solving industrial problems, and it is emerging as a paradigm beneficial to both industry and academia. 

To conclude our examples of Aspen Dynamics simulation of tubular reactor systems, we study a very important industrial process for the production of methanol from synthesis... 

The field of chemical kinetics and reaction engineering has grown over the years. New experimental techniques have been developed to follow the progress of chemical reactions and these have aided study of the fundamentals and mechanisms of chemical reactions. The availability of personal computers has enhanced the simulation of complex chemical reactions and reactor stability analysis. These activities have resulted in improved designs of industrial reactors. An increased number of industrial patents now relate to new catalysts and catalytic processes, synthetic polymers, and novel reactor designs. Lin [1] has given a comprehensive review of chemical reactions involving kinetics and mechanisms. 

Kiparissides, C., Daskalakis, G., Achilias, D.D., Sidiropoulou, Dynamic simulation of industrial poly(vinyl chloride) batch suspension polymerization reactors, Ind. Eng. Chem. Res., 1997, 36,1253-1267... 

Several sophisticated techniques and data analysis methodologies have been developed to measure the RTD of industrial reactors (see, for example, Shinnar, 1987). Various different types of models have been developed to interpret RTD data and to use it further to predict the influence of non-ideal behavior on reactor performance (Wen and Fan, 1975). Most of these models use ideal reactors as the building blocks (except the axial dispersion model). Combinations of these ideal reactors with or without by-pass and recycle are used to simulate observed RTD data. To select an appropriate model for a reactor, the actual flow pattern and its dependence on reactor hardware and operating protocol must be known. In the absence of detailed quantitative models to predict the flow patterns, selection of a model is often carried out based on a qualitative understanding of flow patterns and an analysis of observed RTD data. It must be remembered that more than one model may fit the observed RTD data. A general philosophy is to select the simplest model which adequately represents the physical phenomena occurring in the actual reactor. 

These models require information about mean velocity and the turbulence field within the stirred vessels. Computational flow models can be developed to provide such fluid dynamic information required by the reactor models. Although in principle, it is possible to solve the population balance model equations within the CFM framework, a simplified compartment-mixing model may be adequate to simulate an industrial reactor. In this approach, a CFD model is developed to establish the relationship between reactor hardware and the resulting fluid dynamics. This information is used by a relatively simple, compartment-mixing model coupled with a population balance model (Vivaldo-Lima et al., 1998). The approach is shown schematically in Fig. 9.2. Detailed polymerization kinetics can be included. Vivaldo-Lima et a/. (1998) have successfully used such an approach to predict particle size distribution (PSD) of the product polymer. Their two-compartment model was able to capture the bi-modal behavior observed in the experimental PSD data. After adequate validation, such a computational model can be used to optimize reactor configuration and operation to enhance reactor performance. 

In the previous sections the concept of the effectiveness factor has been discussed. In this section it is discussed in further detail with the aim of extending the concept to industrially important complex reaction networks. The effectiveness factor is the most widely used man-made factor to account (in a condensed, one number manner) for the effect of different diffusional resistances on the actual (or apparent) rate of reaction for gas-solid catalytic systems. Although the use of the effectiveness factor concept in the simulation of catalytic reactors taxes the solution by extra computations, nevertheless it is a very useful tool to account for the complex interaction between the diffusion and reaction processes taking place within the system. Most of the published work (e.g. Weisz and Hicks, 1962 Aris, 1975a,b) deals with the effectiveness factor for the simple irreversible reaction,... 

Curve 5 corresponds to no-heat transfer through the solid and this predicts a hot spot that is far too important. Such a model is no improvement at all with respect to the two-dimensional pseudo-homogeneous model of Sec. 11.7. It is interesting also to note that, for the conditions used in these calculations, the solid temperature only exceeds the gas temperature by 1 or 2°C This is generally so in industrial reactors. Finally, the radial mean temperatures of the two-dimensional models are significantly different from the temperature predicted by the one-dimensional models. Provided the physical data are available the two-dimensional models would definitely have to be preferred for the simulation of this reactor. 

De Falco M, Marrelli L, Basile A (2009) An industrial application of membrane reactors modelling of the methane steam reforming reaction, Chapter 9. In Simulation of membrane reactors. Nova Science Publishers Inc., New York, ISBN 978-1-60692-425-9... 

Kinetic and hydrodynamic analyses, and methods for the calculation of the parameters of industrial reactors are sufficiently developed today [2-6]. Computer simulation is also popular because if we know the kinetic and hydrodynamic parameters of processes and the principles of reactor behaviour, it is not a problem to calculate process characteristics and final product performance. This principle is an adequate tool for the description of low and medium rate chemical transformations with uniform concentration fields and isothermic conditions which are easy to achieve. In this case, it is easy to calculate and control all the characteristics of a chemical process under real conditions. 

Based on naphtha pyrolysis experiments conducted in a bench scale tubular reactor (suitable for the simulation of industrial tubular furnace operations and taking into account the changes of expansion, temperature and the pressure in the reactor), a kinetic model has been developed for the calculation of the degree of de-con osition, the actual residence time, and the severHy of cracking. 

---