Structured reactor models

The application of CFD to packed bed reactor modeling has usually involved the replacement of the actual packing structure with an effective continuum (Kvamsdal et al., 1999 Pedernera et al., 2003). Transport processes are then represented by lumped parameters for dispersion and heat transfer (Jakobsen... 

Arthur D. Little has carried out cost structure studies for a variety of fuel cell technologies for a wide range of applications, including SOFC tubular, planar and PEM technologies. Because phenomena at many levels of abstraction have a significant impact on performance and cost, they have developed a multi-level system performance and cost modeling approach (see Figure 1-15). At the most elementary level, it includes fundamental chemical reachon/reactor models for the fuel processor and fuel cell as one-dimensional systems. 

LPCVD reactor modeling involves many of the same issues of multi-component diffusion reactions that have been studied in the past decade in connection with heterogeneous catalysis. Complex fluid-flow phenomena strongly affect the performance of atmospheric-pressure CVD reactors. Two-dimensional and some three-dimensional flow structures in the classical horizontal and vertical CVD reactors have been explored through flow visual-... 

Miniplant Technology - A Model for the Micro Structured Reactor Plant Concept... 

As miniplant technology must be considered as a model for micro structured reactor plant technology, a closer look into its working principles is very illustrative. All characteristics of a miniplant were described by the inventors of this technology, Dow, in 1979 [28] and BASF [29], which also introduced the combined simulation/ experimentation concept (Figure 4.2). 

This 3-D layout model is a valuable draft but it has to be revised during detailed engineering. The tool is important for the micro world if in the future micro structured reactor plants become more and more standardized. In this case, standardized autorouter tools could help to automate the time-consuming part of detailed engineering in complex micro structured reactor plants. 

This approach was successfully used in modeling the CVD of silicon nitride (Si3N4) films [18, 19, 22, 23]. Alternatively, molecular dynamics (MD) simulations can be used instead of or in combination with the MC approach to simulate kinetic steps of film evolution during the growth process (see, for example, a study of Zr02 deposition on the Si(100) surface [24]). Finally, the results of these simulations (overall reaction constants and film characteristics) can be used in the subsequent reactor modeling and the detailed calculations of film structure and properties, including defects and impurities. 

Reactor model. The structured model of the activated sludge process includes the following concentration variables soluble... 

By means of a model reaction it has recently been proved that in many cases circulating fluidized-bed reactors cannot be characterized by solely considering mixing phenomena [111]. Instead, the presence of mass transfer limitations and bypassing was found to have a significant influence. In analogy to low-velocity fluidized beds a detailed description of the local flow structure within the reaction volume must serve as a basis for appropriate reactor modeling. 

A typical example of a circulating fluidized-bed reactor model has been presented by Schocnfeldcr et al. [112, 116]. Its structure shown in Fig. 21 is based on the definition of four axial zones. Above the bottom zone the splash zone is located. It yields a mixing condition to link the bottom zone with the upper part of the reactor. Due to the internal backmixing this upper section is referred to as the recirculation zone. At the riser outlet, an exit zone on top of the recirculation zone yields a second boundary condition. Here, complete mixing is assumed. 

Chemical reaction models play a major role in chemical engineering as tools for process analysis, design, and discovery. This chapter provides an introduction to structures of chemical reaction models and to ways of formulating and investigating them. Each notation in this chapter is defined when introduced. Chapter 3 gives a complementary introduction to chemical reactor models, including their physical and chemical aspects. 

Sorensen, J. P., and W. E. Stewart, Structural analysis of multicomponent reactor models I. Systematic editing of kinetic and thermodynamic values, AIChE J., 26, 98-104 (1980). 

In the packed-bed reactor, the molar concentrations and temperature at the exterior of the catalyst particle are coupled to the respective fluid-phase concentrations and temperature through the interfacial fluxes given in Eqs. (3.3-1) and (3.3-2). Overall mass and heat transfer coefficients are often used to describe these interfacial fluxes, similar in structure to Eqs. (3.2-1) and (3.2-2). Complete solution of the packed-bed reactor model... 

Me Greavy C., Draper L. and Kam E.K.T., Methodologies for the design of reactors using structured catalysts modelling and experimental study of diffusion and reaction in structured catalysts, Chem. Engng ScL 49 5413 (1995). 

Jiang, Y, Khadilkar, M.R., Al-Dahhan, M.H. and Dudukovic, M.P. (2001), CFD modeling of multiphase flow in packed bed reactors fluid flow, bed structure and modeling issues, AIChE J., submitted for publication. 

Fig.4.5-7. Structure and model of a 3-stage two impinging-stream reactor...

The traditional modeling framework describing reactive flow systems is presented in the following. The basic conservation equations applied in most reactor model analyzes are developed from the concept that the fluid is a continuum. This means that a fluid is considered to be a matter which exhibits no finer structure [168]. This model makes it possible to treat fluid properties at a point in space and mathematically as continuous functions of space and time. From the continuum viewpoint, fluid mechanics and solid mechanics have much in common and the subject of both these sciences are traditionally called continuum mechanics. 

Often the kinetics of the chemical reaction and whether or not the reaction rate is affected by transport limitation are not known a priori. Lab-scale experimental reactors are structured such that they are operated isothermally and can be described by one of three ideal reactor models (ideal batch, CSTR, and plug flow). Isothermal operation is achieved by providing a large beat-transfer surface and maintaing the reactor in a constant-temperature bath. Experiments are conducted at different initial (or inlet) reactant proportions (to determine the form of the rate expression) and at different temperatures (to determine the activation energy). 

An unexpected key benefit from the use of microreactors has been the detailed flow information that is available from their construction. To build optimized channel structures detailed models of the devices have been developed. These models offer the potential to use the information as the basis for reactor modeling. The result is that with detailed reactor models and precise composition measurements from on-line analysis the rate expressions for the chemistry can be extracted from the laboratory data. The potenhal of this is significant in that general laboratory data only gives a sense of what is happening whereas now... 

---