is being used as reactors in practice. Some of the commonly used types of reactor are shown in Fig. 1.2. Although the reactors included in this figure show contacting of two phases (gas-liquid or gas-solid), similar equipment can also be used to carry out reactions involving a single phase (homogeneous reactions) or more than two phases. Several different versions of these four major reactor types are used in practice. By looking at these configurations, one can imagine the complexities of the underlying fluid dynamics in these equipments. A reactor engineer is faced with a host of questions when establishing a relationship between reactor hardware, operating protocol and reactor performance. In this section, some of these questions and the relevant tasks required of a reactor engineer are discussed briefly to bring out the role of flow modeling in the overall activity.

The major questions being faced by a reactor engineer can be grouped into three classes:

1. What chemical transformations are expected to occur?

2. How fast will these changes occur?

3. What is the best way to carry out these transformations?

The first question concerns thermodynamics and chemistry. Knowledge of chemistry and reaction mechanisms is helpful to identify the various possible chemical reactions. Thermodynamics provides models and tools to estimate free energies and heat of formations of chemical compounds from which the energetics of all the possible chemical reactions can be examined. These tools help a reactor engineer to identify thermodynamically more favorable operating conditions. The theories and modeling tools required to carry out these functions are fairly well developed and do not involve any consideration of actual reactor hardware and underlying fluid dynamics. These tools are, therefore, not discussed here. More information on these topics can be found in chemical engineering thermodynamics textbooks (for example, Smith and van Ness, 1959; Sandler, 1998). Thermodynamics provides tools to estimate physical properties (density, solubility, vapor pressure, heat capacity, conductivity, etc.) and the state of transforming species under operating conditions. This information is required for flow modeling. A brief discussion of these issues and the key references are given in Chapter 2.

The second question (estimating how fast the thermodynamically possible chemical transformations will occur) involves a knowledge of chemistry, reaction kinetics and various transport processes such as mixing, heat and mass transfer. Analysis of the transport processes and their interaction with chemical reactions can be quite difficult and is intimately connected to the underlying fluid dynamics. Such a combined analysis of chemical and physical processes constitutes the core of chemical reaction engineering. The overall framework of reaction engineering is briefly discussed here.

The first step in any reaction engineering analysis is formulating a mathematical framework to describe the rate (and mechanism) by which one chemical species is converted into another in the absence of any transport limitations (chemical kinetics). The rate is the mass, in moles of a species, transformed per unit time, while the mechanism is the sequence of individual chemical events, whose overall result produces the observed transformation. Though a knowledge of the mechanism is not necessary for reaction engineering, it is of great value in generalizing and systematizing the reaction kinetics. A knowledge of the rate of transformation, however, is essential for any reaction engineering activity. The rate of transforming one chemical species into another cannot be predicted with accuracy. It is a system specific quantity, which must be determined from experimental measurements. Recent advances in computational chemistry and molecular modeling have led to some successes in making a priori predictions of reaction kinetics (Senken, 1992; Dixon and Feller, 1999). However, in spite of such progress, most of the practical reaction engineering analysis will have to rely on experimental measurements of reaction kinetics (at least in the immediate to intermediate future).

Measuring the rate of chemical reactions in the laboratory is itself a specialized branch of science and engineering. The rate is formally defined as the change in moles of a component per unit time and per unit volume of reaction mixture. It is important that this rate is an intrinsic property of a given chemical system and is not a function of any physical process such as mixing or heat and mass transfer. Thus, the rate must be a local or point value referring to a differential volume of reaction mixture around that point. It is, therefore, essential to separate the effects of physical processes from the measured experimental data to extract information about the intrinsic reaction kinetics. It is a difficult task and has some parallels with the reactor engineering activity in reverse order (measurement of reactor performance-transport processes-fluid dynamics-intrinsic kinetics). More information about chemical kinetics and about laboratory reactors used to obtain intrinsic kinetics can be found in such textbooks as Smith (1970), Levenspiel (1972) and Doraiswamy and Sharma (1984). Assuming that such intrinsic rate data is available, chemical kineticists have developed a number of valuable generalizations to formulate rate expressions including those for catalytic reactions. Various textbooks cover aspects of chemical kinetics in detail (Smith, 1970; Levenspiel, 1972; Froment and Bischoff, 1984). Mathematical models (and corresponding model parameters) of intrinsic reaction kinetics will be assumed to be available to reactor engineers using this book.

Once the intrinsic kinetics is available, the production rate and composition of the products can be related, in principle, to the reactor volume, reactor configuration and mode of operation by solving mass, momentum and energy balances over the reactor. This is the central task of a reaction and reactor engineering activity. The difference between reaction engineering and reactor engineering lies in the treatment of momentum balances or in other words, of the underlying fluid dynamics. In reaction engineering, emphasis is given to reaction-related issues by making simplifications in the underlying fluid dynamics. In this way, it is possible to establish a relationship between the process design of a reactor and the performance of a reactor. Reactor engineering combines reaction engineering with the rigorous modeling of underlying fluid dynamics to establish a relationship between actual reactor hardware and its performance.

In order to understand these aspects, it may be useful to consider the operation of a single-phase reactor of arbitrary type. The microscopic mass balance of a reactant over an element of reactor volume (Fig. 1.3) can be written in the following general form applicable to any reactor type:

accumulation of component p =

rate of change of component <p due + to convection rate of change of component <p due + to dispersion rate of change of component <p due to reaction

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