Choice of reactor

During the development of a chemical process, a choice must be made regarding the type of reactor to be used on a plant scale. Some theoretical considerations and their practical impact on reactor issues are presented here. Choosing the right type of reactor can indeed improve the safety of the process. The considerations are reflected as well in the mode of operation. Reactors are characterized by type of operation (i.e., batch, semi-batch, and continuous). 

A batch reactor is an agitated vessel in which the reactants are precharged and which is then emptied after the reaction is completed. More frequently for exothermic reactions, only part of the reactants are charged initially, and the remaining reactants and catalysts are fed on a controlled basis this is called a semi-batch operation. For highly exothermic reactions and for two-phase (gas-liquid) reactions, loop reactors with resultant smaller volumes can be used. 

Batch (noncontinuous) reactors are characterized by nonstationary conditions, that is, there are composition and heat generation changes during operation. 

In continuous reactor systems, all reactants are continuously fed to the reactor, and the products are continuously withdrawn. Typical continuous reactors are stirred tanks (either single or in cascades) and plug flow tubes. Continuous reactors are characterized by stationary conditions in that both heat generation and composition profiles remain constant during operation (provided that operating conditions remain unchanged ). 

The choice of a reactor is usually based on several factors such as the desired production rate, the chemical and physical characteristics of the chemical process, and the risk of hazards for each type of reactor. In general, small production requirements suggest batch or semi-batch reactors, while large production rates are better accommodated in continuous reactors, either plug flow or continuous stirred tank reactors (CSTR). The chemical and physical features that determine the optimum reactor are treated in books on reaction engineering and thus are not considered here. 

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Since process design starts with the reactor, the first decisions are those which lead to the choice of reactor. These decisions are among the most important in the whole design. Good reactor performance is of paramount importance in determining the economic viability of the overall design and fundamentally important to the environmental impact of the process. In addition to the desired products, reactors produce unwanted byproducts. These unwanted byproducts create environmental problems. As we shall discuss later in Chap. 10, the best solution to environmental problems is not elaborate treatment methods but not to produce waste in the first place. 

Before we can proceed with the choice of reactor and operating conditions, some general classifications must be made regarding the types of reaction systems likely to be encountered. We can classify reaction systems into five broad types ... 

Polymerization reactions. Polymers are characterized by the distribution of molecular w eight about the mean as well as by the mean itself. The breadth of this distribution depends on whether a batch or plug-flow reactor is used on the one hand or a continuous well-mixed reactor on the other. The breadth has an important influence on the mechanical and other properties of the polymer, and this is an important factor in the choice of reactor. 

Figure 2.3 Choice of reactor type for mixed parallel and series reactions when the parallel reaction has a higher order than the primary reaction.

In the preceding section, the choice of reactor type was made on the basis of which gave the most appropriate concentration profile as the reaction progressed in order to minimize volume for single reactions or maximize selectivity for multiple reactions for a given conversion. However, after making the decision to choose one type of reactor or another, there are still important concentration effects to be considered. 

The choice of reactor temperature depends on many factors. Generally, the higher the rate of reaction, the smaller the reactor volume. Practical upper limits are set by safety considerations, materials-of-construction limitations, or maximum operating temperature for the catalyst. Whether the reaction system involves single or multiple reactions, and whether the reactions are reversible, also affects the choice of reactor temperature, as we shall now discuss. 

The choice of reactor temperature, pressure, arid hence phase must, in the first instance, take account of the desired equilibrium and selectivity effects. If there is still freedom to choose between gas and liquid phase, operation in the liquid phase is preferred. 

Having discussed the choice of reactor type and operating conditions at length, let us try two examples. 

The secondary reactions are parallel with respect to ethylene oxide but series with respect to monoethanolamine. Monoethanolamine is more valuable than both the di- and triethanolamine. As a first step in the flowsheet synthesis, make an initial choice of reactor which will maximize the production of monoethanolamine relative to di- and triethanolamine. 

Reactors in the overall process. It should be emphasized that many considerations other than those represented in Figs. 2.9, 2.10, and 2.11 also influence the decision on the choice of reactor. Safety considerations, operating pressure, materials of construction, etc. have a considerable effect on the outcome. 

Choice of reactor. The first and usually most important decisions to be made are those for the reactor type and its operating conditions. In choosing the reactor, the overriding consideration is usually raw materials efficiency (bearing in mind materials of construction, safety, etc.). Raw materials costs are usually the most important costs in the whole process. Also, any inefficiency in raw materials use is likely to create waste streams that become an environmental problem. 

Each stage of particle formation is controlled variously by the type of reactor, i.e. gas-liquid contacting apparatus. Gas-liquid mass transfer phenomena determine the level of solute supersaturation and its spatial distribution in the liquid phase the counterpart role in liquid-liquid reaction systems may be played by micromixing phenomena. The agglomeration and subsequent ageing processes are likely to be affected by the flow dynamics such as motion of the suspension of solids and the fluid shear stress distribution. Thus, the choice of reactor is of substantial importance for the tailoring of product quality as well as for production efficiency. 

The need to satisfy these interrelated and often contradictory factors makes reactor design a complex and difficult task. However, in many instances one of the factors predominates, hence determining the choice of reactor type and the design method. 

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