101 Engineering Of Stirred Reactors

Several types of stirred reactor are used in practice and several ways of classifying these exist. Some of the widely used types of stirred reactors are shown in Fig. 10.1. The stirred reactor shown in Fig. 10.1(a) is a typical multiphase reactor used for carrying out exothermic reactions such as hydrogenations and oxidations. Stirred reactors provide excellent heat and mass transfer characteristics and can handle multiphase systems effectively. Most industrial reactors of this type use more than one impeller and have one or more set of cooling coils. Alternatively, the contents of the reactor are circulated through an external heat exchanger if the geometrical constraints restrict the provision of heat transfer area within the reactor. When per pass conversion of gas is low, gas is recirculated using an external compressor. To avoid the need for external circulation of unreacted gas, gas-inducing type impellers may be used (shown in Fig. 10.1b). It typically comprises a hollow shaft impeller with a stator arrangement to enhance gas induction rate. Numerous variations of these typical configurations are possible and are used in practice. For highly viscous systems, helical ribbon or screw impellers are used either with or without a draft tube (Fig. 10.1c). As mentioned earlier, the reactor engineer has to select the best possible hardware configuration to suit the process under consideration. Some of the industrial processes carried out in stirred reactors are listed in Table 10.1. These processes are classified on the basis of phases handled in the stirred reactor.

Engineering of stirred reactors (see Fig. 1.10 for a general methodology of reactor engineering) begins with the analysis of process requirements and evolving a preliminary configuration of the reactor. This step is based on laboratory study and on reactor models based on idealized fluid dynamics and mixing. In most industrial cases, this step itself may involve several iterations, especially for multiphase systems. It is often necessary to carry out reactor sizing for different values of relevant transport rates (heat transfer, mass transfer) or operating parameters (rotational speed, solid loading and so on). The process of evolving preliminary configuration helps to firm up performance targets for the reactor. Transformation of a preliminary reactor configuration to an industrial reactor proceeds through several steps, some of which

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