102

with2>affles2

10 102 lOi lOf

Nr.2

(c)23ependence2)f2nodel^)arameters2)n2

reactor^eometryindiiperating2;onditions2

with2>affles2

10 102 lOi lOf

Nr.2

(c)23ependence2)f2nodel^)arameters2)n2

reactor^eometryindiiperating2;onditions2

FIGURE 1.6 Flow model for stirred reactors (from Wen and Fan, 1975).

Discussions on flow modeling so far have been more or less restricted to singlephase reactors. However, in a broad range of application areas, multiple phases are involved in chemical reactions (see examples cited by Ramachandran and Choudhari, 1983; Doraiswamy and Sharma, 1984; Kunii and Levenspiel, 1991; Shah, 1991; Dudukovic et al., 1999). Reactors carrying out such reactions are generically termed multiphase reactors. There are several types of multiphase reactors and several methods are available to classify these reactors. One of the simplest methods of

different zones was specified using experimental measurements (from Mann and Mavros, 1982).

classification is based on the presence of phases, such as:

Gas-liquid reactors: Stirred reactors, bubble column reactors, packed columns, loop reactors.

Gas-liquid-solids reactors: Stirred slurry reactors, three-phase fluidized bed reactors (bubble column slurry reactors), packed bubble column reactors, trickle bed reactors, loop reactors.

Gas-solid reactors: Fluidized bed reactors, fixed bed reactors, moving bed reactors.

Some of these reactors are shown schematically in Fig. 1.2. The existence of multiple phases opens up a variety of choices in bringing these phases together to react. Questions like operability, and stability of the flow regime need to be answered. When reactants under operating conditions constitute more than one phase, the need to understand flows and quantitative predictions becomes even more crucial. Each of these reactors exhibits complex fluid dynamics and can be operated in a variety of flow regimes. For example, a gas-solid reactor can be operated in a variety of regimes ranging from a fixed bed reactor (where a bed of solid particles is stationary and gas flows through the voids between the solid particles) to a fast-fluidized bed reactor (where solid particles are transported by the gas phase). Bubble column reactors may be operated in a homogeneous regime (with more or less uniform bubbles and uniform gas volume fraction distribution within the reactor) or in a heterogeneous regime (with wide bubble size distribution and non-uniform gas volume fraction distribution within the column, which leads to significant internal re-circulation). Gas-liquid stirred reactors may also exhibit different flow regimes depending on the type, size and location of the impeller, gas flow rate and impeller speed. As an example, these flow regimes are illustrated schematically in Fig. 1.8. For very low impeller speeds, flow generated by rising gas bubbles dominates the flow generated by the impeller (Fig. 1.8a). In such cases, the gas phase behaves like a plug flow and the liquid phase may exhibit varying degrees of mixedness depending on relative time scales of mass transfer, reaction and mixing. For the other extreme, where flow is dominated by the impeller (Fig. 1.8e), gas bubbles follow liquid streamlines and are dispersed all over the reactor. In such a case, the gas phase behaves as if it is completely mixed. Thus, fluid dynamics and mixing in these multiphase reactors is determined by the operating flow regime. To select an appropriate reactor model, it is therefore essential to know the prevailing operating regime in the reactor (for the given hardware and operating conditions). Some generic multiphase flow regimes are shown in Fig. 1.9. It is not possible to discuss the intricacies of all of these reactors and their operating regimes here. More information may be obtained from the books cited above (and references cited therein) and from Chapters 10 to 13 of this book.

Apart from the flow regimes, several other issues control the performance of these multiphase reactors. For example, in a gas-liquid reactor, the rate of mass

Nf : Minimum impeller speed

Ci increasing N -ยป increasing Qg ctti to avoid flooding Ncd : Minimum impeller speed for complete dispersion Nr : Minimum impeller speed for recirculation

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