Combinations of reactors

Most of the factors in Table 4.1 apply equally to batch and flow vessels. The throughput in a well-designed CSTR is small compared to the internal circulation and does not affect quantities such as power, mixing time, and heat transfer to the jacket. The heat transfer factors in Table 4.1 are discussed in Chapter 5. 

We have considered two types of ideal flow reactor the piston flow reactor and the perfectly mixed CSTR. These two ideal types can be connected together in a variety of series and parallel arrangements to give composite reactors that are 

Different reactors in the composite system may operate at different temperatures and thus may have different rate constants. 

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Simple combinations of reactor elements can be solved direc tly. Figure 23-8, for instance, shows two CSTRs in series and with recycle through a PFR. The material balances with an /i-order reaction / = /cC are... 

What type of reactor best meets the process requirements Are there advantages associated with the use of a combination of reactor types, or with multiple reactors in parallel or series ... 

In order to achieve increases in production capacity or to obtain higher conversion levels, it may be necessary to provide additional reactor volume through the use of various series-parallel combinations of reactors. 

For an autocatalytic reaction, Example 15-10 shows that a recycle PFR operating with an optimal value of R requires the smallest volume for the three reactor possibilities posed. (In the case of a PFR without recycle, the size disadvantage can be offset at the expense of maintaining a sufficient value of cBo (in the feed), but this introduces an alternative disadvantage.) A fourth possibility exists for an even smaller volume. This can be realized from Figure 15.8 (although not shown explicitly), if the favorable characteristics of both normal and abnormal kinetics are used to advantage. Since this involves a combination of reactor types, we defer consideration to Chapter 17. 

These aspects were carefully studied [29,46-48] and analyzed in several macrokinetic models. They are essential for choosing the methods indicated in Fig. 9.3 and constructing the suitable combination of reactor and operation conditions. In Fig. 9.3 the CVD/CVI methods are designated according to the methodology for how a gradient in the chemical potential of the reaction is applied. 

Therefore the moments M, T2 and T3 of the RTD of the whole reactor system are simply the sums of the moments of the RTDs of its constituent parts. This, of course, can be generalised to any series combination of reactors or other flow vessels provided that they are linked together in a statistically independent and non-interactive manner this latter phrase implies that changes in operating conditions of a downstream unit are unable to influence the way in which an upstream unit behaves. 

We have just seen that a combination of CSTRs gives a total residence time that approaches that of a PFTR. Next we consider a combination of reactors involving both CSTRs and PFTRs as sketched in Figure 3-10. 

Figure 3-22 shows possible r versus Cao — Ca curves. What reactor or combination of reactors will give the shortest total residence time for a high conversion ... 

Chemical reactor design For homogeneous flow reactions, a digital computer can determine the optimum combination of reactor type and operating conditions (with G.T. Westbrook). Ind. Eng. Chem. 53, 181-186 (1961). 

As indicated above, a combination of reactor and cyclotron irradiations is used to prepare most radionuclides. While many of these radionuclides are available commercially, some are not. In addition, nuclear structure, nuclear reactions, and heavy-element research require accelerator or reactor irradiations to produce short-lived nuclei or to study the dynamics of nuclear collisions, and so on. One of the frequent chores of radiochemists is the preparation of accelerator targets and samples for reactor irradiation. It is this chore that we address in this section. 

For a 100 times slower reaction, the heat exchange becomes fully uncritical, but the conversion of 99% can only be reached with a volume of 90 m3, which is unrealistic. The situation improves with a higher reactor temperature or with a different combination of reactors cascade of CSTRs or CSTR followed by a tubular reactor. 

The combination of reactor elements is facilitated by the concept of transfer junctions. By this means the Laplace transform can be found for the overall model, and the residence time distribution can be found after inversion. Finally, the chemical conversion in the model can be developed with the segregation and maximum mixed models. 

Figure 1. Reactor behaviour of two different combinations of a plug flow reactor (PFR) and a continuous stirred tank reactor (CSTR). The combinations are denoted in the figure. Fig. la and b are the corresponding E(t) -curves, Fig. Ic and d are the conversion profiles, Fig. le and f are the selectivity profiles along the combination of reactors.

However, it is important for the reader to realize that if the rate of reaction is available solely as a function of conversion, rf,= f(X), or if it can be generated by some intermediate calculation, one can design a variety of reactors or combination of reactors. 

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