The selection of a reactor type

Qiemical reactor development starts when a certain amount of knowledge of a chemical synthesis is available. At this point one wants to reconsider the reaction conditions that have been applied, with the intention of optimization and scale up. One of the first questions concerns the selection of a reactor type, based on the knowledge that is available so far. In later stages of the development process this selection may be reconsidered. The selection of a reactor type for a given chemical reaction comprises four decision steps  

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The importance of "enthalpy management", or how to cope with large amounts of heat of reaction, has been indicated in Chapter 8. The importance of this aspect increases with the scale of the reactor, for obviuous reasons. The consequence is, that requirements for heat transfer may be more decisive factors in reactor design, the larger the scale of operation. So it may be that the selection of a reactor type has to be revised completely when heat transfer is considered. Yet the indicated order of decision mal g may be the most effective. 

In two-phase systems, there may be steep concentration gradients near the interfaces, particularly when rapid reactions are taking place. When mass transfer and chemical reactions can be considered as processes in series, the qualitative effects can be estimated simply (see section 5.3.5). When kinetic data are available, quantitative effects can be calculated. When diffusion and chemical reactions take place in the same zone, calculations become quite complex, but qualitative effects can yet be estimated. This may apply to situations where a reactant dissolves in a liquid and is converted widiin the diffusion layer adjacent to the interface. The dissolving reactant may be introduced as a gas, a solid or a second liquid phase (see section 5.4.2.2). It was shown that in these situations the selection of a reactor type is essential for obtaining a good selectivity (see also section 9.3). Similar effects are encountered with reactions takmg place inside porous solids (see section 5A.3.2), Therefore the structure of solid catalysts may be important in view of process selectivity. 

The selection of a particular type of reduction depends on technical feasibiUty and the economics of the process as well as on physicochemical considerations. In particular, the reducing agent should be inexpensive relative to the value of the metal to be reduced. The product of the reaction, RX, should be easily separated from the metal, easily contained, and safely recycled or disposed of. Furthermore, the physical conditions for the reaction should be such that a suitable reactor can be designed and operated economically. 

I Shunt reactors These are provided as shown in Figure 24.23 to compensate for the distributed lumped capacitances, C , on EHV networks and also to limit temporary overvoltages caused during a load rejection, followed by a ground fault or a phase fault within the prescribed steady-state voltage limits, as noted in Table 24.3. They ab.sorb reactive power to offset the charging power demand of EHV lines (Table 24.2, column 9). The selection of a reactor can be made on the basis of the duty it has to perform and the compensation required. Some of the different types of reactors and their characteristics are described in Chapter 27. 

The treatment of reactor design in this section will be restricted to a discussion of the selection of the appropriate reactor type for a particular process, and an outline of the steps to be followed in the design of a reactor. 

The selectivity of a catalyst is typically optimized towards a reaction type, but some operations required a high level of removal for more than one contaminant. In fact, the treatment of a VGO, for instance, involves the removal of metals, S and N. Depending on the quality of the feed and on the specifications of the desired product, the hydrotreatment may require more than one catalyst. The catalyst can be stacked in a single reactor or disposed in sequential stages, when more than one reactor is available. Stacked-bed reactors with more than one catalyst type are a common practice in HDT. 

I m not talidng about fun you can have at an amusement park, but CRE fun. Now that we have an understanding on how to solve for the exit concentrations of multiple reactions in a CSTR and how to plot the species concentration down the length of a PER or PER, we can address one of the most important and fun areas of chemical reaction engineering. This area, discussed in Section 6.1, is learning how to maximize the desired product and minimize the undesired product. It is this area that can make or break a chemical process financially. It is also an area that requires creativity in designing the reactor schemes and feed conditions that will maximize profits. Here you can mix and match reactors, feed steams, and side streams as well as vary the ratios of feed concentration in order to maximize or minimize the selectivity of a particular species. Problems of this type are what I call digital-age problems - because... 

PTGL-processes (table Ia,b,c) can be characterized by their operating principle (pyrolysis, gasification or liquefaction) and reaction conditions, their type of reactor and method of heat supply and the possible presence of auxiliary systems. The selection of a particular process will ultimately be based on its proven reliability on one hand and on the quality of the process output on the other [ ]. ... 

One of the most important areas for application of concepts discussed in the previous section is the selection of polymerization reactors. The properties of polymers depend on their molecular weight distribution (M WD) and so the design should ultimately use this as its basis. The subject is a vast one, and so only the basic concepts will be briefly discussed. Several excellent reviews now exist, covering various aspects of the area from a chemical reaction engineering viewpoint see Shinnar and Katz, Keane, and Gerrens, [18, 19, 20]. The latter presents a masterful survey of the effects of the choice of reactor type. 

In a recent study Wang and Hofmann (1999) have stressed the importance of nonisothermal rate data. From a simple theoretical analysis they conclude that kinetic and transport data obtained under isothermal conditions in a laboratory reactor cannot logically be used to simulate any other type of reactor. This is because of the behavior of the Lipschitz constant L, which is a measure of the sensitivity of the reaction to different models. It tells us how any two models would diverge at the end of a reactor under different thermal conditions of operation. It is therefore a useful criterion for selecting the best model. It has been shown that L is different for different reactor models ... 

The selection of a soluble boron-free core is based on the studies conducted at the CEA [5]. The main design features allowing for this type of core in SCOR reactor are ... 

The second control rod system foreseen in the MARS reactor is a passive type and causes control rods insertion into the core when the core coolant temperature reaches a selected set value. The operation of this system (called ATSS, Figure IV-6) is based on the differential thermal expansion of a bimetallic sensor located inside the fuel assembly the differential displacement, due to coolant temperature increase, causes the release of a traditional-type control rod cluster. 

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