Choice of Reactor and Design Considerations

The choice of reactor type and its design for a particular reaction netw ork may require examination of trade-offs involving reactor size and mode of operation, product distribution (selectivity), and production rate. If, as is often the case, selectivity is 

kj and kr are first-order rate constants with known Arrhenius parameters (Af, EAj) and (A, EAr), respectively. This situation, with different kinetics, arises in cases of gas-phase catalytic reactions and is further treated from this point of view in Chapter 21. Here, we consider four cases, and assume the reaction is noncatalytic. 

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The remainder of this text attempts to establish a rational framework within which many of these questions can be attacked. We will see that there is often considerable freedom of choice available in terms of the type of reactor and reaction conditions that will accomplish a given task. The development of an optimum processing scheme or even of an optimum reactor configuration and mode of operation requires a number of complex calculations that often involve iterative numerical calculations. Consequently machine computation is used extensively in industrial situations to simplify the optimization task. Nonetheless, we have deliberately chosen to present the concepts used in reactor design calculations in a framework that insofar as possible permits analytical solutions in order to divorce the basic concepts from the mass of detail associated with machine computation. 

The choice of reactor will be very dependent on the requirements of the chemical reaction scheme, the relative importance of mixing and heat transfer, and practical considerations (e.g., the effect of solids in the process materials of construction flexibility). A comparison of the typical performance of different designs is given in Table 5. HEX Reactors are discussed in more depth in Chapter 4. 

Nuclear reactors are designed for production of heat, mechanical and electric power, radioactive nuclides, weapons material, research in nuclear physics and chemistry, etc. The design depends on the purposes, e.g. in the case of electric power production the design is chosen to provide the cheapest electricity taking long term reliability in consideration. This may be modified by the availability and economy of national resources such as raw material, manpower and skill, safety reasons, etc. Also the risk for proliferation of reactor materials for weapons use may influence the choice of reactor type. Many dozois of varying reactor concepts have been formulated, so we must limit the discussion in this chapter to a summary of the main variables, and the most common research and power reactors. Fast reactors and some other designs are discussed in Chapter 20. 

Many hydrocarbon chlorinations that follow free a radical mechanism are associated with the problems of poor selectivity towards the intermediate products. Since most of the time there happens to be a distinct difference between the volatilities of the various chlorinated products, RD can be an appropriate choice of reactor for obtaining better selectivity towards the particular chlorinated product. The important example of commercial relevance is the photochlorination of aromatics such as benzene or toluene [60, 100]. Like hydrogenation, this process is also associated with the use of a non-condensing gas (chlorine) and hence its flow rate would make a significant impact on design considerations. 

The design of a polymerization reactor begins with the selection of the type of reactor (batch, semibatch or continuous) and then proceeds to the sizing and details of the reactor configuration. Only then can the details of operation and control be addressed. To this end, we will begin with a discussion of the basic types of reactors. Ultimately, it will be clear that the choice of reactor type is determined not only by practical considerations such as scale of production and propensity for fouling, but also by the specific polymerization kinetics. More complete discussions of reactor types and their residence time distributions may be found in references [1,2],... 

Having made an initial specification for the reactor, attention is turned to separation of the reactor effluent. In some circumstances, it might be necessary to carry out separation before the reactor to purify the feed. Whether before or after the reactor, the overall separation task might need to be broken down into a number of intermediate separation tasks. Consider now the choice of separator for the separation tasks. Later in Chapters 11 to 14, consideration will be given as to how separation tasks should be connected together and connected to the reactor. As with reactors, emphasis will be placed on the choice of separator, together with its preliminary specifications, rather than its detailed design. 

In industrial practice, the laboratory equipment used in chemical synthesis can influence reaction selection. As issues relating to kinetics, mass transfer, heat transfer, and thermodynamics are addressed, reactor design evolves to commercially viable equipment. Often, more than one type of reactor may be suitable for a given reaction. For example, in the partial oxidation of butane to maleic anhydride over a vanadium pyrophosphate catalyst, heat-transfer considerations dictate reactor selection and choices may include fluidized beds or multitubular reactors. Both types of reactors have been commercialized. Often, experience with a particular type of reactor within the organization can play an important part in selection. 

Many industrial reactions are not carried to equilibrium. In this circumstance the reactor design is based primarily on reaction rate. However, the choice of operating conditions may still be determined by equilibrium considerations as already illustrated with respect to the oxidation of sulfur dioxide. In addition, the equilibrium conversion of a reaction provides a goal by which to measure improvements in the process. Similarly, it may determine whether or not an experimental investigation of a new process is worthwhile. For example, if the thermodynamic analysis indicates that a yield of only 20 percent is possible at equilibrium and a 50 percent yield is necessary for the process to be economically attractive, there is no purpose to an experimental study. On the other hand, if the equilibrium yield is 80 percent, an experimental program to determine the reaction rate for various conditions of operation (catalyst, temperature, pressure, etc.) may be warranted. 

Considerations based on the known physical phenomena can guide the choice of catalyst porosity and porous structure, catalyst size and shape and reactor type and size. These considerations apply both to the laboratory as well as to large-scale operations. Many comprehensive reviews and good books on the problem of reactor design are available in the literature. The basic theory for porous catalysts is summarized in this book and simple rules are set forth to aid in making optimum choices to obtain fully effective catalyst particles, which give the best performance from an economic point of view. 

In general, the procedure for designing a bubble column reactor (BCR) (1 ) should start with an exact definition of the requirements, i.e. the required production level, the yields and selectivities. These quantities and the special type of reaction under consideration permits a first choice of the so-called adjustable operational conditions which include phase velocities, temperature, pressure, direction of the flows, i.e. cocurrent or countercurrent operation, etc. In addition, process data are needed. They comprise physical properties of the reaction mixture and its components (densities, viscosities, heat and mass diffusivities, surface tension), phase equilibrium data (above all solubilities) as well as the chemical parameters. The latter are particularly important, as they include all the kinetic and thermodynamic (heat of reaction) information. It is understood that these first level quantities (see Fig. 3) are interrelated in various ways. 

Biological solids retention time (Oc) has been suggested in this paper as the kinetic based parameter of choice for use in design and control of fiuidized culture continuous fiow biological processes. The value of Oc selected for design of the process, (Oc ), directly determines the volume of reactor needed for a conventional digester system and a given waste fiow (Q) since the value of Oc is equal to the hydraulic retention time (0). The relationship between Oc and the reactor volume for the system with recycle is more complex and involves consideration of the effects of solids recycle rate and recycle solids concentration. 

Therefore for the accurate modelling, design and optimization of fixed-bed catalytic reactors the mass transfer resistance must be taken into consideration. However, it is not always necessary to take all the mass transfer resistances mentioned above into consideration. In fact the choice of the resistances that need to be considered and those that can be neglected represents a crucial decision and requires experience and careful scientific judgement. Of course, some cases are obvious,... 

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