Reactor choice

A rational choice from the variety of reactor types available requires knowledge of reaction kinetics, thermodynamics, and the properties of the reaction mixture throughout the reaction. Table 1 lists characteristic properties of the reactors discussed above, which can serve as guidelines for preliminary reactor selection. Experimental techniques and correlations for evaluation of the various parameters can be found in, for instance Refs 1, and 4-6, and references cited therein. 

The more catalyst present per unit volume of reactor, the higher the rate of reaction can become (expressed as amount converted per unit reactor volume). Therefore, intrinsically slow reactions (reactions that are determined by the kinetic regime, and not by mass transfer) are usually best conducted in a reactor with a large volume fraction of catalyst, such as the trickle-bed reactor. With very active catalysts, on the other hand, where mass transfer dictates the rate of the overall process, slurry reactors are more suitable. 

The rate of mass transfer between gas and liquid, determined by the product of the gas-liquid interfacial area, a, and the mass transfer coefficient, kj, is an important parameter many heterogeneously catalyzed gas-liquid reactions are limited by mass transfer of the gaseous reactant. The greater the product a k, the faster is mass transfer, and therefore, the observed rate of reaction for reactions in which mass transfer is the controlling step, i. e. for intrinsically fast reactions. The largest can be achieved in stirred-tank reactors and jet-loop reactors, so 

Reactor Stirred tank Bubble column Jet-loop 3-Phase fluid bed Trickle-bed 3-Phase Monolith 

Catalyst Mobile small particles 1-200 pm Mobile particles 0.1-5 mm Spheres, extrudates, etc. 1.5-6 mm Blocks with channels covered with catalyst layer 10-150 pm 

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During the manufacturing process, if the grafting increases during early stages of the reaction, the phase volume will also increase, but the size of the particles will remain constant [146-148]. Furthermore, reactor choice plays a decisive role. If the continuous stirred tank reactor (CSTR) is used, little grafting takes place and the occlusion is poor and, consequently, the rubber efficiency is poor. However, in processes akin to the discontinuous system(e.g., tower/cascade reactors), the dispersed phase contains a large number of big inclusions. 

Figure 5.6 Reactor choice for parallel reaction systems. (From Smith R and PetelaEA, 1991, The Chemical Engineer, No. 509/510 12, reproduced by permission of the Institution of Chemical Engineers).

Qualitative Considerations for Reactor Choice, Scaleup and Optimization... 

Before the details of a particular reactor are specified, the biochemical engineer must develop a process strategy that suits the biokinetic requirements of the particular organisms in use and that integrates the bioreactor into the entire process. Reactor costs, raw material costs, downstream processing requirements, and the need for auxiliary equipment will all influence the final process design. A complete discussion of this topic is beyond the scope of this chapter, but a few comments on reactor choice for particular bioprocesses is appropriate. 

Most chiral chemicals are relatively small-scale products (1 to 1000 tonnes per year for pharmaceuticals, 500 to 10000 tonnes per year for agrochemicals) that are usually produced in multipurpose batch equipment This is probably the case for most catalytic reactions described in this chapter however, as a rule very little information on process technology is provided by the manufacturers. Here, we will discuss only briefly the reactor choices for hydrogenation reaction typically carried out in the liquid phase. For a successful implementation the following demands must be met ... 

The book is on kinetics, not reaction engineering It focuses on reactor-independent behavior, that is, on reaction rates under given momentary and local conditions (concentrations, temperature, pressure). Reactor-dependent, global behavior is included only to the extent necessary for evaluation of kinetic experiments, which, of course, require reactors, and in a few instances in which vagaries of multistep kinetics produce uncommon behavior or impact reactor choice. 

The rules have implications for reactor choice and operating conditions in situations in which a desired product undergoes subsequent decay. If the desired reaction is of higher overall order than the decay, selectivity is better in a batch or tubular reactor than in a continuous stirred tank, and in batch or tube it is better at higher charge or feed concentrations. On the other hand, if the decay is of higher order than the desired reaction, the opposite is true. 

Electricity generated at nuclear power stations presently accoimts for some 8.4 EJ y or 2% of global energy use (USDoE, 2003). The technology used is primarily light water reactors, a commercial spin-off from the submarine nuclear-powered propulsion systems introduced in the 1950s. The situation after World War II was characterised by two factors of some importance for the development of nuclear energy and the specific reactor choice ... 

The representation of decision models on the instance level is still an open issue. The simple modeling examples in Figs. 2.33 - 2.35 suggest that increasing complexity hinders the readability of larger and more realistic models, as indicated by a set of decision models addressing the reactor choice in the IMPROVE reference scenario (cf. Subsect. 1.2.2) [228]. Decomposition of decision models into clearer parts is not straightforward due to the complex network... 

The performances of the three reactors are compared for different parameters in Figure 11.40 for a first-order reaction. The importance of the decay parameter X is clearly evident and provides a strong point (among many other issues to be considered) in reactor choice. Sadana and Doraiswamy (1971) and Prasad and Doraiswamy (1974) have extended the comparisons to non-first-order and complex reactions. 

The second step results in a kinetic model for the whole reaction system and a choice of an appropriate reactor based on the reaction kinetics. Criteria for reactor choice will be discussed in Sect. 7.5.1. 

Besides the classical engineering question of reactor choice, the most important point in enzyme reactor design is the aspect of enzyme reuse, either by immobilization or by separation from the product stream. Batch processes without enzyme reuse are only possible if the costs of the biocatalyst are negligible. Different reactor techniques addressing the aspect of enzyme reuse are discussed in the following sections. 

It is a good idea to leave reactor choice to the person to perform the experiments He ll do best with what he is familiar with. Whatever reactor is chosen, the reaction should be conducted at constant temperature, if at all possible. A variation of temperature in the course of the reaction makes the evaluation much more difficult and less reliable. 

We split up the problem of reactor selection into three sub-problems. By making decisions regarding these three separate attributes of the reactor, we obtain the final reactor choice. These three subsets of the reactor are discussed next. 

Table I0.3.a-I Optimum reactor choice for consecutive and parallel reactions. 

The examples of the hydrogenation of glucose to sorbitol and of esters to alcohols demonstrate the dilemma of reactor choice. Formerly, suspension reactors with Raney nickel or copper chromite catalysts were used, but today trickle-bed reactors with novel noble metal catalysts are preferred. The following advantages are claimed for the trickle-bed reactors ... 

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