The Heterogeneous Reactor

As has been mentioned already (see Section 2.4) the resonance capture by in a homogeneous natural uranium reactor is too great to permit even an infinitely large system of this type to attain criticality the maximum possible value of for the reactor is in fact of the order of 0.74. It was 

For a natural uranium reactor, the uranium is typically in the form of cylindrical rods which are located in a regular lattice array, usually in a square or hexagonal geometry, the space between the rods being occupied by moderator. The principal effects on the infinite multiplication factor as a result of adopting the heterogeneous arrangement are as follows. 

In order to take pronounced fine structure effects into account in calculations of the physics of a reactor core, the periodic array of the reactor lattice is divided into a number of identical cells and the fine structure of the flux distribution in each cell is calculated using transport theory. This can then be used to generate cell-averaged group constants which may be used in a multigroup calculation for the whole reactor core. 

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Designed to obtain such fundamental data as chemical rates free of mass transfer resistances or other complications. Some of the heterogeneous reactors of Fig. 23-29, for instance, employ known interfacial areas, thus avoiding one uncertainty. 

The heterogeneous reactors with supported porous catalysts are one of the driving forces of experimental research and simulations of chemically reactive systems in porous media. It is believed that the combination of theoretical methods and surface science approaches can shorten the time required for the development of a new catalyst and optimization of reaction conditions (Keil, 1996). The multiscale picture of heterogeneous catalytic processes has to be considered, with hydrodynamics and heat transfer playing an important role on the reactor (macro-)scale, significant mass transport resistances on the catalyst particle (meso-)scale and with reaction events restricted within the (micro-)scale on nanometer and sub-nanometer level (Lakatos, 2001 Mann, 1993 Tian et al., 2004). 

The effect of different design, operating and physico-chemical parameters on the optimal feed temperature policies of the heterogeneous reactor are presented in this section. Table 7.3 gives base values of the parameters. 

Determining quantum yields requires the assessment of Pa the rate of absorbed photons in the heterogeneous reactor (Cabrera et al., 1996). Semiconductor surfaces are highly reflective (Fox and Dulay, 1993) and therefore eiTors can arise from light back-scattering or forward-scattering from the catalyst particles (Valladares and Bolton, 1993). 

The latter model type describes the experimentally determined relations between dependent and independent variables with the help of statistical methods and neglects the known physicochemical relations. Such models are primarily used on reactor types difficult to describe deterministically. The cell models are composed of specific networks of mixing cells (e.g. stirred reactor cascades) or of combinations of mixing cells and transport cells (ideal tube reactors). The so-called continuum models, however, handle each phase as a continuum. The continuum models are further distinguished as homogeneous and heterogeneous reactor models. In the heterogeneous reactor model, the fluid phases and the solid phase (catalyst) are considered and mathematically described as individual items. 

Unlike homogeneous reactors in which all the reacting chemical compounds are present in one single phase, in the heterogeneous reactors, the reactants are distributed in different phases. Heterogeneous reactors are multiphase reaction vessels that are designed to handle reachons occurring between chemical compoimds present in two or more different phases. Consider a chemical reaction between compoimd A in phase PI and compound B in the phase P2. 

As gas and solid phases are involved in this reaction, any gas-solid-containing equipment such as a fluidised bed can be used to carry out this reaction. Here, the heterogeneous reactor is the fluidised bed reactor (Figure 2.30). 

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