Reactors for Homogeneous Systems

The difference increases with higher conversions and higher reaction orders. 

For series reactions as A— P— R when the goal is the maximization of the intermediate, 

For parallel reactions as A— P, A— R the objective is a desired product distribution. 

Complex reactions can be analyzed by means of simple series and parallel reactions. 

For first-order series-parallel reactions the behavior as series reactions dominates. 

---

Table 2.9 Heuristics Selection of reactor for homogeneous systems.

Reactors Often yes Direct scale-up from the laboratory to the full scale often possible for homogeneous systems. 

So far we have concentrated on homogeneous reactions in ideal reactors. The reason is two-fold because this is the simplest of systems to analyze and is the easiest to understand and master also because the rules for good reactor behavior for homogeneous systems can often be applied directly to heterogeneous systems. 

Gassy systems are untempered. This means that the operation of the relief system cannot control the rate of the runaway reaction, but simply acts to remove material from the reactor. For untempered systems, homogeneous flow in the reactor (see... 

Equation (8.4) is suitable for homogeneous system, equation (8.5) is useful for fluid/solid catalyst reactors, while equation (8.6) is applicable for gas/liquid or liquid/liquid reactors. 

The calculation principle on which the assessment of design for such reactors is based is a substitution of the multi-phase reaction system by a quasi-single-phase model. In two-phase systems both reactants have to get into contact at a certain place. Consequently a reaction and a transport phase are distinguished. If the mass transfer rate from the transport to the reaction phase is veiy fast compared to the actual reaction rate, the process in total is dominated by the reaction kinetics. In order to discriminate this situation from one taking the mass transfer into account, it is referred to as micro-kinetically dominated In this ease all formal kinetic laws presented for homogeneous systems may be applied directly. 

A high scale-up factor in Table 2.1 reflects the ease in scahng up of the reactor. It can be seen that heterogeneons/mnltiphase reactors have substantially smaller scale-up factors than those for homogeneous systems. This is due to the complex nature of scale-dependent diffnsional processes (Section 2.4). 

As interest in large fast reactors began to accelerate after 1960, the importance of Doppler effect became more and more evident, and several groups began to make more careful calculations and re-examine the theoretical basis of the methods. At least for homogeneous systems, this phase of redevelopment of the theory is nearly complete, and there are... 

The current progress of computational fluid dynamics (CFD) enables computational experiments in a reactor equipment to reveal file RTD. A lot of commercial software has recently been developed to carry out CFD calculations, particularly for homogeneous systems, such as CFX and Huent. Typically CFD is used for non-reactive fluid systems, but nowadays even reactive systems can be computed (Baldyga and Bourne 1999). The ultimate... 

Fig. 10-13. Equilibrium fuel concentrations and reactor dimensions for homogeneous systems operating at 280°C and producing 125 Mw electrical power.

These component balances are conceptually identical to a component balance written for a homogeneous system. Equation (1.6), but there is now a source term due to mass transfer across the interface. There are two equations (ODEs) and two primary unknowns, Og and a . The concentrations at the interface, a and a, are also unknown but can be found using the equilibrium relationship, Equation (11.4), and the equality of transfer rates. Equation (11.5). For membrane reactors. Equation (11.9) replaces Equation (11.4). Solution is possible whether or not Kjj is constant, but the case where it is constant allows a and a to be eliminated directly... 

The interpretation is straightforward. At reaction conditions the concentration in the film is lowered by reaction, and, as a consequence, the driving force for mass transfer increases. In a homogeneous system this results in high values of Ha. In a slurry reactor this enhancement can occur if the catalyst particles are so small that they accumulate in the film layer. Table 5.4-4 summarizes expressions for the reaction rate or enhancement factor for various regimes. 

In practice, it is often possible with stirred-tank reactors to come close to the idealized mixed-flow model, providing the fluid phase is not too viscous. For homogenous reactions, such reactors should be avoided for some types of parallel reaction systems (see Figure 5.6) and for all systems in which byproduct formation is via series reactions. 

---