Reaction mixing sensitive

The mixing in the first five cells is illustrated in Figure 15.16. In the fifth cell, after 50 ms, the reactants are very weU mixed. After the fifth ceU, only the temperature must be controlled to keep the reactants weU mixed [30]. The very good mixing properties were also verified with a mixing-sensitive reaction, that is, the mixing-sensitive diazo coupling between 1-naphthols, 2-Naphthols, and diazotized sulfanilic acid [30]. 

We will revisit this topic in Section III when discussing CFD models for mixing-sensitive reactions. Note that while the discussion above applies to RANS turbulence models, the method can be extended to LES by integrating over the SGS wavenumbers (i.e., starting at kc). 

The failure of first-order moment closures for the treatment of mixing-sensitive reactions has led to the exploration of higher-order moment closures (Dutta and Tarbell 1989 Heeb and Brodkey 1990 Shenoy and Toor 1990). The simplest closures in this category attempt to relate the covariances of reactive scalars to the variance of the mixture fraction (I 2). The latter can be found by solving the inert-scalar-variance transport equation ((3.105), p. 85) along with the transport equation for (f). For example, for the one-step reaction in (5.54) the unknown scalar covariance can be approximated by... 

Similarly, when moving from the pilot plant to manufacturing, a process engineer will either choose an existing vessel or specify the design criteria for a new reactor. A necessary condition for operation with a specified reactor temperature profile is that the required jacket temperature is feasible. We have therefore chosen to focus on heat transfer-related issues in scale-up. Clearly there are other scale-up issues, such as mixing sensitive reactions. See Paul [1] for several examples of mixing scale-up in the pharmaceutical industry. 

Guidelines to minimize yield loss in mixing-sensitive reactions on scale-up from bench-scale to industrial reactors. 

The rates of a reaction are determined by rate constants, concentrations of reactants, and temperature. At a given temperature, the rate of a second-order reaction depends on the product of the rate constant and the concentrations of the reactants. The concentration at which the chemical reaction becomes faster than the mixing is the critical concentration at which conversion and yield will be affected by mixing. It must be emphasized that the relative reaction rate at a point in the reactor is proportional to the product of the rate constant and the local concentration. It is recommended that bench and pilot data for mixing-sensitive reactions can be obtained at the same concentrations as they are to be used in the commercial plant. This eliminates concentration as a concern in scale-up. 

The importance of feed rate on yield for a mixing-sensitive reaction was demonstrated in Ref. ". The addition time in a semibatch reaction is often increased on scale-up because of heat transfer limitations. In the case of a mixing-sensitive reaction, the time of addition is increased on scale-up to compensate for the increase in blend time and to maintain the expected molar ratio at the feed point. The minimum feed time to achieve the expected yield is, therefore, scale dependent. Feed times that are too short will result in mesomixing conditions and reduced yield. 

If experiments show a possibility of mixing-sensitive reactions and the rate of addition is important, consider multiple point injections. The feed time will have to be increased in large-scale equipment. 

Boltersdorf, U., Deerberg, G., and Schluter, S. (2000) Computational studies of the effects of process parmeters on the product distribution for mixing sensitive reactions and on distribution of gas in stirred tank reactors. Recent Res. Dev. Chem. Eng, 4, 15. 

Figure 13-11 Mixing configuration for semibatch reactors for mixing-sensitive reactions when in-line reaction systems are not viable. Note the feed directly into the region of highest turbulence and the second impeller used to maintain turbulence and flow in the top third of the tank.

Generally, it is recommended that bench and pilot data for mixing sensitive reactions be obtained at the same concentrations as are to be used in the commercial plant. That eliminates concentration as a concern in scale-up. 

The main benefits of small characteristic dimensions are that they enable large heat and mass gradients compared to conventional equipment leading to significant acceleration of heat and mass transfer rates ]2], Higher selectivity and yields have been observed in particular for mixing-sensitive reactions, and the improved thermal control of exothermic reactions allows safe operation of processes not accessible with conventional equipment due to their restriction in heat transfer capabilities. 

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