Reaction and Mixing Times

Establishing the process sensitivity with respect to the above-mentioned factors is crucial for further scale-up considerations. If the sensitivity is low, a direct volume scale-up is allowed and the use of standard batch reactor configurations is permitted. However, many reactions are characterized by a large thermal effect and many molecules are very sensitive to process conditions on molecular scale (pH, temperature, concentrations, etc.). Such processes are much more difficult to scale up. Mixing can then become a very important factor influencing reactor performance for reactions where mixing times and reaction times are comparable, micromixing also becomes important. 

Table 5.4-24 summarises the various characteristic time constants for reaction and mixing. Instantaneous (very rapid), fast (rapid) and slow (very slow) reactions have been classified based on characteristic time constants (time scales) for mixing and reaction. Denoting the mixing time scale by xm (t99, to, xms, xds, or x ,) reactions can be classified as follows from the viewpoint of competition with individual stages of mixing tm xf => instantaneous TM XR => fast TM XR => slow... 

The intra-cell processes are common to all PDF codes, and are treated the same in both Eulerian and Lagrangian PDF codes.8 On the other hand, inter-cell processes are treated differently in Eulerian PDF codes due to the discrete representation of space in terms of x . In PDF codes, fractional time stepping is employed to account for each process separately. Methods for treating chemical reactions and mixing are described in Section 6.9. Thus we will focus here on the treatment of inter-cell processes in Eulerian PDF codes. 

The competition between reaction and mixing is given by the Damkoehler number Da), which is the ratio between the reaction rate and the local mixing rate, or conversely, the ratio of the characteristic local mixing time tm and the reaction time tr ... 

Mixing time can be varied as discussed above, but reaction and induction time are both system specific. Reaction time and induction time are both concentration dependent, and induction time also is a function of supersaturation (t, j decreases with increasing 5). 

Add 20-pL precipitation mixtures to 5-pL Snapshot reactions and mix three times. 

In a mixture of ideal gases the sound velocity and consequently the resonant frequencies of a resonator depend on the effective specific heat ratio and the average molecular mass of the mixture. Chemical reaction and mixing processes, for example, are normally accompanied with a change of these properties. Therefore, such dynamic processes can be monitored by a repeated measurement of the shift of one of the eigenfrequencies of the resonator as a function of time. 

In order to work safely with multi-injection reactor, an accumulation of the limiting reactant and of heat in the main channel has to be thoroughly prevented. As the considered reactions are mostly Umited by mixing, the time required to mix can be estimated by using the correlation between specific energy dissipation and mixing time. Sufficient residence time should be provided between two injection points to minimize the hot spot. 

The preparation of TPVs involves chemical reactions and mixing of a crosslinldng agent. When TPVs are prepared in a twin-screw mixer, one has to make sure that the crosslinking reaction is complete. The residence time in the twin-screw extruder depends upon several variables, such as ... 

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